The use of Duplex stainless steel has been approved for use on Japanese tankers and vessels by the leading classification society Class NK. Class NK (Nippon Kajo Kyokai) is a well-respected organization that was founded to regulate the shipping industry in Japan. From its earlier role as a promoter of shipping activities, it is now more focused on the technical aspects of the shipping industry.
Their March announcement about approving the NSSC 2120 on chemical tankers was hailed as a milestone decision. Originally, tankers manufactured by Sasaki Shipbuilding Co., Ltd and the Hakata Shipbuilding Co., Ltd used austenitic stainless steel. With the newly approved NSSC 2120, chemical tankers have a greater resistance to corrosion from sea water and other acids. This is because the NSSC 2120 is double the strength than the SUS304 with .2% proof stress.
According to ClassNK, this new development will be good for the industry because it is an upgrade and improvement especially since duplex stainless steel is being used by other countries.
What is Duplex Stainless Steel?
Duplex stainless steel is named as such because it has 2 phases in its structure. Just imagine a marbled cake and that is what you find with the duplex stainless steel. The first phase is the yellow austenitic and the second is the blue ferritic phase. When the steel is melted, it turns to ferritic and as the steel cools down, 50% of the ferritic grain turns into austenitic grains creating that marbled effect.
Duplex stainless steel is a good alternative because it is twice stronger than either ferritic or solid austenitic stainless steel products. It is also ductile and a lot tougher than ferritic although it fails to achieve the same values of the austenitic grades. Its corrosion resistance is almost identical to austenitic stainless steel but at the same time has an almost identical stress corrosion resistance of the ferritic stainless steel. This means it can handle well under elevated and humid temperatures as well as when exposed to chlorides.
Finally, the cost of duplex stainless steel is lower because it contains less nickel and molybdenum.