In heat-treating furnaces, many different types of protective atmospheres are used. This article researches the effect of protective atmospheres on the quality of the surface layer of bolts during the process of heating to reach the temperature of hardening. For this research, we produced specimens that were annealed in the furnace with two different types of protective atmosphere, i.e. in atmospheres of endothermic gas and nitrogen. After hardening and tempering, we measured the hardness of the specimens and investigated the microstructure. We measured the hardness profile from the surface to the inside of the product. We found that the hardness of the surface of the tested product was lower while using protective atmosphere of nitrogen due to the occurrence of ferrite. The depth of the decarburised layer in this atmosphere reached up to 70 mm, where predominantly there was a microstructure of ferrite on the surface, and then, with depth, an increasingly mixed microstructure of ferrite and martensite was found. The depth of the decarburised layer for sample treated in endothermic gas was minimal (i.e. 10 mm) on the surface.