A Model to Predict the Microstructural Constituents after Quenching and Partitioning of Martensitic Stainless Steels

Abstract

Abstract The typical heat treatment of martensitic stainless steels comprises hardening and subsequent tempering. Depending on the application and size of the component, tempering is carried out either at low temperatures (< 300 °C) or at high temperatures (> 500 °C). In this paper, tempering at lower temperatures is examined. First, the austenitizing step is considered in greater detail and an optimized formula for the calculation of the MS temperature of such steel grades is created in order to enable to be modelled. For the calculations, the austenite composition is determined at different austenitizing temperatures using thermodynamic simulation. Furthermore, the transformation of austenite into martensite during quenching is described with the help of the Koistinen-Marburger equation. The second part deals with effects in the material at low holding temperatures. Here, the influence of different hardening temperatures and interception temperatures of the quenching procedure is investigated. There is no complete partitioning at temperatures of 300 °C. Certain tempering processes can also take place, such as the formation of transition carbides, so-called M3C carbides. A typical tempering with formation of stable Cr-rich carbides does not occur at this low temperature. Finally, the calculated results of the model correlate well with microstructural investigations (XRD, LOM). ◼