TWI Industrial Member Report Summary 653/1998
B J Ginn and T G Gooch
While intermetallic phases reduce corrosion resistance of superduplex stainless steels, there is no direct correlation between intermetallic volume fraction and the resultant critical pitting temperature.
Superduplex stainless steels are sensitive to intermetallic precipitation as a result both of the high alloy content and of the rapid diffusion of substitutional elements in the ferrite phase or along ferrite/austenite phase boundaries. The timescale over which precipitation can develop in superduplex stainless steels is of the same order as the thermal cycle experienced during a welding operation. Hence, intermetallics may form at welds in superduplex steels, both in the heat affected zone (HAZ) and weld metal, and this can reduce the weldment corrosion resistance and toughness. The adverse effects of intermetallic formation can be substantially avoided by use of low heat input and interpass temperatures to minimise the time spent on cooling in the intermetallic precipitation temperature range. Certainly, welding can be carried out reliably with little loss of properties from intermetallic formation, and the superduplex steels have found increasing use in oil and gas, petrochemical and other plant.
Nevertheless, because of concern regarding possible loss of service properties welding procedure qualification (WPQ) specifications commonly require metallographic examination and demonstration of negligible intermetallic phase formation in the weld area. Complete freedom from intermetallic phases may be stipulated: this is virtually impossible to demonstrate metallographically, and thus many WPQ specifications require that intermetallic phases should be below a specific volume fraction, typically 1%. Such maximum intermetallic levels essentially reflect what can be achieved without unacceptable restriction on welding conditions and productivity rather than what is necessary to obtain a specific level of corrosion resistance.
- To quantify the influence of intermetallic phase content on pitting corrosion resistance of superduplex stainless steel.