In this masters's thesis, the estimation of hardening depth of the machine part NT.23-DP.77 after the heat treatment of hardening was approximated. In the first part of the thesis, the steel is explained through its basic properties and characterization according to various criteria also the influence of various accompanying elements, non-metallic inclusions and alloying elements on the properties of steel are explained. In the central part of the thesis, the theory of heat treatment of steel is covered, the process of heat treatment of steel and phase transformation of steel due to heat treatment is discussed. Also the process of heat treatment of hardening, properties of hardening and hardenability of steel is described in more detail and Jominy method of assessing the hardenability of steel is explained. In the last part of the thesis, all the steps of performing Jominy experiment and hardness testing are explained in detail, along with all the equipment and tools used. At the end of the thesis, based on the results from experiment and Crafts-Lamont diagrams, a U-curve is constructed for the cases of quenching in oil and quenching in water. The U-curve shows the estimated of hardening depth of the machine part NT.23-DP.77. Base on the results, we can conclude that in both cases the machine part will not be fully hardened and that it does not have the required surface hardness value for the use as a material for bolts and that the machine part must go through heat treatment process of carburisation before hardening.