Redistribution of the substitutional elements during σ and χ phase formation in a duplex stainless steel

While the kinetics of intermetallic phase formation in duplex stainless steel have been extensively studied for a wide range of compositions, relatively little research has been done on the mechanisms by which the phases nucleate and grow. In this research the emphasis was on the diffusional growth of the sigma (sigma) and chi (chi) phases. Intermetallic phase precipitation in a standard DSS type EN 1.4462 was studied in detail at two temperatures: at 850 degreesC where large amounts of a phase are formed, and at 650 degreesC where chi is the main intermetallic phase. The redistribution of the substitutional alloying elements Cr, Mo, Ni, Mn and Si between the ferrite, the austenite and the intermetallic phases was followed by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Mn and Si showed no distinct partitioning between the several phases. The diffusion paths of the other elements were found to be determined by two phenomena. On the one hand, the ferrite to austenite transformation causes Mo and Cr to enrich in the ferritic phase and Ni to enrich in the austenite. On the other hand, Mo (sigma, chi) and Cr (sigma) are removed from the ferrite by intermetallic phase precipitation. At 650 degreesC, strong Mo enrichments appear at the alpha-gamma phase boundaries indicating that grain boundary diffusion could,be an important mechanism for Mo to diffuse to the growing chi phase.