Prediction of Creep-Fatigue Life Based on the Damage Mechanism of Grain Boundary Cavitation and Improvement of Life by the Modification of Carbide Characteristics

摘要

It is understood that grain boundary cavitation is one of the detrimental process for the degradation of austenitic stainless steels that reduces the creep-fatigue life at high temperatures. A new damage function based on a model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed for austenitic stainless steel. This damage function is a combination of the fatigue and creep terms related to the cavitational damage (cavity nucleation and growth) in the life prediction equation and is found to be generally applicable to all the materials in which failure is controlled by the grain boundary cavitational damage. The cavity nucleation factor, P', which is introduced in the creep-fatigue life model, is found to be closely related with the characteristics of grain boundary carbides acting as cavity nucleation sites. The modification of carbide characteristics through grain boundary serration is successfully made by the special heat treatment so that the modified carbides are favorable for cavitation resistance, resulting in a lowered P' value of material. It is observed that the creep-fatigue life is remarkably improved by the modification of carbide characteristics through grain boundary serration.