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

Master Thesis


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University of Cape Town

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 impact behaviour at room temperature is given brief attention. The uniform straining ability of a metastable austenitic stainless steel, such as AISI 301, which is used for stretch forming applications, is promoted by transformation-induced plasticity (1RIP), 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 characterized before and after tensile deformations 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°C. The martensite volume fraction present after a true tensile strain of 0.3 was measured, and the work- hardening behaviour of the alloys was characterized up to the point of maximum uniform elongation. All the alloys considered, showed fully austenitic microstructures at the solution treatment temperature (1050°C), and the indications are that 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. The austenite stability is shown to affect the extent to which the TRIP behaviour occurs in the experimental alloys. In particular, the greatest ductility is provided by alloys containing approximately 5 wt% nickel and nitrogen contents in the range 0.14 - 0.16 wt%. This ductility is shown to be comparable to the AISI 301 alloy when deformed at 20, 60, and 120°C. The addition of nitrogen results in much increased strength compared to AISI 301, due to the interstitial solution hardening effect of nitrogen in austenite, and to a greater extent in martensite. Furthermore, pronounced serrated flow is identified in the experimental alloys when deformed at 60°C, whereas the behaviour is absent in AISI 301. Both these factors need to be considered when taking into account the formability of the experimental alloys. This study illustrates the potential for obtaining much cheaper 1RIP alloys relative to AISI 301, and at the same time indicates some of the limitations associated with alloying element substitution.

Bibliography: pages 91-96.