Processing of submicron grain 304 stainless steel

Abstract

A novel thermomechanical processing technique for the synthesis of bulk submicron grain (grain size ഠ200 nm) 304 stainless steel is reported. This ingot-metallurgical technique requires a total deformation of only 95%, and the key steps to this processing technique involve (i) formation of ultrafine dislocation cell structure, and (ii) the conversion of dislocation cells into grains with medium to high misorientation by grain boundary sliding.Materials with submicron grain microstructure exhibit significantly improved mechanical properties over their course grain counterparts.1 These properties include higher strength, improved formability and ductility, and reduced superplastic temperature.2 However, due to metastable nature of these microstructures, submicron grain materials are difficult to produce in bulk form and are usually produced by special powder metallurgical techniques. 3,4 There is a considerable interest in nonpowder metallurgical techniques for the production of these materials, because the problem of residual porosity can be completely avoided by the use of these techniques.1 Here, we describe a novel processing technique for the production of a submicron grain (200 nm) 304 stainless steel (304 SS) by an ingot metallurgical route. This processing technique does not require any powder precursors and requires a total deformation of only 95%. The key steps to this technique involve (i) generation of a dislocation cell structure of same size (or finer) to finally desired grain size, and (ii) activation of grain boundary sliding in this microstructure, leading to conversion of dislocation cells into grains with high angle boundaries. This processing technique is based on our previous work where we observed the collapse of microstructure in Fe -28Al-2Cr intermetallic compound from 80 nm to 10 nm during room temperature compressive deformation. 4 The starting material for this investigation was a commercial grade 304 stainless steel (304 SS) with an average grain size of ഠ200 mm. The x-ray diffraction (XRD) pattern of the as-received stainless steel is shown in Fig. 1. The microstructure consists of a mixture of bcc a) Present address: Gillette R&D, Gillette Park, Boston, Massachusetts 02106-2131.and fcc phases as peaks from both bcc and fcc phases are visible in the x-ray diffraction pattern. The 304 SS rod (3͞8″ in diam) was initially rolled 63% at room temperature to an ingot with square cross section. This ingot was further rolled 65% at liquid nitrogen temperature, which resulted in reduction of cross section to 0.120 in. 3 0.120 in. The ingot obtained in this manner will be referred to as SS8 in the rest of this paper.The microstructure of the SS8 ingot was examined by transmission electron microscopy (TEM) and x-ray diffraction in both directions, i.e., in the planes parallel and perpendicular to the axis of rolling. X-ray diffraction scan of this ingot as shown in Fig. 1 reveals that the microstructure consists entirely of bcc phase. The microstructure of SS8 ingot as examined by TEM is shown in...

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