The influence of nickel-nitrogen ratio on the deformation behaviour of austenitic stainless steels

Abstract

This study examines the effect that a partial substitution of nickel with nitrogen has on the deformation behaviour of a metastable austenitic stainless steel, AISI 301. The effect on the tensile deformation behaviour is studied in detail at various temperatures, and the effect on the impact behaviour at room temperature is given brief attention. The uniform straining ability of a metastable austenitic stainless steel, such as AISI, which is used for stretch forming applications, is promoted by transformation-induced plasticity (TRIP), which depends on the manner in which deformation-induced martensite forms during straining. This includes both the rate of martensite formation, and the stage at which the martensite is formed. In particular, incipient necking is resisted when martensite forms gradually and selectively, preventing the formation and propagation of micronecks and microcracks. The microstructures of ten alloys, each having a type 301 base composition, but systematically varying nickel-nitrogen ratios, were characterised before and after tensile deformation, using optical and electron microscopy as well as X-ray diffraction techniques. Tensile tests were performed on solution treated specimens at temperatures of 0, 20, 60 and 120 degrees Celsius. The martensite volume fraction present after a true tensile strain of 0.3 was measured, and the work-hardening behaviour of the alloys was characterised up to the point of maximum uniform elongation. All the alloys considered showed fully austenitic microstructures at the solution treatment temperature 1050 degrees Celsius, and the indications are that the nitrogen is fully dissolved. The austenite stability of the alloys however varies at room temperature. Alloys containing approximately 5 wt% nickel, with a maximum nitrogen content of 0.28 wt%, contain up to 97% retained austenite, whereas alloys with 3.4 wt% nickel and the same maximum nitrogen content, contain only up to 63% retained austenite.