ANALYTICAL STUDY ON FATIGUE-STRENGTH REDUCTION FACTOR OF SMALL-DIAMETER SOCKET-WELDED PIPE JOINTS

Four-point bending and rotating bending fatigue tests were conducted o n socket-welded joints made of carbon, stainless, and Cr-Mo steels for clarification of the effects of diameter, welding pass sequence and p ost-weld heat treatment (PWHT) on fatigue strength. The results were e valuated quantitatively. Fatigue strength of socket-welded joints was found to strongly depend on weld pass sequences in fillet welds, this being possibly due to large change in residual stress distribution at roots and toes. The effects of residual stress were thus examined quan titatively by comparison of fatigue strength of PWHT stress-free speci mens with that of as-welded specimens. By the modified Goodman's metho d, the lowest S-N curve corresponding to maximum tensile residual stre ss and the highest S-N curve corresponding to maximum compression resi dual stress were obtained for different steels and diameters. Conventi onal S-N data of socket-welded joints were situated between these two limiting curves. Based on the lowest curve, fatigue strength reduction factors of socket-welded joints were proposed.