EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Quenching and Partitioning (Q&P) is not a typical heat treatment for martensitic stainless steels, but variation of the interception temperature and subsequent holding at low “tempering” temperatures (<500°C) has a significant impact on these steels due to partitioning effects.

With the help of physical models, the microstructure after Q&P heat treatment can be predicted. In comparison to low-alloyed advanced high strength steels, martensitic stainless steels are characterized by higher alloying contents, especially by additions of nitrogen. In order to describe the onset of the martensite formation during quenching from the austenitic region, a new MS formula was determined in this study. For this purpose, MS temperatures at four different austenitizing conditions were determined for several steels and the corresponding austenite composition was calculated using ThermoCalc®. The influence of different databases on the calculation was taken into consideration.

The transformation of austenite to martensite during quenching was observed by dilatometry and mathematically described using the Koistinen-Marburger equation. The k-value for this equation was also redetermined in the course of this work for the selected materials.

In the present model, partial partitioning was assessed instead of full partitioning as considered by constrain carbon equilibrium model. Additionally, nitrogen was taken into account for the partitioning. Another factor considered in the model is not only the chemical stabilization of austenite via C- and N-partitioning, but also the mechanical, which was derived from the dilatometric curves.

Complete Q&P heat treatment cycles were conducted on a dilatometer using different austenitizing and interception temperatures. The samples were subsequently analysed, in order to determine the retained austenite content and it was confirmed partial partitioning taking place during isothermal holding at tempering temperature. The calculated results of the model were compared with experimental data from dilatometer samples and showed good agreement.