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A study is made of steel 15Kh2MFA and its resistance to low-cycle deformation, inelastic characteristics, and mechanical properties, as well as the parameters of surface microcracks formed after cyclic loading of specimens of this material. The steel is studied in two states: initial and embrittled. It is established that, with allowance for the change in the non-dangerous component of energy, the total energy of fatigue fracture is invariant relative to the number of cycles to crack nucleation. It is shown that increases in the number of loading cycles and total energy dissipation are accompanied by an increase in the mean length and density of surface macrocracks and a decrease in the mean distance between them.

1. The results show that at temperatures of 293 and 623 °K the fatigue crack growth rate in the 15Kh2MFAA and 15Kh2MFA steels coincides in the middle section of the KDFF. At the same time, the threshold stress intensity factor Kth for 15Kh2MFAA steel is higher than for 15Kh2MFA steel. The increase of the stress ratio from 0.1 to 0.7 reduces the fatigue crack growth rate in both steels. 2. The fracture toughness characteristics Kfc1, Kfck, KIc (KQ5%) of the 15Kh2MFAA and 15Kh2MFA steels increase with increase in test temperature and in the temperature range 250–623°K they reach the upper plateau. In the low-temperature range (>123°K) the characteristics Kfc1, Kfck, KIc converge but the distinctive lower plateau does not form. 3. In the low-temperature range (123–200°K) the cyclic nature of loading reduces the fracture toughness characteristics Kfc1, Kfck, and σfc1, σfck in comparison with the values of KIc and σcmax in static loading. 4. At temperatures below 150°K the fracture resistance in static loading of 15Kh2MFAA steel is lower than that of 15Kh2MFA steel, and in the temperature range 150–623°K it is higher than that of 15Kh2MFA steel. Below 243°K the fracture resistance in cyclic loading 15Kh2MFAA steel is higher than that of 15Kh2MFA steel, and in the temperature range 243–623°K it is lower than that of 15Kh2MFA steel.

The influence of preliminary (before crack origin) cyclic plastic deformation by tension-compression (Rɛ = ɛmin/ɛmax = −1 on the brittle fracture resistance of 15Kh2MFA heat resistant steel in two conditions was investigated. The preliminary loading was done at 293 K and the fracture at 123 and 293 K. It was established that in the case of relatively short preliminary deformation the change in static fracture toughness is caused by a change in yield strength as the result of the Bauschinger effect. However, with an increase in cyclic deformation the total irreversibly dispersed specific energy of inelastic deformation and the structural changes in the material are the determining factor. A physically based method is proposed for prediction of the influence of preliminary cyclic plastic deformation on the brittle fracture resistance of cyclically softening steels.