Short-time TMT - Thermomechanical short-time treatment of high-alloy martensitic stainless steels (MSS) for improved energy efficiency in steel sheet processing

• Applicants: Dr.-Ing. Thomas Grund; Dr.-Ing. Andreas Kunke
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 529708992

In the project, a thermomechanical treatment (TMT) for sheets of martensitic stainless steels (MSS) characterized by very short austenitization times is being studied. This will provide a new approach for MSS sheet hardening with reduced energy and resource consumption. The aim of the project is to provide a TMT of short total duration (short-time TMT) for MSS in order to enable high deformation degrees in sheet processing with shortest process times possible while securing specified material properties of the treated steels. In a subsequent technological implementation, this would result in reduced energy consumption and better material yields (due to reduced surface oxidation). To reach this goal, fundamental research is required. Due to the thermodynamic disequilibrium state resulting from the rapid temperature change during heating and quenching, the state of dissolved alloying as well as accompanying elements shifts, and, in consequence, so do the phase transformation temperatures of the treated steels. Therefore, the established models for TMT of MSS are not directly transferable to short-time TMT. Therefore, the sheet material X46Cr13 used in the project is heated by means of inductive rapid heating at rates of at least 100 K/s up to its austenite region, where it is simultaneously deformed without holding time and subsequently quenched. Forming steps in both the stable and metastable austenite regions are investigated. All short-time TMTs are realized with the aid of a forming simulator and a deformation dilatometer in order to gain insights into the phase transformation behavior as well as the deformation behavior. Based on the precipitation state and the retained austenite content, the influence of the heating rate and the austenitizing temperature on the solution state of the alloying as well as the accompanying elements is described by investigations using SEM, EDX or XRD, and by thermophysical calculations. Accompanying mechanical and chemical characterizations lead to the identification of a process window for a forming process, by which both the forming capacity of the MSS sheet can be significantly increased and its application properties at room temperature improved. In a final examplary implementation, a demonstrator tool and process for the production of thermomechanically treated deep-drawn parts in combination with rapid heating are designed and tested, thus creating the added value of the project beyond the existing state of the art in science and technology.

• DFG Programme: Research Grants