Enhanced ultra-cryogenic impact toughness in 9 wt% Ni steel through lamellar microstructure refinement

Abstract

The research aims to effectively enhance the low-temperature toughness of 9 wt% Ni steel, which is used in storage containers for alternative energy sources such as liquefied natural gas. To achieve this, a lamellarizing heat treatment was applied to 9 wt% Ni steel, initially manufactured using the conventional quenching-tempering (QT) process, thereby introducing a quenching-lamellarizing-tempering (QLT) process. The Charpy impact toughness of the 9 wt% Ni steel was then evaluated under two extreme cryogenic conditions: 196 °C and −253 °C. The 9 wt% Ni steels subjected to the QLT treatment demonstrated exceptional Charpy impact toughness values of 238 J at −196 °C and 217 J at −253 °C. This study presents an examination of the microstructural and micro-hardness evolution throughout the various stages of the QLT heat treatment process. A comparative investigation was undertaken by contrasting these results with samples treated using quenching (Q), quenching-tempering (QT), and quenching-lamellarizing (QL) treatments. The QLT treatment induces a substantial volume fraction of retained austenite, combined with a strategically distributed micro-hardness profile. In this profile, the soft phase interspersed among hard phases can alter the crack propagation path and absorb energy, thereby enhancing toughness. This synergy contributes to the exceptional Charpy impact toughness observed at −253 °C. These results suggest that the introduction of the QLT heat treatment is an effective method for enhancing the cryogenic impact toughness of 9 wt% Ni steel at temperatures below −196 °C, compared to the conventional QT process.