Mechanical Properties of nickel free high nitrogen austenitic stainless steels

摘要

The fine grained structures of Al steel with high strength and good ductility has been obtained by means of cold rolling and subsequent annealing. The Hall-Petch dependency for both yield stress and tensile strength was found to be valid for the grain size range. The micro hardness of the steel increased with decreasing grain size, according to Hall-Petch equation. The ductility of cold rolled steel decreased when the grain size was refined in the present works. By increasing test temperature, a decrease of tensile strength and yield strength was found for the steel. The ductility was improved with increasing the test temperature in the range of 77K to 573K. Above 573K, the elongation of the steel decreased. The tensile deformation of the three steel during uniform plastic deformation was well described by the modified Ludwik equation. The strain hardening n_1 values of the three steels investigated decreased when the crosshead speed was increased, which implies that high nitrogen steels is more active under low crosshead speed. The high nitrogen stainless steels investigated showed a ductile brittle transition. The change of fracture patterns of high nitrogen austenitic stainless steels was dimple- > shallow dimple- >mixture of quasi-cleavage facet and dimple- > cleavage facet. Typical river patterns which are the typical feature of cleavage fracture were observed at 77K. The river patterns started at grain boundary or crossed small-angle boundary to another grain. The transgranular and intergranular fracture were main fracture modes at low temperature.