ELECTROSLAG WELDING: THE EFFECT OF SLAG COMPOSITION

ON MECHANICAL PROPERTIES

b y

JAMES S. MITCHELL B . S c , Queens U n i v e r s i t y a t K i n g s t o n , 1973

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPL IED SCIENCE

I n t h e Depa r tmen t

o f

METALLURGY

We a c c e p t t h i s t h e s i s a s c o n f o r m i n g

t o t h e r e q u i r e d s t a n d a r d

THE UNIVERSITY OF BR IT ISH COLUMBIA

A u g u s t 1977

© James M i t c h e l l , 1977

In p resent ing t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements fo r

an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that

the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study.

I f u r t h e r agree t h a t permiss ion for e x t e n s i v e copying of t h i s t h e s i s

f o r s c h o l a r l y purposes may be granted by the Head of my Department or

by h i s r e p r e s e n t a t i v e s . I t i s understood that copying o r p u b l i c a t i o n

of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be al lowed without my

w r i t t e n p e r m i s s i o n .

Department of

The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5

Date 7

ABSTRACT

Previous studies of the properties of electroslag weld metal have

been done using electroslag remelted ingots made under welding conditions.

This procedure assumes the e l ec tr i ca l and thermal regimes of these pro­

cesses to be equivalent. To test this assumption an experimental program

was devised in which the remelted metal of an ingot and weld made with

each of three slag systems was analysed and the mechanical properties

examined. The results show that each process imparted different properties

to the remelted metal by alloy and inclusion modification. Consequently

the above assumption was proved inval id . Special consideration was given

to the effect of inclusion composition and overall distr ibution toward

mechanical properties.

i i

TABLE OF CONTENTS

Page

ABSTRACT . i i

TABLE OF CONTENTS . . i i i

L I ST OF FIGURES v i

L I ST OF TABLES v i i i

L I ST OF SYMBOLS i x

ACKNOWLEDGMENTS . . . x

C h a p t e r

I INTRODUCTION 1

1.1.1 The ESW P r o c e s s 1

I . 1 .1 .1 A p p l i c a t i o n s o f ESW . . . . . 1

1.1.2 P r o p e r t i e s o f ESW . 4

1.1.3 G e n e r a l P r o p e r t i e s o f Welds . . . . . . . . . . . . . 6

1 .1 .3 .1 Weld M e t a l 6

1 .1 .3 .2 The HAZ . . 8

1.2.1 S l a g C o m p o s i t i o n 9

1 .2 .1 .1 L i q u i d S l a g C h e m i s t r y 9

1 .2 .1 .2 S l a g R e a c t i o n s 10

1 .2 .1 .3 S l a g E l e c t r o c h e m i c a l R e a c t i o n s 16

1 .2 .1 .4 C o m p a r i s o n o f ESR-ESW

S l a g R e q u i r e m e n t s . . . 17

1 .2 .1 .5 S l a g C h a r a c t e r i s t i c s 19

1 .2 .1 .6 W e l d i n g C o n s i d e r a t i o n s 21

i i i

C h a p t e r Page

1.2.2 I n c l u s i o n s and M e c h a n i c a l P r o p e r t i e s 22

1 .2 .2 .1 I n c l u s i o n s 26

1 .2 .2 .2 M e c h a n i c a l P r o p e r t i e s 29

1.2.3 Gas P o r o s i t y 31

1.2.4 Heat D i s t r i b u t i o n and S t r u c t u r e 33

1.3.1 Summary 35

1.3.2 S t a t e m e n t o f t h e P r o b l e m 37

1.3.3 E x p e r i m e n t a l 38

I I EXPERIMENTAL AND RESULTS . 40

I I . 1 M a t e r i a l s . 40

I I . 1 . 1 S t e e l 40

I I . 1.2 S l a g 40

1 1 . 1 . 2 . 1 C a F 2 43

1 1 . 1 . 2 . 2 A 1 2 0 3 . 43

1 1 . 1 . 2 . 3 CaO 43

I I . 1.2.4 S i 0 2 43

I I . 2 A p p a r a t u s 44

I I . 3 P r o c e d u r e 46

1 1 . 3 . 1 C h e m i c a l A n a l y s i s 46

1 1 . 3 . 2 M e t a l l o g r a p h i c A n a l y s i s 50

11 . 3 . 3 M e c h a n i c a l T e s t i n g . . . 56

I I I DISCUSSION 71

I I I . l S l a g E f f e c t s on A l l o y C o m p o s i t i o n 71

I I I . 2 S l a g E f f e c t s on I n c l u s i o n P o p u l a t i o n 76

i v

C h a p t e r P a e e

111.3 I n c l u s i o n D i s t r i b u t i o n 83

111.4 M e c h a n i c a l P r o p e r t i e s 86

IV CONCLUSIONS 92

BIBLIOGRAPHY 9 5

APPENDIX I 98

APPENDIX I I 1 0 1

APPENDIX I I I • 102

v

LIST OF FIGURES

F igu re Page

1 Schematic of E l e c t r o s l a g Welding Process 2

2 H y d r a u l i c P ress Housing 3

3 E l e c t r o s l a g P r o d u c t i o n of a Rotor 3

4 H o r i z o n t a l C r o s s - s e c t i o n of M e l t Bw 7

5 E f f e c t of S lag B a s i c i t y on R e s u l f u r i z a t i o n R a t i o . . 13

6 S lag B a s i c i t y vs S u l f u r Content of ESR Ingots f o r V a r i o u s Remel t ing Power Modes 15

7 E l e c t r o d e T i p Current D e n s i t y vs F i l l R a t i o of most E l e c t r o s l a g Processes 18

8 R e l a t i o n s h i p of S h e l f Energy and Volume F r a c t i o n of I n c l u s i o n s 23

9 E f f e c t o f Second Phase Volume F r a c t i o n on T o t a l S t r a i n to F a i l u r e 24

10 E f f e c t of I n c l u s i o n Shape on Mechan ica l A n i s o t r o p y of an as R o l l e d , Low Carbon S t e e l 25

11 Free Energy of Format ion of Some M e t a l S u l f i d e s . . 27

12 Example of C e n t r e - l i n e C r a c k i n g 34

13 Macros t ruc tu re of an ESR Ingot . 36

14 Exper imenta l ESW Apparatus Used 39

15 P u b l i s h e d S p e c i f i c a t i o n s of A l l o y Welten 80-C . . . 41

16 E l e c t r o d e Guide Assembly, Schematic 45

17 Schematic of ESW C o n f i g u r a t i o n Before S t a r t i n g , S lag Removed . . . . . . 47

18 S e c t i o n i n g of ESR Ingot and ESW M e t a l f o r M e t a l l o g r a p h i c and Chemical A n a l y s i s 49

v i

Figure Page

19 Inclusion Distribution of ESR Ingots 52

20 Inclusion Distribution of ESW Metal 53

21 Apparatus Used to Deep Etch Samples for S.E.M. Survey . . 55

22 Orientation of Impact Specimens Cut from ESR

Ingot 59

23 Orientation of Impact Specimens Cut from ESW . . . 60

24 to 29 Impact Energy, Lateral Contraction and Fracture

Appearance Data 61 to 66

30 Instrumented Impact Oscillograph and Tracing . . . 68

31 Anodic Polarization Curves for Pure Iron 75 32 Relationship of [Al] and [0] in Equilibrium with

Alumina of Various Act iv i t i e s 78 33a X-Ray Energy Analysis of an Inclusion in C i . . . . 89

33b X-Ray Energy Analysis of an Inclusion in Cw . . . . 90

v i i

LIST OF TABLES

Table Page

1 Example of Weld Metal Properties of Heavy Section Joints . . . . . 5

2 Commercial Fluxes used for Electroslag Processes 20

3 Chemical Analysis of Alloy Welten 80-C, as

Received 42

4 Remelting Conditions of Ingots and Welds . . . . . . 48

5 Chemical Analysis of Ingot and Weld Metal 51

6 Inclusion Area Fraction, Spacing and Diameters . 54

7 Inclusions Observed on Deep Etched and Ductile

Fracture Surfaces 57

8 Observed Morphology of MnS Type Inclusions 58

9 Instrumented Impact Energies of CVN Specimens

Tested at 170°C 70

10 Electroactive Surface Areas . 73

11 Summary of Impact Test Data 87

12 Value of. n for I n i t i a l Slag. Compositions 101

v i i i

PRINCIPAL SYMBOLS

A in metal phase

A in slag phase

A as a pure l iquid

A as a pure sol id

act iv i ty of specie A in slag

act iv i ty of specie A in metallic solution

free energy change

standard free energy change

free energy change at specified temperature

equilibrium constant

parts per mi l l ion

temperature

weight percent

ix

ACKNOWLEDGMENTS

I w o u l d l i k e t o s i n c e r e l y t hank my r e s e a r c h s u p e r v i s o r s

D r . A . M i t c h e l l and D r . E.B. Hawbo l t f o r t h e i r a s s i s t a n c e and g u i d a n c e

t h r o u g h o u t t h i s s t u d y .

The f i n a n c i a l a s s i s t a n c e o f a N a t i o n a l R e s e a r c h C o u n c i l S c h o l a r s h i p

i s g r a t e f u l l y a c k n o w l e d g e d .

x

CHAPTER I

I n t r o d u c t i o n

I.1.1. The E l e c t r o s l a g Welding Process

E l e c t r o s l a g we ld ing (ESW) i s the d e p o s i t i o n o f a f i l l e r meta l s u p p l i e d

as an e l e c t r o d e through a l i q u i d s l a g to e f f e c t f u s i o n of m e t a l l i c members,

u s u a l l y p l a t e s , see f i g u r e 1. Energy f o r the process i s d e r i v e d from

e l e c t r i c r e s i s t a n c e hea t ing o f the l i q u i d s l a g . The cu r r en t pa th i s

between the e l e c t r o d e and parent m e t a l l i c p l a t e s .

In p r a c t i c e the process i s l i m i t e d to a few o r i e n t a t i o n s and e l e c t r o d e

c o n f i g u r a t i o n s . Weld meta l i s l a i d i n p l ace by g r a v i t y , thus we ld ing can

o n l y proceed i n the v e r t i c a l or near v e r t i c a l d i r e c t i o n . The e l e c t r o d e

must be of s u f f i c i e n t c ross s e c t i o n to accommodate l a r g e e l e c t r i c cu r r en t s

and supply weld meta l to the j o i n t . T h i s f a c t o r and a l s o the a l ignment

i n the gap, to prevent shor t c i r c u i t i n g are c r i t i c a l . These c o n d i t i o n s set

a minimum to the weld gap and m a t e r i a l t h i cknesses tha t may be used.

However there i s no maximum gap or t h i c k n e s s a p p l i e d to the p roces s .

Thus e l e c t r o s l a g we ld ing i s p r i m a r i l y used i n j o i n i n g m a t e r i a l of l a r g e

c ross s e c t i o n w i t h one pass .

I.1.1.1. A p p l i c a t i o n s of ESW

E l e c t r o s l a g we ld ing has been used f o r the assembly of c a s t i n g s to

produce l a r g e machine p a r t s . H y d r o e l e c t r i c t u r b i n e s , pump housings and press

frames have been cas t i n s e c t i o n s and welded i n t o f i n a l p roducts ( F i g s . 2 , 3 ) .

1

2

ELECTRODE

RUN - OUT BLOCK

PARENT PLATE

SLAG BATH

METAL POOL

WELD METAL

STARTING SUMP

FIG. I •• S C H E M A T I C OF E L E C T R O S L A G

WELDING P R O C E S S .

FIG. 2 : HYDRAUL IC P R E S S HOUS ING. R E F . 4 3 .

FIG. 3= E L E C T R O S L A G P R O D U C T I O N OF A

ROTOR . R E F . 5 4 .

4

Also the fabrication of rotors has been accomplished by electroslag welding

a series of electroslag refined ingots prior to forging, eliminating much

of the waste encountered in conventional production practice.

As with a l l technology popular with industry, the reason for the

adaptation of electroslag welding i s economic. A weld of large cross

section may be constructed with a one pass technique resulting in a saving

of welding hours. The soft thermal profi les characteristic of the process

reduces the need for preheating. These advantages, despite the necessity

of a post heat treatment required in some cases, are the reason why the I

process is being used. l'

1.1.2. Properties of ESW

Mechanical properties required of large section welds are different

from those of thinner welds. Tensile strength and elongation specifications

vary with weld metal thickness, see table 1, usually the greater the weld

section, the lower are these requirements. Bend tests are also performed

on welded structures. This is a simple estimate of weld performance and

is confined to thinner weld assemblies. The results from this type of test

are empirical, the outer fiber elongation and nature of cracking only

are reported. Another test often specified for welds is the impact test.

This test requires that a minimum absorbed impact energy be obtained from

areas within the welded joint at specified temperatures. Again although

this test is empirical, some relations to structurejand processing

have been correlated.

As a result of the so l id i f i cat ion sequence i t experiences, weld metal

has a cast structure. Of this , the grain size and nature of non-metallic

TABLE I : EXAMPLE OF WELD METAL PROPERTIES OF HEAVY SECTION

JOINTS. FROM REF. I .

ASTM A 242 or A44I ASTM A58 I 3

MATERIAL (in.) THICKNESS (mm)

<75 < 19

.75-1.5 19-38

1.5-4 38-102

4 - 8 102- 203

< 4 < 102

4 - 5 102-127

5 - 8 127-203

MIN. lO'PSI U.T.S. I 0 e Kg/m2

70 49

67 47

63 44

60 42

70 49

67 47

6 3 44

MIN. IO sPSI Y. S. I O e Kg/m2

50 35

46 32

42 30

40 28

50 35

46 32

42 30

MIN. ELONGATION % 22 22 2 4 ' 2 4 ' 2 1 ' 21 ' 21 '

DEDUCT 0 . 5 % FROM ELONGATION ( UP TO 3.0 % ) FOR EVERY

0.5 in.(l3mm) INCREASE IN THICKNESS ABOVE 3.5 in ( 89mm) .

6

inclusions have a major influence on mechanical properties. Since joint

design and metal composition may alter the weld structure, they are chosen

in such a way that the welded joint meets the required minimum

specifications.

1.1.3. General Properties of Welds

Welding of steel is often performed to jo in materials that have been

shaped by plast ic deformation. The weld metal structure is then very

different from that of the parent material (Fig. 4). The weld metal was

l iquid when la id in place and w i l l have a cast structure. The thermal

cycle imposed by this process onto the parent metal w i l l result in a heat

affected zone (HAZ) adjacent the weld metal. The structures of this zone

are a function of the heat treatment properties of the parent steel and

the amount of thermal input to the welding process.

1.1.3.1. Weld Metal

The sequence of events that occur in the weld pool after i t has been

la id in place can be described by so l id i f i cat ion theory. Heat i s lost from

the pool to the joint and other heat sinks, causing so l id i f i ca t ion to

in i t i a t e from these areas. As grains grow' into the weld pool, segre­

gation of some constituents in solution occurs and these compounds are last

to freeze. The resulting structure of electroslag weld metal consists of c o l ­

umnar grains with non-metallic inclusions situated in the interdendritic spaces.

Other inclusions may precipitate from sol id solutions as the metal cools

leaving an even dispersion of fine particles throughout the melt.

7

FIG. 4= HORIZONTAL C R O S S - S E C T I O N OF MELT B w

8

P r o p e r t i e s of weld meta l are r e l a t e d to the cas t s t r u c t u r e .

C h a r a c t e r i s t i c o f t h i s i s r e l a t i v e l y h i g h d u c t i l e to b r i t t l e t r a n s i t i o n

temperature i n d i c a t i n g b r i t t l e f r a c t u r e b e h a v i o r .

1 . 1 . 3 . 2 . The HAZ

The heat a f f ec t ed zone of a weld i s not sub jec t to the chemica l p r o ­

p e r t i e s of the s l a g as i s the weld m e t a l . However i t i s a product of the

thermal schedule of the we ld ing p roces s . Thus when p r o p e r t i e s of a welded

s t r u c t u r e are determined t h i s zone i s a l s o ana ly sed .

The HAZ may be subd iv ided i n t o two main r e g i o n s . The area adjacent

the weld m e t a l , which i s exposed to temperatures r ang ing between the s o l i d u s

and A ^ , w i l l c o n t a i n s t r u c t u r e s o f r e c r y s t a l l i z a t i o n and g r a i n growth. In

e l e c t r o s l a g we ld ing the area near the weld i s exposed to s u f f i c i e n t thermal

energy and t ime tha t some i n c l u s i o n s w i l l d i s s o l v e and r e p r e c i p i t a t e upon

c o o l i n g . Other i n c l u s i o n s , the more r e f r a c t o r y v a r i e t y , may spherodize or

remain unchanged. In e i t h e r case , the r e s u l t i s a coarse r g r a i n s i z e and

i n c l u s i o n p o p u l a t i o n .

