Abstract
In phase thermomechanical fatigue (TMF) in the temperature range 573–973 K (300–700°C) and isothermal fatigue behaviour at 973 K in air were studied for type 316 stainless steel using smooth cylindrical specimens machined from base metal, weld metal, and the weld joint (cross-weld). In all joint specimens, fatigue failure occurred in the weld metal region. The lifetimes of weld metal and joint specimens were almost equal and were always inferior to those of base metal specimens. In the base metal, the effect of strain rate on the isothermal fatigue life was not very significant. Although TMF lifetimes were always a little shorter than the isothermal fatigue lifetimes in base metal, the difference was small for the same mechanical strain range and similar strain rate. This may be because the fracture mode for both types of loading was of a similar mixed type. Conversely, a dramatic reduction in lifetime was observed in weld metal and joint specimens under TMF in comparison with isothermal fatigue. This was attributed to the additional damage caused by many independent subsurface cracks at σ phase boundaries and linkage of these cracks with the surface crack, leading to rapid crack propagation. The δ ferrite in the weld metal completely transformed to σ phase in both the isothermal fatigue and TMF tests.
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