The fatigue strength of box welds was tripled by using low transformation temperature welding wire and postweld heat treatment (PWHT). The improved fatigue strength was obtained due to the stress ratio effect resulting from compressive residual stress. The low transformation temperature welding material which contains 10 mass % Cr and 10 mass % Ni expands during the final cooling process of welding. This expansion of weld metal induced the compressive residual stress around the welded part. The PWHT at 720°C for 1 hour and subsequent cooling in air induced the transformation from austenite to martensite at the same time for three passes. That is, the volume expansion on three passes by PWHT became large compare with that on final pass in as-welded condition, and the compressive residual stress could be induced efficiently.