For the a l l o y o f t h i s r e g i o n the c o o l i n g c y c l e of the we ld ing procedure

w i l l l e ad to a m i c r o s t r u c t u r e which may be de sc r i bed by the cont inuous

c o o l i n g t r ans fo rma t ion temperature (CCT) b e h a v i o r . The l o n g thermal c y c l e s

of e l e c t r o s l a g we ld ing r e s u l t i n sha l low quenching g r a d i e n t s . When these

are superimposed onto the a p p r o p r i a t e CCT diagram, the m i c r o s t r u c t u r e , i n

theory , may be p r e d i c t e d d i r e c t l y . T h i s s t r u c t u r e , r e s u l t i n g from the so f t

thermal p r o f i l e s , and the a l t e r e d i n c l u s i o n morphologies combine and r e s u l t

i n a genera l d e r a t i n g o f p r o p e r t i e s i n t h i s a r e a .

9

The second r e g i o n of the HAZ i s tha t which i s exposed to temperature

below A l . The s t r u c t u r e here r e f l e c t s the e f f e c t o f tempering neares t the

warm annealed r e g i o n to s l i g h t r ecovery f u r t h e r from the w e l d .

1 . 2 .1 . S l ag Composi t ion

Reac t ions between s l a g and me ta l have been the sub jec t of many i n ­

v e s t i g a t i o n s i n t o s t e e l r e f i n i n g p r a c t i c e . The i n f l u e n c e of s l a g compos i t ion

2

on s t e e l chemis t ry i s w e l l understood . Consequently the e f f e c t on s t e e l

p r o p e r t i e s imparted by the s l a g v i a the a l l o y compos i t ion i s v e r y impor tan t .

For example, a s l a g may be designed to promote the r e d u c t i o n o f oxygen i n t o

the meta l r e s u l t i n g i n a l a r g e ox ide or gas p o r o s i t y f r a c t i o n i n the s o l i d i f i e d

a l l o y . These c o n s t i t u e n t s l e a d to reduced mechanica l p r o p e r t i e s and are

desc r ibed i n the f o l l o w i n g s e c t i o n s .

Cont ra ry to c o n v e n t i o n a l s tee lmaking p r a c t i c e the i n t e r a c t i o n of s l a g ,

meta l and atmosphere fo r the e l e c t r o s l a g processes i s not w e l l unders tood.

Many thermodynamic parameters such as temperature, hydrodynamic behav io r and

a c t u a l s l a g compos i t ion are d i f f i c u l t to measure. Of these , s l a g compos i t ion

has a major e f f e c t on the l i q u i d s t e e l chemis t ry and the re fo re the meta l

p r o p e r t i e s .

1 . 2 . 1 . 1 . L i q u i d S lag Chemistry

Ca lc ium f l u o r i d e , CaF2» i s the b a s i s of most s l a g systems used fo r the

e l e c t r o s l a g p rocess . I t has the d e s i r a b l e p r o p e r t i e s of good e l e c t r i c a l

c o n d u c t i v i t y , reasonable v i s c o s i t y and w h i l e i t w i l l d i s s o l v e most o x i d e s ,

i t i s not a component of r e d u c t i o n - o x i d a t i o n r e a c t i o n s at e l eva ted tempera­

t u r e s . E s s e n t i a l l y , CaF 9 behaves as an i n e r t so lven t i n t o which s l a g

10

compounds as lime (CaO), s i l i c a (Si02) and alumina (Al^O^) w i l l dissolve.

Once in solution the acid (oxide-ion acceptor) components, s i l i c a and 3

alumina, w i l l undergo strong acid-base reactions to form stable ions . The

s i l i c a reactions, 5102 + 0"2 = S i 0 3 ~ 2 1-1

to

-2 -2 -4 5103 +0 = S i0 4 1-2

and similar alumina reaction,

A1 2 0 3 + 0~2 = 2A102" 1-3

4

are believed to y ie ld ions stable in basic slags . The existence of these

species in CaF 2 melts has been veri f ied by freezing point depression studies

on the appropriate systems"'. The source of oxygen to the above reactions

is the base type slag reactions discussed below.

Basic compounds (oxide-ion donors) dissolve in CaF 2 slag by ionization. +2 -2

Lime in solution exists as the cation Ca and anion 0 . Thus when

describing the influence of each compound on the slag composition, or the

act iv i ty of these compounds in solution, i t i s important to consider the

actual ionic specie present in the melt. 1.2.1.2. Slag Reactions

For the major alloying elements in steel , thermochemical relations

are not sufficient to explain their mass transfer between electrode and

ingot . These elements include Cr , S i , Mn, Co, Cu and Ni and may be

present in quantities large enough to result in fast oxidation-reduction

reactions at the electrode/flux or metal pool/flux interface. In the present

11

work 60 Hz power was u t i l i z e d , thus any direct current effects, such as

gross concentrations gradients in the slag at the various slag interfaces,

do not occur. However i t is postulated that i f the concentration of an oxidiz-

able element in the metal (ingot or electrode) were large enough, the sl ight

60 Hz polarization effect would be sufficient to oxidize that element.

Thus a concentration gradient would develop at the anode. This occurs with

iron by the corrosion reaction

Fe = F e + 2 + 2e~ 1-4

The result of reaction 1-4 is a layer of slag saturated in (FeO) about the

anode^. Since the electrode/flux interface exhibits a smaller interfac ia l

area than the pool/flux interface, i t develops a higher current density.

In effect the electrode/flux interface becomes the s i te of (FeO) saturation

during AC melting. An overal l conclusion from the above phenomenon i s

that oxidation at the anode can not take place i f the anodic potential of

the reaction is greater than that of reaction 1-4. Also, i f surface

depletion of an alloy occurs at the electrode i t i s by a mechanism of

anodic corrosion in preference to iron. Chromium has a lower polarizing

potential than iron and has been observed to develop a depletion gradient

at the electrode tip^. For other major alloying elements, l ike Si and Mn,

that have polarization potentials less than that of i ron , the reaction

rate at the separate slag/metal interfaces i s important. If the concen­

tration of the al loy element is high enough the rate of the appropriate

oxidation-reduction reaction may also be high. When the equilibrium co­

efficient i s near to one, as i t i s for

(FeO) + [Mn] = (MnO) + [Fe] 1-5

12

then the difference in temperature between the electrode and metal pool

w i l l result in oxidation at the electrode and a nearly equal rate of

reduction at the pool interface.

In practice, qualitative predictions of the effect of slag chemistry

can be made using simple thermodynamics. The slag basicity ra t io , or "V"

rat io , applied to slag systems in steelmaking has proved useful in analysing

the effect of slag composition on metal chemistry. Early experiments have

concluded a sulfur distribution factor to be related to the acid and base g

constituents of a slag system (Fig. 5). This data indicates basic slags

lead to low sulfur contents in steel . The same work concluded CaF^ and FeO

contents of a slag and the slag temperature had l i t t l e effect on the de-

sulfurization rat io . Also concluded was that MnO, MgO and CaO are equally 4

good desulfurizers. Reinterpreting these and other results , Turkdogan has

shown sulfur distribution between iron and basic slags may be written as [S] + (O"2) = (S~2) + [0] 1-6

In slags common to the electroslag process the dissolved oxygen in the slag

is related to the (CaO) content. The presence of lime in commercial CaF^ q

systems can not be avoided . Products of the desulfurization reaction,

(CaO) + [S] = (CaS) + [0] 1-7

(CaS) along with calcium (Ca) are highly soluble in CaF 2 slags making them . * 10,11 excellent desulfurxzation systems

Alternately the existence of the less stable oxides l ike (FeO),

(SiO,,) and (MnO) in high proportions in the slag results in an increased oxygen

content of the metal pool 1 0'" 1" 2 . Simple equilibrium thermodynamics indicates

the reactions

1 3

FIG. 5 = EFFECT OF SLAG BASICITY v ON

DESULFURIZATION RATIO. FROM REF. 4 .

14

(FeO) = Fe + [0] 1-8

(Si0 2) = [Si] + 2[0] 1-9

(MnO) = [Mn] + [0] 1-10

are preferred in place of, for example, alumina reduction.

The influence of each component to the slag chemical react iv i ty has

13 14

been described as a function of the acidity ' . Slag compounds considered

to be acidic are those which u t i l i z e the oxygen ion content and produce

more complex metal ions. Reactions 1-1, 1-2 and 1-3 describe acid type

reactions. Basic components are those which increase the oxygen ion

act iv i ty in slags. These include the afore mentioned lime dissolution

reaction CaO = (Ca + 2 ) + (0~2) 1-11

and

MgO = (Mg + 2) + (0 - 2 ) 1-12

The relative influence of these compounds has been empirically established.

In order of increasing acidity and relat ive strength the common acid

constituents are Fe 20.j, 2 A ^ O ^ , 2 S i0 2 and 4 P 20^. The numbers refer to

the moles of base necessary to neutralize one mole of the respective acid

2

compound . CaO and MgO are basic constituents of equal strength and CaF 2

and FeO are considered neutral.

Slag basicity i s only one factor affecting desulfurization, polarity

of the metal pool and electrode w i l l also alter the sulfur removal

mechanism^"* (Fig. 6). In practice electroslag welding i s carried out in

either AC or DC mode, the latter being subdivided into DC electrode

negative (straight polar i ty) , and vice-versa. Electrochemical aspects of

1 5

F I G . 6 S L A G B A S I C I T Y v s S U L F U R C O N T E N T O F

E S R I N G O T S F O R V A R I O U S R E M E L T I N G

P O W E R M O D E S . F R O M R E F . 1 5 .

16

DC remelting have been investigated and in general steels processed this

way are found to be inferior to AC processed steel . In AC remelting the

concentration polarization of ionic slag species at the electroactive

surfaces i s re lat ive ly small. This sl ight polarization is imposed by a

net current rect i f icat ion resulting from a saturated zone of (FeO) in the

slag in contact with the electrode. For DC remelting polarization i s

at a maximum. The ensuing electrochemical reactions result In a large

number of oxide inclusions in the remelted metal and induce the oxidation

of many alloy components.

1.2.1.3. Slag Electrochemical Reactions

16

Careful experiments performed by Be l l have led to the conclusion

that simple chemical reactions between slag and metal cannot account for

many of the compositional changes imparted to processed steels. U t i l i z i n g

c lass ica l thermochemistry the computed equilibrium temperature of a slag

species interacting with the metal i s much higher than temperatures ex-12

periences in the electroslag process . The reduction of alumina is an

example of this . Thus electrochemical reactions were employed and some

workers consider them responsible for as much as 75% of the overal l chemical

interaction.

Another condition supportive of the above observations i s that chemical

equilibrium during electroslag processing i s not attained. The work of

Fraser i s evidence the rate controll ing steps in this process i s mass

6

transport . Thus in electroslag ref ining, a slower process than ESW, true

equilibrium does not occur. Thus thermochemistry cannot account for the

behavior of reactive species during processing, except in giving a qual i ­

tative indication of the direction of a chemical reaction.

1 7

The o v e r a l l e f f e c t o f s l a g r e a c t i o n s r e s u l t s i n an oxygen and s u l f u r

c o n t e n t o f t h e w e l d m e t a l p o o l w h i c h c an be r e l a t e d t o t h e s l a g c h e m i s t r y .

The l e v e l s o f t h e s e components i n t h e m e t a l a r e however n o t c a l c u l a b l e .

1 . 2 . 1 . 4 . ESR-ESW C o m p a r i s o n

I n v i e w o f t h e above c h e m i c a l phenomenon a c o m p a r i s o n o f t h e e l e c t r o ­

s l a g w e l d i n g and r e f i n i n g t e c h n i q u e s f o l l o w s .

R e f i n i n g i s c a r r i e d ou t i n an e l e c t r i c a l l y i n s u l a t e d m o l d , t h e c u r r e n t

p a t h b e i n g be tween t h e two a c t i v e m e t a l / s l a g s u r f a c e s , t h r o u g h t h e s l a g

mass. W e l d i n g c an be c o n s i d e r e d a l i v e mo ld ESR p r o c e s s . The o n l y d i f f e r e n c e

i s a p a r t o f t h e mo ld i n t e r a c t s e l e c t r i c a l l y and c h e m i c a l l y w i t h t h e s l a g .

The c u r r e n t p a t h i s s h o r t and t h u s t h e t h e r m a l and e l e c t r o c h e m i c a l b e h a v i o r

o f w e l d i n g i s v e r y d i f f e r e n t f r o m ESR. A l s o , i n t h e r e f i n i n g p r o c e s s t h e

p o o l / s l a g i n t e r f a c e i s an e l e c t r o a c t i v e s i t e . The b u l k o f t h e m e t a l p o o l

i s a v a i l a b l e t o i n t e r a c t w i t h r e a c t i o n p r o d u c t s d e v e l o p e d a t t h i s i n t e r f a c e .

I n e l e c t r o s l a g w e l d i n g t h e most e l e c t r o a c t i v e s i t e i s a d j a c e n t t h e e l e c t r o d e .

T h i s i s c o o l e r t h a n t h e s l a g / p o o l i n t e r f a c e o f ESR i n g o t s and l a c k s t h e

f l u i d b u l k a s a s o u r c e o f r e a c t a n t s . Thus t h e p o o l / s l a g i n t e r f a c e o f t h e

e l e c t r o s l a g w e l d i n g p r o c e s s i s e s s e n t i a l l y t h e s i t e o f t h e r m o c h e m i c a l

r e a c t i o n s o n l y .

P r e v i o u s l y t he e f f e c t o f s l a g s y s tems on t h e m e c h a n i c a l p r o p e r t i e s

3

o f r e m e l t e d s t e e l has been i n v e s t i g a t e d . However a p r o b l e m a r i s e s when

t h e s e r e s u l t s , d e r i v e d i n a s m a l l ESR mo ld u s i n g w e l d i n g w i r e and h i g h

r e s i s t a n c e s l a g s , a r e c o r r e l a t e d t o w e l d m e t a l p e r f o r m a n c e . The i m p l i c a t i o n

t h a t t h e e l e c t r o d e - i n g o t d i a m e t e r r a t i o i s t h e o n l y r e l e v a n t v a r i a b l e

r e l a t i n g w e l d i n g t o r e f i n i n g c anno t be s u p p o r t e d w i t h r e s p e c t t o t h e above

argument ( F i g . 7 ) . The c o n t r i b u t i o n o f t h e e l e c t r o a c t i v e mo ld t owa rd

18

I.Oi T r——i r

.8 o

m

AREA PROCESS REF. r.

BAR <

WIRE I

ESR 51 ES R 52 ESW 47 ESR 53 ESW 52 ESW 47 ESW 3

AY

_L

1.0 2.0 3.0 80 90

C U R R E N T D E N S I T Y ( A / m m 2 )

100 no

FIG. 7 »'• E L E C T R O D E TIP CURRENT DENSITY vs

FILL RATIO OF MOST E L E C T R O S L A G

P R O C E S S E S .

19

weld metal properties in ESW is significant and cannot be ignored.

1.2.1.5. Slag Characteristics

A l l slag systems must perform several necessary functions to the

electroslag process, ESR or ESW. Slags used for welding are very different

from those of refining operations, see table 2. In either process pro­

perties of density, v iscosi ty , surface tension, vapour pressure, specific

heat and e lec tr ica l conductivity influence the selection of an appropriate

slag. The other slag requirement i s chemical reactively which is imparted

by the composition. Since current profi les in the slag bath are different

between the welding and refining processes the slags used for each are

different. The most pronounced difference being e l ec tr i ca l conductivity.

To decrease slag conductivity welding slags contain a greater proportion

of SiC^ and A ^ O ^ and less CaF2 than slags common to ESR. The presence

of s i l i c a and alumina increases the complex ion fraction of the melt and

thus basicity decreases. These ions are less mobile than the smaller

oxygen ion they consume. Since current conduction i s a function of ionic

transport in these slags the conductivity decreases. Also the decrease of

basicity l imits the refining action of welding slags, but generally this

is not an important feature of selection.

Slag f lu id i ty and s tab i l i ty are necessary to succesful electroslag

welding and these can be adjusted to some extent with the CaF^, MgO, and

CaO contents. Also a necessary part of welding slags important to heat

transfer i s the surface and interfac ia l tension existing between the slag/

atmosphere and slag/metal surfaces. Ideally these properties should be

low, as they are for acid type slags. Hence, MnO, FeO and again MgO and

Si0 o are present in welding slag systems.

