Comparison of the fatigue performance of ferrite–pearlite and ferrite–bainite dual-phase steels

Abstract

Recent demands for large vessels and deep water exploration have increased the requirements for thicker plates to ensure the sufficient strength of such structures. The use of thicker plates, however, has an adverse influence on the fatigue performance. To enhance the fatigue life, post-weld treatments, such as weld-toe grinding, TIG (tungsten inert gas) dressing, hammer peening, and high-frequency mechanical impact (HFMI) treatments are commonly employed. On the other hand, these techniques require additional fabrication time and production cost. Therefore, there is keen interest with respect to thick steel plates with improved fatigue strength. This study examined the fatigue characteristics of conventional steel (Ferrite–Pearlite, F–P steel) and Ferrite–Bainite dual-phase steel (F–B steel). The fatigue behavior of both steels was investigated by a series of fatigue tests with three different types of welded joints. In case of gusset and cruciform-welded joints, fatigue strength of F–B steel is higher than that of F–P steel. F–B steel is known to exhibit improved fatigue performance associated with the existence of bainite microstructure. In addition, the fatigue data in this study are validated against the existing literatures. Based on the fatigue test results, a modified fatigue design curve is suggested from this study. In order to discuss the effects of fatigue crack path, the fatigue crack growth rate (FCGR) of both steels was investigated in terms of Paris’ law. Finally, microstructural analysis was conducted to discuss the fatigue performance.