Effect of Hold Time and Thermal Aging on Elevated Temperature Fatigue Crack Propagation in Austenitic Stainless Steels.

Abstract
The effects of combined cyclic and static loading on fatigue crack propagation in thermally aged and unaged Types 304, 316, 321, and 348 stainless steels were investigated at 427 and 593 C. The Types 304 and 316 steels were evaluated in both the solution annealed and cold worked conditions. The results show that aging and hold time produce no significant effect on crack propagation rate at 427 C. At 593 C, the effects of aging on crack propagation performance were most significant for the Type 316 steel and related to thermomechanical history. Thermal aging decreased the crack propagation rate in solution annealed Type 316 steel at 539 C and increased the propagation rate in cold worked material. The effect of hold time at 593 C was to increase crack propagation rate with increased hold period in Type 316 steel. In the cold worked Type 316 steel, thermal aging reduced the magnitude of the hold time effect. SEM examination of the fracture surfaces of both solution annealed and cold worked Type 316 specimens tested at 593 C revealed a transition from transgranular to intergranular failure mode with increasing hold time in the unaged condition. After thermal aging, the failure mode was transgranular for all hold times. TEM study of the aged and unaged microstructures indicated the presence of profuse grain boundary precipitates identified as M23C6. Consideration was given to influence of both environmental effects and the intergranular precipitates on the transition in failure mode.

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