Thermal aging effect on elevated temperature deformation mechanisms of 316L stainless steel weld metal by small punch test

Abstract

The effects of long-term thermal aging on the microstructural evolution and its elevated temperature deformation behavior of 316L stainless steel weld metal were studied using the small punch test at 350 °C. The mechanical responses at the microstructural scale were investigated to better understand the mechanism of the influence of long-term thermal aging on the mechanical properties of the 316L weld metal. The results indicated that the hardness and mechanical strength of the 316L weld metal increased slightly with extended thermal aging time at 400 °C, which was primarily attributed to spinodal decomposition and G-phase precipitation in the ferrite phase. The hardening of ferrite increased the deformation incompatibility between the ferrite and austenite phases. Slip bands were blocked at the phase interface, leading to preferential cracking along the phase interface. In contrast, in the as-received 316L weld metal, slip bands could extend between two phases by means of multi-system slip, and cracks tended to crack along austenite grain boundaries. Notably, the deformation mechanism of austenite phase exhibited significant changes due to long-term thermal aging. The as-received austenite deformed through dislocation slip and twinning due to its lower stacking fault energy. After long-term thermal aging, the primary mechanisms governing the plastic deformation of austenite shifted to cross slip and multiple slip. In addition, a smaller dislocation cell structure (about 200 nm) was formed in the deformed austenite after long-term thermal aging, compared to a larger dislocation cell structure (about 500 nm) in the deformed austenite before aging. In contrast, the deformation mechanism of the ferrite phase in the 316L weld metal remained consistent, dominated by slip deformation both before and after long-term thermal aging.