high temperature properties of a new wrought austenitic steel

23. - 25. 5. 2012, Brno, Czech Republic, EU

HIGH TEMPERATURE PROPERTIES OF A NEW WROUGHT AUSTENITIC STEEL

Tomáš VLASÁK1, Jan HAKL1, Pavel NOVÁK1, Anna VÝROSTKOVÁ2

1SVÚM a.s., Podnikatelská 565, 190 11 Prague 9, Czech Republic

2 Slovak Academy of Sciences, Institute of Materials Research, Košice, Slovakia

Abstract: Austenitic steels for boiler superheater tubes are intended for regions where the metal

temperature is in the interval of 620°C to 680°C. This application has higher requirements for adequate

creep characteristics and corrosion resistance of the used materials. The paper focuses on the creep

properties studied on the new wrought austenitic steel BGA4 (23Cr-15Ni-6Mn-1,5W-2,5Cu-0,3V-0,5Nb-

Mo,B,N) developed by the British Corus company. The dependencies of the rupture strength, strength for

specific creep strain and minimum creep rates were evaluated on the basis of the long term creep tests

carried out at temperatures between 625°C and 725°C. Metallographical analyses are also a part of the

work.

Keywords: creep, austenitic steel, metallography.

1. INTRODUCTION

Ferritic and martensitic steels can be used in the power industry at temperatures of up to 650°C, austenitic

steels at 620-680°C and nickel alloys at higher temperatures. Austenitic steels are applied in the end

portions of superheater pipes where high resistance to corrosion together with sufficient creep properties is

generally required. These materials can be divided into four groups according to Cr contents [1]:

steels containing 15 % of Cr, steels containing 18 % of Cr,

steels containing 20-25 % of Cr,steels containing higher Cr contents.

Besides Cr, these steels contain increased quantity of Ni (usually within the range of 10 to 25 %) and,

moreover, they are alloyed with any of the below listed elements: C, Mo, Mn, W, V, Nb, B, Cu, Ti a N.

Alloying with Mn together with N has an austenite formation effect (partial substitution of Ni), Cu precipitates

in a phase increasing creep resistance [2], B modifies grain boundaries (increases their strength) and other

elements combine with C so that they are carbide forming.

Paper will deal with the BGA4 austenitic steel, which is material developed by CORUS company from Great

Britain [3]. As to creep properties, it is similar to Esshete 1250 (15Cr10NiMnMoVNbTi) or NF 709

(20Cr25NiMoNbTi) steels and it is very similar to the SAVE 25 steel (23Cr18NiWNbCuN) as to chemical

composition. Medium chemical composition of these steels is presented in Table I according to [4-6].

Table 1 Chemical composition of some austenitic steel (wt.%)

Steel C Si Mn Ni Cr Mo W V Nb Ti Cu N

Esshete 1250 0,12 0,5 6 10 15 1 - 0,2 1 0,06

NF 709 0,15 0,5 1,0 25 20 1,5 - - 0,2 0,1

SAVE 25 0,10 0,1 1,0 18 23 - 1,5 - 0,45 3 0,2

2. EXPERIMENTAL MATERIAL

The CORUS company supplied the BGA4 material – identified as CORUS CODE H4F53 – with chemical

composition stated in Table II. The supply included rods with length of 160-300 mm with diameter of

approximately 20 mm that had been rolled from original material with cross section with diameter of 183 mm.

The rods were subjected to an ultrasound test the aim of which was to identify internal defects. This test did


not reveal any defects that could be affecting consequent results. The experimental material was heat

treated using the 1 200°C/20 min/water procedure at the manufacturer. Testing specimens with specific

dimensions of 5x25 mm were made and creep tests carried out in SVÚM.

Table 2 Chemical composition of BGA4 steel (wt.%)

C Si Mn P S Cr Mo Ni B Nb V N Cu W

0,11 0,49 6,10 0,02 0,024 22,9 0,14 15,4 0,007 0,61 0,31 0,185 2,70 1,49

The creep tests were commenced so that the parameters at temperature range from 625 to 725°C and

stress of 90 – 330 MPa. The creep tests have been carried out on air at a constant load in the SVÚM a.s.

laboratory accredited in accordance with EN ISO/IEC 17025. Deformation time change has also been

measured.

3. RESULTS OF THE CREEP TESTS

3.1. CREEP STRENGTH

The dependence of stress on the Larson-Miller parameter (PLM) was assessed at first. A following regression

model was used for assessment [7]

2321log LMLM PAPAA , (1)

where PLM=T.(logtr+A4), is stress (MPa), T is temperature (K), tr is time to rupture (h), A1-A4 are material

constants.

Fig. 1 illustrates the

assessed master curve and

the results of creep tests.

Fig. 2 provides another

illustration where the

dependence of time to

rupture on stress for

temperatures from 625 to

725°C is drawn. The A1-A4

parameters are listed in

Table III.

