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HeatTreatmentTechnology

Dr.SantoshS.Hosmani

DEPT. METALLURGY & MATERIALS SCIENCE,

COLLEGE OF ENGINEERING, PUNE

The role of alloying elements in Steels

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3

Classification of iron alloy phase diagrams:

(Ref.: Wever, Archiv fr Eisenhttenwesen 2, 193, 19281929)

Fe-Ni system

Ni Mn Co

4


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5C, N, Cu, Zn, Au

6

, , , ,

Ti, V, Mo, Cr

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B,

Ta, Nb, Zr8Ref.: Book by H.K.D.H. Bhadeshia & R.W.K. Honeycombe


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9Ref.: Book by H.K.D.H. Bhadeshia & R.W.K. Honeycombe

Effect on Fe-C phase diagram

Austenitesta i izinge ements:Ni,Mn,Co,C,N

Ferritestabilizingelements:Cr,Si,Mo,W,V

A1temperature is lowered by the austenite-formers (e.g. Ni,

Mn, Co) and raised by the ferrite-formers (e.g. Cr, Si, Mo,

W, V).

A chrome steel containin 12% Cr and 0.4% C re uires a

higher austenitizing temperature than a eutectoid carbon

steel, whereas a 3% Ni steel will already begin to

.

This state of affairs is clearly of great practical importance

w en ese s ee s are e ng use a empera ures aroun

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Effect on Fe-C phase diagram

Influence of allo in element addition on eutectoid tem erature and

eutectoid carbon content is displayed in following figure:

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What are our observations from the followin hase-dia rams ?

The amount of Ti required to close the -phase loop is much less thanthe re uired for Cr in binar Fe-allo s

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Hardening effects of alloying elements in solid solution in fully

annealed ferrite

Here, factors affecting hardness are:

and solvent atoms,

Concentration of solute atoms

Elastic modulus of solute

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Classification of alloying additions

A useful grouping is based upon the effect of the element on:

(a) the stability of the carbides and

(b) the stability of the austenite/ferrite.

1. Elements which tend to form carbides:Cr, W, Ti, Nb, V, Mo and Mn.

2. Elements which tend to ra hitise the carbide:Si Co Al and Ni.

Only a small proportion of these elements can be added to the steel

before graphite forms during processing.

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15


. , , .

These elements alter the critical points of iron in a similar way to

carbon by lowering the A3point, thus increasing the range in which

austenite is stable and the also tend to retard the se aration of,

carbides.

They have a crystal lattice (f.c.c.) similar to that of -iron in- .

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4. Elements which tend to stabilise ferrite: Cr W Mo V Si and Ti.. , , , , .

These elements are more soluble in -iron than in -iron. They

diminish the amount of carbon soluble in the austenite and thus tend

to increase the volume of free carbide in the steel for a given carbon

content. On the binary equilibrium diagram of these elements withpure iron the A3point raised (although it may be lowered initially), until

the two points merge to form a closed gamma loop.

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re

peratu

CrTe

18Fe CAnimation: S.S. Hosmani

One convenient way of illustrating quantitatively the effect of an alloying element

-


ternary system the -phase field boundaries for increasing concentration of a

particular alloying element. This is illustrated in Figure below for titanium andchromium, from which it can be seen thatjust over 1wt% Ti will eliminate the-

oop, w e w r s requ re o reac s po n. er ernary sys ems

can be followed in the same way, e.g. in FeVC, vanadium has an effect

intermediate between that of titanium and of chromium.

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,

amount of each of these elements

the austenite phase disappears

-

point down to room temperature.

No critical points exist and such

steels e. . 18% chromium irons

are not amenable to normal heat

treatment, except recrystallisation

after cold work.

This effect, however, can be

counteracted by adding certain

elements like 2% of nickel is

FeCrsystem

Here, in Fe-Cr phase diagram, we

added to the 18% chromium

stainless steel to enable it to berefined by normal heat-treatment;

can no e a , or r . , e

ferrite phase becomes stable over

the entire temperature range up to

the melting point. If C is present

carbon has the same effect. ,

is required to nullify carbons

austenite stabilizing effect.20


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Stainless steels are categorized into four groups on the basis of

composition and crystal structure:

ferritic,

austenitic,

martensitic and

duplex stainless steels.

-

addition.

The effect of other elements present in Cr-Ni steels can be

expressed as Ni equivalent (if they stabilize the austenite) and as Cr

equivalent (if they stabilize ferrite):

. u. noequ va ent+++++=

W0.751.5Ti1.75Nb5.5Al5V1.5Mo2SiCrequivalentCr +++++++=

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TheSchaeffler dia ramde icts the hases resent in the allo as

a function of Ni and Cr equivalents:

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- - ,

effect of ferritic and austenitic stabilizing alloying elements should

be taken into account.

mani

S.S.Hos

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Formationofduplexmicrostructureduringsolidificationofthemelt

Du lexstainlesssteelssolidif 100%ferrite ferrite .

Uponcoolingtheferritestartstopartiallydecomposeintoaus en e a nuc ea esan grows rs a egra n

boundariesofferrite,followingfavorablecrystallographic

orientationsinsideofthegrains.

Asthetemperaturelowerstheferritecontentdecreases

asausteniteincreases(andcarbidesandseveralintermetallicphasesma also form durin coolin :de endin u on coolin rate

Man literature refers to denote ferrite in DSSs asferrite rather than ferritebecause DSSs

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contain ferrite which is resulted (transformed) from liquid andnotfrom austenite (refresh memory for

FeC diagram).


