Deep Cryogenic Treatment and Nitriding of 42CrMo4 Steel*

Abstract

Abstract The paper presents the combination concept of a deep cryogenic treatment (DCT) with a nitriding process of 42CrMo4 steel. The DCT process at −180 °C was carried out immediately after the steel quenching, before subsequent tempering and nitriding. As a reference mode, an involving conventional heat treatment process as hardening, tempering and nitriding was carried out. A multi-stage gas nitriding process at 520 °C/32 h was used. Properties of nitrided steel subjected to DCT differed significantly from the properties of steel subjected to conventional nitriding process. After heat and thermo-chemical treatment involving DCT, samples had a higher surface hardness (increase of approx. 17 %), higher impact strength (increase of approx. 15 %) and higher wear resistance (a 25 % reduction of max. wear in a 3 rolls-cone system). The properties of deep cryogenically treated and nitrided steel resulted from the forming of a thicker white layer on the steel surface. In comparison with the layer thickness obtained after the conventional treatment, the white layer was thicker by about 160 % (6.5 μm vs. 2.5 μm). The thickness of the diffusion zone for both treatment variants was similar (approx. 300 μm). It was stated that deep cryogenic treatment by causing, among others, the refinement of the matrix grain and increasing the dislocations density, affects the number of nucleation sites of nitrides and carbonitrides during forming of the white layer, which increases the layer thickness.