Affect of steel cleanliness to fatigue behaviours of the low alloy steels

Steel is an important material with a wide range of usage area and in recent years, the most important issue for steel making process is to produce clean steel. During steel making process, the basic inclusions are Al_2 O_3 and MnS for Aluminum-killed steels. Calcium treatment is used to produce clean steel in Aluminum-killed steels. In this study, fatigue strengths of the low alloy steels was examined with untreated and calcium treatment applied samples. Firstly, yield loads of low alloy steels for untreated and steel cleanliness applied materials were determined before the fatigue tests. Fatigue tests were performed under the constant amplitude axial loads and load values were determined according to yield loads. In the experiments conducted, load ratio was taken as 0,1 and the maximum load values were determined as 18 kN, 16,5 kN and 15,5 kN. According to the fatigue test results, S-N (stress-life) diagrams was created for untreated and steel cleanliness applied samples. The interpretation of the test results in an optical microscope and scanning electron microscope (SEM) was used. As a result of the study, fatigue strength of steel cleanliness applied samples is close to fatigue strength of untreated samples, although a lower amount was measured. It was observed that the inclusions which are contained in the steel material affect the fatigue life, depending on their shape, size, quantity and distribution within the structure. Keywords: Steel, Steel cleanliness, Fatigue, S-N diagram, Microstructure