Duplex stainless steels (DSS) are gaining popularity for use in corrosive environments because of their excellent stress corrosion cracking resistance in CO2/C1-environments and their high yield strength. The high strength of these steels relies on the presence of a two-phase mixture of austenite and ferrite. Resistance to corrosion is mainly derived from high levels of Cr, Mo and N, but in many cases galvanic coupling is also believed to be beneficial.
There is a growing concern in welding of DSS that slow cooling conditions can result in the formation of various solid-state transformation products in addition to austenite and ferrite. The presence of transformation products such as sigma, nitrides, laves, etc., is detrimental to both the corrosion performance and the mechanical properties. Among these precipitates, sigma-phase is of particular interest. It is a thermodynamically stable phase below 900 Β°C and due to its high hardness it reduces toughness significantly, resulting in brittle weldments. This problem is generally overcome in the production of plates by quenching from high solution heat-treatment temperatures (e.g. 1150Β°C). However, in welding, continuous cooling is often unavoidable and hence the cooling rate is of primary importance in determining whether sigma-phase forms. Fortunately, the formation of sigma-phase is relatively sluggish because of the large tetragonal unit cell of 32 atoms.
Despite the sluggishness of the transformation, the formation of sigma-phase can be encouraged if cooling conditions and microstructural variations favour nucleation. For example, alloying additions, such as Cr, Mo and W, accelerate the formation of sigma-phase by enlarging the sigma-phase field on