Study of Fatigue Strength of Fine-Grained Austenitic Steel 08Cr18Ni10Ti Produced by Rotational Forging

The study was performed on 08Cr18Ni10Ti corrosion-resistant steel with a two-phase ultrafine-grained (UFG) structure formed by rotary forging (RF) at room temperature. Bars with a diameter of 6 mm exhibit high tensile strength and a non-uniform hardness distribution across the bar cross-section. The influence of annealing temperature on the microstructure parameters and mechanical properties of UFG steel was determined. It was shown that TiC particles form in the annealing temperature range of 450–550°C, the reverse transformation of martensite into austenite occurs in the range of 550–650°C, and recrystallization processes begin at annealing temperatures above 700°C. Fatigue tests on smooth cylindrical specimens using the “bending with rotation” scheme were performed at room temperature, at a frequency of 50 Hz. Fractographic analysis of fractures was performed using scanning electron microscopy. The fatigue curves were analyzed using the Basquin power equation: σa = AN–k. It was shown that RF and annealing lead to a decrease in the coefficient A in the Basquin equation. It was found that coarse-grained and UFG steels have the same fatigue limit: the physical fatigue limit based on 107 cycles is 500–550 MPa, and the conditional fatigue limit, calculated using the Basquin equation, is σ–1 = 150–155 MPa based on 108 cycles. It is shown that the dependence of σ–1 on the annealing temperature of UFG steel is non-monotonic with a maximum. Fatigue test results were analyzed using a model of plastic deformation at the crack tip.