4171) FATIGUE BEHAVIOR OF WELDED JOINTS IN A FERRITIC STAINLESS STEEL SUS444

Fatigue behavior was studied on welded joints of a ferritic stainless steel, SUS444. Push-pull fatigue tests were conducted using plate specimens with the weld zone at the center of gauge section. Fatigue crack propagation (FCP) tests were also conducted using two types of CT-specimens with a starter notch in the heat-affected zone (HAZ) and in the weld metal. The fatigue strength of the welded specimens decreased remarkably compared to that of the base specimens, SUS444. In order to clarify the cause of the reduction in fatigue strength, additional fatigue tests were carried out using annealed welded specimens and smooth welded specimens in which the excess weld was removed. The fatigue strength of the annealed welded specimens was slightly improved, indicating that residual stress exerted a little influence on the fatigue strength of the welded specimens. On the other hand, the fatigue strength of the smooth welded specimens was nearly the same as that of the base specimens. Based on these results and FEM analysis of the welded zone, it was concluded that the decrease in fatigue strength of the welded specimens was attributed to the stress concentration at the toe of weld. In the welded specimens, multiple cracks were generated at the toe of weld and then coalesced, but the crack growth could not be measured owing to the shape of the excess weld. Early crack growth was measured using the smooth welded specimens. In early crack growth region, the discernible difference was found between the base specimens and the smooth welded specimens, i.e. the crack growth rates of the base specimens were faster than those of the smooth welded specimens. With increasing crack size, the da/dN-∆K relationships for small cracks gradually approached the da/dN-∆Keff relationships for long cracks and then coincided with them.