Effects of the grain size on the corrosion behavior of refined AISI 304 austenitic stainless steels

Abstract

Although there have been many studies on fine grained ferritic steels, only a few research reports are available on refined austenitic stainless steels and, in particular, on the influence of the grain size on the corrosion resistance of this class of material [1, 2]. The grain size of ferritic steels can be easily induced by phase transformation, but in austenitic alloys, following the absence of a phase transformation, the grain diameter is usually controlled by recrystallization after cold working [3]. This method is mainly affected by the working temperature, amount of deformation and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [4] but this method seems to be limited. In a previous paper [5] we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel of ca. 1 μm average grain size was obtained by applying the reverse transformation of martensite to austenite on subzeroworked steel annealed at low temperatures. Up to now, the corrosion behavior of such ultrafinegrained austenitic stainless steels has not been reported. This paper deals with the corrosion behavior, especially general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained AISI 304 stainless steel. Results are compared with those of similar measurements on standard AISI 304 steel. The chemical composition of the AISI 304 stainless steel, obtained from a commercial batch, is shown in Table I. After subzero working down to 90% thickness reduction, the material was subjected to the following four heat treatments in order to obtain different microstructures: annealing at 800 ◦C for 160 s and 900 s (specimens A and B respectively) and at 1000 ◦C for 10 s and 600 s (specimens C and D respectively). The grain sizes corresponding to the above specimens, as measured by automatic image analyzer, are shown in Table II. The typical microstructures of the 1 μm and 50μm specimens are shown in Fig. 1. Tensile properties of the specimens are shown in Fig. 2. Ultimate tensile stress and 0.2% yield stress increase with decreasing grain size, according to the Hall Petch relation [6]. Steel materials were machined into corrosion test specimens of 15 × 15 × 1 mm. The specimen surface was polished by using increasingly finer abrasive papers, starting with a 300 grit paper and finishing up with