Fatigue performance of friction stir welded weathering mild steels joined below A1 temperature

Abstract

Hot cracks are prone to occur at the welded joints of weathering steel with high phosphorus contents and drastically reduce the fatigue strength. The present study evaluated the fatigue performance of the weathering steels SMA490AW and SPA-H joined by friction stir welding (FSW) at low temperature (below A1), which can inhibit the segregation of phosphorus. The high-cycle fatigue experiments were conducted on the base metal and welded joint, and the strain distribution was measured by digital image correlation (DIC). The different ratcheting behaviors influenced by the applied maximum stress were analyzed. The S-N curves for the two weathering steels were fitted and compared to the recommended design curves. It is found that the fatigue strength of the welded joints is higher than the design values assessed by nominal stress. The base metal and FSW joints of the weathering steels have almost identical fatigue strength, indicating a favourable weldability. The fatigue fracture of the FSW-joined specimens all occurred in the base metal region due to the high strength of the weld nugget and absence of geometry discontinuities or heat affected zone (HAZ) softening.