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Effect of coating on the low-cycle-fatigue behavior of CF8C-Plus at 800 C

Effect of coating on the low-cycle-fatigue behavior of CF8C-Plus at 800 C. Deepak Kumar , S. Dryepondt, A. Shyam, B. A. Pint, E. Lara-Curzio Materials Science & Technology Division Oak Ridge National Laboratory MS&T-2010, Houston, USA October 17 th to 21 st , 2010. Acknowledgements :

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Effect of coating on the low-cycle-fatigue behavior of CF8C-Plus at 800 C

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  1. Effect of coating on the low-cycle-fatigue behavior of CF8C-Plus at 800C Deepak Kumar, S. Dryepondt, A. Shyam, B. A. Pint, E. Lara-Curzio Materials Science & Technology Division Oak Ridge National Laboratory MS&T-2010, Houston, USA October 17th to 21st, 2010 Acknowledgements : J. L. Moser, T. Geer, L. Walker, Y. Zhang 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. exhaust manifold turbo-housing • Replacement for SiMo cast-iron diesel engine exhaust components • Replacement for CF8C steel gas turbine structural components

  3. CF8C-Pluscast stainless steel was developed for applications in power generation that require higher temperature capability and reliability • Some Candidate Alloy Compositions (wt%) • CF8C (Cast equivalent of 347): Fe-19Cr-10Ni-0.07C-1.0Nb-1Si-0.7Mn • CF8C-Plus: Fe-19Cr-12Ni-0.07C-0.07Nb-0.4Si-4Mn-0.3N • SiMo Cast Iron: Fe-3.45C-4Si-0.6Mo-0.3Mn • Ni-Resist: Fe-2Cr-35Ni-0.5Mn-5Si-1.9C 550C to 850oC CF8C-Plus has better creep-rupture resistance than other candidate alloys Shingledecker et al., JPVT, 2009, 131, 051404

  4. Materials for applications in high-temperature power generation will be exposed to aggressive environments (e.g., H2O, CO, CO2, SO2, H2S) Fe-25Cr-19Ni cyclic oxidation 100-h cycle 800C air+10 vol%H2O Maziasz et al., Proc. GT2010, ASME Turbo Expo 2010: June 14-18, 2010, Glasgow, Scotland CF8C-Plus is susceptible to environmental degradation in aggressive environments which are rich in water vapor.

  5. Materials for applications in high-temperature power generation will be exposed to aggressive environments (e.g., H2O, CO, CO2, SO2, H2S) Zhang et al., Surf. Coat. Tech., 188 (2004) pp. 35 Fe3Al coating by CVD 700C, Air + 10vol% H2O The use of alumina-scale forming coatings can improve the oxidation resistance of CF8C-Plus in aggressive environments.

  6. Major concerns: Long term coating/substrate compatibility Degradation of mechanical properties • Loss of surface Al concentration below the critical level required for Al2O3 formation • Spallation of the coating due to • thermal stresses due to CTE mismatch • Kirkendall voids at the coating/substrate interface 10, 100-h cycles, 800C, Air + 10vol%H2O 15, 100-h cycles, 800C, Air + 10vol%H2O

  7. Major concerns: Long term coating/substrate compatibility Degradation of mechanical properties creep, 650C in air P91 (Fe-9Cr-2W) reduction in creep-rupture life Aguero et al., Surf. Coat. Tech., 201 (2007) pp. 6253

  8. Major concerns: Long term coating/substrate compatibility Degradation in mechanical properties TMF, cycle= 200C to 650C, alloy 122 (Fe-11Cr-2W) Change in TMF life Aguero et al., Surf. Coat. Tech., 201, 2007, 6253

  9. Objective: Determine the effect of coating on low-cycle-fatigue behavior of alloy CF8C-Plus at 800C water cooled grip Furnace upper heater thermocouple Strain controlled, Fully reversed cycle lower heater extensometer water cooled grip Temperature gradient in the gauge length = 5C Test started with ~1h of soaking at 800C

  10. Coating was applied by pack cementation process* Pack: Masteralloy + 2 wt% NH4Cl + Al2O3 Heat treatment: 900C, 6h, Ar *Coating was applied by pack-cementation process by Dr. Y. Zhang’s research group at Tenn. Tech. Univ., Cookeville, TN

  11. Characterization of the as-coated sample Fe,Al rich outer layer Al rich oxide Al, N 20µm Ni, Al rich substrate Al Fe Ni O N

  12. Cracks and voids were present in the outer layer even in the as-coated form top view of the coating crack void 100µm crack crack void Outer layer Outer layer Inner layer 5µm Inner layer 10µm void substrate AlN NiAl

  13. Coated sample is stiffer than uncoated sample 800C @0.05 % 1/s, laboratory air coated -FeAl uncoated Inner layer substrate -Al2O3 50µm -FeAl 1µm Tensile segment of first fatigue cycle Calculated effective E for coated sample: 110GPa

  14. ~15% reduction in fatigue life due to coating Macro-crack initiation coated uncoated

  15. Higher strain energy in coated sample resulted in lower fatigue life Hysteresis loop in steady state area  strain energy stored per unit volume

  16. Faster rate of cyclic hardening in coated sample coated substrate uncoated • Residual stresses? • Dislocation motion inhibition via dislocation-dislocation, dislocation-precipitate interactions?

  17. Fatigue crack started from the surface 1mm 1mm final separation at room temperature coated uncoated • Similar failure behavior in the coated and uncoated samples

  18. Inter-dendritic crack-growth in uncoated and trans-dendritic crack growth in coated uncoated inter-dendritic crack fracture surface 500µm stress inner layer substrate coated trans-dendritic crack 500µm 100µm

  19. Summary test condition: 800C,  = 0.5%, R = -1, laboratory air • Pack-cementation coating reduced the low-cycle-fatigue life of CF8C-Plus by ~15% because of higher strain energy available for macro crack-initiation. • Coated sample was stiffer than the uncoated sample and had higher apparent yield strength. • The initial cyclic hardening rate of substrate increased due to coating. • Predominant crack propagation mode changed from inter-dendritic in the uncoated sample to trans-dendritic in the coated sample.

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