Laser surface hardening: A simulative study of tempering mechanisms on hardness and residual stress

Although laser surface hardening of AISI 4140 has been an established process for years, inaccuracies still exist with regards to process prediction, leading to the use of the trial-and-error method for parameter optimization. The proposed FE-simulation model aims to improve the hardness and residual stress prediction of laser surface hardening simulations by regarding tempering effects occurring during the process. The proposed phase-dependent thermal-mechanical FE-simulation is validated by a comparison to experimental data and used for a parameter study analyzing the effects of different initial tempering states and additional post process tempering methods. The results are evaluated regarding changes to the phase fractions, hardness and residual stress states. The implementation of tempering effects showed a 30 HV0.1 decrease in hardness for the hardened zone (HZ) due to self-tempering effects and a significant decrease in hardness for the transition zone (TZ) and heat affected zone (HAZ), leading to a local hardness that can be lower than the base material hardness. Modeling tempering mechanisms during laser surface hardening leads to a significant improvement in prediction of the resulting hardness and residual stress states. This enables a simulation based process optimization regarding the initial tempering state as well as additional post process tempering steps.