Laser hardening of thin walled parts with cryogenic cooling

The surface properties of functional components have a significant influence on the wear resistance or fatigue behavior and therefore on the lifetime of the part. To improve these surface properties, one common industrial process is laser hardening. The aim of the process is to heat the material locally over the Ac3 temperature staying below the melting temperature and to get martensite by self-quenching. However, in case of thin walled parts, the volume of the material is not large enough to transport the heat fast enough out of the process zone for self-quenching. To realize a sufficient cooling and to get simultaneous a high depth of hardening, the approach of using an active cryogenic cooling for laser hardening of thin walled parts was investigated. As a result of the cryogenic cooling with liquid CO2, the temperature decreased fast enough under the martensite start temperature that the microstructure changed to martensite. Due to the position of the localized cooling point in relation to the position and feed rate of the laser spot, parts with a thickness of 1 mm and 3 mm have been hardened. As a consequence of the process parameters and the cryogenic cooling, the achieved depth of hardening of the thin walled parts are in range between 0.2 mm and 1 mm. This paper will present in detail this innovative approach and the results.