Effect of Temperature of Spark Plasma Sintering on the Development of Oxide Compound in Fe-25wt%Ni-17wt%CrAustenitic Stainless Steel

Abstract

To investigate the effect of sintering temperature of spark plasma on the development of oxide compound in Fe-25wt%Ni-17wt%Cr austenitic stainless steels (ASS).The ingredient samples were high purity powder of Fe, Ni and Cr and mixed by through milling process for 5 hours. Compacting the three elements under 30MPa and vacuum was carried out in a spark plasma sintering (SPS) machine at temperature of 900 and 950 o C for 5 minutes. The microstructure of Fe-25wt%Ni-17wt%Cr ASS including oxide compound layer was examined through an Optical Microscope (OM), a Scanning Electron Microscope (SEM) and a Transmission Electron Microscope (TEM) equipped with Energy Dispersive Spectrometers (EDS), and also by Raman Spectroscopy.The crystal structure of austenite grains in the matrix was identified using X-Ray Diffractometer. Although, the SPS can consolidate nano alloy, the Fe-25wt%Ni-17wt%Cr ASS consists of matrix of fine austenite, g-FeNi grains and particles of a'-Cr. Sintering temperature at 950 o C increase O content in the surface of Fe-25wt%Ni-17wt%Cr ASS that mainly related to increasing the distribution of very fine a'-Cr particles as CrO compound beside of increasing the trapped air bubbles. The Raman spectra of Cr-rich M 2 O 3 was identified (M: Metal) at sintering temperature of 950 o C, while the Fe-rich M 2 O 3 and pure Fe 3 O 4 without significant amount of dissolved chromium or nickel in a main band at 671 cm −1 was identified at 900 o C. The existence of g-Fe 2 O 3 and a-Fe 2 O 3 was also successfully detected from the Raman.Changes in the number of particles with high Cr content or a'-Cr in the matrix with fine grains of g-FeNi formed during the process of sintering by spark plasma makes more oxide compounds in the Fe-25Ni-17Cr ASS at 950C as compared to sintering temperature at 900 o C. The oxide compound is formed from the reaction of O and Cr content in a'-Cr particles during process of spark plasma sintering.