TWI Industrial Member Report Summary 505/1995
R N Gunn
A number of superduplex stainless steels have been developed, based on a 25%Cr, 7%Ni composition, and containing up to 4%Mo and 0.3%N, some with additions of Cu and/or W. These steels are currently being selected for a wide range of applications, due to their excellent resistance to general, pitting and crevice corrosion and to chloride stress corrosion cracking (SCC). As a result they can offer trouble free service in a variety of corrosive media, which, combined with their high strength-to-mass ratio, allows for savings to be made both in terms of structural weight and maintenance costs. However, the high alloy content of the steels renders them susceptible to the formation of intermetallic phases during exposure to elevated temperatures (i.e. 300 to 1050°C), which results in a reduction in both corrosion and toughness properties.
Some degree of intermetallic formation may be expected in the heat affected zone (HAZ) of a weld, especially if the thermal cycle is prolonged. The current programme of work aims first to compare the relative sensitivity to intermetallic precipitation of new and established 25%Cr duplex grades, and second to evaluate the influence of varying welding conditions. The first phase is presently reported. Representative alloys were subjected to isothermal heat treatments, followed by assessment of chloride pitting resistance and impact toughness. The trends observed were related to the material microstructure and extent of intermetallic formation.