Investigation on fatigue strength of cut edges produced by various cutting methods for high-strength steels

Due to the interest in effective light steel constructions, high-strength steels have gained importance. Different thermal cutting processes are frequently used in the metal processing industry. Besides the weld seams, free cutting edges gain technical and economic relevance as locations for potential fatigue cracks. In this investigation, fatigue tests were carried out on 8-mm-thick samples of S355M and S690Q steels at a stress ratio of R = 0.1. The cutting methods used were oxygen, plasma, laser, and waterjet cutting. Quality improvement methods, like shot peening, grinding, and cutting speed reduction, were applied for some series. The surface roughness was measured to classify the specimens into quality groups according to ISO 9013. The cut edge condition was also characterized by hardness and residual stress measurements. The investigation shows that all tested series exceed the FAT100 class and can be classified in FAT125. Specimens ranged in quality group 2 of ISO 9013 according to the roughness achieve FAT140 regardless of cutting technology or material. According to the ISO 9013, most of the specimens are classified in the quality group 2 and group 3. Fatigue strength results are significantly different in one quality group. No prediction can be made. ISO 9013 has a weak connection to fatigue strength. Quality improvement methods have a significant influence on the fatigue strength and can increase it. Due to reduced cutting speeds, the roughness decreases also. It results in an increase of the fatigue strength in all tested series in this study. In order to make a prediction of the fatigue performance, the standard has to be specified and the cutting process as well as the steel strength should be considered.