Thesis of this paper is steel “improvement“. The selection of heat treatment parameters was made in order to achieve the set requirements in the form of steel hardness. The first part of the paper, i.e. chapters 1 and 2, cover the theoretical part of the improvement process. Steel improvement is distinguished into two processes: quenching and tempering. In the sub-chapters Quenching and Tempering, the theoretical basis of these procedures and microstructure changes that occur are set, as well as the way of selecting the parameters of heat treatment. Likewise, each process is completed with tables and diagrams that corroborate the theoretical part. Furthermore, in the theoretical part, improvement steels are also defined. The mechanical properties of this most common group of steels for improvement as well as their properties and application are defined. In addition, a table of the most common steels for improvement and proper selection of them based on their dimension and conventional yield strenght has been set up. Finally, Chapter 4 explains the heat treatment procedure that has been carried out. The request that needs to be met is explained. Object which was subjected to heat treatment was defied as well ast he parameters that were necessary for the heat treatment and after that the hardness control was carried out. In addition, the type of equipment used is explained.