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Duplex stainless steels (DSSs) are in between the austenitic and the ferritic grades,

SomeoftheimportantaspectsofDSSs

combining the best mechanical and corrosion resistance (especially, pitting, stress

corrosion cracking) properties of both.

s a resu o e r g mec an ca s reng , goo erma c on uc v y an exce en

corrosion resistance DDSs are extensively used both in pulp and paper industries, inchemical and petrochemical plants. They also find some applications in food and

biomedical fields as well.

The wide use of DSSs is closely connected to their specific microstructure, formed by

roughly 50%50% austeniteferrite ratio: higher yield and ultimate tensile strength than

the austenitic grades, with good ductility and toughness. But, on the other hand, the

microstructural anisotropy of the hot rolled materials can result in variability of

mechanical properties, such as tensile strength and fracture toughness.

Theaustenite+ferrite matrixisattainablebycombiningvariousphasestabilizingelements.

CrandMo:ferritestabilizers.Ni(andN):austenitestabilizers.

Some economical advantages as a result of lower nickel content than the austenitic

grades.25

Pittingresistanceequivalentnumber(PREN):

PREN=%Cr+3.3x%Mo+17x%N

Figure:NominalPRENvaluesfordifferentstainlesssteels

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[$$]

[$$]

Ref.[$$]:D.S.Bergstrom,J.J.Dunn,J.F.Grubb,W.A.Pratt:PatentNumberUS20036551420B1(2003).

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Nickelfreeduplexstainlesssteels

s we nown a n r ogena oy ng n s ee pro uces a var e y o excep ona proper es

such as high strength, high ductility and resistance to stress corrosion cracking (SCC).

Nickel could be completely replaced by nitrogen in order to enhance SCC resistance and

reduce the alloying element cost.

A much lower nitrogen content is needed to maintain a 50% austenite phase compared

with the necessary addition of nitrogen to reach a 100% austenitic microstructure.

Except for nonmagnetic applications, nickelfree DSS are definitely promising in regard to

their properties and cost, and thus are suitable, for example, as reinforcing steels or

.

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Table: Chemical Composition, Austenite Content and Calculated PREN Values of the DSSs.

, ,

10%Mn and 0.35%N.

SteelD103contains additionally about 3%Mo together with the further elevated nitrogen

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concentration of 0.45% in oder to improve the corrosion resistance.


Figure: The calculated nitrogen section ofthe phase diagram of alloyFe-22Cr-10Mn-N Figure: The calculated nitrogen section ofthe phase diagram of alloy Fe20Cr10Mn

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y ermo- ac. 3MoNby ThermoCalc.

D 10 D 10-3


Figure: The dependence of the ferrite

content on annealing temperature.

In comparison with commercial Duplex 2304 and Duplex 2205, the D10 and D103 alloys

show amore stable austenite content at high temperatures:

Due to its potent austenitic stabilizationand rapid diffusion ratein FeCr alloys, nitrogen insuch nickelfree DSS increases the microstructural stability at high temperatures.

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Alsocompare phase diagrams on previous slide with the phase diagram on Slide # 48 or 49 .


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Table: T icalRoom-tem erature Mechanical Pro ertiesof the Investi atedAllo s. .


.

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Table: Results of Pitting and Crevice Corrosion Tests of the Experimental DSS.

Tccc: critical crevice corrosion temperature


havepoorCrevice Corrosion Resistance.

With respect to corrosion properties, D103 alloy seems to be better choice than D10

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alloy; and properties are somewhat comparable to Duplex 2205.


Table: Typical Composition and Costs of DSS and Austenitic Stainless Steels.

D10 and D103 Steels possess the highest ratio of PRE value to alloying cost due to a high

nitrogen content and the absence of nickel.

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ProblemsassociatedwithDSSs(Warnings)

, , ,

temperature processing and can degrade the mechanical and corrosion properties of the

DSS.

During welding of DSS parts, such undesirable phases can form in heat affected zone (HAZ).

Therefore, welding of DSS parts is challenging task.

DSSs are characterized bytwo embrittling temperature ranges(Cshaped curves) which exhibitseveral secondary phases, carbides and nitridesprecipitation at different holding times:

em r tt ement

a spinodal decomposition of the a ferrite in two

phases: an Crrich phase and anFerich phase. nucleation and growing of NiSiMo rich f.c.c. G

36Figure:TypicalTTTdiagramforDSSs

phase, characterised by a very slow precipitation

kinetic (the overall concentration in Gforming

elements increases from 40 to 60% between 1000

and 30000 h at 350C).


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The stability of a given secondary phase usually depends on Fe, Cr, Mo and Ni contents in

.

The effect of alloying elements on the formation of those phases is indicated in the

following Figure:

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BSE and X EDS analysis

Howtoidentifysecondaryphasesinmicrographs?

can help in identification.

Figure: BSE micrograph of CD3MWCuN Figure: BSE micrograph of CD3MN

a oy annea e at . ontrast

difference can be seen between and .

EDS scanning assured the brighter phase

asenveloped by.

ta en at a ter ay eat

treatment.

38Forverysmallsize(tiny)particles:TEMcan,ofcourse,helpinidentification

IdentifythefollowingtypesofStainlesssteelsusingtheirmicrostructures:

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