Effect of loading frequency, temperature, and cycle asymmetry on the endurance of heat-resistant steels 1Kh2M and Kh18N9. II

Abstract

1. The application of a mean tensile load in tests of low-alloy steel causes a reduction in the endurance of the specimens throughout the investigated ranges of frequency and temperature. With an increase in loading frequency, endurance remain constant or increases slightly at all values of asymmetry. In tests of an austenitic chromium-nickel steel which undergoes strain hardening, the opposite effects are seen: At room and elevated temperatures, the application of a mean tensile stress of (0.2–0.4)σu may increase endurance by 15–20% compared to the case of symmetrical loading. The strain-hardening effect is especially evident with a reduction in loading frequency, so that the effect of the latter on endurance is inversed: in symmetrical loading, an increase in frequency increases endurance at room and elevated temperatures, while in asymmetrical loading endurance may decrease with an increase in loading frequency at elevated temperatures. 2. To evaluate endurance for the asymmetrical loading of low-alloy and austenitic steels at normal and high frequencies and room (in air) and elevated temperatures, the following well-known linear relation (Goodman curve) may be used: $$\sigma _\alpha = \sigma _{ - 1} \left( {1 - \frac{{\sigma _m }}{{\sigma _u }}} \right)$$ . The error which is possible with such an estimate increases the safety factor. 3. The fatigue strength of the investigated materials is lower in an air-water medium or in tap water than in air. The application of a mean tensile load within the range (0.15–0.25)σu sharply increases the damaging effect of the medium. The rate of decrease in endurance drops with a further increase in the mean load.