Influence of the heat treatments on the corrosion resistance of a duplex stainless steel manufactured by powder metallurgy

Duplex austenite-ferrite stainless steels were manufactured by sintering mixtures of austenitic (Type 316L [UNS S31603]) and ferritic (Type 434L) atomized powders. Although different Type 316L/434L ratios were prepared, the present work centered on the 50% ferrite-50% austenite sintered steel. Solution treatment was performed to homogenize the microstructure. A microstructural study of the solutionized material was performed to evaluate the microstructure, proportion and shape of porosity, and ferrite percentage. The materials were isothermal-treated in the range of 700 degrees C to 1,000 degrees C for 10, 30, and 60 min and water-quenched to study the microstructural transformations that occurred during heat treatment and their influence on the intergranular corrosion resistance. Microstructural studies using light microscopy, scanning electron microscopy, x-ray diffraction, and nanoindentation tests showed the presence of an austenite transformation in the austenite/ferrite grain boundaries, originated by Ni diffusion, after heat treatment at temperatures up to 800 degrees C. The method used to evaluate the sensitization to the intergranular corrosion was the electrochemical potentiokinetic reactivation (EPR) procedure. The results of the electrochemical tests were related to the microstructures that formed at the different heat treatments, although the porosity level was found to be the determining factor in the corrosion sensitizing of these kinds of duplex steels.