Influence of Heat Treatment Time and Temperature on the Microstructure and Corrosion Resistance of Cast Superaustenitic Stainless Steels

The influence of heat treatment time and temperature on the microstructure and corrosion resistance of cast stainless steel alloys CN3MN (UNS J94651) and CK3MCuN (UNS J93254) were investigated with a combination of microscopy techniques and immersion corrosion testing. The as-cast alloys exhibited significant residual microsegregation of Mo across the dendritic substructure and the presence of sigma (σ) phase in the interdendritic regions. Complete dissolution of the sigma phase and homogenization of Mo occurred after a heat treatment of 1,205°C for 4 h for castings with an average dendrite spacing of 35 μm. A heat treatment of only 1,150°C for 1 h was sufficient to produce similar results in welds with a smaller dendrite spacing of 10 μm. The use of the 1,205°C/4-h treatment restored the corrosion resistance of the cast alloys to a level that is comparable to the wrought counterpart alloys when evaluated using the ASTM G48 Method A test. This improvement is attributed to homogenization of Mo and dissolution of the σ phase. Calculated curves are proposed to estimate the combination of time and temperature needed for effective heat treatment of castings with various dendrite