The topic of this diploma thesis is the observation and determination of the decarburization depth of the test sample PG.22-DR.85 according to the standard HRN EN ISO 3887. The diploma thesis includes a theoretical part related to the performed procedures of heat treatment and testing of properties and chemical composition of the machine part. After the theoretical part, the practical part is described in detail. The theoretical part consists of 4 chapters. The first chapter presents the definition of heat treatment and what such procedures are intended for, and the decarburization process is described. The second chapter deals with devices designed to heat the test samples or other objects to certain temperatures that may vary according to the requirements of the client or researcher. The process of austenitization is described in the third chapter. In the continuation of the third chapter, the phase transformations in steel that take place by the implementation of quenching of the material are described in more detail. The last chapter of the theoretical part refers to the methods of testing the properties of materials, ie. methods that try to determine or prove the occurrence of decarburization in the machine part. In the last part of the thesis, in the fifth chapter, the practical part of the task is described. At the beginning, the implementation of the processes of heat treatment of quenching and softening is described, followed by metallographic analysis, hardness testing and finally spectrographic analysis. All these methods have been successfully used to prove decarburization and certain methods have revealed the actual depth of decarburization very precisely or with little error.