Local Residual Stress Distributions Induced by Repeated Austenite-Martensite Transformation via Laser Surface Hardening of Steel AISI 4140

Abstract

The effects of laser surface hardening of steel samples on the microstructure and residual stresses were determined for single as well as multiple laser pulses. Samples made of steel grade AISI 4140 were hardened by means of a high-power diode laser (HPDL) system using either single or multiple laser pulses resulting in single as well as repeated austenite-martensite transformations. The hardening was carried out in a specially designed process chamber allowing laser surface treatment in inert atmosphere in order to avoid oxide scale formation. The residual stress distributions in lateral and in depth direction were analysed by means of X-ray diffraction for samples hardened by up to 27 laser pulses. Residual stress analyses were carried out by means of the sin²y- method. The results indicate the extension of the hardened zone in lateral and in depth direction with an increase in the number of applied laser pulses. This evolution is connected with significant changes in the local residual stress distributions.