Duplex and high alloy stainless steels – corrosion resistance and weldability

Conventional 18Cr–8Ni austenitic stainless steels have excellent resistance to corrosion in mildly aggressive environments, good mechanical properties, and are readily weldable. They are therefore used in a wide range of areas, including the food, petrochemical, and pharmaceutical industries. A major disadvantage to the use of austenitic stainless steels however is the drastically reduced resistance to pitting or crevice attack in aqueous environments containing chlorides. Furthermore, at temperatures above 65°C they are susceptible to failure from chloride stress corrosion cracking (SCC). Improvements in pitting or SCC resistance may be achieved by alloying additions, producing the high alloy austenitic stainless steels. Alternative solutions are to employ high Cr and Mo ferritic stainless steels, or the two phase, or duplex, ferritic–austenitic stainless steels. The choice of alloy depends on several factors, such as mechanical properties, economics, and weldability. The last property may be critical since all three stainless types, the duplex and high alloy austenitic and ferritic steels, are less readily weldable than the conventional 18Cr–8Ni grades. The various stainless steel types are discussed, with emphasis given to the welding precautions necessary to ensure adequate weld area properties.

MST/629