| Hot ductility of 18Cr-8Ni stainless steels(type 304) containing high nitrogen was investigated in terms of fracture initiation mechanisms during high temperature deformation. The effects of nitrogen, sulphur and δ-ferrite contents on fracture mode changes were determined. It was found that, with the addition of nitrogen, the major fracture initiation mode at 1100℃ changed from brittle cracking along δ/γ interfaces to ductile cavity or micro-void formations along the austenite grain boundaries. It is due to decrease in residual δ-ferrite content with increase in nitrogen content. TEM analyses of the hot deformed microstructure showed that the transition from homogeneous dislocation tangled structure to planar array of dislocations occurred with the increase in nitrogen content. This planar slip behavior markedly reduced hot ductility. The fracture initiation mechanism was also found to be largely affected by the content of sulphur impurity, particularly in the high-nitrogen-containing steels. In these steels, a small increase in sulphur content resulted in intergranular fracture with micro-void formation along austenite grain boundaries at the temperatures below 1200℃. |
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