20

T A B L E 2 : COMMERCIAL FLUXES USED FOR ELECTROSLAG PROCESSES

DESIGNATION COMPOSITION ( W T . %)

USES DESIGNATION

C o F , A l , 0 5 C o O S i O ; M g O T i O j M n O orMca

USES

XI A A 60 26 10 4 EXPERIMENTAL

X 2 A A 6 0 17 2 0 3 ' ••

Y 4 A 4 0 4 3 • 7 10 II

— 7 5 5 10 5 5 II

— • 3 0 3 0 3 0 10 TOOL STEEL

— • 3 0 4 0 2 0 10 II

Y 2 A 3 0 3 4 2 6 10 II

T 1 C 5 0 3 4 4 12 II

— 8 0 5 10 5 II

— 5 5 2 5 15 2 3 DIE STEEL

Y 3 A 3 0 3 4 17 6 1 3 RENE 41

— - 3 5 "5 10 5 3 5 COLD START

A N 8 16 14 5 3 5 6 2 4 WELDING

AN 2 2 2 0 2 0 15 2 0 IS 10 »

A N 2 5 3 5 12 5 3 4 0 5

F e 2 0 j II

A N 10 2 0 2 0 1 0 2 0 3 0 —

F T s 7 5 3 3 4 7 17 2 5 WELDING

A N 8 M 15 5 8 3 8 32 2

N0;0 II

B V 8 4 2 2 6 2 4 8 II

21

There has been l i t t l e attempt by industry to develop slags for each

al loy that has been electroslag welded or remelted. However, some al loy

categories and types of electroslag welding have been assigned various

slag designations related to the composition of major flux components.

Only recently has one a l loy , Rene 41, been al lotted the slag Y3A"^, see

table 2.

1.2.1.6. Welding Considerations

Electroslag welds are usually made with wire as the consumable

electrode. To achieve the required mechanical properties from the weld

metal region of the joint the al loy of the f i l l e r wire used i s different

from that of the parent material. A less frequently used type of electroslag

welding used primarily for the manufacture of semi-finished products i n ­

corporates a bar electrode often fabricated from the al loy of the parent

stock used in the jo int . Current density at the consumable bar electrode

approaches the values encountered in ESR. This is unlike the high values

observed with consumable wire ESW, see figure 7. Since electrode to

plate gaps are small for bar electrode welding some features of welding

slags, i . e . low conductivity, may be required. Alternatively due to the

s ignif icantly lower electrode current densities, slag chemistries similar

to ESR systems may be used. The diversity and economic advantages of wire

fed ESW are lost when using bar electrodes but a wider range of welding

conditions and slag chemistries are possible with the la t ter . Another

consideration with wire welding practice is the commercial ava i lab i l i ty of

wire of the appropriate chemistry. This res tr ic t s precise matching of

mechanical properties throughout a welded jo int . Also the e l ec tr i ca l and

22

mechanical i n s t a b i l i t y i s a r e s t r i c t i o n . Large cu r r en t s are passed through

a f e r rous a l l o y w i r e , t y p i c a l l y 3 mm i n d iameter , i n t o the s l a g phase.

Current d e n s i t y at the e l e c t r o d e t i p i s ve ry h i g h and s l a g des ign must

a l l o w fo r t h i s . Problems w i t h w i r e feed and e l e c t r i c a l connec t ion f i x -

t u r i n g have been met w i t h s p e c i a l l y designed machines, some o f which are

ve ry complex.

1 . 2 . 2 . I n c l u s i o n s and Mechan ica l P r o p e r t i e s

Many workers propose the m a j o r i t y i n c l u s i o n p o p u l a t i o n i n ESR i n g o t s

are a r e s u l t of the p r e c i p i t a t i o n of n o n - m e t a l l i c s as the a l l o y s o l i d i f i e s " ^ ' '

They m a i n t a i n no i n c l u s i o n present i n the consumable e l e c t r o d e remains

i n t a c t to j o i n the i n g o t . The same arguments may be a p p l i e d to the weld

meta l of an e l e c t r o s l a g w e l d . As a r e s u l t of s l i g h t l y ove rhea t ing the

parent meta l du r ing we ld ing some of t h i s m a t e r i a l mel t s and j o i n s the p o o l

wi thout pa s s ing through the s l a g . Thus i t i s conce ivab l e tha t some i n c l u s i o n

types , the more r e f r a c t o r y v a r i e t y , w i l l en ter the weld m e t a l . In g e n e r a l ,

the i n c l u s i o n f r a c t i o n of the remel ted meta l of e i t h e r process has been

observed to be a f i n e d i s p e r s i o n of p a r t i c l e s l e s s than ten microns i n

s i z e .

The mechanica l behavior of s t e e l i s s i g n i f i c a n t l y a f f e c t e d by the

i n c l u s i o n f r a c t i o n . I n g e n e r a l , i n c l u s i o n s have an adverse e f f e c t as shown

i n f i g u r e s 8-10. However not a l l i n c l u s i o n s r e l a t e the same e f f e c t to

mechanical p r o p e r t i e s . In s tee lmaking p r a c t i c e a p o l i c y i s made to min imize

the more harmful i n c l u s i o n types as not a l l can be avo ided .

FIG. 8 ; E F F E C T OF INCLUSION SHAPE ON MECHANICAL ANISOTROPY

OF AN AS ROLLED , LOW CARBON S T E E L . FROM REF. 2 2 . U>

24

EFFECT OF SECOND PHASE VOLUME

FRACTION ON TOTAL STRAIN TO F R A C T U R E .

FROM REF. 21 .

2 5

F I G . 1 0 » R E L A T I O N S H I P O F S H E L F E N E R G Y A N D

V O L U M E F R A C T I O N O F I N C L U S I O N S .

F R O M R E F . 2 0 .

26

1.2.2.1. Inclusions

Inclusions are the product of precipitation reactions from a l iqu id

or sol id solution. In section 1.2.1. the existence of inclusion forming

elements in the melt is expounded. Using the theory of c lass ica l nu­

cleation, as the dendritic so l id i f i ca t ion front advances into the metal

pool i t traps regions of stagnant l iqu id between the dendritic array. As

freezing progresses, the concentration of segregation products increases in

these regions. The high melting point inclusions, l ike alumina, w i l l be

precipitated f i r s t . With continued cooling and concentration of segre­

gated species, the lower melting point oxides and sulfides w i l l precipitate.

The later inclusion types w i l l nucleate at existing inclusions resulting

in duplex type inclusions of which oxy-sulfides are an example. The last

form of Inclusion to evolve i s a non-metallic precipitation from sol id

solution. This type of inclusion, often sulfides, are extremely small and

evenly dispersed. The precipitation of iron sulfides i s one case and occurs

along primary grain boundaries. In some circumstances this may form a

23

fi lm l ike inclusion . This leads to low strength and d u c t i l i t y , and by

alloy design enough higher melting sulfide forming elements are added to

the al loy to precipitate in preference to iron from the melt. The standard

free energy of formation of some sulfides i s shown in figure 11. The most

common element added for this i s manganese. Recently more reactive sulfide

formers, Zr , La and Ce, have been introduced as these form very stable

sol id sulfides in the melt. Since manganese is the principal sulfide

inclusion-forming addition to commercial steels i t has been the subject of

many investigations.

0

- 2 0 h

-160" • • • — : ' ~ — " 0 500 I'000 1500

TEMPERATURE (•C).

FIG. II F R E E E N E R G Y OF F O R M A T I O N O F S O M E

M E T A L S U L F I D E S . F R O M REF. 2 4 .

28

Many workers have examined the fo rmat ion and morphology of MnS i n -

25-33

e l u s i o n s i n cas t s t e e l s . A code f o r the v a r i o u s shapes of manganese

s u l f i d e p a r t i c l e s observed was f i r s t proposed by Sims and has been r e t a i n e d

throughout the l i t e r a t u r e . The morpho log i ca l types are c l a s s e d :

Type I - i n d i v i d u a l s u l f i d e s or o x y - s u l f i d e p a r t i c l e s , g l o b u l a r i n

shape and evenly d i spe r sed throughout the m a t r i x .

Type I I - rod shaped s u l f i d e s , o f t en c l u s t e r e d and d e s c r i b e d as

e u t e c t i c . Type I I I - i n d i v i d u a l s u l f i d e s , angular ( c r y s t a l l o g r a p h i c ) i n appear-

26

ance. The e f f e c t o f a l l o y elements common i n s t e e l s and the s o l i d i f i c a t i o n

29

r a t e on the fo rmat ion of manganese s u l f i d e s has l e a d to a gene ra l con ­

c l u s i o n tha t the shape o f MnS i n c l u s i o n s i s r e l a t e d to the s o l u b i l i t y of 28

s u l f u r i n l i q u i d s t e e l . Where the s o l u b i l i t y becomes depressed , i . e . by the i n f l u e n c e of s o l u t e s as ca rbon , s i l i c o n and aluminum, the s u l f i d e

type I I I w i l l form as i t p r e c i p i t a t e s a t h ighe r temperatures than type

25

I or I I . A l l manganese s u l f i d e s form i n the l i q u i d occupying the i n t e r ­

s t i c e s of a d e n d r i t i c s o l i d i f i c a t i o n f r o n t . S l i g h t l y g rea te r s o l u b i l i t i e s

of s u l f u r w i l l render type I I i n c l u s i o n s . The fo rmat ion mechanism of these

i s u n c l e a r . Sims concludes MnS forms as a cont inuous f i l m over the pr imary

g r a i n boundaries which l a t e r breaks i n t o rods as the s t r u c t u r e c o o l s . Other

26 30 33

workers ' ' d e s c r i b e type I I fo rmat ion as a monotec t ic r e a c t i o n due to

the depressed f r e e z i n g p o i n t imparted to the manganese s u l f i d e by a l l o y

i m p u r i t i e s . The o r i g i n o f type I s u l f i d e s i s g e n e r a l l y agreed upon by a l l

workers . These have been observed as g l o b u l a r p a r t i c l e s and the i n t e r -

f a c i a l sur face t e n s i o n between the l i q u i d manganese s u l f i d e and i r o n i s

h i g h . They conclude the s u l f i d e p r e c i p i t a t e s as a l i q u i d from the segregated

29

l i q u i d meta l a long the d e n d r i t i c f r o n t . T h i s p r e c i p i t a t i o n r e a c t i o n i s

a ided by the e x i s t e n c e of s o l i d p a r t i c l e s , u s u a l l y ox ide i n c l u s i o n s , onto

which they nuc lea t e from the segregate . The temperature o f t h i s r e a c t i o n

i s below the l i q u i d u s of s t e e l , t he re fo re these s u l f i d e s c o n t a i n i m p u r i t i e s .

1 . 2 . 2 . 2 . Mechan ica l P r o p e r t i e s

The b u l k of study on MnS i n c l u s i o n s has been toward t h e i r e f f e c t on

the mechanica l behavior of worked s t e e l s . G e n e r a l l y t h e i r c o n t r i b u t i o n 21 22 2/j

toward a n i s o t r o p y of p r o p e r t i e s has been repor ted (see f i g u r e 9) ' '

In cas t s t r u c t u r e s manganese s u l f i d e s have been observed a t pr imary g r a i n

25 26

boundaries and i n t e r d e n d r i t i c spaces ' . The e f f e c t they Impart to

mechanical p r o p e r t i e s i s r e l a t e d to t h e i r o v e r a l l d i s t r i b u t i o n and shape.

For s t e e l o f a g iven i n c l u s i o n volume f r a c t i o n , evenly d i s t r i b u t e d

equiaxed f i n e p a r t i c l e s are favoured i n s p i t e of an i n c r e a s e i n the abso lu te

number. Large or c l u s t e r e d i n c l u s i o n format ions are d e t r i m e n t a l to optimum

mechanica l p r o p e r t i e s as are non-equiaxed i n c l u s i o n s where i s o t r o p i c be­

h a v i o r i s r e q u i r e d . Thus manganese s u l f i d e types I and I I I are d e s i r e d

i n p l ace of type I I . Al though a l l s u l f i d e s are l o c a t e d i n the area between

pr imary g r a i n s o f cas t s t r u c t u r e s , type I I occup ies the l a r g e s t area f o r a

g iven t o t a l s u l f i d e volume. Th i s c rea tes a g rea te r p lane of weakness than types I or I I I and t h u s l y type I I i s the most d e l e t e r i o u s to mechanica l

35

p r o p e r t i e s . Baker and Char l e s have observed in t e r connec t ed c o l o n i e s of

type I I s u l f i d e s on the f r a c t u r e sur face of cas t s t e e l . T h i s fo rmat ion

p rov ides a cont inuous c rack pa th through the m a t r i x r e s u l t i n g i n i n t e r -

g ranu la r f r a c t u r e . Th i s has o n l y appeared i n s t e e l s w i t h r e l a t i v e l y h i g h

s u l f u r l e v e l s (about 0.28 wt . %). In a l l cases the s i t u a t i o n becomes worse

30

when hot working leads to a further increase in the area to volume ratio

and imparting greater anisotropy to mechanical behavior.

The overall effect of second phase particles within the metal matrix

is to present a point of discontinuity to stress. For non-metallic i n ­

clusions the inclusion matrix in ter fac ia l bond and thermal expansion

coefficient relat ive to the matrix are important in determining the magni-

36 tude of this stress . Manganese sulfides have a large coefficient of thermal

—6 —1 —6 —1 expansion (18 x 10 °C ) , larger than steel (12.5 x 10 °C average) for

37

the temperature range less than 800°C . As a cast al loy cools the MnS

inclusions w i l l contract to a volume less than the cavity i t occupies.

Since interfac ia l cohesion i s low a void may form and so reduce tensile

stresses within the inclusion. This has been observed in the typical ly

larger type I formations rendering these inclusions generally the second

most detrimental to properties. The effect of the above inclusions and those with expansion coefficients

less than steel (e.g. A ^ O ^ and calcium aluminates) on the matrix has been

38

discussed in terms of fracture mechanics . The inclusion s ize, shape

and influence on the surrounding material was considered as a defect in

the matrix. The importance of tesselated stresses about inclusions of

smaller thermal expansion coefficients than steel i s reported by Brooksbank

36 37

and Andrews ' as a mode of crack i n i t i a t i o n . However they also report

inclusions with larger coefficients, l ike MnS, may also be considered defects

and impart a greater c r i t i c a l crack size to the applied stress. The

absolute values they calculate from plain strain fracture mechanics for

various materials i s much larger than the size of inclusions found in

practice. Alternatively steel tending toward ductile rupture w i l l fracture

31

by void formation, growth and coalescence. Manganese sulfides are usually

the larger species of inclusions in commercial steels, thus retaining the

greater volume fraction of the inclusion population. Since they exist in

a contracted state and have a low inter fac ia l bond strength they are easi ly

separated from the matrix forming a void. This occurs at very small

39

strains . A subsequent increase in strain leads to void growth and

coalescence.

1.2.3. Gas Porosity

Porosity in an electroslag ingot i s the result of the gaseous element

content of the l iquid steel becoming insoluble during so l id i f i ca t ion and

precipitating as a bubble. The thermochemistry of gases in steel has been

2,14

dealt with in the l i terature and only the source of these gaseous

elements to the metal pool w i l l be reviewed.

An advantage of the electroslag process i s that the remelted steel i s

never in direct contact with the atmosphere. Gaseous species picked up by 12

the metal must f i r s t be combined in the slag. Hawkins et a l show high

nitrogen pick up by an ESR ingot at the beginning of a melt, but lower

(50 ppm) levels throughout the remainder of the ingot regardless of melting

mode. Nitrogen is soluble in calcium fluoride slags but i t s absorption

rate onto the metal pool surface i s slow.

The hydrogen content of remelted metal i s derived from three sources:

i hydrogen present in the electrode

i i hydrogenous compounds present in the slag prior to the start

of melting

i i i the water vapour pressure of the gas phase in contact with the

slag

32

The hydrogen content o f the e l e c t r o d e w i l l be r e t a i n e d i n i n g o t s of

40

e l e c t r o s l a g mel ts processed i n contac t w i t h a i r . The h i g h l e v e l of

hydrogen p i c k - u p a t the s t a r t o f an e l e c t r o s l a g mel t i s a t t r i b u t e d to the

water content o f hydrated s l a g compounds. The decrease i n hydrogen l e v e l

as an ingo t i s produced i n d i c a t e s the hydrogen from t h i s source i s con ­

sumed i n the e a r l y stages of r e m e l t i n g . As the hydrogen content of the

b u l k of an ingo t i s constant a s teady s t a t e c o n d i t i o n i s assumed. Thus a

p s e u d o - e q u i l i b r i u m between hydrogenous spec ies i n the gas phase and meta l

poo l becomes the predominant p i c k up mechanism. The s l a g compos i t ion

d i r e c t l y a f f e c t s the extent to which the second and t h i r d sources o f

hydrogen a f f e c t the remel ted m e t a l . I n p r a c t i c e hydra ted compounds o f CaF^

and e s p e c i a l l y CaO are the p r i n c i p a l sources of hydrogen. These compounds

y i e l d the v e r y s t a b l e (OH ) i o n i n the s l a g . The m o b i l i t y o f t h i s i o n i n

s l a g has been assumed to c o n t r o l the r a t e of change o f hydrogen c o n c e n t r a t i o n

40

i n the p o o l . O v e r a l l , hydrogen i s not cons idered too severe a problem to

ESW. The hydrogen conta ined i n the weld me ta l has s u f f i c i e n t t ime to

d i f f u s e away p r i o r to the j o i n t r each ing room temperature.

