This article deals with the effect of quenching and tempering on the hardness and microstructure of 42CrMo4 steel. All investigated samples were water quenched from an austenitizing temperature of 820 °C. After quenching, the samples were tempered at 610 °C, for 30 minutes and for 60 minutes. After the heat treatment, the cross-sectional hardness of the samples was measured. The hardness was measured using the Vickers tester and a test force of 98,1 N (HV10). The highest average hardness was achieved after quenching with values of 657,3 HV10 on the surface and 487,6 HV10 in the core. Samples that were tempered for 30 and 60 minutes showed an average surface hardness of 364,3 HV10 and 330 HV10 and an average core hardness of 332,3 HV10 and 303,3 HV10, respectively. The microstructure was investigated by the use of light optical microscope.
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