Fig. 1 Creep rupture strength of BGA4 steel Fig. 2 Time to rupture dependence on

temperature and stress of BGA4 steel

10

100

1000

20000 21000 22000 23000 24000

PLM=T.(log(tr)+A4), [K,h]

Str

ess [

MP

a]

Master curve

625°C

650°C

675°C

700°C

725°C

100

1000

10000

100000

50 100 150 200 250 300 350

Stress [MPa]

Tim

e t

o r

up

ture

[h

]

625°C

650°C

675°C

700°C

725°C

Table 3 Material constants of regression models (1), (2) and (3).

Model (1) Model (2) Model (3)

A1 3,665931E+00 B1 7,164867E+00 C1 -1,870272E+01

A2 5,086161E-05 B2 -2,769734E-04 C2 -3,781084E+00

A3 -5,189760E-09 B3 1,241528E-09 C3 -3,922192E+00

A4 1,980094E+01 B4 1,736592E+01 C4 -1,240277E+01

C5 1,047203E+03

C6 5,671428E-06


Fig. 3 Creep curves of BGA4 steel

3.2. STRENGTH FOR SPECIFIC CREEP STRAIN

Records of creep strain were evaluated using the model [8].

)))(

0

0

tg

mc

MK

Ntg

)2exp(1

2exp(1))((

)

rt

t 2

0 10)(

TE

T

EEETE 3

21 exp)(

where c is total creep strain (%), t is time (h), tr is time to

rupture (h), 0 is initial deformation (%), is stress

(MPa), T is temperature (K), K,M,N,m, E1-3 are material

constants. Examples of some evaluated creep curves are

illustrated in Fig. 3.

The evaluated creep curves allowed specifying

temperature and stress dependencies of the specific creep

strain. The identical equation (1) in the form

21

3

1

211log LMLM PBPBB , (2)

where )(log 411

1 BtTPLM , 1 is stress (MPa), T1 is

temperature (K), t1 is time to creep strain 1% (h), B1-B4 are

material constants given in v Tab.III., was used. The result

of assessment of 1% creep strength for strain 1% is

illustrated in Fig. 4.

3.3. CREEP RATE

The model according to [9] was used to assess the creep rate.

,)sinh(log11

log)sinh(log11

loglog 6

5

463

5

21 TCCT

CTCCCT

CC (3)

where is minimum creep rate (%/h), is stress (MPa), T is temperature (K), C1-6 are material constants

shown in Tab.III. The assessed creep rate is illustrated in Fig. 5.

Fig. 4 Comparison rupture strength and strength

for 1% strain of BGA4 steel

Fig. 5 Minimum creep rate of BGA4 steel

0

2

4

6

8

10

0 1000 2000 3000 4000 5000

Time [h]

Str

ain

[%

]

650°C/280MPa

650°C/260MPa

650°C/240MPa

650°C/210MPa

0

2

4

6

8

0 2000 4000 6000 8000 10000 12000

Time [h]

Str

ain

[%

]

700°C/200MPa

700°C/180MPa

700°C/150MPa

700°C/130MPa

700°C/115MPa

10

100

1000

20000 21000 22000 23000 24000

PLM=T.(log(tr)+A4), [K, h]

Str

ess [

MP

a]

Rupture strength

Strength for 1% creep strain

0,0001

0,001

0,01

50 100 150 200 250 300 350

Stress [MPa]

Min

imu

m c

reep

rate

[%

/h]

625°C

650°C

675°C

700°C

725°C


Fig. 6 Shaeffler’s diagram of microstructure state

of BGA4 steel

Fig. 7 Specimens chosen for metallography

20% Ferrite

40% Ferrite

10% F

errite

100% Ferrite

80% Ferrite

0

4

8

12

16

20

24

28

32

0 4 8 12 16 20 24 28 32 36 40

Creq=Cr+2Si+1.5Mo+5V+5.5Al+1.75Nb+1.5Ti+0.75W, [wt %]

Ni e

q=

Ni+

Co

+0.5

Mn

+0.3

Cu

+25N

+30C

,

[wt

%]

A+M

Martensite

F+MFerrite

Austenite

A+F

A+M+F

BGA4

Creq=27,83

Nieq=27,19

4. METALLOGRAPHY

Delta ferrite should not be present in the BGA4 steel

structure. This structural component could be

transformed relatively soon to a sigma phase during

exposure at higher temperatures, which would result

in making steel brittle. Occurrence of delta ferrite can

be predicted using the Schaeffler’s diagram [10] which

is shown on Fig. 6. The equivalents of Cr and Ni

contents can be calculated from the equations [11, 12]

Ceq= Cr+2Si+1.5Mo+5V+5.5Al+1.75Nb+1.5Ti+0.75W, Nieq=Ni+Co+0.5Mn+0.3Cu+25N+30C,

where contents of individual elements are in weight

percentage.

The equivalent contents for the evaluated BGA4 steel

are following: Creq=27.83 %, Nieq=27.19 %.It is

apparent from Fig. 6 that delta ferrite should not be

present in the BGA4 steel. This structural component

has not been detected during our metallographic

investigation.

