ICME-Based Process and Alloy Design for Vacuum Carburized Steel Components with High Potential of Reduced Distortion

Carburized steel components are usually quenched from a hardening temperature, which lies in a complete austenitic phase, to room temperature. This leads to a microstructure comprised of mostly martensite plus bainite giving rise to unwanted heat-treatment-induced distortion. However, having a soft phase of ferrite dispersed throughout the microstructure can be quite effective in this regard. This is attributed to the capability of ferrite in accommodating the plasticity resulted from austenite-to-martensite transformation expansion. In the context of this work, it is demonstrated that how a proper selection of chemical compositions and a hardening temperature can greatly suppress the associated distortion. Hence, in order to systematically design a new steel alloy which fits to the above mentioned conditions, an ICME-based methodology has been employed. Thus, a series of calculations have been carried out by means of the well-known thermodynamic-based software Thermo-Calc® and the scripting language of Python. The austenite to ferrite phase transformation kinetics is also captured by the software DICTRA® generating a virtual TTT (Time-Temperature-Transformation) diagram which is subsequently utilized for further finite element simulations in the software Simufact.forming®. The carburizing process, the following phase transformations and the effect of the developed microstructure on the final distortion are simulated in macro-scale through Simufact.forming. The finite-element-based results of the Simufact.forming have in turn been enhanced by the results of the above-mentioned thermodynamic-based computational tools. At a later stage the simulation outcomes are experimentally validated by employing Navy C-Ring specimens.