Microhardness of the microstructural phases of laser hardened steels

This study investigated the microhardness and microstructure of different steels hardened by a fibre laser. These samples included rolled steel, quenched and tempered steel, annealed alloyed steel and conventionally through-hardened steel. Microhardness (HV0.01) was measured in martensite, pearlite, ferrite and cementite structures at different depths below the laser-irradiated surface. The microhardness test results were compared with the conventional macrohardness (HV5) results. Increase of carbon content caused higher microhardness of martensite, although this influence was weaker than the effect on macrohardness. The grain size of rolled ferritic-pearlitic steels had distinct effect on surface microhardness. The macrohardness of quenched and tempered steel might be markedly influenced by the homogeneity of alloy contents. In-grain indentation showed that cementite is around 150 HV harder than pearlite. High Cr content may increase the microhardness on the surface and up to a depth of about 0.6 mm. Soft annealed alloyed steels achieved high surface hardness but poor hardened depth despite high alloy content. Dispersed granular pearlite did not affect the microhardness of soft annealed steel. The macrohardness of the base material was close to the microhardness of the softer phase structure. The measured microhardness was about 100-250 HV higher than the macrohardness.