Free oxygen l e v e l s i n e l e c t r o s l a g remel ted meta l must be kept low

to avo id the format ion of blow h o l e s . The presence of d e o x i d i z i n g elements

i n most a l l o y s i s s u f f i c i e n t to prevent t h i s c o n d i t i o n . However to o b t a i n

m a t e r i a l s of h i g h c l e a n l i n e s s , low ox ide con ten t s , the oxygen l e v e l of an

ingo t must be reduced. When s t e e l i s mel ted through a c i d s l a g s the f o r ­

mation of carbon monoxide blowholes i s p o s s i b l e . The r e a c t i o n S i 0 2 + 2[C] - [ S i ] + 2 CO 1-13

i l l u s t r a t e s the importance of the i n t e r a c t i o n between [ C ] , [ S i ] and oxygen.

Here d i s s o l v e d s i l i c o n i s the d e o x i d i z i n g element.

33

The relationship between dissolved oxygen in the l iquid metal and the

42

(FeO) content of the slag has been the subject of previous research

The ingot oxygen content increases as the slag (FeO) content. Electroslag

processes exposed to a ir develop a higher (FeO) fraction than those shielded

from a i r . Thus the atmosphere i s a source of oxygen to the ingot. Alternate 16

oxygen sources include alumina equilibrium between slag and metal (A1 20 3) = 2[A1] + 3[0] 1-14

The effect of porosity on mechanical properties has been treated in the

same way as inclusions. A hole, unlike an inclusion, w i l l have an inter­

fac ia l bond strength and elast ic modulus of zero. Inclusions l ike manganese

sulfides are also treated this way (see section I .2 .2 .2 . ) .

1.2.4. Heat distribution and Structure

The mechanical properties of an electroslag welded joint are greatly

affected by the heat flow conditions under which i t was produced. In con­

strained electroslag welds so l id i f i ca t ion cracks result from tensile forces

acting on the weld centre during fusion. In unconstrained welds cracking may

result from a deep metal pool prof i le producing columnar grains oriented h o r i ­

zontally. This pool prof i le has been described as a "shape factor" and is a

43

function of slag composition and the welding schedule . Contraction of the

weld metal just after so l id i f icat ion is greatest along the axis of columnar

crystals . For horizontally oriented grains these forces are applied to the

last region to freeze creating a centre l ine crack, see figure 12, and poor

mechanical properties.

Heat flow in electroslag refining i s very different from welding. The

mechanical behavior of ingot metal reflects the orientation and size of

the dendritic grains as compared to ESW metal. The remelted ingot metal is

34

FIG. 1 2 = E X A M P L E OF C E N T R E - L INE C R A C K I N G

F R O M R E F . 4 3 .

35

subjec t to slower c o o l i n g p r o f i l e s as the p r i n c i p a l heat s i n k i s the base .

Thus l a r g e mos t ly a x i a l l y - o r i e n t e d g r a i n s r e s u l t and cen t r e c a v i t y fo rmat ion

does not e x i s t (see f i g u r e 1 3 ) .

Heat i s generated i n the e l e c t r o s l a g process by an ohmic power drop

through the s l a g . The magnitude of energy l i b e r a t e d i n the s l a g i s a

f u n c t i o n of s l a g c o n d u c t i v i t y which i s a p rope r ty o f compos i t i on . A d d i t i o n s

of a lumina and s i l i c a to c a l c i u m f l u o r i d e decrease the c o n d u c t i v i t y and thus

smal le r c u r r e n t s are r e q u i r e d to generate enough heat to mel t s t e e l . Lime

a d d i t i o n s make an i n s i g n i f i c a n t change to the r e s i s t i v i t y of these systems.

When r e l a t i n g p r o p e r t i e s of welds made w i t h a v a r i e t y o f processes

i t i s u s e f u l to compare the c o o l i n g p r o f i l e s of the weld m e t a l . ESR and

ESW have long meta l c o o l i n g curves as compared to tha t o f some arc weld

meta l d e p o s i t s . The c o o l i n g g r a d i e n t s w i l l determine the type o f second

phase seg rega t ion and pr imary g r a i n s i z e and are thus r e l a t e d to mechanica l

p r o p e r t i e s .

1 . 3 .1 . Summary

Al though they are s i m i l a r p rocesses , e l e c t r o s l a g we ld ing and r e f i n i n g

have some ve ry d i f f e r e n t c h a r a c t e r i s t i c s . Both processes i n v o l v e r e m e l t i n g

an e l e c t r o d e through l i q u i d e l e c t r i c a l l y conduc t ive s l a g . The meta l and

s l a g of the ESR mel t I s h e l d w i t h i n a s o l i d , t h e r m a l l y and e l e c t r i c a l l y

i n s u l a t i n g s l a g s k i n . On the o ther hand, the weld meta l and s l a g o f an

ESW are i n d i r e c t con tac t w i t h a p a r t of the mold , the parent p l a t e s , and

i n t e r a c t w i t h i t . The i n f l u e n c e s these d i f f e r e n c e s have toward the cu r r en t

p r o f i l e s of each process are ve ry profound. T h i s process d i f f e r e n c e l eads

FIG. 13= M A C R O S T R U C T U R E OF AN E S R INGOT

37

to d i s s i m i l a r i t i e s i n the thermal and e l e c t r o c h e m i c a l behavior of each

p roces s , and consequent ly i n the meta l p r o p e r t i e s . 3

From the survey on mechanica l p r o p e r t i e s the o n l y i n v e s t i g a t o r to

compare e l e c t r o s l a g Ingots to welds assumed the remel ted meta l o f an ESR

i n g o t made w i t h we ld ing - type s l a g was e q u i v a l e n t to weld me ta l o f an ESW.

Th i s i m p l i e s tha t d i f f e r e n c e s i n e l e c t r i c a l and thermal regimes impart

the same i n f l u e n c e to the remel ted meta l of both p rocesses . Indeed, a

system of ESW s l a g q u a l i f i c a t i o n has been proposed based on t h i s assumption.

However, t h i s i s c o n t r a r y to the c o n c l u s i o n s of the r ev iew and should be

r e s o l v e d . From the survey of s l a g composi t ions we may conclude there i s a

s i g n i f i c a n t d i f f e r e n c e between s l ags used f o r r e f i n i n g and w e l d i n g . Thus

an ingot and weld of each o f the s l a g systems should be s t u d i e d .

Remelted meta l compos i t ion i s r e l a t e d to s l a g compos i t ion f o r both

p rocesses . Minor element c o n s t i t u e n t s of the a l l o y may be mod i f i ed by

i n t e r a c t i o n w i t h the s l a g . S ince mechanica l p r o p e r t i e s are dependent upon

the a l l o y element con ten t , they too are a l t e r e d by s l a g c o m p o s i t i o n .

Of g rea te r s i g n i f i c a n c e i s the e f f e c t of s l a g compos i t ion on the i n c l u s i o n

cha rac te r of the remel ted m e t a l . Consequently some ingo t and weld meta l

p r o p e r t i e s should be ana lysed and the e f f e c t o f s l a g compos i t ion be

determined.

1 .3 .2 . Statement o f the Problem

From the fo rego ing d i s c u s s i o n i t i s c l e a r two important ques t ions i n

the area of e l e c t r o s l a g we ld ing technology remain to be r e s o l v e d . F i r s t , i s

i t c o r r e c t to assume tha t a weld and an i n g o t , mel ted through s i m i l a r s l a g s ,

w i l l possess the same mechanica l p r o p e r t i e s ? I f t h i s statement i s i n c o r r e c t ,

38

then one present standard method of slag qual i f icat ion is in error. Second,

may we assume that an electroslag weld has properties which are primarily

controlled by the inclusion content? If so, then the slag composition has

a direct influence on weld properties and may be optimized accordingly.

The program summarized below was undertaken in order to resolve these

questions.

1.3.3. Experimental

An electroslag weld and ingot were made with each of three slag

systems. The slag composition varied between that used for welding, one used

for refining and a third system which was of neither extreme. The re­

melted metal of each process was analysed chemically and metallographically

and an assessment of mechanical properties was done. These tests and the

results are described in the following sections.

A l l material for this study was obtained from a single plate of steel .

From this electrodes for both processes were cut and the parent plates

for welding were obtained. The actual slag systems used were:

A. 80 wt 7, CaF 2 , 20 wt % A l ^

B. 55 wt % CaF 2 , 35 wt % A l ^ , 10 wt % CaO

C. 55 wt % CaF 2 , 15 wt % A l ^ , 15 wt % CaO, 15 wt % S i 0 2 .

F IG. 1 4 - E X P E R I M E N T A L E.S.W. A P P A R A T U S USED

CHAPTER I I

E x p e r i m e n t a l and R e s u l t s

I I . 1 . M a t e r i a l s

1 1 . 1 . 1 . S t e e l

A s i n g l e p l a t e 1.5 i n c h e s (38 mm) t h i c k o f an a l l o y d e s i g n a t e d W e l t e n

80-C p r o d u c e d by t h e N i p p o n S t e e l C o r p o r a t i o n was u s ed i n a l l e x p e r i m e n t s .

T h i s m a t e r i a l was c u t t o make e l e c t r o d e s f o r w e l d i n g and r e f i n i n g and t h e

p a r e n t p l a t e s e c t i o n s o f t h e w e l d a s s e m b l i e s . W e l t e n 80-C i s a quench and

tempered h i g h s t r e n g t h l o w a l l o y s t r u c t u r a l s t e e l w i t h good w e l d a b i l i t y .

The g e n e r a l c h a r a c t e r i s t i c s and c h e m i c a l a n a l y s i s o f t h i s a l l o y p u b l i s h e d by

the p r o d u c e r a r e shown i n f i g u r e 15 and t a b l e 3 . The m e c h a n i c a l p r o p e r t i e s

o f t h i s m a t e r i a l a r e g e n e r a l l y e q u i v a l e n t t o ASTM A-514 and A - 5 1 7 . A l l

c u t t i n g was done u s i n g a band saw t o a v o i d any h e a t i n g e f f e c t s f r o m f l a m e

c u t t i n g . S u r f a c e s expo sed t o t h e e l e c t r o s l a g p r o c e s s were g round f r e e

o f o x i d e .

1 1 . 1 . 2 . S l a g

S l a g c o m p o s i t i o n s were made by m i x i n g c r u s h e d and w e i g h e d s amp le s o f

t h e v a r i o u s s l a g c o n s t i t u e n t s . . A f t e r m i x i n g t h e s l a g was s t o r e d i n a d r y i n g

oven a t a t e m p e r a t u r e j u s t above 100°C u n t i l r e q u i r e d . S l a g s u s ed i n t h i s

p rog ram were ba sed on c a l c i u m f l u o r i d e ( C a F „ ) , f r o m E l d o r a d o N u c l e a r , w i t h

40

41

WEL-TEN 80C exhibits excellent resistance to stress cor­rosion cracking because it does not contain nickel, an element that contributes to H.S stress corrosion cracking. Its high notch toughness at low temperatures and other

good properties make WEL-TEN 80C an ideal material for semi-refrigeration pressure vessels for ethylene and other chemicals.

Si

0 .15 -0 .35

Mn

0.60-1 .20

Chemical

Composition m a x

0.18

Heat Treatment

Available Thickness Range, mm

Yield Point, min kg/mm2 (psi)

Tensile Strength, kg/mm= (psi)

Thickness, mm

P max

0.030

Mechanical Properties

Elongation

Bending Properties (180° bend radius)

Notch Toughness

6 to 13, excl.

13 to 21, excl.

21 to 40, incl.

Thickness, mm

6 to 32, incl. Over 32 to 40, incl.

Thickness, mm

Over 12 to 40, incl.

Carbon Equivalent

0.62 See Nole

Maximum W e l d Hardness (to be tested when the value of carbon equivalent given at right is exceeded)

Nole. Carbon equivalent is calculated by the following formula: C • "«Mn • " 2 ) 5 i -'/(oNi-'-'/sCr ' - '/(MO-'-'/HV

S Cu max

0.030 0 .15 -0 .50

Cr M o max

B max

Thickness, mm

Over 12

0 .70 -1 .30 0.60 0.006

Quenched and Tempered

6 to 40, incl.

70 (100,000)

8 0 - 9 5 (113,800-135,100)

?i. min Test Specimen

16 JIS No. 5

22 )IS No. 5

16 JIS No. 4

Ratio of Bend Radius to Specimen Thickness

1.5 2.0

Charpy 2 mm V-Notch Impact Value

3.6kg-m (26 ft-lb) at - 1 5 C

Hv

450 max

FIG. I 5 ' PUBLISHED SPECIFICATIONS OF A L L O Y

W E L T E N 8 0 - C .

TABLE 3 CHEMICAL ANALYSIS OF ALLOY WELTEN 8 0 - C 1.5 inch ( 3 8 m m ) THICK P L A T E , AS RECEIVED .

SOURCE OF

OATA C Mn Si P S Ni Cr Cu Mo A l Nb V B

SPECIFICATION 0.18 (max)

0.6 1.2

0.15 0.35

0.03 (mox)

0.03 (mox)

0.70 1.30

0.15 0.50

0.60 (max)

.006 (max)

HEAT SHEET 0.12 .89 0.24 Oil .004 0.83 0.22 0 .33 0.04 .001

SPECTROGRAPHS

DETERMINATION 0.12 0.85 0.29 .014 .005 0.02 0.75 0.25 0.29 .076 .013 I

ho

43

a d d i t i o n s o f a l u m i n a ( A ^ O ^ ) f r o m N o r t o n Company, l i m e (CaO) f r o m Dynami t

N o b e l and s i l i c a (S102) f r o m G e n e r a l E l e c t r i c . A t r i p l e beam b a l a n c e was

u sed t o w e i g h t h e s l a g compounds w i t h an a c c u r a c y w i t h i n two p e r c e n t .

1 1 . 1 . 2 . 1 . C a l c i u m F l u o r i d e

C a l c i u m f l u o r i d e was c r u s h e d and s i e v e d t o - 6 +48 mesh, a s i z e by

e x p e r i e n c e f ound t o be a c c e p t a b l e f o r t h e a p p a r a t u s . C h e m i c a l l y , l i m e i s

t h e m a i n i m p u r i t y and c an be f ound a t a l e v e l o f 500 ppm. T h i s may v a r y

between m e l t s . T r a c e s o f s i l i c a and i r o n o x i d e s have been d e t e c t e d b u t

a r e o f l i t t l e con sequence i n s l a g s y s tems c o n t a i n i n g d i s s o l v e d o x i d e s . The

maximum i m p u r i t y a n a l y s i s o f c a l c i u m f l u o r i d e , a s r e c e i v e d , i s g i v e n b e l o w

i n w t . p e t . :

C h l o r i n e ( C l ) .005

S u l p h a t e (SO^) .01

I r o n (Fe) ' .005

Lead (Pb) .005

S i l i c a ( S i 0 2 ) ' -°5

1 1 . 1 . 2 . 2 . A l u m i n a

E l e c t r o f u s e d a l u m i n a o f 99 .9% p u r i t y and a g r a n u l a r s i z e o f 16 mesh

was u s e d .

1 1 . 1 . 2 . 3 . C a l c i u m O x i d e (L ime)

R e c r y s t a l l i z e d l i m e o f 99 .5% p u r i t y was u s e d .

1 1 . 1 . 2 . 4 . S i l i c a

C l e a r f u s e d q u a r t z o f 99 . 8% p u r i t y was o b t a i n e d by c r u s h i n g c l e a n

a n a l y t i c t u b i n g t o a s i z e o f - 6 +48 mesh. The m a i n i m p u r i t i e s a r e l i s t e d

44

below as an average in ppm:

5

2

50

CaO 7

MgO 2

4

4

II.2. Apparatus

9

The U.B.C. electroslag r i g , as described by Etienne , was readily

adapted to both ingot and weld production. A water cooled 3 inch (76 mm)

internal diameter copper mold was used to make an ESR ingot of approximately

10 inches (254 mm) in length. The mold and water jacket were e l ec tr i ca l ly

insulated from the process resulting in a current path between the electrode

and base assemblies. The dimension of the electrodes used for refining was

1.5 x 0.75 inches (38 x 19 mm) in section and about 50 inches (1.27 mm) long.

For welding a fixture was instal led which supported the parent plate

assembly such that the electrode bar and weld gap were aligned. The

electrode was positioned in the gap by a r o l l guide assembly that fastened

to the support f ixture. This device (Fig. 16) was e l ec tr i ca l ly insulated

from the weld assembly thus preventing a short c i rcu i t between the electrode

and plates. Water cooled, copper faced jackets were positioned on each side

of the gap to contain the slag bath. E l e c t r i c a l connection to the parent

plates resulted in a current path between the electrode and plates.

45

BOLT CLAMP IN A SLOT PERMITS ADJUSTMENT

ELECTRICAL INSULATOR

h - W E L D SUPPORT 8 ALIGNMENT JIG

ELECTRODE

SPRING LOADED ROLLS

JUL

RUN-OUT BLOCKS

7

FIG. 16 = E L E C T R O D E GUIDE

( NOT TO S C A L E ) .

