Effect of Post-weld Heat Treatment on Fatigue Reliability of Super-duplex-stainless-steel Weldments

• Published in: ISIJ International Vol. 57; no. 7; pp. 1228 - 1232
• Main Authors: Udo, Ryujiro, Numakura, Hiroshi
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 01.01.2017
• Subjects: fatigue strength; post-weld heat treatment; sigma embrittlement; sigma phase; welded joint
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2017-079

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.