1. An increase in test temperature above room temperature leads to a decrease in crack growth rate in steels 15Kh2NMFA and 15Kh2MFA on the low-amplitude section of the fatigue curve and an increase in this rate in steel 15Kh2NMFA on the medium-amplitude section. 2. Intergranular fracture is active as a fracture micromechanism at room and low test temperatures and crack growth rates greater than 10−6 mm/cycle. The contribution of the intergranular mechanism to fatigue crack formation in the steels at room temperature is greatest (up to 40% of the total area of the fracture surface) on the medium-amplitude section of the curve, at values of Kmax ≈ 20 MPa·√m. 3. An increase in test temperature from 273 to 673 and 623°K, respectively, for steels 15Kh2NMFA and 15Kh2MFA leads to disappearance of intergranular fracture due to a reduction in the grain-boundary strength of the steels and to an increase in the SIF at which secondary microcracks appear.