A S S E M B L Y , SCHEMAT IC

46

E l e c t r o d e s w i t h a c r o s s s e c t i o n o f 1.0 x 0.75 i n c h e s (25 x 19 mm) and a

l e n g t h o f a p p r o x i m a t e l y 50 i n c h e s (1 .27 mm) were u sed t o f i l l w e l d i n g gaps o f

I. 13 i n c h e s (29 mm) s p r e a d a t t h e b o t t o m w h i c h opened t o 1.25 i n c h e s (32 mm)

a t t h e t o p . A t t he b a s e o f t h e a s s emb l y a s t a r t i n g sump made f r o m t h e

same a l l o y a s t h e p l a t e s was w e l d e d i n t o p l a c e ( F i g . 1 7 ) . E l e c t r o d e b a r s

f o r b o t h p r o c e s s e s were TIG w e l d e d d u r i n g t h e i r f a b r i c a t i o n t o make e l e c t r o d e s

o f s u f f i c i e n t l e n g t h .

I I . 3. P r o c e d u r e

The r e m e l t i n g p rog ram o f i n g o t s and w e l d s was done u s i n g l i n e f r e q u e n c y

(60 Hz) AC power . To s t a r t t h e p r o c e s s DC power was a p p l i e d t h r o u g h a

compact p l a c e d be tween t h e e l e c t r o d e and sump o r b a s e p l a t e . The c y l i n d r i c a l

compact measured 1.5 i n c h e s (38 mm) i n d i a m e t e r and abou t 1.5 i n c h e s (38 mm)

i n h e i g h t . I t c o n s i s t e d o f 60 grams o f - 325 mesh c a l c i u m f l u o r i d e m i xed w i t h

about 200 grams o f s t e e l t u r n i n g s . When a m o l t e n s l a g p o o l was e s t a b l i s h e d

t h e power mode was s w i t c h e d t o AC . A l l e x p e r i m e n t s were c a r r i e d o u t i n

c o n t a c t w i t h t h e a t m o s p h e r e , no a r g o n o r o t h e r gas c o v e r was u s e d when

mak ing e i t h e r i n g o t s o r w e l d s . O n l y m a t e r i a l f r o m t h e AC m e l t z o n e , e x ­

c l u d i n g about 0.5 i n c h e s (12 mm) o f t h e . t o p , was r e t a i n e d f o r a n a l y s i s and

m e c h a n i c a l t e s t i n g . The r e m e l t i n g c o n d i t i o n s o f t h e ESR and ESW e x p e r i m e n t s

a r e shown i n t a b l e 4.

I I . 3 . 1 . C h e m i c a l A n a l y s i s

The i n g o t s and w e l d s were s e c t i o n e d a s shown i n f i g u r e 1 8 . Samples

f o r c h e m i c a l and m e t a l l u r g i c a l a n a l y s i s were t a k e n f r o m t h e c e n t r a l r e g i o n

o f e a ch m e l t . The c h e m i c a l d e t e r m i n a t i o n s were done u s i n g an ARL e m i s s i o n

47

RUN-OUT BLOCK

STARTING COMPACT

SUMP

ELECTRODE

(25) (32)

FIG. 17 SCHEMATIC OF E S W CONFIGURATION B E F O R E START ING, S L A G R E M O V E D . (NOT T O S C A L E )

T A B L E 4 : REMELTING CONDITIONS OF INGOTS ( i ) a WELDS (w).

MELT * VOLT

(V)

AMPS

(A)

ELECTRODE FEED RATE SPEED MASS

( cm/ sec ) ( g/ sec) x .01

INITIAL SLAG MASS

( grams)

RATE OF SLAG ADDED DURING MELT (g/min)

I A 1 2 5 1050 6.6 3 . 8 7 0 0 0

B i 3 0 8 5 0 7.6 4 . 3 7 0 0 QO

I C • 2 7 1 0 0 0 6.7 4 . 3 7 0 0 0

A w 2 4 1150 11.2 4 . 3 160 5

B w 2 4 IO5.0 9 .2 3.5 160 5

C w I 2 4 [ 1 0 5 0 11.0 4 . 2 160

5

* SLAGS « A - 8 0 % C o F 2 , 2 0 % A l 2 0 3

B - 55 % CaF 2 , 35 % A l 2 0 3 , 10 % CaO

C - 55 % CaF 2 , 15 % A l 2 0 s , 15 % CaO , 15 % S i 0 2

i - INGOT

w - WELD

49

FIG. 1 8 * SECTIONING OF E S R INGOT AND ESW METAL FOR METALLOGRAPHIC AND CHEMICAL ANALYSIS.

50

s p e c t o g r a p h o f t h e E . S .Co . f a c i l i t y a t C o q u i t l a m , B r i t i s h C o l u m b i a . These

spec imens were about 1.0 i n c h (25 mm) s q u a r e by 0.25 i n c h e s (6 mm) t h i c k .

The s u r f a c e a n a l y s e d was f i r s t g round by t h e s p e c t o g r a p h o p e r a t o r t o comp ly

w i t h t h e i r e x p e r i m e n t a l p r o c e d u r e s . A l t e r n a t e s u l f u r and oxygen c o n t e n t s

were d e t e r m i n e d u s i n g L e c o s u l f u r and oxygen a n a l y s i n g e q u i p m e n t . T a b l e 5

l i s t s t h e s e r e s u l t s a l o n g w i t h t h e p l a t e a n a l y s i s .

I I . 3 . 2 . M e t a l l o g r a p h i c A n a l y s i s

Spec imens f o r m e t a l l o g r a p h i c s t u d y were p o l i s h e d i n t h e t r a n s v e r s e

p l a n e , p e r p e n d i c u l a r t o t h e a x i s o f t h e i n g o t o r w e l d . T h i s s u r f a c e was

c h o s e n as i t b e s t r e v e a l s i n t e r d e n d r i t i c i n c l u s i o n f o r m a t i o n s . A q u a n t i t a t i v e

image a n a l y s e r , Q u a n t i m i t 720, was u sed t o c o u n t and s i z e i n c l u s i o n s o v e r

an a r e a t h a t t o t a l l e d a p p r o x i m a t e l y one s q u a r e m i l l i m e t r e . The s u r v e y was

c o n d u c t e d a t a m a g n i f i c a t i o n o f 1220 X . P a r t i c l e s s m a l l e r t h a n 0.5 m i c r o n s

i n d i a m e t e r c o u l d n o t be c o u n t e d a c c u r a t e l y and were e x c l u d e d f r o m t h e r e s u l t s .

S i z e d i s t r i b u t i o n d a t a a r e shown f i g u r e s 19 and 20. The i n c l u s i o n a r e a

f r a c t i o n , a v e r a g e i n t e r p a r t i c l e s p a c i n g and d i a m e t e r s a r e l i s t e d i n t a b l e 6.

The same spec imens u sed f o r t h e i n c l u s i o n s u r v e y were deep e t c h e d

i n an a p p a r a t u s shown s c h e m a t i c a l l y i n f i g u r e 2 1 . The e t c h a n t was 50 v o l . %

n i t r i c a c i d i n w a t e r and t h e b a t h t e m p e r a t u r e was n o t a l l o w e d t o e x ceed 15°C.

E t c h i n g t i m e s v a r i e d be tween 8 and 10 m i n u t e s . A f t e r e t c h i n g e a c h s pec imen

was q u i c k l y r i n s e d i n a c o l d w a t e r b a t h t h e n d r i e d u s i n g a l c o h o l . T h i s

p r o c e d u r e removed about 0.5 mm o f m e t a l f r o m t h e s u r f a c e and l e f t t h e

i n c l u s i o n s i n t a c t . U s i n g a s c a n n i n g e l e c t r o n m i c r o s c o p e (ETEC A u t o s c a n )

t h e i n c l u s i o n s were o b s e r v e d and t h e i r c o n s t i t u e n t e l e m e n t s a n a l y s e d w i t h a

m u l t i - c h a n n e l x - r a y ene r gy a n a l y s e r ( O r t e c mode l 6 2 0 0 ) . The e l e m e n t s

TABLE 5 : CHEMICAL ANALYSIS OF INGOT a WELD METAL

u c i T SPECTOGRAPHIC DETERMINATION " L E CO "

M t L I C Mn S i P S Ni Cr Cu Mo A l Nb S 0

A : .12 .83 .22 .014 <.00l .02 .81 .25 .30 .040 .012 .001 4 0

A w .12 .87 .25 .014 .004 .02 .81 .25 .30 .047 .012 .005 119

B i .11 .80 .18 .014 <.00l .02 .80 .25 .30 .036 .012 .001 4 2

B w .12 .84 .27 .015 .005 .02 .80 .25 .30 .086 .012 . 0 0 8 103

j C i I"1 2 .81 .24 .013 .001 .02 .80 .24 .29 .013 .Oil . 0 0 2 3 4

[ C w .12 .84 .29 .015 .004 .02 .80 .24 .30 .058 .Oil . 0 0 6 8 9

| P L A T E .12 .85 .29 .014 .005 .02 .79 .25 .29 .076 .013 .011 4 5

NOTE 5 A l l values are wt . -% ( except Oxygen analysis , which is ppm ) .

FIG. 19= INCLUSION DISTRIBUTION OF E S

C i

(>6 u= II. %)

j i u= 2 3 4 SIZE (M)

INGOTS.

5 0 r

o 00 40 H ^

CD C 30 H

30

O z

5 20

A w

(>6JJ « 1.0 %.)

10

J L 1

B w

(>6 » * 4.6 %)

J L T~1 i 0 1 2 3 4 5 6 0

SIZE (p)

I 2 3 4 5 6

SIZE (p)

C w

(>6 )X » 3.6 %)

' • ' 1 L 0 1 2 3 4 5

SIZE (p)

FIG. 2 0 : INCLUSION SIZE DISTRIBUTION OF E S W M E T A L

T A B L E 6 = INCLUSION AREA FRACTION , SPACING AND D I A M E T E R S .

INCLUSION «

MELT A R E A FRACTION

(%)

AVERAGE SPACING

(u)

DIAMETER MEAN AVERAGE

(u) (u)

A i . 0 6 2 6 5 1.6

A w .166 4 4 1.4 2 . 0

B i . 0 7 3 71 1.4 2 .2

B w .174 6 4 2.1 3 . 0

C i . 0 9 4 8 9 1.5 3.1

Cw . 2 4 2 6 0 2.1 3 . 3

55

SPECIMEN HOLDER

SPECIMEN

r

L— THERMOMETER

COOLING BATH

MAGNETIC STIRRER

FIG. 21 = APPARATUS USED TO DEEP ETCH S A M P L E S FOR S.E.M. SURVEY .

56

found t o make up t h e i n c l u s i o n s i n t h e s e s amp le s a r e l i s t e d i n t a b l e 7.

The above s c a n n i n g e l e c t r o n m i c r o s c o p e was u s e d t o examine i n c l u s i o n s

on t h e d u c t i l e f r a c t u r e s u r f a c e o f i m p a c t s p e c i m e n s . These s pec imens were

c h a r p y i m p a c t b a r s t e s t e d a t 1 70°C . The r e s u l t s o f t h i s s u r v e y a r e a l s o

l i s t e d i n t a b l e 7.

F o r t h e s e i n c l u s i o n s u r v e y s , no e l e m e n t o f a t o m i c number l e s s t h a n 13

(a luminum) was d e t e c t e d by t h e x - r a y ene r g y a n a l y s e r . A l s o , d u r i n g t h e

s u r v e y t h e t y p e o f t h e manganese s u l f i d e i n c l u s i o n s were n o t e d ( see t a b l e 8 ) ,

a l t h o u g h s u l f i d e s g e n e r a l l y were a s m a l l f r a c t i o n o f t h e i n c l u s i o n p o p u l a t i o n .

I I . 3 . 3 . M e c h a n i c a l T e s t i n g

S t a n d a r d Cha rpy V n o t c h i m p a c t t e s t b a r s c o n f o r m i n g t o ASTM E-24

s p e c i f i c a t i o n s were c u t f r o m t h e i n g o t s and w e l d s i n t h e o r i e n t a t i o n shown

i n f i g u r e s 22 and 2 3 . F o r e a c h m e l t t h e d u c t i l e t o b r i t t l e t r a n s i t i o n

t e m p e r a t u r e (DBTT) c u r v e s were d e t e r m i n e d . Each p o i n t r e p r e s e n t s a s i n g l e

t e s t . F rom t h e b r o k e n i m p a c t b a r s t h e f r a c t u r e a p p e a r a n c e t r a n s i t i o n

t e m p e r a t u r e (FATT) and t h e l a t e r a l c o n t r a c t i o n (LC) c u r v e s were d e r i v e d

( F i g u r e s 24 t o 2 9 ) .

The f r a c t u r e a p p e a r a n c e d a t a was o b t a i n e d b y mak i n g a t r a c i n g o f t h e

d u c t i l e r e g i o n s o f t h e f r a c t u r e s u r f a c e s f r o m e n l a r g e d p h o t o g r a p h s . U s i n g

a q u a n t i t a t i v e image a n a l y s e r (Quan t imet 720) t h e d u c t i l e a r e a f r a c t i o n s

were d e t e r m i n e d . An e s t i m a t e o f e r r o r u s i n g t h i s method i s l e s s t h a n t e n

p e r c e n t .

A t r a v e l l i n g m i c r o s c o p e was u s ed t o measure t h e c o n t r a c t i o n o f s h e a r

l i p s on i m p a c t b a r s . A c c u r a c y o f t h e measurement was g r e a t e r t h a n FATT

d a t a . However s i n c e no a l t e r n a t e m e a s u r i n g o f c o n t r a c t i o n c o u l d be u sed t o

c heck t h i s d a t a , an e s t i m a t e o f e r r o r i s d i f f i c u l t t o a s s e s s .

57

T A B L E 7 « INCLUSIONS O B S E R V E D ON D E E P ETCHED AND DUCTILE FRACTURE S U R F A C E S .

MELT DEEP ETCH

X-RAY PEAKS SIZE

FRACTURE

X-RAY PEAKS SIZE

A i Al 10-15 Al.Zr 2-5 AI.Zr.Mn.S 2 -4

Al 6-8 AI.Mn.S 7-12 AI,Zr,Mn, S 3-5 Mn,S 3-4

A w Al 3 -6 AI.Mn.S 5-10 Al.Zr , Mn , S 5 - 8 Mn.S 5 -10

AI,Mn,S 5 -6 AI.Zr.Mn.S 4 - 6 Al, Zr .S i , Mn.S 3 - 6 Mn.S 2 -8 Mn.S .S i 5 - 6

B i

Al 3 - 5 Al.Zr 3 - 4 AI.Zr.Mn.S 3-5 AI.Mn.S 2 - 3

Al 4 - 8 AI.Mn.S 7-10 Al,Si,Mn.S 2 - 5 Al.Zr.Si.Mn.S 2-5

B w Al 5-12 AI.Mn.S 2 - 8 Mn, S 2 - 3

Al 2- 8 AI.Mn.S 3 - 6 Mn.S 2 - 5

C i

Al 7-13 A l , Mn , S 3 - 5 Mn, S 3 - 5

Al 5 - 7 AI.Mn.S 5 - 6 S i .Mn.S 1-2 Mn.S 2-5 Mn.S.Cr.S i 2-3

Cw Al 2 -5 AI.Mn.S 2 - 7 Mn, S 2 - 5

AI.Si.Mn.S 3 -5 Si.Mn.S 3 -4 Mn.S 4 - 8

58

TABLE 8 : OBSERVED MORPHOLOGY OF MnS

INCLUSIONS.

MELT MnS TYPE REMARK

Ai I NOTE 1

Aw I PURE I NOTE 1 II PURE

B i I PURE I NOTE 1

B w I PURE H PURE

Ci I PURE I NOTE 2 r t PURE

Cw i PURE i NOTE 3 u PURE

NOTE I : SULFIDES WERE ASSOCIATED WITH PARTICLES CONTAINING Al AND Zr .

NOTE 2- SULFIDES WERE ASSOCIATED WITH PARTICLES CONTAINING Al .

NOTE 3 = SULFIDES WERE ASSOCIATED WITH PARTICLES CONTAINING Al AND Si .

59

INGOT AXIS

FIG. 2 2 ' ORIENTATION OF IMPACT SPECIMENS

CUT FROM E S R INGOT.

6 0

WELD

AXIS

FIG. 2 3 • ORIENTATION OF IMPACT SPECIMENS

CUT FROM E S W .

61

FRACTURE APPEARANCE (%BRITTLE) o o o cb o O O

L A T E R A L CONTRACTION (mm)

u o

UJ cc

< or UJ

a.

ui

C V N E N E R G Y (ft.-lb.)

FIG. 2 4 = IMPACT E N E R G Y , L A T E R A L C O N T R A C T I O N and

F R A C T U R E A P P E A R A N C E for A i

62

F R A C T U R E A P P E A R A N C E ( % B R I T T L E ) o o o

CO o (0

o o CM

L A T E R A L CONTRACT ION ( m m )

o

C V N E N E R G Y (ft.-lb.)

