Increasing the fatigue strength of high-strength steel grades

Abstract

The paper considers the issue of increasing the fatigue strength of high-strength steel grades. Based on the results of experimental measurements of the fatigue strength limit (σ–1) of spring steel grades, we analyzed the effect of tensile strength, ratio of the yield strength during shear and the fatigue strength limit. The absence of statistical relationship between fatigue strength limit and tensile strength (σ–1 ≠ f (σu)) was established. The ratio τt / σ–1 is the stress concentration coefficient (SCC), which is closely related to the tensile strength of steel. From the theoretical analysis, it follows that in the presence of the same morphological type and size of non-metallic inclusions (NMI) in steel, relationship of SCC with the strength properties of steel is functional. Spread of its actual values is associated with the presence of various morphological types and sizes of NMI in the metal. Each morphological type of NMI is characterized by corresponding physical and mechanical properties (modulus of elasticity, tensile strength and various SCC). SCC increases both with an increase in the strength of steel and with an increase in diameter (thickness) of NMI. It was established that the intensity (rate) of the increase in SCC depends on the size and elastic modulus ЕMNI of NMI (ratio of mass fractions of SiO2 and Al2O3 oxides in NMI). The average intensity of the change in SCC obtained by processing experimental data corresponds to similar indicators for NMI: 13 % SiO2 ; 87 % Al2O3 (4.0 μm thick); 20 % SiO2 , 80 % Al2O3 (5.0 μm thick); 25 % SiO2 ; 75 % Al2O3 (7.0 μm thick). According to the obtained connections, dimensions of NMI and their morphology are approximately indicated, which make it possible to increase the fatigue properties of spring steels grades in the tensile strength range from 1200 to 2000 MPa. To increase the fatigue life of steel (especially in high-strength condition), it is recommended to use the technology of aluminum-free metal deoxidation during smelting. At the same time, a favorable morphology of NMI with SCC less than 1.0 is provided. Formation of a fine-grained structure of steel after heat treatment is obtained in the absence of aluminum during deoxidation with small additives of vanadium, niobium or titanium.