TWI Industrial Member Report Summary 594/1997
R N Gunn
Background
Several grades of 25%Cr superduplex stainless steels are marketed, of varying composition, most notably in the Mo and W contents: one grade contains 4%Mo, 0%W (UNS S32750), another 3.5%Mo, 0.6%W (S32760) and a third 3%Mo, 2%W (S39274). Both Mo and W are known to increase the pitting resistance of stainless steels in chloride-bearing media, but unfortunately, also stabilise deleterious intermetallic compounds when exposed to elevated temperatures. Most published data on intermetallic formation have been produced using isothermal heat treatment and most agree that significant intermetallic contents are formed after about 100sec at around 900°C. This is considered a long time in the welding context, and yet, during the early 1990s, intermetallic phases were known to form in superduplex weldments and, in particular, in the heat affected zones (HAZs) of thin-walled tubes, several millimetres from the fusion boundary. These observations questioned the assumption that HAZ intermetallic phase formation could be predicted from isothermal heat treatment, and also the significance of the intrinsic differences between weld thermal cycles and an isothermal heat treatment. Earlier work at TWI showed that the thermal cycles experienced in the HAZ of duplex welds could be predicted using a finite element approach. The current report describes thermal cycles computed for welds which were known to have precipitated intermetallic phases. Different 25%Cr superduplex stainless steel grades were subjected to these thermal cycles, using a weld thermal simulator, to establish whether this technique produced similar intermetallic formation, as seen in practical welds.
Objectives
- To establish whether the time to form intermetallic phases during isothermal heat treatment is similar to that during simulated weld thermal cycles.
- To determine whether intermetallic phases seen in real multipass welds can be induced by thermal simulation.