U.A. Sachadel, P.F. Morris, P.D. Clarke

Tata Steel Europe

8th International Charles Parsons Turbine Conference

5-8 September 2011, Portsmouth, UK

Design of 10%Cr Martensitic Steels for

Improved Creep Resistance

in Power Plant Applications

2

For high efficiency and low CO2 emissions high steam temperatures are necessary in fossil fired power plants

Steels could compete with Nickel base alloys in application for components that are operating up to 650°C

620°C is the limit of operating temperature for current steel grades

The best currently available steel is 9%Cr Steel 92 (9%Cr, 2%W, 0.5%Mo)

There is a need of the development of steel suitable for operating under such condition

Average

worldwide

620°C

steam turb.

Average

Germany

700°C

steam

turbine

Efficiency [%]

Sp

ec

ific

CO

2 e

mis

sio

ns

[g

/KW

h]

Background

3

Development of long-term creep resistant steel for power plants of new generation

Criteria:

100,000 h average stress rupture strength of 100 MPa at well in excess of 620°C

Steam oxidation resistance at 650°C (10-12 wt.% Cr steel)

Low thermal expansion coefficient (excludes austenitic steels)

Objective

4

Current maximum temperature 620°C

Where have we got to? 620°C!!

Elements Used for Alloy Development

5

Principles of Current Work

• Systematic approach to alloy design

• Based on existing knowledge

• Use of thermodynamic modelling packages

• Optimise heat treatment

• Maximise alloy in solution on hardening

• Develop high volume fraction, smallest size, stable precipitate

dispersion on tempering

6

Tempered martensite

Abe, 2004 & 2008

Development

in creep

Why current 10-12%Cr tempered martensitic steels fail at

650ºC?

Precipitation and coarsening of CraVbNbcNd Z-phase

consuming existing fine (V,Nb,Cr)(C,N) [MX] or

(Cr,V,Nb,Fe)2(C,N) [M2X]

Coarsening of (Cr,Fe,W,Mo)23C6 carbides [M23C23]

Coarsening of FeaWbCrcModSie Laves phase

Inhomogeneous recovery near prior austenite grain boundaries

and lath/block boundaries of martensite

Laves phase

Not an issue

in 9%Cr

steels

Also in 9%Cr

steels

Creep strength loss in current 10-12%Cr steels at 650°C

7

Optimise C, N, B, Nb, V, Ta additions and heat treatments to control M23C6, MX, M2X and Z-phase

Optimise W, Mo and Cu to control Laves phase

Optimise Cr/Ni equivalents to avoid δ-ferrite (Cu, Co and Ni additions) but with a minimum of 10% for oxidation resistance

“Nitride strengthened” martensitic steel [dominant nanoparticles: M2X / MX]

Proposed Solutions

8

Wt% Steel A Steel B

Cr 10.0 10.0

Si 0.25 0.25

N 0.05 0.05

C 0.02 0.02

Ta 0.1

Nb 0.05

V 0.2 0.2

B 0.001 0.001

W 1.5 1.5

Mo 0.5 0.5

Cu 1.5 1.5

Co 1.5 1.5

Ni 0.5 0.5

Mn 0.3 0.3

Steam oxidation resistance

Solid solution strengthening / Laves phase

Single phase matrix (no δ-ferrite)

Stabilization of M23(C,B)6

Increased nitride content

Control of Laves phase

Decreased carbide content

Stable nitride strengthening effect (MX/M2X)

Use of Nb has implications for nuclear

applications

Design of chemical compositions

9

Effect Details Improvement in creep life compared

to Steel 92

High SHT

(Morris et al., 2010)

Increase in the SHT temperature from 1150

to 1200°C – needed for nitride precipitates

70%

at 650°C/110MPa

Fast cooling

(Yamada et al, 2002)

Application of fast cooling (WQ) from SHT

temperature instead of air cooling

90%

at 650°C/120MPa

Low Tempering

(Igarashi et al., 2001;

Sawada et al., 2008)

Lowering tempering temperature from 765-

770°C down to 550°C

90%

at 650°C/120MPa

Combined:

