Influence of austenitisation and transformation hardening on the residual stress development of laser beam treated components made of 20MnCr5

In steel forming process chains residual stresses affect the final form of work pieces. This may have a detrimental effect, e. g. in the case of released residual stresses during laser welding of a shaft-hub connection, or may be used purposefully, as in the case of a selective application of residual stresses in thermal pre-stressing. During final heat treatment or rather hardening, distortion is essentially changed. In order to control or exploit the effect of residual stresses, they must be determined at first and their formation has also to be understood. One often neglected influence on the residual stresses is the dependence of the local flow stress, which strongly changes due to the structural transformation. Especially for processes with a high heating and cooling rate the incomplete austenitisation limits the structural hardening during cooling. For both of the applications mentioned above, it will be demonstrated how this influence can be taken into account based on a model and how this influence affects the absolute quantity of residual stresses. The modeled shape changes and residual stresses results are substantiated using speckle photography, laser triangulation, x-ray and neutron diffraction measurements.