Papers by Keyword: Sigma

Abstract: It is known that heat treatment (HT) highly affects the properties of base metal (BM) and fusion zones (FZ) of duplex stainless steel (DSS). In fact, it may give unwanted structure changes. Duplex stainless steels SAF 2205 welded joint was subjected to thermal cycle at temperature of 850^◦C at holding times 1, 3, 5 and 7 hours. The influence of heating cycles and concentration of corrosive medium on the corrosion properties and microstructure of 2205 alloy was the objective of this work. It was found that process led to noticeable decrease in the corrosion resistance of BM and FZ specimens; moreover the decrease was large in BM than FZ. It was also found that sigma phase (σ) precipitated in the different zones of the structure. σ phase volume fraction was found to increase with increasing the holding time of HT, and its increase is larger in BM. Corrosion resistance was found to be oppositely related to σ phase formation. Secondary austenite phase (γ₂) was also precipitated and its volume fraction in FZ was found to increase with increasing the holding time of HT and decreased in BM.

Abstract: The aim of this work is to study the precipitation mechanism of the intermetallic phases present in duplex stainless steels (UNS S32205 and UNS S32750), as well as to find out the most suitable method for detecting and analyzing accurately these secondary phases, particularly Sigma-phase, Chi-phase, nitrides and carbides. The samples were characterized after a solution annealing at 1080^oC followed by an isothermal treatment at 830^oC from 1 min to 9 h, with the purpose of figuring out the mechanism of chi-phase nucleation and nitrides formation in relation with the sigma-phase. The study has two main objectives: 1) to find out the most suitable technique for the detection, identification and quantification of the secondary phases, obtaining the best results with the combination of field emission scanning electron microscopy (FESEM) and backscattered electron detector (BSE) in comparison with the optical microscopy (MO); 2) to study the influence of the chemical composition on the nucleation mechanism of the intermetallic phases. It has been concluded that molybdenum balance content in chi-phase related to sigma phase is close to two, consequently the kinetics of nucleation and growth of these phases is remarkably faster when this alloying element content in the steel is higher. Chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments carried out to the specimen wherein chromium nitrides role is a favorable site for the nucleation of sigma and chi phases.

Abstract: The characteristics of microstructures in friction stir (FS) weld of 304 austenitic stainless steel were examined. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallized and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-boundaries. Electron microscopic observations revealed that ferrite and sigma phases were formed in austenite matrix in the SZ during friction stir welding (FSW).