FIG. 2 5 ' IMPACT E N E R G Y , L A T E R A L C O N T R A C T I O N

and F R A C T U R E A P P E A R A N C E f o r A w

a o

UJ cr

< cr

o_ UJ

63

F R A C T U R E A P P E A R A N C E (%BRITTLE) o o o

00 o 10

O

-T-O <\l

L A T E R A L CONTRACTION (mm)

C V N E N E R G Y (ft.-lb.)

FIG. 2 6 « IMPACT E N E R G Y , L A T E R A L C O N T R A C T I O N

and F R A C T U R E A P P E A R A N C E f o r B i

o o

or Z> y-< or UJ o.

Ul

6 4

F R A C T U R E A P P E A R A N C E (%BRITTLE) O o o o o y oo I D v CM <-

i i 1 i i 1 i i i 1 i L A T E R A L CONTRACT ION ( m m )

O O in O m i f ) < S Q h- «n co o O

- T — - i T i T 1 r 1 r — — i r 1 \ ?

C V N E N E R G Y <ft.- lb.)

FIG. 2 7 •• IMPACT E N E R G Y , L A T E R A L C O N T R A C T I O N

a n d F R A C T U R E A P P E A R A N C E f o r B w

65

FRACTURE APPEARANCE (%BRITTLE) 2 o o o o _ S 00 «0 * (M ^

I I I I I I I I I I *1 LATERAL CONTRACTION ( mm )

CVN ENERGY (ft.-lb.)

FIG. 28 « IMPACT ENERGY , LATERAL CONTRACTION

and FRACTURE APPEARANCE for Ci •

66

F R A C T U R E A P P E A R A N C E (%BRITTLE) o o o o o r

O OO <o ™ . r 1 1 1——i 1 1 1—"~i 1

L A T E R A L CONTRACT ION ( m m )

C V N E N E R G Y ( f t . - lb. )

FIG. 2 9 • I M P A C T E N E R G Y , L A T E R A L C O N T R A C T I O N

ond F R A C T U R E A P P E A R A N C E f o r C w .

67

Impact specimens of each melt were fractured at a temperature well into

the ducti le region of the DBTT curve. An instrumented drop weight impact

tester was used and the energy of fracture analysed. A l l specimens were

tested at 170°C, using a s i l i con o i l bath as the heating medium. An

oscillograph of each test recorded the impact transducer voltage output

from semiconductor strain gauges as i t varied with time during the fracture

event (Fig. 30). This was interpreted as s tr iker , or tup, load versus

time from internal electronic cal ibrat ion. Using a planimeter the area

under the load-time trace was analysed. The areas representing crack

in i t i a t ion and propagation were taken as the areas before and after the

maximum load point respectively. This and the following treatment were

44

procedures followed by Server for dynamic toughness evaluation of post

yie ld fracture.

A f i r s t approximation to the energy, Eo, represented by the area, A,

under the load-time curve may be found by Eo = A-v I I - l

This applies to the areas corresponding to fracture i n i t i a t i o n , propagation

or the total event. The striker velocity, v, at the time of impact with

the specimen i s

v = ( 2 . g - d ) 0 , 5 II-2

2 2

where g is the acceleration due to gravity (32.172 ft/sec , 9.806 m/sec )

and d is the mass drop height. Throughout this calculation the effects of

a ir resistance and f r i c t i ona l losses were assumed to be negligably small and

thus omitted. The value of Eo determined this way implies a constant striker

velocity during fracture. For b r i t t l e fracture, change in striker velocity

0.5 m sec. / div.

FIG. 3 0 = I N S T R U M E N T E D

TRAC ING .

IMPACT O S C I L L O G R A P H AND

69

i s ve ry s m a l l and thus any e r r o r to Eo w i l l a l s o be v e r y s m a l l . However,

where the absorbed energy i s l a r g e , as i t i s d u r i n g d u c t i l e r u p t u r e , a

c o r r e c t i o n to Eo must be a p p l i e d . The c o r r e c t e d energy, E c , may be determined

by

Ec = E o ( l - - Eo/4Es) I I - 3

where Es i s the i n i t i a l k i n e t i c energy of the s t r i k e r a t impact and has the

va lue

Es = m«g«d I I - 4

The va lue of m, the r e s t mass of the w e i g h t - s t r i k e r assembly, was 3.14 l b (m)

or 1.42 K g . The c o r r e c t e d f r a c t u r e energ ies are l i s t e d i n t a b l e 9.

70

TABLE 9 ' INSTRUMENTED IMPACT ENERGIES OF C V N SPECIMENS TESTED AT 170 °C .

MELT NO. OF TESTS

ENERGY f t . - lb . ( N - m ) MELT NO. OF TESTS

INITIATION PROPAGATION TOTAL

Ai 5 30.6 (41.5) 93.4(126.6) 124.0

(168.1)

A w 6 26.1 (35.4) 62.6 (84.9) 88.7

(120.3)

Bi 4 29.1 (39.5) 79.1 (107.2) 108.2

(146.7)

B w 5 26.8 (36.3) 68.6 ( 93.0) 95.4

(129.3)

C i 4 30.1 (40.8) 91.3 (123.8) 121.4

(164.6)

C w 5 28.8(39.0) 82.1 (III.3) 110.9

(150.4)

CHAPTER I I I

D i s c u s s i o n

S lag compos i t ion does not have a d i r e c t e f f e c t on mechanica l p r o p e r t i e s .

These however are r e l a t e d by the e f f e c t s l a g compos i t ion has on the a l l o y

and i n c l u s i o n contents of the remel ted m e t a l , and the subsequent e f f e c t of

these two parameters on mechanica l p r o p e r t i e s .

I I I . l . E f f e c t s on A l l o y Composi t ion

D i f f e r e n c e s i n a l l o y compos i t ion appear i n t a b l e 5. W i t h i n the accuracy

o f the spec tographic de t e rmina t ion used the elements C, C r , Cu , Mo and Nb

are the same as the p l a t e a l l o y con ten t . Lower va lues o f manganese and

s i l i c o n were observed i n a l l i n g o t s w h i l s t the weld me ta l contents of these

elements remained r e l a t i v e l y unchanged. A s i m i l a r t rend was observed w i t h

aluminum. Only the s u l f u r and oxygen a n a l y s i s show the same d i f f e r e n c e as

above, but no r e l a t i o n s h i p between these and the p l a t e contents can be

e s t a b l i s h e d from the da t a .

The mechanism of a l l o y l o s s to the s l a g has been de sc r i bed i n s e c t i o n 1.2.

Since a g rea te r l o s s of the more o x i d i z a b l e elements such as Mn and S i

was observed i n the i n g o t s remel ted through a l l s l a g systems, the s l a g com­

p o s i t i o n d i f f e r e n c e s are not r e s p o n s i b l e f o r t h i s e f f e c t . The magnitude o f

the d i f f e r e n c e i n r e a c t i v e element l o s s i s d i f f e r e n t f o r the v a r i o u s s l a g s

71

72

u s e d . S i n c e t h e a b s o l u t e a c c u r a c y o f t h e s p e c t o g r a p h i c a n a l y s i s t e c h n i q u e

i s unknown, i n d i v i d u a l c o m p o s i t i o n a l d i f f e r e n c e s be tween t h e i n g o t and

w e l d o f s p e c i f i c s l a g s y s tems c a n n o t be commented o n .

A n o t h e r mechan i sm t h a t w i l l e x p l a i n t h e d i f f e r e n c e i n a l l o y l o s s

between t h e ESR and ESW p r o c e s s i s p o s t u l a t e d . F o l l o w i n g t h e work o f

Beynon^ on t h e e l e c t r o c h e m i c a l e f f e c t s o f t h e e l e c t r o s l a g p r o c e s s , some

v e r y s i g n i f i c a n t d i f f e r e n c e s be tween w e l d i n g and r e f i n i n g may be e x p e c t e d .

The e l e c t r o s l a g r e f i n i n g t e c h n i q u e u sed was an i n s u l a t e d mo ld p r o c e s s .

The e f f e c t o f t h e d i f f e r e n c e i n t h e m e t a l - s l a g i n t e r f a c i a l a r e a s r e s u l t s i n

a g r e a t e r c u r r e n t d e n s i t y a t t h e e l e c t r o d e / s l a g t h a n a t t h e m e t a l p o o l / s l a g

i n t e r f a c e . The a p p r o x i m a t e i n t e r f a c i a l a r e a s o f t h e s e s u r f a c e s a r e l i s t e d

i n t a b l e 10. I n e l e c t r o s l a g w e l d i n g t h e p a r e n t p l a t e s c o n s t i t u t e a p a r t

o f t h e mo ld and a r e e l e c t r i c a l l y a c t i v e . The m e t a l on t h i s s u r f a c e a t t a i n s

a t l e a s t t h e l i q u i d u s t e m p e r a t u r e o f t h e a l l o y . As shown i n t a b l e 10 t h e

d i f f e r e n c e s o f t h e i n t e r f a c i a l a r e a s , and t h u s c u r r e n t d e n s i t i e s , a r e

g r e a t e s t f o r e l e c t r o s l a g w e l d i n g . The p o l a r i z a t i o n r e s u l t i n g f r o m e l e c t r o ­

c h e m i c a l r e a c t i o n s a t t h e s e s u r f a c e s w o u l d d e v e l o p a DC p o t e n t i a l be tween

them and t h e e l e c t r o d e w o u l d become t h e anode . S i n c e t h e m a g n i t u d e o f

t h i s p o t e n t i a l i s r e l a t e d t o t h e c u r r e n t d e n s i t y d i f f e r e n c e be tween t h e

e l e c t r o d e and m e t a l s i t e s i t w i l l be g r e a t e r f o r e l e c t r o s l a g w e l d i n g .

As d i s c u s s e d i n s e c t i o n 1.2. t h e p o l a r i z a t i o n p o t e n t i a l d e v e l o p e d a t

t h e e l e c t r o d e w o u l d no t e x ceed t h a t o f i r o n o x i d a t i o n ,

Fe = F e + 2 + 2e~ I I I - l

F o r s u f f i c i e n t l y h i g h c u r r e n t d e n s i t i e s t h e s l a g i n c o n t a c t w i t h t h e e l e c t r o d e

t i p w o u l d s a t u r a t e w i t h ( FeO) . T h i s b e h a v i o r was o b s e r v e d by Beynon i n t h e

n o n - l i n e a r c o r r e l a t i o n be tween p o l a r i z a t i o n p o t e n t i a l and c u r r e n t d e n s i t y

73

T A B L E 10 : E L E C T R O A C T I V E S U R F A C E A R E A S .

P R O C E S S E L E C T R O D E

i n 2 ( c m 2 )

LIQUID M E T A L S U R F A C E

i n 2 ( c m 2 )

RATIO

ESR (i) 1.8 (11.4) 6.5 (41.9) 3.7

ESW (w) 0.8 (5.2) 5.6 (36.3) min. 7.0

6.0 (38.7) max. 7.5

* ESTIMATED FROM MELT Bw .

SCHEMATIC OF E L E C T R O D E TIP P R O F I L E .

ESR E S W

74

( F i g . 3 1 ) . The r e s u l t o f t h e f o r e g o i n g argument i s t h a t t h e s l a g a d j a c e n t

t o t h e e l e c t r o d e t i p i s h i g h i n o x i d a n t , ( F e O ) , f o r b o t h e l e c t r o s l a g

r e f i n i n g and w e l d i n g . Thus r e a c t i v e a l l o y l o s s s h o u l d appea r i n t h e d a t a

o f b o t h p r o c e s s e s . A s t h i s was n o t o b s e r v e d t h e p a r t i c u l a r o x i d a t i o n

mechanisms o f manganese and s i l i c o n s h a l l be d i s c u s s e d .

S u f f i c i e n t thermodynamic i n f o r m a t i o n e x i s t s t h a t t h e e q u i l i b r i u m

c o n s t a n t s o f t h e Mn and S i d e o x i d a t i o n r e a c t i o n s may be c a l c u l a t e d . The

p r o p o s e d r e a c t i o n s a r e :

(FeO) + [Mn] = (MnO) + Fe I I I - 2

2(FeO) + [ S i ] = ( S i 0 2 ) + 2 Fe I I I - 3

= ^ M n O ) • > e ] I I X _ 4

1 1 1 1 a ( F e 0 ) • a [ M n ]

1 1 1 - 3 a [ S i ] \"(FeO) /

The above e q u i l i b r i u m c o n s t a n t s r e d u c e t o

K I I I - 2 = a ( M n ° ) / a [ M n l 1 1 1 - 6

K I I I - 3 = a ( S i 0

2 ) / a t S i ^ 1 1 1 - 7

i f t h e s l a g a t t h e e l e c t r o d e i s c o n s i d e r e d s a t u r a t e d w i t h ( FeO ) . U s i n g d a t a

45 46 f r o m Bod swo r t h and K u b a s c h e w s k i t h e r e l a t i o n s h i p be tween t h e e q u i l i b r i u m

c o n s t a n t and t e m p e r a t u r e may be c a l c u l a t e d ( s ee a p p e n d i x A I ) .

l o g K I I 3 ; _ 2 = (6400/T) - 2.8 I I I - 8

l o g K i ; [ I _ 3 = (19200/T) - 6.8 I I I - 9

75

FIG. 31 « ANODIC POLARIZATION {if) C U R V E S FOR P U R E IRON IN C a F 2 + A l 2 0 3

S L A G S . FROM R E F . 7 .

76

B o t h e q u i l i b r i u m c o n s t a n t s d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e . F r a s e r "

has shown t h e t e m p e r a t u r e d i f f e r e n c e be tween t h e e l e c t r o d e / s l a g and p o o l /

s l a g i n t e r f a c e s o f t he ESR f u r n a c e w i l l d e v e l o p a s i t u a t i o n where t h e r a t e

o f manganese r e d u c t i o n a t t h e h i g h e r t e m p e r a t u r e p o o l i n t e r f a c e i s n e a r l y

e q u a l t o t h e o x i d a t i o n r a t e a t t h e e l e c t r o d e . T h e r e ha s been no p r e c i s e

measurement o f t e m p e r a t u r e s d u r i n g e l e c t r o s l a g w e l d i n g . However some

47

w o r k e r s have o b s e r v e d t h e s l a g t e m p e r a t u r e d u r i n g ESW t o b e much h i g h e r

t h a n t h e s l a g t e m p e r a t u r e o f r e f i n i n g p r o c e s s e s . T h i s b e i n g t h e c a s e , t h e

above e q u i l i b r i u m c o n s t a n t s a r e s m a l l e r f o r w e l d i n g t h a n r e f i n i n g . Thus

more o x i d a t i o n o f manganese and s i l i c o n was e x p e c t e d d u r i n g t h e r e f i n i n g

p r o c e s s t h a n w e l d i n g . T h i s d i s c u s s i o n r e l i e s on c l a s s i c a l t h e r m o ­

dynam ic s and e q u i l i b r i u m c o n d i t i o n s may n o t be a c h i e v e d . The o v e r a l l t r e n d

however a c c o u n t s f o r t h e d i f f e r e n c e i n o x i d a t i v e a l l o y l o s s be tween t h e

p r o c e s s e s .

I I I . 2 . E f f e c t on I n c l u s i o n P o p u l a t i o n

O t h e r e l e m e n t r e c o v e r i e s were s i m i l a r t o t h a t o f manganese and s i l i c o n .

However t h e mechan i sms o f mass t r a n s p o r t f r o m t h e e l e c t r o d e t o t h e m e t a l

p o o l were d i f f e r e n t . . A luminum and p o s s i b l y z i r c o n i u m were p r e s e n t i n t h e

e l e c t r o d e p r i m a r i l y a s s t a b l e o x i d e s . The t i m e and t e m p e r a t u r e , abou t 100°C

s u p e r h e a t , a r e i n s u f f i c i e n t t o p e r m i t d i s s o l u t i o n o f t h e s e i n c l u s i o n s i n

t he e l e c t r o d e t i p o r d r o p l e t zones d u r i n g r e m e l t i n g . A l t e r n a t i v e l y t h e s e

i n c l u s i o n s pa s s i n t o t h e s l a g where t h e y d i s s o l v e . C o m b i n i n g t h i s mechan i sm

and t h e p r i n c i p l e s o f o x i d a t i o n f r o m t h e p r e c e d i n g s e c t i o n , many workers " * " * * ' " ^

c o n c l u d e t h e r e a r e no i n c l u s i o n s I n t h e r e m e l t e d m e t a l t h a t o r i g i n a t e d i n

t h e e l e c t r o d e . I n c l u s i o n s d e t e c t e d i n an i n g o t a r e t h e r e s u l t o f p r e c i p i t a t i o n

77

r e a c t i o n s i n t h e l i q u i d o r l i q u i d - s o l i d (mushy) r e g i o n o f t h e m e t a l p o o l .

