Effect of Impurities Spacing on Fatigue Strength Coefficient

Abstract

The influence of impurities present in steel on fatigue strength has been the subject of research conducted for many years. Despite a large number of studies, there is no unambiguous explanation of the influence of impurities on the fatigue life of steel. The interpretation of the results becomes more complicated as the ductility of the steel increases. For this reason, most of the research concerns low-ductility hard steels. In addition, the studies presented in the literature mostly concern laboratory conditions, which the authors of the papers have tried (with varying degrees of success) to adapt to industrial research. There are a few studies on the influence of impurities in steel on the fatigue resistance factor. The coefficient k is the result of the fatigue strength zg divided by the hardness of the steel. With its help, it is possible to determine the fatigue strength depending on the hardness of the steel. In the presented work, an attempt was made to determine the impact of impurities of different sizes and located at different distances from each other on the fatigue strength coefficient. The analysis was carried out at seven heats made in industrial conditions. Melting was carried out in electric furnaces with a capacity of 140 tons. Steel from all melts was subjected to desulfurization. Samples with a diameter of 18 mm were taken. The samples were hardened from the austenitizing temperature of 880 °C. To diversify the microstructure and mechanical properties, the steel was tempered at temperatures from 200 to 600 °C. After heat treatment, the samples were subjected to rotational bending. Based on the tests, it was found that the fatigue strength coefficient k depends on the size of the impurities and the distance between the inclusions. A difference in the specific k-factors was noted depending on the microstructure of the steel.