TWI Industrial Member Report Summary 624/1997
By R N Gunn and C S Wiesner
Duplex and superduplex stainless steels have become common engineering materials, and are utilised generally in the as-welded condition. Over the years, the toughness of welded joints has been assessed and optimised, but there has been uncertainty over the correlation between fracture toughness requirements and the conventional weld procedure qualification (WPQ) tests, such as Charpy V notch. In consequence, impact energy requirements have frequently been set at conservative, high levels, which may be difficult to meet with normal fabrication practice. Recent Belgian work concluded that achievement of an average absorbed energy of 35J would normally afford sufficient safeguard against unstable fracture in service: surface breaking defects with a depth of 3mm and a length of 15mm could be tolerated in weld metal of 40mm thickness, and even larger defects at the fusion line for service temperatures down to -40°C. Analogous studies at TWI recommended that a Charpy energy requirement of 40J at the minimum operating temperature, for weldments up to 50mm thickness, will ensure adequate defect tolerance and could form a basis for toughness requirements to be implemented for these steels in fabrication codes. Duplex stainless steels can be prone to the formation of embrittling intermetallic phases, given the wrong heat treatment or inadequate weld procedure. Previous TWI work studied these effects and concluded that even small intermetallic contents could have a dramatic effect on toughness. However, the welds described in the Belgian work were made under controlled conditions, consistent with good industrial practice, and were not expected to contain intermetallic precipitates. The TWI studies considered the effects of intermetallic precipitates on weld metal toughness and reached the conclusion that the Charpy/CTOD correlation was not affected by the presence of intermetallic phases up to about 3% volume fraction. For the Charpy/CTOD correlations established to be of general applicability, it is necessary to ensure that intermetallic phases have a similar effect on both toughness criteria also for parent steel and HAZ microstructures. In addition, an increased database would enhance the confidence in the previous findings. Accordingly, the aim of the current work was to evaluate the influence of different intermetallic contents in a superduplex parent steel on CTOD and Charpy V notch tests.
To evaluate the effect of intermetallic precipitation on both fracture toughness and impact test results in parent steel and HAZs of superduplex stainless steel.