SIGMA-PHASE PRECIPITATION AND ITS EFFECT ON THE MECHANICAL-PROPERTIES OF A SUPER DUPLEX STAINLESS-STEEL

A super duplex stainless steel containing nominally (by weight) 25 Cr, 7 Ni, 3.5 Mo and 0.25 N has been investigated for sigma phase precipitation and its effect on mechanical properties at elevated temperatures. The specimens were solution treated and some of these were annealed between 700 and 1050 degrees C for times ranging between 1 and 60 min. Selective etching, optical microscopy, X-ray diffraction, bulk extraction and chemical analysis have been used to identify the austenite, ferrite and sigma phases, and to construct a time-temperature-precipitation diagram for sigma phase formation. Hot tensile tests and subsequent fractography were performed on specimens containing a wide range of sigma phase volume fractions. The results indicate the following: the incubation time for sigma precipitation is about 5 min at 850-900 degrees C; the ductility of specimens deformed at about 600 degrees C is greatly reduced by sigma phase precipitation; the effects of this phenomenon increase with the sigma phase volume fraction and exhibit quite a marked temperature dependence; at higher temperatures, e.g. 900 degrees C, sigma phase particles in the steel are realigned along the shearing direction during deformation and the final fraction appears ductile.