Microstructural Stability of Nickel-base Alloys for Advanced Power Plant Applications

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1 Microstructural Stability of Nickel-base Alloys for Advanced Power Plant Applications P J Ennis, A Strang, S P Gill, H V Atkinson University of Leicester G M McColvin Alstom Power Thermal Products, Rugby

2 Alloys for steam power plant 1920: temperatures 350 o C, carbon steels, design based on yield strength 1929: Creep of steel at high temperatures, F H Norton e min = ks n It is safe to say that many steam installations are running today with the superheater tubes at 850 F (450 o C) and plants are contemplated where the temperature will run to 900 F (480 o C) or even 1000 F (530 o C). By 1980s: temperatures o C, 1Cr0.5Mo, 2.25Cr1Mo, 12Cr1Mo 1990s: temperatures up to 600 o C, 9Cr1Mo 21st century: 700 o C and higher, nickel-base alloys are required

3 Alloys Alloy designation Alloy 230 Alloy 617 Alloy 282 Alloy 718Plus Commercial names HAYNES 230 alloy, INCONEL alloy N06230 INCONEL alloy 617, Nicrofer 5520Co HAYNES 282 alloy Allvac 718Plus

4 Chemical compositions Alloy Cr Mo Co Al Ti C W Nb < CCA Plus

5 Heat treatments Alloy Heat treatment 230 solution anneal 1170 o C 617 solution anneal 1170 o C 617CCA solution anneal 1170 o C min/1150 o C wq, 8 h/800 o C ac 718Plus 1 h/970 o C, 5 h/788 o C furnace cool to 675 o C, hold 8 h, ac

6 h stress rupture strength, MPa Stress rupture strengths of Alloys 230, 617, 282 and 718Plus Alloy 718Plus Alloy 617 Alloy 230 Alloy temperature, o C

7 stress, MPa Stress rupture strengths of Alloys 617 and 617CCA CCA Totemeier and Ren Larson-Miller Parameter (1.8T)(20 + log t)/1000

8 Microstructure and hardness after ageing at 700 o C Thermocalc predictions of phases present at 700 o C

9 Microstructure of Alloy 617, h 700 o C

10 Hardness and volume fraction of phases in aged Alloy 617 condition Hv 20 TiN M 23 C 6 g m st + 10 h/670 o C nd 574 h/650 o C nd h/650 o C h/700 o C h/700 o C nm 3.7 st solution treated nm not measured nd not detected

11 mole fraction, % 20 Thermocalc predictions for Alloy 617 at 700 o C ASTM max ASTM mean ASTM min CCA max CCA min 0 TiN M 23 C 6 (Ni,Co,Fe) 7 Mo 6 g' s TiN M23C6 (Ni,Co,Fe)7Mo6 g' s

12 phase mole fraction, % Thermocalc predictions for Alloy 617 aged at 700 o C Ti g' 0.6 Ti m phase m phase M 23 C g' phase Al content, wt %

13 Microstructures of Alloys 230 and 282 after exposure at 700 o C Alloy h 700 o C Alloy h 700 o C

14 Phases found in Haynes 230 and 282 alloys Alloy M 23 C 6 M 12 C M 6 C g m 230 x x x 282 x x x

15 Vickers hardness, Hv Hardness of Alloy 230 after ageing 700 o C o C 650 o C 600 o C exposure time, h

16 Vickers hardness, Hv20 Hardness Alloy o C 700 o C exposure time, h

17 Alloy Al + 5Co 0.35 Al + 3Fe 0.5 Al 0.35 Al 0.2 Al spec max spec mean M 23 C 6 P phase mole fraction of phase, %

18 mole fraction, % Thermocalc predictions of phases in Alloy 282 aged at 700 o C mean min max 0 M23C6 (Ni,Co,Fe)7Mo6 gamma prime sigma

19 Microstructure of Alloy 718 Plus aged 6000 h at 700 o C

20 hardness, Hv st + twostage aged Hardness of Alloy 718Plus 650 o C 700 o C 750 o C st exposure time, h

21 Thermocalc predictions for Alloy 718Plus aged at 700 o C s M 23 C 6 g ' Ni 3 (Al, Nb) mole fraction, %

22 Summary Alloys 230, 617 and 282 exhibit increasing hardness after ageing at 650 and 700 o C Alloy 718Plus exhibits slight hardening at 650 o C, slight softening at 700 o C and rapid softening at 750 o C Thermocalc predicts some phases that have not yet been found experimentally There may be scope for optimisation of the alloys for service at 700 o C

23 Composition ranges for Alloys 617, 617CCA, 230, 282 and 718Plus alloy Cr Co Mo C Al Ti Fe others <0.6 <3 <1 Si, <1Mn, <0.4N, <0.006B 617CCA <1.5 <0.3Si, <0.3Mn, <0.05N, B < < W, Si, Mn, 0.015B, La < Si, <0.3Mn, <0.005B, <10ppm N 718Plus Nb, <0.35Si, <0.35Mn, W, B

24 Maximum service temperatures Alloy 740 Alloy C263 Alloy 282 Alloy MA956 Alloy 718Plus Alloy 617 Alloy 230 AISI 316 P92 AISI 347 P Cr1Mo 1Cr0.5Mo maximum service temperature based on a h stress rupture strength of 100 MPa