Thermal treatment and properties relationship with brittle phases in duplex and superduplex stainless steels

Abstract

Duplex stainless steels are used primarily in the chemical industry and in oil refinery plant, as their microstructure consists of 50% austenite and 50% ferrite, an excellent combination of mechanical properties is ensured. In this Project the changes produced in the microstructure have been studied, as well as the variation of mechanical properties in duplex (UNS S32205), laminated superduplex (UNS 32750) and tube-shaped superduplex stainless steels when they are subject to thermal treatments, in a temperature range close to the one used in the industrial processes, between 600-1000ºC. In this precise range of temperatures, intermetallic phases, such as sigma phase or chi phase, can precipitate and hereby deteriorate their mechanical properties. Although many studies have been performed to understand this process submitting these steels to long duration thermal treatments, very few have been conducted for short time duration in order to analyse the initial steps of the precipitation of these phases, situation that can already occur during the shaping process. The thermal treatments were conducted at 830ºC during 4,6,8,13,17 and 25 minutes, in order to promote the precipitation of the phases inducing embrittlement, sigma and chi. The objective was thus to find the relationship between the growth of these phases and the increase of hardness that these steels suffer after the thermal treatments, and thereby to get a comparative evaluation of the three different steels used (two of them in shape and two of them in grade). The study of the microstructure was carried out using the high resolution scanning electron microscope (Field Emission SEM). Using a backscattered electron detector and EDS (energy dispersive spectroscopy) microanalysis we were able to observe and characterize all the different phases. The obtained results show the formation and growth of these phases in the three types of steels. The formation of the sigma and chi phases is located on the grain boundaries and is caused by the diffusion of chrome and molybdenum from the ferrite phase. Nonetheless, whilst sigma phase grows towards the inside of the ferrite, we do not observe a significant growth in the chi phase remaining in the phase boundaries. Moreover, the fast kinetics of the formation of these phases in the laminated super duplex steel is remarkable compared to the ones in the other two steels. Finally, the mechanical behaviour of each sample was studied by the Vickers microhardness testing, corroborating that with a higher amount of theses brittle phases, the material hardens and increases their strength, although decreasing the ductility.