I n e l e c t r o s l a g w e l d i n g i t i s c o n c e i v a b l e t h a t a s e cond s o u r c e o f i n c l u s i o n s

t o t h e w e l d m e t a l e x i s t s . B e s i d e s t h o s e f o rmed by p r e c i p i t a t i o n , some

i n c l u s i o n s f r o m t h e m e l t e d , o r u n d e r c u t , f a c e s o f t h e p a r e n t p l a t e s w i l l

e n t e r t h e w e l d m e t a l d i r e c t l y . The more r e f r a c t o r y v a r i e t y l i k e a l u m i n a

a r e i n t h i s c a t e g o r y . The number o f i n c l u s i o n s w h i c h e n t e r t h e w e l d m e t a l

p o o l t h i s way w i l l be a f u n c t i o n o f t h e i n c l u s i o n p o p u l a t i o n and e x t e n t

o f u n d e r c u t t i n g o f t h e p a r e n t p l a t e s . T h i s a c c o u n t s f o r some o f t h e

i n c l u s i o n p o p u l a t i o n o b s e r v e d i n t h e w e l d m e t a l . The m a j o r i t y , b o t h o x i d e s

and s u l f i d e s , a r e a p r o d u c t o f p r e c i p i t a t i o n r e a c t i o n s d u r i n g s o l i d i f i c a t i o n .

The o c c u r e n c e o f a l u m i n a i n c l u s i o n s i n ESR i n g o t s was s t u d i e d by

B e l l ^ . He c o n c l u d e d s m a l l e v e n l y d i s t r i b u t e d a l u m i n a i n c l u s i o n s a r e a

n e c e s s a r y c o n s t i t u e n t o f i n g o t s t h a t had been r e m e l t e d t h r o u g h a s l a g c o n ­

t a i n i n g a l u m i n a . The c a l c u l a t e d e q u i l i b r i u m d i a g r a m f o r t h e r e a c t i o n

A 1 2 0 3 = 2 [A1] + 3 [ 0 ] 111-10

l o g K J H . J Q = ( -64000/T) + 2 0 . 5 I I I - l l

i s shown i n f i g u r e 32. Each l i n e r e p r e s e n t s t h e p r e d i c t e d e q u i l i b r i u m a t

1700°C and 2000°C f o r v a r i o u s a l u m i n a a c t i v i t i e s i n s l a g s . F o r a g i v e n

a l u m i n a a c t i v i t y t h e v a l u e o f t h e e q u i l i b r i u m c o n s t a n t i s p r o p o r t i o n a l t o

t h e t e m p e r a t u r e . S i n c e t h e s l a g t e m p e r a t u r e f o r w e l d i n g i s h i g h e r t h a n

r e f i n i n g , t h e m e t a l p o o l o f t h e w e l d i n g p r o c e s s c a n be e x p e c t e d t o c o n t a i n a

g r e a t e r q u a n t i t y o f d i s s o l v e d a luminum and o x y g e n . T a b l e 5 shows t h i s

d i f f e r e n c e was o b s e r v e d i n a l l s l a g s y s t e m s .

A s i m i l a r d i s c u s s i o n may be u sed t o d e s c r i b e t h e s u l f u r e q u i l i b r i u m

between t h e m e t a l p o o l and s l a g p h a s e . H i s t o r i c a l l y t h e mechan i sm o f s u l f u r

78

FIG. 32 = R E L A T I O N S H I P OF [Al] AND [o] IN

EQUIL IBR IUM WITH A L U M I N A O F VAR IOUS

ACT IV IT IES : F R O M R E F . 16 .

79

removal has been treated differently. For lime containing slags the

24

proposed desulfurization reaction is

(CaO) + [S] = (CaS) + [0] 111-12

A G ° = +12.7 Kcal . (at 1600°C) 111-13 The equilibrium constant for this reaction i s

. *<CaS) • a [0] m . u

i n - 1 2 a(CaO) • a [ s ]

In practice, sulfur removal has been analysed as a relationship between an

empirically derived excess base quantity and the sulfur part i t ion between

the slag and metal phases. The excess base quantities calculated for the

slags used in this study are negative (see appendix A l l ) . For these values

the sulfur part i t ion , (S)/[S], would be close to unity. However some de­

sulfurization occurs, the extent of which cannot be determined by this type

of calculation.

The composition of the slag w i l l change during the electroslag process.

Lime is present in a l l calcium fluoride based slags. In this study i t is

conceivable that another source of lime was the formation of (CaO) from

slag-air reactions. Other compounds picked up by the slag during remelting

include (FeO). The source of oxygen to this slag constituent i s also the

a i r . Thus the operating composition of the slag w i l l be different from the

i n i t i a l composition. The effect of these additions to a slag i s to increase

'the slag basicity . The amount by which the basicity increases is i n ­

calculable as many essential variables, such as s lag-air interaction and

anodic reactions, are unknown. For this reason the calculated excess base

quantity certainly does not relate to the desulfurization capacity of the

slag systems used.

80

The s u l f u r contents of t a b l e 5 show a genera l decrease as a r e s u l t

of both e l e c t r o s l a g p rocesses . T h i s o b s e r v a t i o n conforms to the above

argument tha t a l l s l a g systems used i n t h i s s tudy to some extent remove

s u l f u r from the e l e c t r o d e s t ock .

S ince bo th we ld ing and r e f i n i n g was c a r r i e d out exposed to a i r , s u l f u r

4 48

r e a c t i o n s between the s l a g and gas phase e x i s t . Many workers ' have

cons idered the s u l f u r d i s t r i b u t i o n r e a c t i o n h S2+ (0~ 2 ) = h 0 2 + ( S - 2 ) 111-15

at low oxygen i o n a c t i v i t i e s , as i n a c i d s l a g systems, t h i s r e a c t i o n favours

the format ion of gaseous s u l f u r . When s l a g - a i r systems are used the f o r -

4

mation of SO2 i s most p robab le . Turkdogan de f ined a s u l f i d e c a p a c i t y of

s l a g by the term C s , C s = N s ' ( P 0 2 / p S 2 ) % IH-16

where Ns i s the atomic f r a c t i o n of s u l f i d e ions i n the s l a g and ?Q2 and 7^

are the p a r t i a l p ressures of oxygen and s u l f u r r e s p e c t i v e l y i n the gas

phase. From s l a g - m e t a l and gas-metal e q u i l i b r i a he c a l c u l a t e d

l o g C g = - (2660/T) - 1.276 + v 111-17

The term v i s a constant r e l a t e d to s l a g compos i t i on . By i n s p e c t i o n , the

s u l f i d e c a p a c i t y of the s l a g i nc r ea se s as the temperature i n c r e a s e s . Thus

the h ighe r temperature s l a g s of the we ld ing process have a g rea te r s u l f i d e

c a p a c i t y , and l e s s s u l f u r i s t r a n s f e r r e d to the a i r than i n the ESR p roces s .

In a l l cases the weld me ta l con ta ined more s u l f u r than the i n g o t . T h i s

cannot be due e n t i r e l y to d i l u t i o n of the remel ted e l e c t r o d e meta l by the

l i q u i d s t e e l from undercut areas of the parent p l a t e . An es t imate of the

f r a c t i o n of weld meta l made up by the mel ted parent p l a t e was about 33

81

percent f o r the worst case , melt Bw (Appendix A I I I ) . I f d i l u t i o n were

the o n l y cause of h ighe r s u l f u r contents i t would have to be about 70

percent to account f o r the d i f f e r e n c e between the weld and i ngo t a n a l y s i s

of the B experiment . Impl ied i s the assumption tha t there i s no s u l f u r

r e f i n i n g of the parent p l a t e meta l by the s l a g o c c u r s . R e a l i s t i c a l l y some

d e s u l f u r i z a t i o n would be expected r e q u i r i n g the d i l u t i o n to be h i g h e r . 4

F o l l o w i n g Turkdogan 's a n a l y s i s the above d e s u l f u r i z a t i o n r e a c t i o n

(111-12) i s e s s e n t i a l l y the oxygen-su l fu r exchange r e a c t i o n ,

[S] + (0~ 2 ) - (S~ 2 ) + [0] 111-18

45

From Bodsworth the s tandard f ree energy o f r e a c t i o n i s

AG° = 17,200 - 9.12 T c a l . / m o l e 111-19

and l o g K J J J ^ Q - - (3750/T) + 1.996 111-20 -2

The r e a c t i o n favours the p r o d u c t s , (S ) + [ 0 ] , as the temperature i nc rea se s .

Thus the h ighe r temperature we ld ing process would r e s u l t i n g rea te r s u l f u r

removal from the meta l i f e q u i l i b r i u m were ach i eved . S ince t h i s was not

observed, a d e s u l f u r i z a t i o n r e a c t i o n more a p p l i c a b l e to the e l c t r o s l a g

process must be used.

F r a s e r approached d e s u l f u r i z a t i o n of s t e e l by combining the above

oxygen-su l fu r exchange r e a c t i o n w i t h the o x i d a t i o n of i r o n Fe + [0] = (FeO) 111-21

+2 -2

The component (FeO) was cons idered to be i o n i z e d as (Fe ) and (0 ) . The

r e s u l t i n g r e a c t i o n i s [S] + Fe = ( S - 2 ) + ( F e + 2 ) 111-22

-2 +2 T r e a t i n g (S ) as (CaS) and (Fe ) as (FeO) the e q u i l i b r i u m cons tant was

82

calculated as

K . TII-22 (CaS) • a(FeO)

a [ s ] 111-23

also

K . = exp (2330/T - 4.07) 111-24 TII-22

This mechanism provides a decrease in desulfurization rate as the temperature

increases. Equilibrium conditions are however implied which may not be

the case and differences in the reaction constants at 1675°C and 2000°C are

too small to account for the observed difference in sulfur removal.

A factor which does affect the above desulfurization reaction is the

(FeO) content of the slag. Because of polarization and the electrochemical

behavior of slag-metal reactions (see section I I I - l ) , welding slags contain

a higher (FeO) proportion than ESR slags. Since this source of (FeO) i s

greater than that of the desulfurization reaction, the act iv i ty of (FeO) in

the slag i s controlled by this mechanism. For both processes sulfur removal

is suppressed by this alternate source of (FeO). Since refining slags

contain less (FeO) than the corresponding ESW slags, more sulfur i s

extracted from ESR processed steel than from weld metal. The resulting

sulfur content of the weld metal w i l l be greater than the corresponding ingot

but less than the i n i t i a l a l loy.

The same difference in the slag (FeO) content between processes

accounts for the different metal oxygen contents. Added to this mechanism

are the reduction reactions of the more reactive al loy elements oxidized at

the electrode/slag interface (reactions III-2 and III-3). The amounts of

83

oxygen in the remelted metal that originate from these sources, and the

aluminum equilibrium reaction (111-10), cannot be determined from these

data. However, since the slags of the welding processes develop higher

contents of these oxidized al loy elements i t i s evident that the weld metal

w i l l reflect this with higher oxygen analysis as shown in table 5.

III.3. Inclusion Distribution

The effect which slag composition has on the remelted metal i s also

observed by the inclusion character in the materials under study. Prior

to so l id i f i cat ion the weld metal pool contains a greater concentration of

inclusion forming elements than the ESR metal pool. The weld metal contains

s l ightly more Mn and Si and much more A l , S, and 0. This observation

explains the difference between inclusion area fractions of table 6. For

a l l slag systems, the weld metal held a greater area fraction of inclusions

than the ingot metal. Thus features unique to electroslag welding, such

as higher operating temperatures and different slag electrochemical reactions,

result in a larger inclusion content.

The inclusion counts summarized in table 6 show a pattern which is only

related to the slag composition. These data indicate that inclusion area

fraction, size and possibly spacing, are related to slag bas ic i ty . The

excess base quantity of each slag is determined in appendix II . From these

calculations the slag system A (80% CaF 2 , 20% Al^O^) was the most basic and

C (55% CaF 2 , 15% A l ^ , 15% CaO, 15% Si0 2 ) the most ac id ic . The metal pro­

cessed with slag composition A, either ingot or weld, may be considered the

cleaner steel . This material had the least area fraction and size of

inclusions. The material remelted through slag system C had the greatest of

84

these two parameters. The inclusion distr ibution of slag B processed metal

f e l l between A and C. Note the above trend is applied to ingot and weld

metal considered separately. Also, an underlying assumption implied through­

out this correlation i s that slag compounds generated by each process have

an equal effect on the slag basicity of the individual melts. Specif ical ly

that the (CaO) and (FeO) contents impart an identical influence to the

slag acid-base reactions for a l l systems. This assumption is reasonable.

The slag preparation and melting procedures were the same in a l l experiments.

Thus no difference in extra lime should develop between runs. Also the

(FeO) generated is related to the electrochemical characteristics d i s ­

cussed in section III-2, and was constant between electroslag welds and

ingots respectively. Thus as slag basicity increases, so does the general

cleanliness of the remelted steel .

The sulfur and oxygen analyses of table 5 s l ight ly correlate to the 25 3

calculated slag excess base quantities. A conclusion of many researchers '

i s that the type of manganese sulfide inclusion observed in steel i s

related to the rat io of oxygen to sulfur analysis. This i s also applicable

to these results . Remelted metal with [0]/[S] greater than 0.04 contained

exclusively type I MnS inclusions. Experiments A i and Bi f e l l in this range.

In the remaining experiments the remelted steel contained both type I and

II inclusions and the [0]/[S] rat io ranged from 0.024 to 0.01. As this

ratio decreases further the morphology of MnS inclusions would be expected

to change to a population of only type II and eventually type III . For

pract ical purposes the appropriate values of [0] and [S] were not attained

and thus the higher sulfides not observed. Generally, the manganese su l ­

fides were a small fraction of the total inclusion count. This was

irrespective of the absolute numbers obtained for the various experiments

85

and is a qualitative assessment as a segregated inclusion distr ibution

analysis was not included in the procedure.

Unlike the MnS inclusion survey the composition of the remaining

inclusions was not completely known. Major elements detected from these

inclusions indicate their composition and variety dif fer between the

processes and slag systems used. In a l l cases aluminum bearing inclusions

were assumed to be alumina. Also inclusions that contained only aluminum

and zirconium, melts A i and B i , were assumed to be the oxides of these

elements. The composition of the remainder of the inclusions surveyed

could not be determined from the data. Some particles analysed indicated

a homogeneous composition while others show areas of local concentration

differences throughout the inclusion.

Table 7 l i s t s the elements detected in inclusions observed on both

deep etched and fracture surfaces. Although most elemental combinations

found in inclusions are l i s t e d , some were less frequently found than others.

Inclusions that contained zirconium and in the one melt, C i , chromium

were rare. Also inclusions greater than eight microns were rare, but

easily detected because of their size. S i l i con bearing inclusions were

also rare in the A and B slag processed materials but accounted for a

large fraction of the inclusions observed in the C slag processed ingot and

weld metal. This was expected as the-C slag composition contained 15 weight

percent s i l i c a and thus the s i l i c a act iv i ty of this slag would be greater

than the others. The s i l i c a reduction reaction,

(SI0 2) = [Si] + 2[0] 111-25

obeys the same arguments applied to alumina reduction (section 1.2). Thus an

increase in slag s i l i c a act iv i ty is related to an increase in the dissolved

86

s i l i con in the metal pool and s i l i c a type inclusions would result from

precipitation reactions during so l id i f i ca t ion .

III.4. Mechanical Properties

The data of figures 24 to 29 i s summarized in table 11. There was

much scatter among the transition temperature values and as such a

correlation may not be drawn from these. The only consistent data is the

upper shelf energy and a correlation between this parameter and the slag

systems used w i l l be discussed.

For every experiment the impact energies of the electroslag refined

ingot metal was greater than that of the weld metal. The correspondingly

lower inclusion area fraction of the ingot metal substantiates the trend

shown in figure 10. Generally as the inclusion content decreases, the

upper shelf impact energy increases. This correlation i s true for both

fracture in i t i a t ion and propagation energy (see table 9). However the

greatest difference between the weld and Ingot metal properties was in the

propagation energies. In every experiment the change in propagation

energy was greater than the corresponding change in i n i t i a t i o n energy.

Thus the inclusion content of steel has a greater effect on crack propa­

gation than on i n i t i a t i o n .

Another trend apparent in table 11 was that the upper shelf energy

increased as the acidity of the slag through which the steel was remelted

increases. The impact energies of the C melt steel was greater than the

A melt steel , ingot and weld metal considered separately. This confl icts

with the above conclusion that the upper shelf energy is adversely affected

by the inclusion content of steel . The C slag processed material had the

87

T A B L E II • SUMMARY OF IMPACT TEST DATA.

MELT 5 0 % F A T T 1

C O

D B T T L C C V N

(°C) (°C)

S H E L F ENERGY (ft-lb.)

A i 8 8 9 4 9 8 121

Aw 9 8 9 5 9 8 8 6

B i 9 7 9 9 117 129

B w 8 6 8 8 109 9 4

C i 100 8 8 9 7 1 3 5

C w 103 100 117 1 12

88

highest inclusion content of the study. However in the l i terature a

mechanism is proposed that accounts for this phenomenon and i t has been

applied to this data.

