Study on the Influence of Element Partitioning on the Cryogenic Toughness of Intercritical Quenched 9Ni Steel

To study the promotion mechanism of the quenching, intercritical quenching, and tempering process on the cryogenic toughness of 9Ni steel, the evolution characteristics of reversed austenite in the process were investigated. The mechanism of the phase transformation of reversed austenite under the partitioning of C, Ni and Mn elements was described. The experimental results showed that the composition of Ni and Mn elements is active in the intercritical quenching process. In particular, when the content of Ni in the austenite phase increased from 9.19% to 13.46%, the volume fraction of reversed austenite increased from 3.6% to 5.6%. The interstitial C atoms formed the Snoek-Kê-Köster peak, whose activation energy increased from 1.13 eV to 1.24 eV. The element partitioning promoted the formation of reversed austenite. However, intercritical quenching can lead to an even distribution of martensitic lath bundles, with smaller length and spacing. The presence of more nucleation sites can facilitate the formation of reversed austenite. After tempering treatment, a thin film of austenite can easily form along the martensite lath, resulting in improved plasticity and toughness of 9Ni steel.