This master’s thesis aimed to establish the approximate depth of hardenability of the machine part DP.19-DP.46 that was made from 16MnCr5 steel. The first part of the thesis deals aims to explain the theoretical background, i.e. the types of steel and the phase changes among them. This part also explains the role that the alloying elements have on the properties of steel. The first part also describes some of the heat treatment processes, such as quenching, hardening and hardenability. This is followed by the description of several methods that are used to estimate the hardenability and the procedure of their usage. The thesis also lists and describes various methods that test material hardness. The procedure for conducting the Jominy end quench test for hardenability on the standard Jominy sample is also explained in detail. This test represents the most practical and the most frequently used method for testing hardenability due to its simplicity and low cost. The Jominy test is conducted by quenching the sample and then testing its hardness. The second, experimental part of the thesis deals with estimating the depth of hardenability of the machine part DP.19-DP.46 using the aforementioned Jominy test. Firstly, the characteristics of the 16MnCr5 steel that was used in the experiment, are described. A graph based upon the data obtained by testing the Jominy sample was then created. Eleven Crafts-Lamont diagrams were then used to draw the line that marks the flow of hardness in the cross section of the machine part DP.19-DP.46 for the case of oil quenching. It was then concluded that the machine part was completely hardened when quenched in oil with the quenching intensity H = 1, while on the other hand it was not sufficiently hardened when quenched in oil with the quenching intensity H = 0,35.