Metallographic study was carried with 11 samples

identified in Fig. 7. The samples were prepared for

examination using usual methods of grinding and

polishing. The investigation was carried out using a

Zeiss-Neophot light microscope. An agent consisting

of 10 ml of HNO3+10 ml of acetic acid+15 ml of HCl+5

drops of glycerine was used for sample surface etching characterizing the initial condition. All the other

samples were etched using an agent consisting of 2g of CuCl2 + 40ml of HCl + 60ml of ethanol.

The samples had an austenitic structure. The size of grain was assessed using a comparison method with

100x magnification and it was found out to be 4.5 according to ASTM E112. The dimension of 75 m

corresponds to this value. The effect of the parameters of thermal exposure on grain coarsening was not

apparent; the size of grains of the investigated samples was constant.

The structure does not contain precipitate in the initial condition; the grain boundaries and the boundaries of

twins are sharp (see Fig. 8.). This figure also shows the changes that occur at creep exposure. The

structures corresponding to growing of time up to fracture are arranged here. However, these values are re-

calculated for maximum temperature of austenitic steels application (680°C). It can be seen that precipitate

is created inside of grains already after a short exposure when compared with the initial condition. This

precipitate coarsens with time under temperature. Growth of thickness of the depleted zone along the grain

boundaries is another change. This change was assessed using photographs of individual statuses (from 10

to 15 statuses). As can be seen in Fig. 9, the change of dimensions of the depleted zone on the boundaries

of grains has an opposite character than creep resistance of BGA4 steel. It means that the size of the

depleted zone could be used to estimate creep lifetime of real parts.

10

100

1000

19000 20000 21000 22000 23000 24000PLM=T.(log(tr)+A4), [K,h]

Str

ess [

MP

a]

Initial

state YO13

625°C

315MPa

1130h

YO6

675°C

175MPa

2524,25h

YO10

700°C

130MPa

4854h

YO7

675°C

150MPa

9377,75h

YO5

675°C

200MPa

2045,25h

YO18

675°C

230MPa

869,75h

YO14

625°C

260MPa

5642h

YO17

725°C

100MPa

6791,25h

YO15

725°C

110MPa

2334,25

h

YO11

700°C

115MPa

10857,25h


a) Initial state b) 625°C/315MPa/1130h

(tr680°C=43h) c) 625°C/260MPa/5642h

(tr680°C=194h)

d) 675°C/200MPa/2045h

(tr680°C=1514h) e) 675°C/150MPa/9378h

(tr680°C=6886h) f) 700°C/130MPa/4854h

(tr680°C=16471h)

g) 700°C/115MPa/10857h

(tr680°C=37469h) h) 725°C/110MPa/2335h

(tr680°C=35247h) i) 725°C/100MPa/6791h

(tr680°C=107827h)

Fig. 8 Changes of BGA4 steel microstructure caused by temperature exposition

Fig. 9 Changes of depleted zone on grain boundary in relation to creep strength

Fig. 10 Area closed to fracture of specimen YO 14 (625°C/260MPa/5642h)

It has also followed from the metallographic analyses that the boundaries of grains represent weak points of

the structure. This is apparent from Fig. 10 where the zone close to the fracture area is shown. Fracture

takes place exclusively on the boundaries.

Study of existence of structural phases using the CAMEBAX MICRO electron micro-analyser was carried out

complementarily to the described metallographic investigation. The samples YO5, YO10 (relatively short time

exposures) and YO15, YO17 (the longest exposures) were analysed. Presence of carbides of the M23C6 and

M6C types on the grain boundaries was proved. Existence of carbides of the MC type (containing mainly Nb)

Load direction

1

10

100

20000 21000 22000 23000 24000

PLM=T.(log(tr)+C), [K,h]

Wid

th o

f d

ep

lete

d z

on

e o

n

gra

in b

ou

nd

ary

[m

m]

10

100

1000

Str

ess [

MP

a]

Width of zone Master curve

625°C 650°C

675°C 700°C

725°C

50m 50m 50m

50m 50m 50m

50m 50m 50m

200m


was detected inside of grains. MC carbides were found in the samples YO15 and YO17 even on boundaries.

Sulphides of the MnS type were also detected inside of grains.

Existence of a phase on the Cu basis (precipitate) was also detected inside of grains. We did not succeed in

finding this phase in the samples YO5 and YO10, however, we found them in the samples YO15 and YO17.

When viewing Fig. 8, we can say that the precipitate coarsens considerably with time of exposure at

temperature. The size of the precipitate particles must be 1-2 m in order we could distinguish the difference

between a particle and surroundings when using our micro-analyser. This can be assumed in case of the last

two mentioned samples.

5. CONCLUSION

The investigation completed can be summarised as follows:

a) Creep tests of the BGA4 steel at temperatures of 625-725°C with times to rupture of 104 h were

carried out. The results of investigation obtained are illustrated in Figs. 1-5.

b) The metallographic study proved changes in the microstructure that take place as a result of creep

exposure. The main pieces of knowledge are illustrated in Figs. 6 and 8 - 10.

ACKNOWLEDGEMENTS

This work was supported by Ministry of Education, Youth and Sports of Czech Republic-1P05

OC020.

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high temperature properties of a new wrought austenitic steel

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