Induction hardening of sintered steels
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Motivation
In an inductive surface hardening process, the near-surface region of a component is hardened by eddy currents from an electromagnetic field and the resulting Joule heat. The microstructure of the boundary layer can be specifically adjusted by selecting suitable process parameters. The advantages of this process include the short process times, the efficient energy transfer and thus low energy costs, the low distortion due to limited heated areas and the good reproducibility of the microstructure setting, at least for conventional steels.
With sintered steels, however, induction hardening brings specific challenges. The risk of cracking is much higher in sintered components than in similar melt-metallurgical variants. Due to pores, the thermal conductivity of the material decreases with increasing porosity. As a result, high thermal gradients are generated, since the major part of the heat output (depending on the working frequency) takes place in a small area close to the surface. These thermal gradients generate high residual stresses in the component, which have a particularly critical effect on the risk of cracking due to the increased number of defects (pores) and the reduced strength of sintered components. Due to the lack of understanding of the relationships between process parameters and hardness, residual stresses, microstructure and cracking in sintered components, the process capability is not sufficient yet despite the high potential.
Objectives
- Identify the essential relationships between material properties, process settings and the metallographic and mechanical results of induction surface hardening for sintered steel components.
- Identify optimal variation space for material and process conditions in terms of crack risk, hardness and residual stress profiles.
Project content
- Material characterisation: Investigation of the most important material properties that influence the inductive surface hardening of components made of sintered steel and their dependencies. This includes e.g. the characterisation of the phase transformation behaviour, the flow properties and the electrical-magnetic properties.
- Component investigation: Systematic variation of the material-related (porosity, carbon concentration, alloy composition) and process-related (settings of the heating phase, quenching and tempering treatment) influencing factors of induction surface hardening; determination of the correlations of these influencing variables with the results of the heat treatment: tendency to crack formation, residual stresses, hardening depth, surface hardness, etc.
- Finite element modelling: development of a numerical model to predict the metallographic (microstructure development) and mechanical (residual stress and hardness profiles) results of inductive surface hardening of components made of sintered steels as a function of the material-related and process-related parameters.
Funding information: AVIF Research Association Project A325
Induction surface layer hardening of sintered gears
Left: inductor and gear (schematic)
Center and right: hardened microstructure at a glance and in the tooth root area