Effects of long-term thermal aging on mechanical properties and microstructural evolution of 17–4 PH stainless steel in simulated thermal conditions for nuclear applications

Abstract

This study examined the effects of long-term thermal aging on the mechanical properties and microstructure of 17–4 PH stainless steel (SS) at temperatures from 300 °C to 400 °C for up to 12,000 h. Mechanical tests, including hardness, strength, and impact toughness tests, were conducted, along with microstructural analysis using transmission electron microscopy. The results indicated that aging at 400 °C leads to early embrittlement and a decrease in mechanical strength after 10,000 h of exposure, due mainly to spinodal decomposition and G-phase formation. At 350 °C, the formation of a G-phase was observed at the boundary between Cu precipitates and martensite matrix after 5,000 h, contributing significantly to the rapid decrease in toughness, but the hardness and mechanical strength were only minimally affected. In contrast, at 300 °C, the mechanical strength increased more gradually, with only spinodal decomposition influencing the mechanical behavior. In particular, slight softening was observed during the first 1,000 h at 300 °C and 350 °C because of carbon diffusion that promoted the growth of niobium and chromium carbides, weakening the martensitic matrix. This study highlights the significant role of microstructural evolution, particularly the relationship between the formation of the G-phase and impact toughness, in determining the long-term mechanical properties of 17–4 PH SS under prolonged thermal aging under simulated thermal conditions for nuclear applications.