Mechanism-based modelling of tempering phenomena in steels

• Applicants: Professor Dr.-Ing. Christoph Broeckmann; Dr.-Ing. Martin Hunkel; Professor Dr.-Ing. Volker Schulze
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259019485

To improve the operational capability of hardened components the process of martensitic hardening is classically supplemented by a subsequent tempering process. After the martensitic hardening, the material in its as-quenched condition has an extreme non-equilibrium state and lacks of toughness. In addition, destructive residual stresses can be generated. During tempering the material progresses towards its equilibrium by diffusion of carbon, carbide precipitation, dissolution of retained austenite as well as residual stress relaxations. As a consequence of these mechanisms, the material receives ductility required for many applications and the life time of the heat treated component will be increased. Although modelling and simulation of heat treatment is well-established in many fields of application, tempering is rarely considered despite its particular importance regarding the resulting performance characteristics of hardened steel parts. The objective of this project is to investigate the tempering and the preceding martensitic hardening process as well as their interactions and hence to develop a physically based model describing the occurring mechanisms. The characteristics of interests in modelling are the resulting microstructure, hardness, mechanical properties and residual stresses as well as the distortion of the parts. The different processes are to be investigated at different steels and heat treatments, whereat certain processes predominate in each case. The formation of carbides and mechanical properties are focused by the IWM of the RWTH Aachen based on the through hardening of the high-alloyed steel X40CrMoV5-1. Interactions between the formation of martensite, the stabilization of austenite and tempering processes which already occur during quenching (autotempering) as well as their effects on the subsequent tempering are analysed using the through heat treated bearing steel 100Cr6 at the IWT Bremen. The inhomogeneous microstructure in consequence of short time austenitization and short time tempering, as occurring in inductive surface hardening, are investigated using the tempering steel 42CrMo4 at the IAM-WK of the KIT. Finally, based on these collaborations a modelling is enabled containing all single effects during tempering.

• DFG Programme: Research Grants