THE KINETICS AND MECHANISM OF THE FATIGUE FRACTURE OF SAMPLES OF 40H AND 38H2N2MA STEELS

It is known that the most of breakdown fractures of the structures or machine parts are related either to the fatigue fracture or brittle fracture caused by the fatigue crack. For this reason, to reduce the possibility of the fatigue crack initiation and development, the expensive medium and highly alloyed steels are often used. This paper presents the comparative analysis of the parameters of the fatigue fracture of samples of low alloyed 40H steel and medium alloyed 38H2N2MA steel. The fatigue tests of 10×15×80 mm prism samples made of 40H steel with the U- and V-shaped stress concentrators and of 38H2N2MA steel samples with the U-shaped stress concentrator were carried out on the Instron 8802 installation at the temperature of 20 °C according to the three-point bending scheme with ⱱ=10 Hz, R=0.1 and various ΔР values. Steels were tested after the quenching in oil and the subsequent heating to 300 °C. The microrelief of fractures was studied using the SIGMA scanning electron microscope of the ZEISS Company. It is identified that the stress concentrator shape in the samples influences significantly the time to the fatigue crack initiation. The straight-line section of the kinetic diagrams of the fatigue fracture of 40H and 38H2N2MA steels almost completely coincide, although the coefficient n in the Paris’s equation for the 38H2N2MA steel is slightly higher than for the 40H steel. The microfractografic studies have shown that the propagation of a fatigue crack in the 38H2N2MA steel was associated with the more ductile fracture mechanism than in the 40H steel. Thus, it is determined that the low alloyed 40H steel is equal to the medium alloyed 38H2N2MA steel in its fatigue characteristics.