Inductive hardening and tempering of graded porous powder metallurgical steels

• Applicant: Professor Dr.-Ing. Volker Schulze
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Materials in Sintering Processes and Generative Manufacturing Processes ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 458684492

Powder metallurgically produced steels have established themselves in recent years in industrial applications thanks to advantages such as near-net-shape forming. Today, pre-alloying can be used to produce powders that could be heat-treated with most heat treatment processes for regular steels. Induction hardening, in which rapid, low-distortion hardening with residual compressive stresses in the surface layer leads to a positive effect on the service life of the component, is now a common process, but has hardly been used to date for PM steels. In the case of sintered steels, however, the abrupt quenching combined with the porosity shows a serious tendency to crack formation and a fundamental change in the transformation kinetics both in the hardening and tempering steps. This leads to drastic changes in the process parameters, which are difficult to predict. Today, process simulations and material models are used for efficient process development of surface hardening. While adapted models exist for regular steels, these are missing for PM steels. A direct application of known models is not possible due to the unknown interaction between porosity and the thermo-physical properties that are decisive for the process flow. Even for the tempering of surface hardened components, which is common after hardening, there are hardly any experimental investigations which consider processes such as precipitation formation with regard to the peculiarities of PM steels. Accordingly, there are no dedicated simulation models for this either.The aim of this project is therefore to close this glaring knowledge gap regarding the kinetic and thermo-physical effects and to develop an efficient model for the entire heat treatment process by comprehensively investigating the porosity-dependent material and microstructure properties.The project will perform extensive measurements with different thermo-physical and metallographic methods of the state after inductive surface hardening and subsequent tempering. An Astaloy CrA with added carbon will be used as material. The aim of the investigation is the porosity-dependent characterisation of thermo-physical quantities, phase transformations and the development of mechanical properties during surface hardening and tempering. The experimental results will then be converted into kinetic and material models, from which a finite element process model of inductive hardening and subsequent tempering will be constructed.Finally, a parameter study will be carried out with the help of this model, which will allow the influence of different process parameters on the resulting component condition to be investigated and the resulting predictability of the process result to be validated.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Stefan Dietrich