Spatially resolved temporal stress evolution during laser surface spot hardening of steel

The time-dependent stress evolution, the resulting residual stresses as well as the microstructure of the heat treatable low alloyed steel AISI 4140 induced by laser surface spot hardening was investigated systematically by means of synchrotron X-ray diffraction. In‐situ stress analyses with a time resolution up to 100ms were carried out at the synchrotron beamlines P05@PETRAIII(*), DESY, Hamburg and PDIFF@ANKA, Karlsruhe, by the application of the measurement and evaluation approach for very fast X-ray diffraction stress analyses. During the laser surface spot hardening using a homogenization working optic with a spot size of approx. 8×8mm2 at a maximum temperature TA,max of 1150°C and heating/cooling rates vheat/cool of 1000 K/s time-resolved diffraction data were collected for various measurement positions inside and outside of the processed zone aiming to analyze the different origins for residual stress build-up. The in-situ tests were supplemented by X-ray residual stress analyses and microscopical investigations of the microstructure subsequent to the laser hardening process (ex-situ analyses).