Morphology and Structure of alpha ' Formed in a Duplex Steel Z 3 CND 22-05

During isothermal heat treatments between 400-550 deg C, the ferritic and austenitic steels show a structural hardening. The origin of this is dependent on the presence of a miscibility gap in the Fe--Cr system. The delta ferrite at high temperatures gives birth to two cubic centered phases, a delta ferrite, low in Cr, and an alpha ' phase, rich in Cr. To study the morphological characteristics and the structure in the formation of the alpha ' phase in Z 3 CND 22-05 (Fe, 22 Cr, 5 Ni, 3 Mo, 0.03 carbon, 0.08 nitrogen), samples were treated at 1375 deg C and then reheated at temperatures between 450-800 deg C for times up to 336 h. The samples were examined under an electron microscope. It was found that < 600 deg C the ferritic matrix decomposed to form the alpha ' phase. Above 600 deg C, the precipitation of the alpha ' phase was accompanied by the formation of austenite with some sigma . The alpha ' phase appeared as both platelets and spheroids. It is believed that Mo is the determining factor. Photomicrographs.