Real-time stress evolution during laser surface line hardening at varying maximum surface temperatures using synchrotron X-ray diffraction

Abstract

Multiple laser surface line hardening experiments on flat samples, made of tempering steel SAE 4140, were carried out under variation of the maximum control temperature in the range of 850 °C to 1150 °C at the sample surface. For each temperature, time resolved X-ray diffraction measurements using synchrotron radiation were performed at beamline P05@PETRA III at DESY (Deutsches Elektronen Synchrotron) in Hamburg, Germany. The samples were line hardened using a 4 kW High Power Diode Laser unit at a constant laser feed of 800 mm/min with a specific laser optics under pyrometer control of the maximum surface temperature. A special designed process chamber with 4 symmetrically attached fast silicon micro strip line detectors allows for stress analysis during the process according to the sin2ψ method in single exposure mode with a measurement frequency of 50 Hz. As a result of the time resolved analyses the elastic strains were separated from thermal ones and near surface local stress evolutions, longitudinal and transverse to the laser track direction in the center of the processed zone, were determined. The in situ experiments were complemented by high spatially resolved post-process residual stress analyses using conventionally generated X-rays and by metallographic investigations. The results are carefully discussed regarding mechanisms of local stress formation during the laser hardening process and their dependence on the maximum control temperature. The temporal stress course during laser surface line hardening is presented for the first time in both surface parallel directions, where temperature differences in the process zone lead to significant differences in the resulting residual stress profiles. In addition to the common validation of process simulations by comparison with experimentally determined data from the final state after processing, e.g. the resulting local residual stresses, the current work provides real-time data from in situ experiments during laser surface hardening for the validation of temporal stress courses of numerical process simulations.