A new method for combined thermo-mechanical treatment of steel 0.4 % C, 1.5 % Cr, 1.5 % Ni, 0.5 % Mo is developed. Thermo-mechanical processing includes hot forging and rapid cooling to the temperature range between the beginning and end of martensitic transformation. The final stage of heat treatment is heating up to the temperature of bainite transformation in which the remaining austenite transforms into a bainite structure and previously formed martensite tempers. In comparison with well known techniques of thermal treatment, the developed thermo-mechanical treatment increases the impact toughness of the steel by 2 times and fatigue crack resistance by 6 times (with equal values of strength parameters). The technological process of high-temperature thermo-mechanical treatment is recommended for treatment of high-strength blank forging.

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Processes of carbon steels fracture under their repetitive dynamic loading according to the compression scheme have been studied. It has been ascertained that the main cause of fracture of cyclically compressed specimens is crack initiation and propagation on exposure to tensile internal stresses forming at the crack tip during the external load removal stages. The crack growth rate is maximal at the initial stage of its advancement near the notch. When the crack propagates depthward the specimen, its rate decelerates and weakly depends on the current specimen cross section. A model of the steels fatigue fracture process under repetitive dynamic loading has been proposed. It has been revealed that medium-and high-carbon steels quenched and tempered at 300 °С have long life duration.

The structure formation features of steel with 0.4 C, 1.5 Cr, 1.5 Ni, 0.5 Mo (wt. %) after high-temperature thermomechanical treatment with martensite-bainite transformation of austenite was investigated. It was found that a mixed structure consisting of alternating elongated areas of tempered martensite and lower bainite was formed by steel treatment. The proposed treatment can significantly increase impact strength and fracture toughness of steel. This technological process is mostly effective for strengthening alloy steels with increased resistance of under-cooled austenite.

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