## 1 Introduction  
Today, to produce various parts of industrial machines--mill rolls, shafts of power-generating equipment--structural steels are used alloyed with different elements (Cr, Ni, Mo, V), such as 38 CrNi3 MoV steel, which is investigated in this work. This steel has a high stability of austenite in the range of 400-500 \({}^{\circ}\)C [1, 2, 3, 4, 5]. Studies on the mechanical properties and microstructure of 38 CrNi3 MoV steel have been carried out in a significant number of works [1, 2, 3, 4, 5]. For steels with high austenite stability, a step-like mode or isothermal cooling is often used [3]. It was proposed that isothermal treatment of 38 CrNi3 MoV steel from the intermediate (\(\gamma\)+\(\alpha\))-area should be used to increase its wear resistance [5]. The nearest steel analogues, 35 NiCrMoV12-5 (DIN) and 4340 (AISI), are used for special-purpose products, and were studied in previous works [6, 7]. The reliability of finished products from alloy steels depends significantly on the homogeneous distribution of structural components and the unavoidable content of retained austenite (RA), which can reach 10-15% after quenching. Alloying elements shift the finish temperature (M\({}_{\text{f}}\)) of martensite transformation below zero. Therefore, the high-temperature tempering does not reduce RA to zero. Hence, increasing the operational stability of finished products made of carbon alloy steels while obtaining the most favorable combination of the mechanical parameters (hardness, wear resistance, and toughness) is an important scientific and technical task. It is well known that deep cryogenic treatment (DCT) can significantly increase the properties of alloy steels [8; 9; 10; 11]; thus, it is used as a final treatment for many technologies and industries, such as materials science [12; 13], tool and steel industries [14; 15; 16; 17; 18], manufacturing of rolls [19], and automotive industry [20]. This process uses sub-zero temperature, below \(-125\)\({}^{\circ}\)C, which is created by liquid nitrogen [18], as the preferred cooling medium. Both gaseous and liquid coolants are applied, but steel sample interaction with a liquid coolant creates higher internal stresses. The achieved changes in the steel parts are almost permanent and irreversible under normal thermal conditions. In addition, DCT is a low-cost, environmentally friendly, non-toxic, and safe technology. The influence of DCT at a temperature of \(-196\)\({}^{\circ}\)C and subsequent tempering at 200 \({}^{\circ}\)C on the microstructure of tungsten carbide (WC-Co) rock cutting bits is investigated in [21]. Different durations of low-temperature treatment are applied (from 6 to 36 h, with 6 h steps). The authors explained the observed phase shift in a binder (Co) by the differences in the thermal expansion coefficient and thermal shear stress. The optimum duration of DCT based on the wear resistance of samples was determined as 24 h.  
The AISI D6 tool steel properties (microstructure, impact toughness, and wear resistance) subjected to DCT is investigated in [22]. Different routes were considered: quenching-tempering, quenching-tempering (single and double)-DCT. The authors concluded that double tempering after DCT produces the best mechanical properties and the same wear resistance due to the precipitation of fine carbides. The soaking time (2 and 24 h) was considered less influential than double tempering on the final properties. The influence of the DCT effect on microscale abrasion of AISI D2 tool steel is represented in [23]. The best wear resistance was achieved when the DCT was performed immediately after tempering. The authors relate this effect to the transformation of RA in martensite and the formation of fine secondary carbides. The wear and fatigue properties of H21 tool steel subjected to DCT are investigated in [24]. The DCT step (\(-185\)\({}^{\circ}\)C) was applied before and after double tempering (540 \({}^{\circ}\)C) with subsequent soft tempering (100 \({}^{\circ}\)C).