36 37

Brooksbank and Andrews ' conclude not a l l inclusions are d e t r i ­

mental to mechanical properties in steel . They studied the effect of

thermal expansion of inclusions on the tessellated stresses within the

matrix in contact with an inclusion. A similar conclusion by Lyne and

49

Kosak was that resulfurized bearing steel contains re lat ive ly fewer

oxide type inclusions. These are the source of stress concentration sites

and are deleterious to fatigue p r o p e r t i e s ^ A l s o they conclude the

oxide inclusions encapsulated in sulfides are less effective stress

raisers than the single phase oxides. This is consistent with the findings

of Brooksbank and Andrews. They measured sulfide inclusions as having a

greater thermal expansion coefficient than steel . Thus the tessellated

stresses about an oxide inclusion surrounded by a sulfide would be reduced.

This argument can be applied to the present study. Although no inclusions

encapsulated in sulfides were found, their existence may not be discounted.

Some inclusions were observed to have a second phase component forming a

part ia l or thin coating about an inner phase (see F i g . 33). The composition

of these phases could not be separated with the technique employed.

Slightly higher sulfur contents and lower oxygen contents of the more

acidic slag processed ingot and weld metals would lead to the acceptance

of the Lyne and Kasak conclusion.

A less conjectural argument assumes the composition of inclusions

changes as the slag through which the steel was processed becomes more

acidic . The actual inclusion compositions could not be determined but the

ALUMINUM SULFUR

CO

LL. T O W v x

FIG. 33 a = X-RAY ENERGY ANALYSIS OF AN

INCLUSION IN C i . ( D E E P ETCHED

5 0 % H N 0 3 , 1200 X )

FIG. 33 b X-RAY ENERGY ANALYSIS OF AN

INCLUSION IN C w . ( D E E P ETCHED

50 % H N 0 3 , 6 0 0 0 X )

91

observations of table 7 indicate the existence of elements such as s i l i con

in the inclusions of melts B and C. From table 5 the oxygen contents of

the ingot and weld metal, notably the weld metal, decreased as the slag

acidity through which i t was processed increased., This simultaneous

decrease in oxygen levels and increase in the sulfur content and mechanical

performance conforms to the conclusions of other w o r k e r s ^ T h e lower

oxygen levels may be direct ly interpreted as fewer oxide inclusions, whether

they be aluminates or the less detrimental s i l i ca tes . The higher sulfur

levels impart a beneficial effect described in the previous paragraph.

Both sulfides and some s i l i cates are known to have a greater inclusion to

matrix cohesion than alumina and aluminate inclusions'^." For this reason

some improvement to fatigue properties has been reported for resulfurized

and acid processed steels.

The mechanism relating impact properties and inclusion character was

not the theme of this study. The general inverse relationship between

inclusion content and ductile impact energy has been reaffirmed. However

the detrimental effect of each type of inclusion and how they relate to

properties is important. As the inclusion composition i s a function of the

slag composition through which the steel was remelted, then the slag

composition does effect mechanical properties.

CHAPTER IV

Conclusion

Different al loy recoveries and inclusion contents were observed i n

the remelted metal of the welding and refining processes. The effect on

impact properties was attributed to these parameters and the effect of

inclusions on ductile fracture was reaffirmed. Since the only variable

in this investigation was the welding and refining process the various

process characteristics were studied, speci f ical ly the slag temperature

and reactor geometry.

The slag temperature of welding was higher than refining although a

precise value was not known. Relationships between temperature and equ i l i

brium constants provide a mechanism for greater manganese and s i l i c o n

recovery and aluminum and oxygen deposition in the weld metal than the

equivalent ingot. The reactor geometry influences the electrochemical

reactions at the l iqu id metal and electrode reaction s i tes . A greater

difference of current density characteristic to electroslag welding result

in higher polarization potentials between the active sites. In iron

systems the compound (FeO) i s the product of this polarization and thus th

welding slags contain a higher fraction (FeO). This compound in the slag

suppresses the desulfurization reaction and contributes to an increase in

oxygen deposition.

These two process characteristics resulted in greater inclusion

contents and lower impact values for the weld metal.

92

93

The ducti le impact energies varied as the inclusion content and type.

As the inclusion content increased for a given slag composition, the upper

shelf impact energy decreased. Instrumented impact testing showed the frac ­

ture propagation energy to be most sensitive to inclusions. However as the

inclusion composition changed, notably as the sulfur and s i l i con levels i n ­

creased, higher impact energies were observed between welds or ingots

respectively. This reaffirms prior studies that the thermal expansion of

inclusions or the cohesive strength of the inclusion-matrix interface, or

both, affect mechanical properties. Thus chemical composition must be con­

sidered when relat ing inclusion content to properties.

The effect of slag on mechanical properties was considered i n two ways.

F i r s t l y the temperature and (FeO) content determine the extent of oxidation

at the electrode, introduction of elements, such as aluminum, which form

oxide inclusions, and the suppression of the desulfurization reaction.

Thus the inclusion volume fract ion i s related to these parameters. Secondly

the effect of slag acid-base behavior on slag/metal reactions was con­

sidered. For small changes in slag ac idi ty l i t t l e difference i n al loy

recoveries was observed. However differences in inclusion composition

were related to ac idi ty and the part icular slag composition. The presence

of s i l i c a i n the slag and the resulting population of the less detrimental

s i l i c a type inclusions is an example of th i s .

From the results of the study we conclude that:

a. the mechanical properties of ingots and welds made using ident ica l

slags and melting procedures are not the same. Hence an ESW slag cannot be

qualif ied by testing ingots remelted through i t . The slag can only be

qualif ied by producing and testing electroslag welds.

94

b. the inclusion population of electroslag welds influences mechanical

properties. However the relat ion is not a simple one and due account

must be taken of the inclusion composition as well as the inclusion content.

BIBLIOGRAPHY

1. A m e r i c a n W e l d i n g S o c i e t y " W e l d i n g H ighway and R a i l w a y B r i d g e s " , AWS D2.0 - 6 9 .

2. " The P h y s i c a l C h e m i s t r y o f I r o n and S t e e l M a k i n g " , Ward R . G . , Edward A r n o l d P u b . , 1962 .

3. Zeke J . , " M e t a l l u r g i c a l P r o c e s s e s D u r i n g E l e c t r o s l a g M e l t i n g E x p l a i n e d f r o m t h e V i e w p o i n t o f I o n T h e o r y " f r o m Second I n t . Symp. on ESR T e c h n o l o g y , C a r n e g i e - M e l l o n I n s t . , 1969.

4. Tu r kdogan E .T . , J . I . S . I . , JL79, 147 , 1955.

5. K o j i m a H. Masson C . R . , C a n . J . Chem. , 47^ 4 2 2 1 , 1969.

6. F r a s e r M .E . , Ph .D . T h e s i s , U n i v e r s i t y o f B . C . , 1974.

7. Beynon G . , Ph .D . T h e s i s , U n i v e r s i t y o f B . C . , 1971 .

8. G r a n t N . J . , Ch ipman J . , T r a n s . A IME, 1 67 , 134 , 1946 .

9. E t i e n n e M . , Ph .D . T h e s i s , U n i v e r s i t y o f B . C . , 1970.

10. Hawk ins R . J . , D a v i e s M.W., J . I . S . I . , 209 , 226 , 1971 .

11 . Roge r s P . S . , e t a l . , " M e t a l - M o l t e n S a l t S o l u t i o n s " , i n P h y s i c a l C h e m i s t r y o f P r o c e s s M e t a l l u r g y , P t . I I , e d . S t . P i e r r e G .R . , I n t e r s c i e n c e P u b . , 1959.

12 . Hawk in s R . J . , e t a l . , " R e l e v a n c e o f L a b o r a t o r y E x p e r i m e n t s t o t h e C o n t r o l o f C o m p o s i t i o n i n P r o d u c t i o n - S c a l e E SR " , i n E l e c t r o s l a g R e f i n i n g , I S I P u b . , p . 2 1 , 1973.

13. " B a s i c Open H e a r t h S t e e l m a k i n g " , e d . De rge G . , 3 r d E d . , A IME, 1964.

14. " E l e c t r i c F u r n a c e S t e e l m a k i n g " , e d . S ims C . E . , V o l . I I , A IME, I n t e r ­s c i e n c e P u b . , 1963.

15 . Swinden D . J . , " P a r a m e t e r s A f f e c t i n g t h e B e h a v i o r o f S u l f u r D u r i n g E SR " , i n C h e m i c a l M e t a l l u r g y o f I r o n and S t e e l , e d . B u c k l e E.R. and Hawk ins R .J . ' , I S I , 1973 .

16. B e l l M .M. , M a s t e r s T h e s i s , U n i v e r s i t y o f B . C . , 1971 .

17. N a f z i g e r R . H . , U.S. Bu r eau o f M i n e s n o . 669 , p. 5 5 , 1976.

95

96

18. Liddle J . F . , "Removal of Inclusions during ESR", in Chemical Metallurgy of Iron and Steel, ed. Bickle E.R. and Hawkins R . J . , ISI, 1973.

19. Mitchel l A . , Ironmaking and Steelmaking, JL, (3), 172, 1974.

20. Gray M . J . , Wilson W.G., Pipeline and Gas J . , 198, (12), 50, 1972.

21. Gladman T . , et a l . , JISI, 208, 172, 1970.

22. Mikelich J . L . , et a l . , JISI, 209, 469, 1971.

23. Matsubara K . , Trans ISIJ, 6, 29, 1966.

24. "Principles of Extractive Metallurgy", Rosenquist T . , McGraw-Hill Pub l . , 1974.

25. Sims C . E . , Trans AIME, 215, 367, 1959.

26. Baker T . J . , Charles J . A . , JISI, 210, 702, 1972.

27. "Non-Metallic Inclusions in Steel", part I I , Kiessling R. and Lang N . , ISI, SR90, 1966.

28. Bigelow L . K . , Flemings M . C . , Met. Trans. B . , 6B, 275, 1975.

29. Mohla P . P . , Beech J . , JISI , 207, 177, 1969.

30. Fredriksson H . , H i l l e r t M . , Scand. J . Metallurgy, 2, 125, 1973.

31. Barnsley B . P . , J . Australian Inst, of Metals, 14, (2), 65, 1969.

32. Marish S., Player R . , Met. Trans. , 1, 1853, 1970.

33. Fredriksson H . , H i l l e r t M . , JISI, 209, 10.9, 1971.

34. Bernard G . , et a l . , Metals Tech. , 2, 512, 1975.

35. Baker T . J . , Charles J . A . , JISI, 211, 187, 1973.

36. Brooksbank D . , Andrews K.W., JISI, 210, 246, 1972.

37. Brooksbank D . , Andrews K.W., JISI, 206, 595, 1968.

38. Kiessling R . , Nordberg H . , "Influence of Inclusions on Mechanical Properties of Steel", i n Production and Application of Clean Steels, ISI Pub., 1972.

39. Gladman T . , "Cavitation at Sulfides During Deformation", i n Sulfide Inclusions in Steel, ASM Pub. no. 6 in Materials Series, 1974.

97

40. Akira Masui, "Mathemetal Model for Hydrogen Behavior in ESR", in Fi f th Int. Symposium on Electroslag and Other Remelt. Tech. , pt. I , p. 284, 1974.

41. Chipman J . , "The System Fe-C-Si-0", in Steelmaking: The Chipman Conference, ed. E l l i o t J . F . , MIT Press Pub., 1962.

42. Kojima Yasushi, Trans. ISIJ, 15, 397, 1975.

43. "Electroslag Welding", ed. Paton B . E . , AWS Pub., New York, 1962.

44. "Strength and Toughness Evaluation from an Instrumented Impact Test", Server W.L. , et a l . , Effects Tech. Corp. , TR74-29R, Cal i forn ia , 1974.

45. "Physical Chemistry of Iron and Steel Manufacture", Bodsworth C , Longmans Pub., London, 1963.

46. "Metallurgical Thermochemistry", Kubaschewski 0. , Pergamon Press Pub., Oxford, 1967.

47. Bacon G.W., Unpublished data.

48. St. Pierre G.R. , Chipman J . , Trans. AIME, 206, 1474, 1956.

49. Lyne C M . , KOSAK A. , Trans. ASM, 61, 10, 1968.

50. Tricot R . , "Relative Detrimental Effect of Inclusions on Service Properties of Bearing Steels", in Production and Application of Clean Steels, ISI, 1972.

51. Suzumoto T . , Sugihara T . , Proceedings of the fourth International Symposium on ESR Processes, ISIJ, p. 115, 1973.

52. Data from this work.

53. Mitchel l A . , unpublished data.

54. Paton B . E . , "New Method for Fabrication of Cyl indr ica l ESR Ingots into Shapes Suitable for Production of Rotors", in Electroslag Refining, ISI, p. 105, 1973.

t

Appendix I

A . I Calculated Equilibrium Constants for Si and Mn

The equilibrium constants at 1600°C for the reactions

[Mn] + {FeO} = Fe + {MnO}

[Si] + 2{FeO} = 2Fe + {Si02}

61 62 have been calculated with the data from Bodsworth and Kubaschewski

The oxidation of manganese was f i r s t considered. For the reaction:

{Mn} + h02 = <MnO> AI 1

the standard free energy change i s

AG° = - 95,400 + 19.7 T AI 2 ref.62

To change sol id MnO to a l iqu id product,

<MnO> = {MnO} AI 3

the standard free energy change i s expressed by:

AG° = 10,700 - 5.2 T AI 4 ref.61

The solution of manganese in l iquid iron i s :

[Mn] = {Mn} AI 5

and: <

AG° = 9.11 T AI 6 ref.61

Combining the relations AI 1, AI 3 and AI 5 the resultant manganese oxidations

reaction i s

98

99

[Mn] + h02 = {MnO} AI 7

The standard free energy of reaction is found by combining the equations AI 2,

AI 4 and AI 6

AG° = - 84,700 + 23.6 T AI 8

For the iron oxidation reaction

{FeO} = Fe + h02 AI 9

the standard free energy change is

AG° = 55,620 - 10.8 T AI 10 ref.62

Adding reactions AI 7 and AI 9 the oxidation of manganese by iron oxide may be

expressed as

{FeO} + [Mn] = [Fe] + {MnO} AI 11

and the free energy change i s found by combining AI 8 and AI 10

AG" = - 29,080 + 12.8 T AI 12

The equilibrium constant represents the rat io of the reactants and products

as written and i s expressed as

a r „ _ T • a

fFe] ' a{MnQ} A I 13

[Mn] * 3{Fe0}

The equilibrium constant i s related to the standard free energy of reaction

AG0 = - 2.303 RT log K

The value of R, the gas constant, i s 1.98 c a l ' ° C _ 1 ' m o l 1 . Thus the e q u i l i ­

brium for manganese oxidation was related to temperature by

100

The same analysis was done for the oxidation of s i l i con

[Si] + 2{Fe0} = <Si02> + 2[Fe] AI 14

AG° = - 87,960 + 31.1 T AI 15

The relationship between the s i l i con oxidation equilibrium constant and

temperature is

19200 /- Q A T i t log K g i = — ^ 6.8 AI 16

Due to inava i lab i l i ty of applicable data the reaction i s written for sol id

s i l i c a as the product. In the slag systems used s i l i c a i s very soluble and

should be treated as l iqu id dissolved in a slag. These considerations would

not alter the fact the equilibrium constant decreases as the reaction

temperature increases.

101

Appendix II

A.II Excess Base Values of Slags

Excess base quantities were calculated for the i n i t i a l slag com­

position of each experiment. The relat ion between excess base and slag

composition was taken from Bodsworth^, page 429.

E.B. = n CaO + n MgO + n MnO - 2n S i0 2 - 4n P 2 0 5

- 2n A1 2 0 3 - n ^&2°2

Here, n represents the number of moles of the oxide i n 100 grams of slag.

The value of n for each oxide and the corresponding excess base quantity

has been calculated and l i s ted in table 12.

TABLE 12

VALUE OF n FOR INITIAL SLAG COMPOSITIONS

Slag A1 2 0 3 CaO S i0 2

E.B.

A .196 - .39

B .343 .178 - .51

1

C .147 .267 .250 - .53

102

A p p e n d i x I I I

A . I l l Weld M e t a l D i l u t i o n

D i l u t i o n o f w e l d m e t a l i n m e l t Bw:

From F i g u r e 4 t h e a r e a o f w e l d m e t a l was measured u s i n g a p l a n i -

m e t e r . T h i s f i g u r e i s a p h o t o g r a p h o f a h o r i z o n t a l c r o s s s e c t i o n f r o m t h e

uppe r end o f w e l d Bw. O t h e r w e l d s , Aw and Cw, had l e s s u n d e r c u t t i n g t h a n

Bw and t h i s a n a l y s i s was n o t a p p l i e d t o them. F rom t h e l o c a t i o n o f f i g u r e 4

2 2 t h e a r e a o f t he gap p r i o r t o w e l d i n g i s 1.5 x 1.25 = 1.88 i n ( 12 .1 cm ).

2 2

The measured c r o s s s e c t i o n o f t h e w e l d m e t a l was 2.81 i n ( 18 .1 cm ).

Thus t h e d i l u t i o n o f t h e e l e c t r o d e m e t a l by t h e p l a t e i s

100 ( 2 . 81 - 1 . 88 )/2 .81 = 33%