Residual stresses and distortion due to the heat treatment process simulation and experiment

Mechanical properties of steel parts can be improved by steel hardening. This process is influenced by several parameters. Besides the geometry of the part and the chemical composition of the material the austenitizing and the cooling conditions determine the development of residual stresses and distortions remaining after the martensitic hardening. Residual stresses have a major influence on the life cycle and distortions require remachining which augments production costs. The processes responsible for residual stresses and distortions cannot be observed in situ during an experiment. Therefore simulation is used to determine the development of temperature, microstructure, deformation and stresses during the heat treatment process. For the modelling and the simulation of the heat treatment process knowledge about heat conduction, phase transformation, mechanical behaviour of the material and their couplings such as transformation plasticity is necessary to achieve a realistic simulation model. Therefore detailed input data is indispensable. One of the most important effects determining the development of deformations and stresses is the heat transfer between the surface of the part and the quenchant. The major problem here is the determination of the heat transfer coefficient. So far, for rotation-symmetrical parts such as cylinders, the heat-transfer coefficient was assumed to vary only along the direction of the rotation-axis which led to very good results. For non-rotation-symmetrical parts such as cubes this cannot be applied as the heat transfer coefficient varies over the surface. In this work numerical and experimental results of cylinders quenched in a gas nozzle system are discussed taking into account the transformation plasticity. An outlook on the simulation of the heat treatment process of more complex geometries will be given.