Influence of Partial Replacement of Co with Cu on Isothermal Transformation Kinetics in Ferritic/Martensitic Heat‐Resistant Steel

Abstract

This study examines the influence of partial replacement of Co with Cu on isothermal transformation kinetics in 10Cr-based heat-resistant steels. Two kinds of heat-resistant steels, having similar chemical compositions except for Co and Cu contents, are prepared and isothermally heat-treated at 710–790 °C up to 240 h after austenitization. During isothermal heat treatment, austenite is decomposed via two different reactions, γ → α + M23C6 and γ → α; the cellular precipitation (γ → α + M23C6) nucleates at prior austenite grain boundaries or the interior of prior austenite grains and grows toward the interior of austenite, followed by γ → α transformation originating from C depletion in the remaining austenite. A time–temperature–transformation diagram exhibits that the partial replacement of Co with Cu not only decelerates the isothermal transformation kinetics but also lowers the nose temperature of transformation. The calculation of the driving forces for the successive reactions reveals that the increase of Cu content predominantly reduces the driving force for both γ → α + M23C6 and γ → α reactions, contributing to the lower nose temperature and sluggish transformation kinetics as Co is partially replaced with Cu.