Influence of post-weld heat treatments on microstructure and mechanical properties of AISI 431 martensitic stainless steel friction welds

The relative effects of different post-weld heat treatments (PWHTs) on microstructure and mechanical properties of friction welded martensitic stainless steel type AISI 431 were studied. The weld microstructure consists of acicular martensite in equiaxed prior austenite grains and with heat treatment, the martensitic microstructure experiences coarsening. It was observed that retained austenite content decreases with an increase in the PWHT tempering temperature. The tensile properties of welds under different PWHTs were comparable to those of parent metal in the respective treatments. Welds exhibited higher notch tensile strengths than the parent metal in the respective heat treatment condition. Double tempering (670°C + 600°C) led to maximum reduction in hardness. Welds exhibited poor impact toughness in the as welded condition. The improvement in impact toughness was found satisfactory in welds subjected to double tempering, although did not match to the level of the parent metal at respective conditions. The mechanism and reasons for the observed behaviour have been discussed, correlating the microstructure, fracture features and mechanical properties.