A variational concept and its experimental calibration for temperature‐induced rate‐dependent phase transformations in steel

Abstract

In forming processes of metals, different physical phenomena are observable which may alter the material properties and behavior severely. These observations can be attributed to changes in the surface layer of the material. This research is a part of the transregional collaborative research center SFB/TRR 136 which deals with surface layer modifications in 42CrMo4 steels and aims to determine process independent correlations between material loads and material modifications. These correlations are called process signatures. The present work is concerned with investigating thermally induced solid-solid phase transformations in steels. An example of a predominantly thermal process is the induction hardening process. In this process, an initially ferritic-pearlitic structure is at first heated until reaching a temperature that lies above the temperature at which homogeneous austenite is formed. During this heating process, the steel undergoes diffusion-driven phase transformations which are heating-rate-and time-dependent. After sufficiently long incubation, a homogeneous austenitic microstructure is obtained. Then, the specimen temperature is lowered at a very high cooling rate. This invokes high-temperature rates on the microstructure and results in an austenitic-martensitic phase transformation, see e.g. [1]. Based on the Principle of the Minimum of the Dissipation Potential (PDMP) as presented in [2], the austenitic-martensitic transformation has been widely discussed in the context of shape memory alloys. We present a material model which is also able to show the ferritic-austenitic transformation. The individual phase transitions are modeled using a rate-and transformation-process-dependent formulation of the dissipation potential. The presented material model is finally tested with numerical simulations.