High SHT + low

Tempering

(Morris et al., 2010)

Increase in the SHT temperature from 1060

to 1150°C + decrease in tempering

temperature from 780°C down to 660°C

250%

at 650°C/110MPa

Design of heat treatment for new steels - objectives:

Dissolution of precipitates and no δ-ferrite formed on SHT

Fast cooling from SHT to avoid precipitation

Tempering optimised to ensure fine distribution of MX/M2X particles

Improvement in creep life due to the HT

10

Steel variant

Maximum temperature for

100% γ

(C)

Dissolution temperature (C)

Temperature A1

(C)

Steel A 1210BN MX (MN)

7631197 1212

Solution HT temperature: 1200°C

(confirmed no δ-ferrite formed at

1200°C)

Tempering temperature range: up to 740°C

MX

δγ

Solution HT temperature

Solution HT temperature

0

10

20

30

40

50

60

70

80

90

100

400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Temperature (C)

Wt

% P

ha

se

Liquid

Ferrite

Austenite

Cu

M23C6

MN

M(C,N)

BN

a)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Temperature (C)

Wt

% P

ha

se

Cu

M23C6

MN

M(C,N)

a)

Heat treatments: thermodynamic calculations, JMATPro 4.1

(Steel A)

11

Solution heat treatment at 1200°C avoids the formation of delta ferrite and reduces the

fraction of undissolved particles in the steels

This should result in an increased fraction of precipitates on tempering, when

cooling from SHT is sufficiently fast

The tempering temperature should not be higher than 740°C (A1 temp)

Lowering the tempering temperature can result in a finer size distribution of all

precipitates and in the change of nano-precipitate (M2X instead MX), which is

considered to be beneficial for creep resistance (delay in formation of Z-phase)

The minimum temperature of the last step of tempering is estimated at 660°C, in order to

give some microstructural stability at the application temperature of 650°C

In previous work tempering temperature of 660°C resulted in excellent creep life of

improved Steel 92

If tempering can be done in steps then lowering the tempering temperature of the first

step of tempering could result in even finer distribution of all precipitates, with extended

stability of the microstructure during service ensured by controlled coarsening in the

second step of tempering at 660°C

Heat treatment

The results of the calculations are not suitable for the prediction of the composition change of

M2X with tempering temperature

Temperature of 700°C was chosen as the most suitable to extract some data for all investigated

steels

Calculations indicate that M2X phase at 700°C could be mainly Cr, V and Nb nitride

When compared with MX, fraction of M2X should be higher by factor 1.7

M2X

MX

Cr

N

Ta

V

C

M23(C,B)6Cr

C

Mo

Fe

Composition of MX

Fraction of MX

(wt.%)

600°C 660°C

at 600°C:

V47Ta4Cr3N46

at 660°C:

V45Ta4Cr4N46C

0.28

Composition of M23(C,B)6

Fraction of M23(C,B)6

(wt.%)

600°C 660°C

at 600°C:

Cr61Fe7Mo9WMnC21{46ppm B}

at 660°C:

Cr59Fe10Mo8WMnC21{70ppm B}

0.34 0.32

Composition of precipitates vs. temperature (Steel A)

13

Steel Compositions

Steel

Chemical composition, wt.%

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

Steel

A*

Nom. 0.02 0.25 0.30 - - 10.0 0.5 0.50 - 0.001 1.5 1.5 0.05 - 0.1 0.20 1.5

Cast 0.034 0.25 0.35 ** 0.015 10.1 0.52 0.52 0.008 0.0012 1.5 1.5 0.056 - 0.1 0.21 1.5

Steel

B*

Nom. 0.02 0.25 0.30 - - 10.0 0.5 0.50 - 0.001 1.5 1.5 0.05 0.05 - 0.20 1.5

Cast 0.032 0.32 0.34 <0.01 0.013 10.1 0.53 0.52 ** 0.0013 1.5 1.5 0.061 0.06 - 0.21 1.4

