Microstructure degradation in stainless steel weld metals due to thermal and mechanical histories

In order to develop a long term creep-fatigue evaluation method of stainless steel weldment, that incorporates the degradation of weld metal, the authors have looked into the mechanism of micro damage in the structure of type 308 and 316 weld metals that had been subjected to thermal and mechanical histories using transmission electron micrography. The results obtained were as follows: (1) Carbide(M sub 23 C sub 6 ) and Laves phase that precipitated in delta -ferrite phase were transformed into sigma -phase being more stable at high temperatures after long time heating. (2) While delta -ferrite phase remained in the microstructure at 550 deg C even after the change of its composition due to the transformation, > 600 deg C, it disappeared after the complete transformation into sigma or austenitic phase. (3) The rate of dissolution of delta -ferrite phase under cyclic stresses for fatigue or creep-fatigue failure was higher than the one under constant stress for creep failure; it was also increased by a factor of 10-100 in comparison with an unloaded case. (4) The dissolution of delta -ferrite phase led to initiation and propagation of cracks at the interfaces of sigma and remaining delta -ferrite or austenitic phase, causing a difference of mechanical properties between them.