Non-destructive characterisation of laser-hardened steels

Surface laser hardening of steels is a well-established industrial technology. The surface of the specimen is scanned with a high-power laser beam along the trace to be hardened. Rapid cooling due to the non-heated surroundings of the laser-treated zones leads to local hardening of the material. The non-destructive evaluation of some mechanical properties (hardness, residual stresses, etc) of the hardened region and the heat-affected zone are of technological importance.

In the present work, non-destructive electromagnetic tests were performed on sets of samples of SAE 1045 (C45), SAE 4140H (42CrMo4) and SAE 4150 (50CrMo4) steels, which had been subjected to laser hardening. The examined specimens were laser-treated at different beam powers and scanning speeds. The hardened areas are 10 mm broad strips; the residual stress, hardness and hardness depth of the layers were varied by changing the laser treatment parameters.

The NDT experiments were carried out with the 3MA-II (Multiparameter Micromagnetic Microstructure and Stress Analysis) equipment developed by the Fraunhofer Institute for Nondestructive Testing (IZFP), Saarbrücken. This contribution presents the results derived from the 3MA-II scans of the specimens. The variation of the measured parameters along a line perpendicular to the hardened strip can be observed. Changes in the lattice parameter and the presence of precipitates were studied by means of X-ray diffraction analysis.