Influence the NON-METALLIC INCLUSIONS on BENDING FATIGUE STRENGTH OF medium-carbon STRUCTURAL STEEL melted in an electric furnace

Non-metallic inclusions are one of the factors that influence the fatigue strength of steel. Although steel has a relatively small number of non-metallic inclusions, those impurities have a considerable impact on the material's technological and strength parameters, in particular fatigue strength and life. The study was performed on 7 heats produced in an industrial plant. Fourteen heats were produced in 140 ton electric furnaces. The experimental variants were compared in view of the applied melting technology and heat treatment options. The results were presented to account for the correlations between the fatigue strength coefficient during rotary bending, the diameter of and spacing between submicroscopic impurities. Equations for calculating the fatigue strength coefficient at each tempering temperature and a general equation for all tempering temperatures were proposed. Equations for estimating the fatigue strength coefficient based on the relative volume of submicroscopic non-metallic inclusions were also presented. The relationship between the fatigue strength and hardness of high-grade steel vs. the quotient of the diameter of impurities and the spacing between impurities were determined. The analyzed material was one grade of medium-carbon structural steel. The proposed linear regression equations supported the determination of fatigue strength coefficient and bending fatigue strength as a function of hardness taking into account impurities. The proposed equations contributes to the existing knowledge base of practices impact of impurities with various diameters and spacing between non-metalic inclusion on fatigue strength.