The goal of our work was to investigate the effect of the structure, the treatment method, the thickness of the hardened layer, the hardness of the surface and the core, and other factors on the cyclic-loading strength of the construction steels 14KhGN, 19KhGN, 20KhGNM, 40KhGNM, and 30KhM. We cut samples from a rod of diameter 24-34 mm of hot-rolled high-grade steel and prepared a series of samples for testing.In Table 1, we present the results of chemical analysis and mechanical testing of the steel samples as supplied.In accordance with the technological parameters commonly used at the Volga Automobile Plant, the samples were subjected to chemical and heat treatment in continuous direct-fired units from the Khol'kroft and Aikhelin company. Carbonitriding of the steels was done in an end-gas atmosphere with CH 4 and NH 3 additives at 860-880°C followed by interim cooling down to 800-820°C and quenching in MZM-26 oil at a temperature of 110-120°C. Samples of 30KhM steel were subjected to subsequent tempering at 540-560°C. In the same units in which carbonitriding was carried out, we carburized samples of 19KhGN steel in the temperature range 840-930°C. Then the samples were subjected to quenching and tempering at the temperatures used for carbonitriding. The length of the carburizing cycle was 13 h, and the length of the carbonitriding cycle was no more than 6 h; i.e., in the second case, the chemical and heat treatment cycle is shortened by 50-60%. In the same units in which we carried out the chemical and heat treatment, the samples were also subjected to bulk quenching at 830-880°C and tempering at 160-180°C. The tempering temperatures for 40KhGNM and 30KhM steels were 560-620°C and 520-560°C, respectively.The samples were tested for static tensile strength (GOST 1497-84), multiple cyclic loading fatigue , and cyclic-loading toughness (RD 50-345-82) [1]. The hardness of the surface was measured by the Rockwell method with a diamond cone for a load of P = 294 N and by the Vickers method with a diamond pyramid for P = 9.807 N. The hardness of the core of the samples was determined by the Rockwell method for P = 1471 N, and also with a steel ball for P = 981 N. The microstructure of the samples before and after testing was studied using a metallographic microscope.In order to avoid fracturing the hardened layers by mechanical treatment after carrying out the chemical and heat treatment, the samples were made without extra allowances, resulting in additional demands on their shape and size. In order to avoid warping, the samples were made with a smooth transition in radius from the core of their working part to the edges. Furthermore, we used a simple device for securing the samples in a vertical position during heat treatment in the continuous unit.Institute of Ferrous Metallurgy, Dnepropetrovsk, Ukraine. Volga Automobile Plant, Tol'yatti, Russia.
