The micromechanisms of fracture in a super-duplex ferritic-austenitic stainless steel (Zeron 100) have been studied for a variety of product forms. Both impact and fracture toughness tests were used over a range of temperatures between room temperature and -196·C. Failure at higher test temperatures (>-80°C) was characterised by ductile micromechanisms of sub-critical crack growth and plastic collapse without significant crack growth. Reduction of testing temperature to below -100°C resulted in Cleavage of the ferrite phase which was nucleated by an increased proportion of twin deformation. This cleavage was associated with near-interfacial regions, although not along the austenite/ferrite boundary itself. Transmission electron microscopy (TEM) of the materials has indicated a layer of increased strain/dislocation density ∼0.5µm into the ferrite grains, which is thought to be associated with this crack path preference. In some orientations and product forms this results in extensive crack bifurcation and splitting. The morphology of the phases was found to have a much greater effect on fracture behaviour than grain size or crystallographic texture variations brought about by differences in processing routes for individual products.
Duplex stainless steels (DSSs) combine the high strength and stress-corrosion cracking resistance of ferritic stainless steels with the high fracture toughness of austenitic stainless steels. It has been established that a 50:50 phase balance offers the optimum combination of properties (Baeslack and Lippqld, 1988). Zeron 100 was one of the first "super-duplex" steels to be developed, which was more highly alloyed with chromium, molybdenum and nitrogen to improve both the corrosion resistance and strength compared to other DSSs (Charles, 1991). The improved properties have resulted in greater use of super-DSSs in the oil and petrochemical industries in harsh environments, including offshore and Arctic applications. Adequate fracture toughness has been ensured in these low temperature applications for all processing routes and fabrication.
