Microstructure change due to thermal and mechanical histories in stainless steel weld metals
Hasebe Shinichi; Asayama Tai
Abstract:As a fundamental study of developing a long term creep-fatigue evaluation method of weldment of stainless steel that incorporates the degradation of weld metal, the authors investigated the mechanism of micro damage in the structure of 308 and 316 type weld metals that had been subjected to thermal and mechanical histories using transmission electron micrography. The results obtained are as follows. (1) M23C6 and Laves phase that precipitated in d-ferrite phase were transformed into s phase that is more stable under high temperatures after long time heating. (2) While d-ferrite phase remained in the microstructure at 550 deg. even after supplying alloying elements to precipitates and changing to Fe rich composition, at above 600 deg. it disappeared after transforming completely to s phase or austenite phase. (3) The rate of dissolution of d-ferrite phase under cyclic stresses for fatigue or creep-fatigue failure was higher than that under constant stress for creep failure, and was accelerated by 10 to 100 times compared with that under no load. (4) The dissolution of d-ferrite phase led to initiation and propagation of cracks at the interfaces of s phase and remaining d-ferrite phase or austenite phase by making a difference of mechanical properties between the phases. Key Words:long term creep-fatigue evaluation, stainless steel, weld metal, d-ferrite phase, micro damage, creep-fatigue crack