Study on copper precipitation during continuous heating and cooling of HSLA steels using electrical resistivity

Electrical resistivity technique was used to study the phase transformation and copper precipitation during continuous heating and cooling of three Cu bearing high strength low alloy (HSLA) steels. Dilatation measurements were performed to compare the results with the resistivity. During heating, the dilatation plot revealed Ac₁ and Ac₃ temperature, while resistivity measurements indicated precipitation of copper in the range of 370–550°C. A method was demonstrated to estimate the amount of copper precipitation during continuous heating. During continuous cooling, the austenite transformation temperatures could be derived from resistivity, which compare well with dilatometry. A hysteresis between heating and cooling curve was noted possibly owing to formation of bainite during cooling. Non-isothermal kinetic analysis of dilatation data during continuous cooling yields an activation energy of 62 kJ mol⁻¹, which could be related to the formation of bainite, whereas higher activation energy of 237 kJ mol⁻¹ obtained from resistivity data may correspond to the diffusion of Cu in iron, associated with the copper precipitation during austenite transformation.