Double-step inter-critical tempering of a supermartensitic stainless steel: Evolution of hardness, microstructure and elemental partitioning

Abstract

The maximum allowed hardness for low carbon martensitic stainless steel components used in the oil and gas industry is 247 HV. Inter-critical tempering is an effective method for hardness control due to the production of stable reverted austenite. By conducting multiple-step tempering cycles, the austenite reversion kinetics can be accelerated and its thermal stability upon cooling can be greatly increased. In this work, supermartensitic stainless steel samples were subjected to two-step inter-critical tempering cycles. First, all samples were heat-treated at 625 °C for 2.5 h to minimize hardness through the maximization of stable austenite. Then, a second stage tempering with temperatures between 560 and 720 °C for 2.5 h was studied. The amount of stable reverted austenite at room temperature increased for second stage temperatures below 625 °C. Between 625 and 670 °C, the amount of reverted austenite notably increased at high temperature but it had limited thermal stability upon cooling. Above 670 °C, all newly reverted austenite was completely unstable during the cooling stage and partial dissolution of the stable austenite obtained after the first tempering cycle also occurred. Interestingly, hardness was mostly insensitive to the stabilization of additional austenite or to the newly formed fresh martensite.