Laser transformation hardening of carbon steel: microhardness analysis on microstructural phases

This study investigates the microhardness and microstructure of different steels hardened by a fibre laser. Rolled steel, quenched and tempered steel, annealed alloyed steel and conventionally through hardened steel were tested. Microhardness (HV0·01) was measured in martensite, pearlite, ferrite and cementite structures at different depths below the laser irradiated surface. The microhardness results were compared with the conventional macrohardness (HV5) results. The grain size of rolled ferritic–pearlitic steels had distinct effect on microhardness. The macrohardness of quenched and tempered steel might be markedly influenced by the homogeneity of alloy contents. In high carbon steel, cementite is ∼150 HV harder than pearlite. Annealed alloyed steels achieved high surface hardness but poor hardened depth. 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.