Effects of Weld Metal and Test Temperature on Fatigue Behavior in Type 444 Stainless Steel Welds

Abstract

In the present study, the effects of weld metal and test temperature on the fatigue behavior in type 444 stainless steel welds were investigated. Two weld metals with different chemical compositions, NbL1 and NbL2, had been used for the MIG butt welding of type 444. NbL2 with Al and Ti showed finer grains in the weld microstructure than NbL1. The hardness of weld and base metals increased by the aging at 500°C due to 475°C embrittlement. At both ambient temperature and 500°C, the tensile strengths of weld metals were higher than that of base metal. Fully reversed axial fatigue tests have been performed using as-welded specimens with the excessive convexities, smooth specimens of welds and base metal at ambient temperature and 500°C in laboratory air. In the smooth specimens of welds, the excessive convexity at the weld metal was removed in order to focus on the microstructural effect. In the as-welded specimen with the excessive convexity, the fatigue strength at ambient temperature decreased compared with the base metal, which could be attributed to the stress concentration at the toe of the weld. By removing the excessive convexity, the fatigue strengths of NbL1 and NbL2 welds were improved, while still lower than that of base metal. Weld defects were responsible for the lower strengths of welds. On the other hand, both welds exhibited higher fatigue strength than base metal at 500°C. The softening of material at 500°C had eliminated the detrimental effect of weld defects on fatigue crack initiation.