Design of 10%Cr Martensitic Steels for Improved Creep Resistance in Power Plant Applications

Transcription

1 Design of 10%Cr Martensitic Steels for Improved Creep Resistance in Power Plant Applications U.A. Sachadel, P.F. Morris, P.D. Clarke Tata Steel Europe 8th International Charles Parsons Turbine Conference 5-8 September 2011, Portsmouth, UK

2 Specific CO2 emissions [g/kwh] Background Average worldwide Average Germany 620 C steam turb. 700 C steam turbine Efficiency [%] For high efficiency and low CO 2 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 2

3 Objective 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) 3

4 Elements Used for Alloy Development Current maximum temperature 620 C Where have we got to? 620 C!! 4

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 5

6 Creep strength loss in current 10-12%Cr steels at 650 C Why current 10-12%Cr tempered martensitic steels fail at 650ºC? Precipitation and coarsening of Cr a V b Nb c N d Z-phase consuming existing fine (V,Nb,Cr)(C,N) [MX] or (Cr,V,Nb,Fe) 2 (C,N) [M 2 X] Coarsening of (Cr,Fe,W,Mo) 23 C 6 carbides [M 23 C 23 ] Coarsening of Fe a W b Cr c Mo d Si e Laves phase Not an issue in 9%Cr steels Also in 9%Cr steels Inhomogeneous recovery near prior austenite grain boundaries and lath/block boundaries of martensite Laves phase Development in creep Tempered martensite 6 Abe, 2004 & 2008

7 Proposed Solutions Optimise C, N, B, Nb, V, Ta additions and heat treatments to control M 23 C 6, MX, M 2 X 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: M 2 X / MX] 7

8 Design of chemical compositions Wt% Steel A Steel B Cr Si N C Ta 0.1 Nb 0.05 V B W Mo Cu Co Ni Mn Steam oxidation resistance Increased nitride content Decreased carbide content Stable nitride strengthening effect (MX/M 2 X) Use of Nb has implications for nuclear applications Stabilization of M 23 (C,B) 6 Solid solution strengthening / Laves phase Control of Laves phase Single phase matrix (no δ-ferrite) 8

9 Improvement in creep life due to the HT Effect Details Improvement in creep life compared to Steel 92 High SHT (Morris et al., 2010) Fast cooling (Yamada et al, 2002) Low Tempering (Igarashi et al., 2001; Increase in the SHT temperature from 1150 to 1200 C needed for nitride precipitates Application of fast cooling (WQ) from SHT temperature instead of air cooling Lowering tempering temperature from C down to 550 C 70% at 650 C/110MPa 90% at 650 C/120MPa 90% at 650 C/120MPa Sawada et al., 2008) 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/M 2 X particles 9

10 Wt % Phase Wt % Phase Heat treatments: thermodynamic calculations, JMATPro 4.1 (Steel A) γ δ a) 100 a) Solution HT temperature Liquid Ferrite Austenite Cu M23C6 MN M(C,N) BN MX Solution HT temperature Cu M23C6 MN M(C,N) Temperature (C) Temperature (C) Steel variant Maximum temperature for 100% γ (C) Steel A 1210 Dissolution temperature (C) BN MX (MN) Temperature A 1 (C) 763 Solution HT temperature: 1200 C (confirmed no δ-ferrite formed at 1200 C) Tempering temperature range: up to 740 C 10

11 Heat treatment 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 (M 2 X 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 11

12 Composition of precipitates vs. temperature (Steel A) N MX Cr M 23 (C,B) 6 V Ta C Cr C Mo Fe Composition of MX Fraction of MX (wt.%) 600 C 660 C Composition of M 23 (C,B) 6 Fraction of M 23 (C,B) 6 (wt.%) at 600 C: 600 C 660 C V 47 Ta 4 Cr 3 N 46 at 660 C: V 45 Ta 4 Cr 4 N 46 C 0.28 M 2 X at 600 C: Cr 61 Fe 7 Mo 9 WMnC 21 {46ppm B} at 660 C: Cr 59 Fe 10 Mo 8 WMnC 21 {70ppm B} The results of the calculations are not suitable for the prediction of the composition change of M 2 X with tempering temperature Temperature of 700 C was chosen as the most suitable to extract some data for all investigated steels Calculations indicate that M 2 X phase at 700 C could be mainly Cr, V and Nb nitride When compared with MX, fraction of M 2 X should be higher by factor 1.7

13 Steel Compositions Steel A* Steel B* Steel 92 Steel 92N Steel * Morris et al., 2010 ** Not measured Chemical composition, wt.% C Si Mn P S Cr Mo Ni Al B Co Cu N Nb Ta V W Nom Cast ** Nom Cast < ** Cast Cast

14 Heat treatment cycles of novel and reference steels Tempering Steel variant (SHT) 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 B (1200 C/1h/Oil quenching) Steel A-1200-T1 Steel A-1200-T2 Steel A-1200-T3 - Steel B-1200-T1 Steel B-1200-T2 - - Steel 92 (1060 C/1h/AC) * Steel T4 * Steel 92 (1150 C/1h/AC) * - Steel 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, N + - (lack of available samples) 15

16 Stress rupture test results Plain Stress Rupture Properties for Steels A, B, 92 & 92N Plain Rupture Life (h) Temp. ( C) Stress (MPa) Aim* (h) Steel A (10%Cr) Steel B (10%Cr) Steel 92 (9%Cr) Steel 92N (9%Cr) b 5792b 5825b 3861b 4001b 4002b 807b 7544b 7973b b 1655b 1409b 2859b 2516b 906b 160b - - * Aim lives based upon P92 with conventional heat treatment 16

17 Stress (MPa) LMP of Creep Data Steel A-1200-T1 Steel A-1200-T2 Steel A-1200-T3 Steel B T1& T2 Steel B T1at 675 C Steel T4 Steel T2 Steel 92N T2 Steel 92N T2 ECCC (2005) C (10 5 h) 625 C (10 5 h) 650 C (10 5 h) LMP (T(C+log t)) x

18 Stress (MPa) LMP of Creep Data Estimate of 100MPa/10^5 Hour Temperature Steel A-1200-T1 Steel A-1200-T2 Steel A-1200-T3 Steel B T1& T2 Steel B T1at 675 C Steel T4 Steel T2 Steel 92N T2 Steel 92N T2 ECCC (2005) 100MPa C (10 5 h) 625 C (10 5 h) 650 C (10 5 h) LMP (T(C+log t)) x

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 19

20 Thank you for your attention 20