CREEP-FATIGUE INTERACTION BEHAVIOR OF TYPE-308 STAINLESS-STEEL WELD METAL AND TYPE-304 STAINLESS-STEEL BASE-METAL

The effects of hold condition (tension-only, compression-only and tens ion-plus-compression holds of 1 min duration) and hold time (up to 10 min in tension) on the low-cycle fatigue behaviour of type 304 stainle ss steel base metal and type 308 stainless steel all-weld metal were i nvestigated at 923 K. All the tests were performed in total axial stra in control in air, employing a strain range of 1.0%. The welds were pr epared by a submerged metal arc welding process. The microstructure of the base metal and the weld consisted of gamma phase and duplex gamma -delta ferrite respectively. The results clearly indicated a reduction in continuous cycling as well as creep-fatigue interaction life of ty pe 308 SS weld metal compared with type 304 SS base metal under identi cal testing conditions. Hold times displayed an effect on life that wa s dependent not only on material condition but also on the position of the hold in the cycle. Fatigue lives recorded for type 308 SS weld in the 1 min hold-time tests were in the order: compression hold --> con tinuous cycling --> tension-plus-compression hold --> tension hold. Fa tigue lives of 304 SS were in the order: continuous cycling --> compre ssion hold --> tension-plus-compression hold --> tension hold. A signf icant reduction in the life of the weld metal was noted on increasing the duration of tension hold time to 10 min. 308 SS weld metal exhibit ed cyclic softening whereas 304 SS base metal showed rapid initial har dening followed by a saturation stage. The observed variations in life are explained on the basis of crack initiation and propagation modes and microstructural changes that occurred during low-cycle fatigue and creep-fatigue interaction testing.