The structural significance of HAZ sigma phase formation in welded 25%Cr super duplex pipework
- TWI, Cambridge (United Kingdom). Dept. of Engineering
- Marathon Oil UK Ltd., London (United Kingdom)
The welding of 25%Cr duplex stainless steel can lead to the formation of sigma phase in both weld metal and heat affected zone (HAZ) regions. It has generally been accepted that this can be avoided by the adoption of appropriate welding procedure controls, generally aimed at reducing heat input and promoting rapid cooling rates. However, experience during pipe spool fabrication for the Marathon East Brae Project has shown that it is extremely difficult to satisfy a welding specification requiring sigma free HAZs. This has proved a particular problem with thin wall pipe welds made in the 2G/5G or 6G fixed positions, where the joint geometry reduces heat flow away from the weld and welding conditions tend to result in the use of higher heat inputs. This paper examines the effect of sigma phase on the fracture toughness of 25%Cr super duplex steel (UNS S32760). It is shown that the CTOD toughness at {minus}20 C decreases as soon as any sigma phase is present and continues to decrease with increasing sigma levels. The toughness of the sigmatized specimens produced by heat treatment was shown to be conservative compared to the toughness measured in the HAZ of 14.2mm and 7.1mm thick pipe weldments, made with welding parameters chosen to enhance HAZ sigma phase formation. Based on the CTOD versus percent sigma level relationship derived from the laboratory specimens, fracture assessment calculations of tolerable flaw sizes were performed. These demonstrated that under the severest design conditions, assuming the maximum flaw sizes which could remain undetected in the pipework, sigma levels up to 2.5% can be tolerated safely. The conservatism of the fracture assessments for predicting the performance of weldments was demonstrated by full scale tensile testing of 2 inch nominal bore x 2.77 mm wall thickness pipe butt welds containing through-thickness circumferential fatigue cracks located in the sigmatized HAZ.
- OSTI ID:
- 33187
- Report Number(s):
- CONF-930641-; ISBN 0-7918-0785-1; TRN: IM9518%%8
- Resource Relation:
- Conference: OMAE `93: 12th international conference on offshore mechanics and arctic engineering, Glasgow (United Kingdom), 20-24 Jun 1993; Other Information: PBD: 1993; Related Information: Is Part Of OMAE 1993: Volume 3, Part A -- Materials engineering; Salama, M.M.; Toyoda, Masao; Liu, S.; Dos Santos, J.F.; Kocak, M.; Williams, J. [eds.]; PB: 434 p.
- Country of Publication:
- United States
- Language:
- English
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