For 304 stainless steel under superimposed creep-fatigue loading conditions, it has been generally accepted that cavity uncleation an growth during hold time degrades the fatigue endurance and the cavities are closely related to the intergranular carbide precipitation. Because the carbon content of 304L stainless steel in about half of that of 304 stainless steel, there is considerably low density and small size of grain boundary carbide precipitates. Therefore, it is expected that the creep-fatigue behaviors of the two materials may be different, and this difference would explain the role of carbide precipitates for the creep-fatigue interaction.
It is well known that, even under the same test temperature, the continuous low-cycle fatigue and creep-fatigue behaviors are significantly changed by the deformation rate and mode. Therefore, for the thorough understanding of the creep-fatigue interaction phenomenon, the investigation of the effects of deformation rate and mode on the low-cycle fatigue behavior is necessary.
In the structural alloys in nuclear reactor, sometimes, the segregation of P at the surface and grain boundary induced by the irradiation was observed. In a recent study on the creep behavior of 304 stainless steel, the segregation of P at the grain boundaries accelerates the cavity nucleation rate. However, there appears to be less information published in the literature for the effects of impurities such as P on the creep-fatigue interaction behavior.
The work reported here examines the low-cycle fatigue behavior of 304L stainless steel at the various deformation modes and investigates the creep-fatigue behavior of commercial and P-dopes 304L stainless steel at 823 K.
All the tests were carried out at 823K in high purity argon atmosphere (matheson 99.999%), the strain wave form was a symmetrical triangular shape of tension and compression and the strain rate was $4×10^{-3}$/s or $4×10^{-5}$/s. For theinvestigation of the phenomenon of creep-fatigue interaction, 30 min hold at tensile peak strain was applied.
From the results of the creep-fatigue tests for 304L stainless steel, it is found that the creep-fatigue interaction of this material is mainly due to the nucleation and growth of grain boundary cavities as in the case of general austenitic stainless steels. That is, the fast nucleation of surface cracks and the accelerated intergranular crack growth due to the grain boundary cavities are the main creep-fatigue interaction.
As a result of the modeling and experiments for the cavity nucleation and growth mechanism under the continuous low-cycle fatigue condition, the fundamental problem of the gap between the nucleation and the growth of cavity in this condition was solved. Moreover, it is found that, under a creep-fatigue interaction condition, the grain boundary cavitation behavior can be changed by the fatigue deformation rate.
Since the fatigue life is shortest under the test condition at which the most dominant grain boundary cavitation occurs, the minimization of grain boundary cavitations is required for the development of more stable elevated temperature structural materials.
On the other hand, from the creep-fatigue test results, the effect of P addition to 304L stainless steel is proven to be beneficial in spite of the lower plastic ductility and the shorter continuous low-cycle fatigue life of P-doped 304L stainless steel than those of commercial 304L stainless steel. Since, in creep-fatigue cycling of this work cavity nucleates and grows during hold time only, we apply the classical nucleation theory to the cavity nucleation during hold time. As a result, it can be considered that the addition of P lowers the nucleation rate of cavity mainly by decreasing the number of grain boundary precipitates. And since the segregation of P at the grain boundary lowers the grain boundary diffusivity of 304L stainless steel, the growth rate of cavity in P-doped 304L stainless steel seems to be decreased. by these low rate of the cavity nucleation and growth, it is considered that the creep-fatigue life of the P-doped 304L stainless steel is higher than that of commercial 304L stainless steel at the same total strain range.
