Investigation of the casting microstructure of a Fe-base, Cr-Ni-Mo duplex alloy

Abstract

The microstructures of Fe-base cast duplex stainless steel specimens, which contain Cr, Ni, Mo and other elements and have been subjected to various heat treatments, were examined with X-ray diffraction, thermal analysis and optical microscopy and their hardnesses were also measured. From the metallographic examinations, it was seen that these specimens included austenite, ferrite, some inter-metallic phases, grain boundary carbides and oxide inclusions. It has been determined that the amount of grain-boundary carbides decreased, and second-phases precipitated in ferrite, in specimens annealed at 950 degrees C for 2 h and then cooled in water and liquid nitrogen. In addition it has been found by X-ray diffraction that the heat treatments applied did not change the types of the phases, but caused lattice deformations in the phases. Furthermore, beside ferrite and austenite, Laves, sigma, G, mu, R, chi, Fe3C, Fe3P, FeS2, MnS phases and M23C6, M7C3, M6C carbides were determined to be in the microstructure. DSC (differential scanning calorimeter) curves showed that some stoichiometry compounds of small amounts occurred between 370 and 470 degrees C, and in specimens cooled in liquid nitrogen after annealed at 950 degrees C for 2 h, a phase with a small percentage dissolving at about 310 degrees C. DTA (differential thermal analysis) curves also showed that austenite phase transformed to ferrite phase completely between 1182 and 1220 degrees C. In addition, it was found that the hardness of the specimens were different. (C) 1997 Elsevier Science S.A.