Preplastic strain effect on chromium carbides precipitation of type 316 stainless steel during high-temperature ageing

Abstract

Type 316 austenitic stainless steel has found wide application in steam-generating plants as piping and superheater tube material as well as structural material in nuclear reactors. Long time exposure of Type 316 stainless steel to elevated temperature (400-900 °C) is known to cause high-temperature embrittlement due to chromium carbides and a-phase precipitating in grain boundaries. Numerous investigations have been published on the mechanical properties and microstructure changes occurring during exposure to high temperatures [1-13]. All of the investigations generally can be divided into two areas: mechanical property loss such as ductility, creep resistance and fracture toughness of the aged material, and phase instabilities during high-temperature exposure such as time-temperature-precipitation (TTP) diagrams. The effect of cold work on ageing is to shift the TTP diagram to shorter times and lower temperatures. The formation of M23C 6 and cr-phase is markedly accelerated [1, 2]. The creep behaviour of several austenitic stainless steels has been studied as a function of prior cold work. In the case of Type 316 stainless steel, Donati et al. [13] showed that improvements in the creep resistance increased with the degree of cold work. However, no investigations exist on the preplastic deformation effect on chromium carbides precipitation in grain matrix and grain boundary during high-temperature ageing of Type 316 stainless steel and then its effects on the room-temperature tensile properties. Since the stainless steel sometimes is deformed before servicing in high temperatures, it is necessary to study the preplastic strain effect of the stainless steel on the microstructure change and mechanical property change during high-temperature exposure. Therefore, the purposes of the present investigation were to study the chromium carbides precipitation in the grain boundary and grain matrix of predeformed Type 316 stainless steel and to determine the predeformation effects on the mechanical property change. Commercial Type 316 austenitic stainless steel was employed for this study. The chemical composition of this material is given in Table I. Cylindrical