2017 Volume 57 Issue 7 Pages 1228-1232
To assess the reliability of weldments of a super duplex stainless-steel (UNS S32750), we have studied how the microstructure and fatigue strength of a welded joint is affected by post-weld heat treatment, focusing on the formation of the intermetallic σ phase during cooling. The microstructure of the welded zone of arc-welded single-V joints was examined by optical microscopy and SEM-EBSD. It revealed that the σ phase appears as the tertiary constituent when cooled from the solution-treatment temperature of 1323 K at rates below 101 K s−1, with its fraction reaching 20% at rates as low as 10−1 K s−1. Fatigue properties were evaluated by plane-bending tests, and were found to be deteriorated correspondingly to the amount of the σ phase. Specimens without heat treatment performed better than any of those subjected to the solution treatment and cooled at a rate in the range between 0.1 to 50 K s−1. To secure fatigue reliability it is advisable that weldments of the material should not be subjected to post-weld heat treatment. If the treatment is necessary for some reasons, the condition should be carefully selected.
