Microstructure and properties of borocarburized 15CrNi6 steel after laser surface modification

The paper presents the results of laser heat treatment (LHT) of the borided layers produced on the carburized 15CrNi6 lowcarbon steel. The two-step treatment carburizing followed by boriding is termed borocarburizing. Laser tracks were arranged by CO2 laser beam as a single track and as multiple tracks formed in the shape of helical line. The microstructure in both cases consists of the following zones: iron borides (FeB and Fe2B) of laser modified morphology, needle-like iron borides, carburized layer with heat affected zone (martensite and alloyed cementite), carburized layer without heat treatment and the substrate (ferrite and pearlite). X-ray microanalysis of the laser modified borocarburized specimen confirmed the presence of the same two types of iron borides (FeB and Fe2B), like those indicated in the as-borided layer. The layer after borocarburizing and LHT has a high microhardness of iron borides, reducing the hardness gradient between the diffusion layer and the substrate in comparison with only borided layer. Probably, the brittleness of this layer is lower. The improved wear resistance of this layer has been found in comparison with borided and borocarburized layers after conventional heat treatment. It is probably result of globular iron boride presence after laser surface modification.