The Effect of Surface Preparation on the Precipitation of Sigma During High Temperature Exposure of S32205 Duplex Stainl
- PDF / 4,074,982 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 164 Views
ODUCTION
DUPLEX stainless steels have received considerable attention in the recent years due their good corrosion resistance and mechanical properties while maintaining a competitive price.[1] Duplex stainless steels are a group of stainless steel alloys where the alloying additions are such that the microstructure is made up of austenite (fcc) and ferrite (bcc). This mixture of phases, usually with a 50/50 distribution, gives this alloy group good corrosion resistance as well as strength when compared to either austenitic or ferritic grades.[1,2] They are extensively utilized in advanced applications where their corrosion resistance and strength make them ideal in aggressive environments, for example, in the gas and oil and chemical industries. Although these alloys are generally used at ambient temperatures, they may be subjected to high temperatures during processing and/or fabrication (e.g., welding). As well as causing the formation of oxide scales, these high temperatures may influence the distribution of phases within the metal such as austenite, ferrite, Chi, and sigma phase, which is known to form even at relatively modest temperatures in these alloys.[2,3,4] Sigma phase is an intermetallic phase containing ~30 pct Cr, 4 pct Ni, and 7 pct Mo.[5] Its formation is usually in the ferrite grains with reports showing that it predominantly forms on the fcc/bcc phase boundaries and bcc triple points.[6,1] After as little as 10 hours at 973 K (700 C),[4] 100 pct of the ferrite can transform to
MARK A.E. JEPSON, MATTHEW ROWLETT, and REBECCA L. HIGGINSON are with the Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, U.K. Contact e-mail: [email protected] Manuscript submitted June 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
sigma phase, although its morphology can vary depending on temperature and prior microstructure. This phase is of particular importance as it can have a deleterious effect on the mechanical properties of the steel, for example, its impact properties.[7,8] The two phases in duplex stainless steel not only have different crystal structures, but also vary in chemistry with the austenite and ferrite stabilizing elements favoring one or other phase. This difference may only be less than 1 wt pct but can lead to differences in the corrosion and high temperature oxidation characteristics of the two phases.[9,10] Although the formation of sigma in the bulk is relatively well documented (e.g., References 3,6,8,11), there appears to be no information on its formation in the surface regions of the alloy. Observations presented here demonstrate the importance of surfaces and surface finish on the initial formation of sigma phase, which may have wider reaching consequences for the assessment of the second phase formation of these alloys and the need for surface preparation control during processing.
II.
EXPERIMENTAL PROCEDURE
The initial material was an as-cast block of S32205 duplex stainless steel, with the nominal composition shown in Table I. For heati
Data Loading...
