In the test condition of the creep-fatigue loading, it has been generally accepted that a reduction of the fatigue life in AISI 304 stainless steel is due to the creep fatigue interaction. This reduction in the fatigue life is thought to be attributed to the interaction between a fatigue damage resulting from the initiation of the fatigue crack at the material surface and its growth and a creep damage resulting from the cavity nucleation and growth at the intergranular carbide precipitates of the bulk materials. Therefore it is expected that the distribution of the intergranular carbide precipitates is closely related to the fatigue life under the test condition of the creep-fatigue loading.
As a result of the studies on the effect of the intergranular carbide precipitates on the creep-fatigue interaction, it has been reported that the thermal aging(the specimen with intergranular carbide precipitates by aging treatment before creep fatigue testing) produced beneficial results(longer fatigue life) relative to the unaged solutionized material(the specimen with no intergranular carbide precipitates), but its reason is not fully clarified.
The present work investigates the effect of the thermal aging on the fatigue crack initiation and the behaviors of the creep-fatigue interaction of solutionized AISI 304 and aged AISI 304 stainless steel at 300K and 873K. All tests were carried out in high purity argon atmosphere(99.998% Ar), the strain wave form was a symmetrical completely reversed triangular shape, and the strain rate was $4×10^{-3}sec^{-1}$. For the investigation of the phenomenon of the creep-fatigue interaction, 10 - 60 min. hold time at tensile peak strain was applied.
From the test results of the crack initiation modes, it is found that fatigue cracks are initiated at the grain boundaries under the test condition of the continuous cycle due to strain incompatibility between adjoining grains, but in the creep-fatigue test, the crack initiation at the grain boundaries is due to the cavity nucleation and growth at the intergranular carbide precipitates. By considering these crack initiation mechanisms, the reason for the surface roughness effect is found to be due to the change in crack initiation sites from the intrinsic sites, grain boundaries, to the extrinsic sites, surface grooves, with test conditions and materials in the continuous cycling, but due to the change in crack initiation and propagation modes with the extent of the grain boundary cavitation process in creep-fatigue tests.
From the test results of continuous cycling, it is believed that intergranular precipitates by thermal aging treatment have no effect on the continuous fatigue process. From the creep-fatigue test results, however, the effect of thermal aging in AISI 304 stainless steel is found to be benefitial relative to solution treatment, and it can be considered that the great amount of small carbides are precipitated at the grain boundaries during creep-fatigue tests in the solutionized material, and they increase the cavity nucleation rate because they can be sites for the cavity nucleation. By this high nucleation rate of cavity in the solutionized material, it is thought that the fatigue life for the aged material is great than that for the solutionized material in the creep-fatigue test at the same test conditions.
On the other hand, from the creep-fatigue test results of AISI 304 and AISI 303L stainless steel, it is thought that the minimization of grain boundary cavity nucleation sites, it is thought that the minimization of grain boundary cavity nucleation sites, carbide precipitates, is a method for the development of the high temperature structural materials because fatigue life is mainly dependent on the cavitation process, specially cavity nucleation rate, in creep-fatigue test.