Steel

92 Cast 0.11 0.33 0.48 0.012 0.005 9.05 0.46 0.22 0.007 0.0056 - - 0.053 0.065 - 0.22 1.85

Steel

92NCast 0.073 0.22 0.48 0.01 0.009 9.01 0.46 0.21 0.005 0.005 - - 0.065 0.068 - 0.20 1.73

* Morris et al., 2010

** Not measured

14

Heat treatment cycles of novel and reference steels

Steel variant (SHT)

Tempering

T1:

600°C/3h/AC

+660°C/3h/AC

T2:

660°C/3h/AC

+660°C/3h/AC

T3:

660°C/6h/AC

T4:

780°C/2h/AC

Steel A (1200°C/1h/Oil

quenching)Steel A-1200-T1 Steel A-1200-T2 Steel A-1200-T3 -

Steel B (1200°C/1h/Oil

quenching)Steel B-1200-T1 Steel B-1200-T2 - -

Steel 92 (1060°C/1h/AC) * - - -Steel 92-1060-T4

*

Steel 92 (1150°C/1h/AC) * - Steel 92-1150-T2 * - -

Steel 92N (1150°C/1h/AC) * - Steel 92N-1150-T2 * - -

Steel 92N (1200°C/1h/AC) * - Steel 92N-1200-T2 * - -

15

Test Conditions – Stress Rupture Tests

Steel

variant

110 MPa at

675°C

122 MPa at

650°C

A, B, 92 + +

92N +

- (lack of

available

samples)

16

Plain Stress Rupture Properties for Steels A, B, 92 & 92N

Stress rupture test results

Temp.

(°C)

Stress

(MPa)

Aim*

(h)

Plain Rupture Life (h)

Steel A

(10%Cr)

Steel B

(10%Cr)

Steel 92

(9%Cr)

Steel 92N

(9%Cr)

1200

600

660

1200

660

660

1200

660

1200

600

660

1200

660

1150

660

1060

780

1150

660

1200

660

650 122 1000 4868b 5792b 5825b 3861b 4001b 4002b 807b 7544b 7973b

675 110 363 1187b 1655b 1409b 2859b 2516b 906b 160b - -

* Aim lives based upon P92 with conventional heat treatment

17

LMP of Creep Data

0

50

100

150

200

250

29 30 31 32 33

LMP (T(C+log t)) x 10-3

Str

ess (

MP

a)

Steel A-1200-T1

Steel A-1200-T2

Steel A-1200-T3

Steel B - 1200 - T1 & T2

Steel B - 1200 - T1 at 675°C

Steel 92 - 1060 - T4

Steel 92 - 1150 - T2

Steel 92N - 1150 - T2

Steel 92N - 1200 - T2

ECCC (2005)

600°C (105h) 625°C (105h) 650°C (105h)

18

LMP of Creep Data – Estimate of 100MPa/10^5 Hour Temperature

0

50

100

150

200

250

29 30 31 32 33

LMP (T(C+log t)) x 10-3

Str

ess (

MP

a)

Steel A-1200-T1

Steel A-1200-T2

Steel A-1200-T3

Steel B - 1200 - T1 & T2

Steel B - 1200 - T1 at 675°C

Steel 92 - 1060 - T4

Steel 92 - 1150 - T2

Steel 92N - 1150 - T2

Steel 92N - 1200 - T2

ECCC (2005)

100MPa

600°C (105h) 625°C (105h) 650°C (105h)

19

Conclusions

• Two novel 10%Cr, low carbon steels have been designed in an attempt optimise long term creep properties in excess of the best currently available from martensitic steels

• The aim is to develop nitrogen-rich precipitates to optimise long term creep performance

• Versions containing both Nb and Ta as the high temperature carbo-nitride forming elements have been studied

• Heat treatments have been optimised to maximise alloy in solution after hardening

• High solution treatment temperatures and low temperature tempering have been used to develop a fine stable dispersion of MX/M2X precipitates

• Creep performance well in excess of conventionally heat treated Steel 92 were obtained

• Based upon relatively short term creep data (5kh) extrapolations based on LMP values suggest a 100MPa/10^5 creep life at temperatures approaching 640°C

20

Thank you for your attention