Phase Transformations in Cast Superaustenitic Stainless Steels

The goal of this investigation was to study phase transformations in cast superaustenitic stainless steels. Experiments were performed to determine the phase transformation behavior for alloys CN3MN and CK3MCuN. Samples were taken from keel bars that were heat treated between 1160 and 1230 A degrees C and then isothermally held for times ranging from 1 min to 2040 h at temperatures in the range of 700-900 A degrees C. The resulting microstructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy and wavelength dispersive spectroscopy. The microstructures, composed primarily of sigma- and Laves-phases within a purely austenitic matrix, showed relatively slow transformation kinetics, with transformation not completed even after the longest anneals. SEM and TEM analysis of superaustenitic samples reveal that precipitation takes place initially on grain boundaries and proceeds intragranularly. The results of these SEM and TEM investigations, along with volume fractions and number densities as a function of time and temperature, will be presented and discussed.