These guidelines have been developed to help shipyards safely and effectively make use of new duplex steels that are increasingly being used in the construction of cargo tanks for chemical carriers.
Duplex stainless steel has a two-phase microstructure containing grains of both ferritic and austenitic phase, giving it a combination of excellent corrosion resistance and high strength with a lower nickel content than austenitic stainless steels, such as SUS304 and SUS316L, ClassNK explained.
The vast majority of the world’s chemical tankers with stainless steel cargo tanks are built at Japanese shipyards, which have traditionally used austenitic steels for cargo tank construction.
Alternative steel
Recently, however, these shipyards are increasingly using duplex stainless steels as an alternative to austenitic stainless steels for their corrosion resistance and high strength properties.
Before shipyards, or builders can use duplex stainless steel in the construction of vessels, they must first obtain approval for the Welding Procedure Specification (WPS) that will be used when welding duplex stainless steel.
It was in order to simplify this process and support the widespread use of these new materials at shipyards worldwide that ClassNK released the ‘Guidelines on the Welding of Duplex Stainless Steels’ on 14th January, 2014.
The new guidelines, which conform to ClassNK rules, provide a comprehensive summary of the requirements for welding procedure specification certification and welding procedure qualification tests, including relevant precautionary measures for the welding of duplex stainless steels.
Speaking on the release of the guidelines, ClassNK material and equipment department general manager, Takeshi Imamura said: “As the world’s leading classification society for chemical tankers with stainless steel cargo tanks, we hope that these new guidelines will help shipyards take advantage of these new materials in developing and building a new generation of safer and more efficient chemical tankers.”