In Situ Synchrotron X-Ray Diffraction Stress Analysis During Laser Surface Line Hardening of Samples with Specific Geometric Features

Multiple in situ X-ray diffraction experiments of temperature controlled laser line hardening processes have been carried out at the German Electron Synchrotron (DESY) in Hamburg, Germany. During the process, the local strain and stress evolution were monitored using synchrotron radiation with a time resolution of 50 Hz with a spatial resolution of less than Ø1 mm. To quantify sample geometry effects on the stress formation and hence the residual stress state in the process zone, different samples with geometric features made of steel grade AISI 4140 were line hardened by means of a high-power diode laser unit using a maximum control temperature of 950 °C at constant laser feed of 0.8 m/min. A specially designed process chamber was used in the experiment, allowing the control of the inert gas atmosphere to avoid surface oxidization. The symmetric application of four fast micro-strip line detectors allows for the real-time measurement of several diffraction lines hkl during the heat treatment process. The thermal and elastic strains were separated and time resolved stress analysis was carried out using a conventional X-ray stress analysis approach. The results were carefully discussed using data obtained by numerical process simulation by finite element method (FEM). It could be shown and explained that geometric features (radii) in the process zone of laser surface line hardening lead to a decrease of resulting residual stresses transverse to the processed workpiece zone