Experiences In Welding 25Cr Super Duplex Stainless Steel For Topsides Hydrocarbon Piping
Authors: Bowden P.L. , Ward J.L. 
Published: October 2008

Experience of welding 26Cr super duplex stainless steel piping spools for Topsides hydrocarbon service is described. During welding procedure qualifications and trials, sigma phase was observed in the low temperature Heat Affected Zone (HAZ) of butt welds in pipe up to 14mm wall thickness.

A fitness for purpose approach was adopted and a fracture toughness testing programme was undertaken to establish the level of HAZ sigma which could be tolerated, whilst still retaining an acceptable degree of overall toughness in pipe butt welds. Corrosion testing demonstrated that adequate resistance to localised corrosion is maintained even when limited amounts of HAZ- sigma phase are present.

The extent of HAZ sigma precipitation was shown to be influenced by the total weld thermal cycle and dictated by primary welding variables, pipe geometry and joint fit- up. Using project specific design data, it was concluded that sigma phase in the low temperature HAZ of 2.5% or less can be tolerated in 25Cr super duplex pipe butt welds. The resulting welding procedure controls needed to ensure this limit was achieved in production welds are described.

INTRODUCTION

The welding of 25Cr super duplex stainless steels requires careful consideration of parent metal and welding consumable chemistry combined with close control of welding variables, in order to achieve acceptable mechanical and corrosion resisting properties. The key to success has rightly been seen as ensuring that satisfactory microstructure are achieved in the weld metal and high temperature Heat Affected Zone (HAZ). Manufacturers' efforts were initially focused on balancing the chemistry of the parent material and welding consumables to ensure optimum overall austenite-ferrite phase balance in the weld and HAZ, thus ensuring adequate low temperature toughness. This presented little problem with solution annealed items. However, welding conditions as well as chemistry determine as-welded microstructure and thus, heat input, interpass temperature controls and sequence or balanced welding techniques are practiced to minimize the formation of undesirable phases such as secondary austenite, carbides, nitrides and sigma phase. Current fabrication specifications for 25Cr super duplex stainless steels generally preclude the presence of such phases, because of their adverse affect on toughness and corrosion resistance. Experience with welding 25Cr super duplex stainless steel seamless pipe on a current UK North Sea Offshore project has shown that sigma phase formation in the low temperature HAZ, (2-5) mm from the fusion line cannot always be avoided, even with very restrictive welding procedures. The effects were pronounced in thin wall pipe, where heat flow away from the weld is slow. Welding trials and extensive metallographic studies led to the adoption of a fitness for purpose approach to justify the acceptance of sigma in the HAZ of as-welded pipe butt welds. Charpy impact and CTOD testing permitted the calculation of tolerable defect sizes based on specific project design data. This, together with satisfactory performance in the ASTM G48 ferric chloride test was used to justify acceptance.. of a maximum volume fraction of 2.5% sigma phase in the HAZ of welding procedure qualification welds.