Low cycle fatigue behavior of alloy CF8C-Plus between 600 to 900 C

1. Low cycle fatigue behavior of alloy CF8C-Plus between 600 to 900C D. Kumar, S. Dryepondt, A. Shyam, P. J. Maziasz, B.A. Pint, E. Lara-Curzio Materials Science and Technology Division, Oak Ridge National Laboratory MS&T-2010, Houston, USA October 17th to 21st, 2010 Acknowledgements: J. L. Moser, T. Geer, L. Walker Research sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program, under contract DE-AC05-00OR22725 with UT-Batelle, LLC

2. CF8C-Plus cast stainless steel is a candidate alloy for advanced diesel engine and gas turbine applications turbocharger housing made out of SiMo cast iron exhaust manifold made out of SiMo cast iron Caterpillar Regeneration System casing made out of CF8C-Plus Solar Turbines 4.6 MW Mercury 50 recuperated gas turbine engine

3. Creep resistance of CF8C-Plus is better than current exhaust alloys • Some Candidate Alloy Compositions (wt%) • CF8C (cast version of 347): Fe-19Cr-10Ni-0.07C-0.8Nb-1Si-0.7Mn • CF8C-Plus: Fe-19Cr-12Ni-0.07C-0.8Nb-0.4Si-4Mn-0.3N • SiMoCast Iron: Fe-3.45C-4Si-0.6Mo-0.3Mn • Ni-Resist: Fe-2Cr-35Ni-0.5Mn-5Si-1.9C Creep, 550C to 850oC CF8C-Plus shows good yield strength Maziasz et al., Proc. GT2010, ASME Turbo Expo 2010, June 14-18, Glasgow, Scotland CF8C-Plus shows better creep-rupture resistance than other candidate alloys Shingledecker et al., JPVT, 2009, 131, 051404

4. Stable microstructure CF8C-Plus commercial, standard CF8C creep tested 850C/23000h creep tested 850C/500h Shingledecker et al., Int. J. Press. Vess. Pip., 84 (1-2), 2007, 21 • Uniform distribution of stable nano-scale MC precipitate at 850C (high from austenitic steel standard) providing good creep resistance

5. CF8C-Plus has good castability due to 4wt% of Mn • CF8C-Plus Fluidity Spiral: • Round-top trapezoid, 3/8” w (bottom), ¼” w (top), ¼” high • Length = linear length of spiral • CF8C-Plus (<0.5Si) and CF8M (1.5Si) have comparable fluidity at equivalent pouring temperatures

6. Objective: Determine the effect of temperature on low-cycle-fatigue behavior of alloy CF8C-Plus Fully reversed , strain controlled T = 600C, 800C, 900C  = 0.5, 1, 1.5, 2 Strain rate = 0.05% 1/s Environment = laboratory air Average surface roughness of the as-received sample ~1µm sample surface roughness measured by optical profilometer

7. Fatigue life decreases with increase in temperature and strain range 0.05% per second, R = -1 • CF8C-Plus shows slightly higher fatigue life than cast austenitic stainless steel, HK-30 (Fe-25Cr-19Ni) between 600C to 800C % strain range 600C 800C 900C cycles to failure

8. Cyclic strain hardening between 600 to 800C and softening at 900C  = 2%  = 0.5% 600C 600C 800C 800C 900C 900C  = 0.5%  = 2% 600C 600C 800C 800C 900C 900C

9. Higher hardening at higher strain amplitudes 900C 600C 1.5% 1% 2% 2% 1% 1.5% 0.5% 0.5%

10. Higher work hardening at 600C than at 900C steady state (broken line) 0.5%, 900C 0.5%, 600C steady state (broken line) first cycle first cycle steady state (broken line) 2%, 900C steady state (broken line) 2%, 600C first cycle first cycle

11. Trans-dendritic crack-growth at 600C and inter-dendritic crack growth at 800C inter-dendritic crack 0.5%, 600C stress fracture surface 0.5%, 800C inter-dendritic crack 0.5%, 900C

12. Summary Test conditions: 600C to 900C,  = 0.5% to 2% , R = -1, laboratory air • Low-cycle-fatigue life of CF8C-Plus is slightly better than cast stainless steel HK30 between 600 to 800C. • Fatigue life of CF8C-Plus reduces with increase in temperature and strain range by 1 to 2 orders of magnitude. • Alloy hardens cyclically at 600C, and softens at 900C for majority of the lifetime. • Fatigue cracks initiate on the surface and propagate trans-dendritically at 600C and inter-dendritically at 800C.