The topic of this bachelor thesis is the procedure of heat treatment of steel by quenching. At the beginnig of the bachelor thesis, the occurrences of carbon in iron alloys are explained, as well as the depedence of mechanical properties of steel in carbon content and a general heat treatment diagram with all associated phases. In the further part of the bachelor thesis, the quenching procedure is described in detail, with all its phases and parameters of the procedures. The choice of extinguishing agent is described, and the methods of extingushing are explained. At the end of the theoretical part of the bachelor thesis, the concepts of hardenability are explained, as well as the surface hardening procedures. In the experimental part of the bachelor thesis, two types of steel, C45 and 42CrMo4. Befor quenching, the microstructure and hardness of the selected steels were examined. After that, the samples were heated to the austenitization temperature and extinguished in water. After quenching, the microstructure nd the hardness of the test samples were examined. It was found tht there was a change in the microstructure and a pronounced increase in hardness.