Evolution of precipitate and its effect on the degradation of impact toughness in a carbon and nitrogen-controlled 316 stainless steel during thermal aging at 650 °C
Xinliang Lv, Shenghu Chen, Lijian Rong
Abstract
Impact toughness degradation and microstructure evolution of a carbon and nitrogen-controlled 316 stainless steel during long-term aging at 650 °C for up to 8800 h were investigated. The degradation of impact toughness during aging can be divided into three stages: a rapid decrease during stage I (the first 400 h), a slow decrease during stage II (from 400 to 3200 h), and the stable stage during stage III (up to 8800 h). P enrichment at the M 23 C 6 /γ interface is induced by the rejection of P during the formation of M 23 C 6 carbide, and the M 23 C 6 /γ interface is preferred location for crack initiation. An increase in the amount of grain boundary M 23 C 6 carbides increases the crack initiation sites, resulting in a rapid decrease of impact toughness during stage I. The ongoing rejection of Ni and Si during the coarsening of M 23 C 6 carbide is beneficial for the nucleation of M 6 C carbide. The micro-crack initiation at the M 23 C 6 /M 6 C and M 6 C/γ interfaces is introduced, and a semi-continuous network of M 23 C 6 carbide along grain boundary promotes the crack propagation, leading to a further decrease of impact toughness during stage II. The nearly continuous precipitate morphology is induced through continued growth of grain boundary M 23 C 6 and M 6 C carbides. The propagation of micro-cracks along the M 6 C/γ, M 23 C 6 /γ, and M 23 C 6 /M 6 C interfaces leads to the mostly intergranular fracture, thus the impact energy tends to be stable during stage III.