Effect of grain size on the corrosion resistance of a high nitrogen-low nickel austenitic stainless steel

Abstract

Nitrogen alloyed austenitic stainless steels exhibit attractive properties such as high levels of strength and ductility, good corrosion resistance and reduced tendency of grain boundary sensitization [1]. The high austenitic potential of nitrogen allows the nickel content in steel to be reduced, offering additional advantages such as cost saving. The production of these low nickel steels is made possible by the addition of manganese that increases the N solubility in the melt and decreases the tendency of Cr2N formation [2]. Although there have been many studies on finely grained ferritic steels (e.g. [3]), only a few research reports are available on refined austenitic stainless steels. The grain size of ferritic steels can be easily refined by phase transformation, but in austenitic alloys, due to the absence of phase transformation, the grain diameter is usually controlled by recrystallization after cold working [4]. In the last case the behavior of the material is affected mainly by the working temperature, working ratio and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [5] but this method seems to be limited. In previous papers [6, 7], we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel with an average grain size below 1 μm was obtained by applying the reverse transformation of martensite to austenite, on subzero-worked steel, annealed at low temperatures. Furthermore, a great increase both in the mechanical [6] and in the localized corrosion [8] resistance was found. In order to further increase the strength, it is possible to combine the effects of nitrogen addition and grain refining. In previous works we analyzed the effect of grain size on the mechanical properties [9] and on the wear resistance [10] of a high nitrogen austenitic stainless steel. This paper deals with the corrosion behavior, in particular general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained high nitrogen austenitic stainless steel. Results are then compared to those of similar measurements on standard AISI 304 steel. The chemical composition of the steel (hereinafter HN) and of the AISI 304 steel under consideration is shown in Table I. After cold working down to 80% thickness reduction, the material was subjected to four different heat