The class of steels for hardening and tempering, alloyed with chrome, nickel and molybdenum, standardized in Europe (four steel grades), USA (10 grades) and Russia (9 grades) is highly diversified and is especially interesting in the production of machine parts having an extremely large range of dimensions. The heat treatment features of these steels are particularly attractive: high bainitic hardenability and a good tem perability etc. The correlation between the hardness achieved after high tempering on products made from these steels, their equivalent diameter and the heat and time parameters of tempering can be explained by means or Jominy samples test for products with equivalent diameters equal to or less than 100 mm, or by the results obtained through the method of simulation of oil cooling (Pavaras–Gheller method) for products with equivalent diameters higher than 100 mm. In this paper, based on experimental results, these correlations are customized for a steel group representative as is 34CrNiMo6.