Microstructures of Austenitic Stainless Steel 56Fe25Ni16.6Cr0.9Si0.5Mn Solid-treated with Different Cooling Rates

Abstract

A new synthesized 56Fe25Ni16.6Cr0.9Si0.5Mn austenitic stainless steel (ASS) was produced through casting and then annealing and normalizing at 1100 °C, followed by cooling at different rates. Microstructures of the samples were studied by using X-ray and neutron diffractometers, scanning, and transmission electron microscopes. The stainless steel had a face center cubic structure (X-ray diffraction profile). The microstructure of ASS consists of γ-austenite matrix and high Cr carbide particles in the interior grains and grain boundaries. It seemed that the annealing process affected information of γ-austenite grain size in the ASS, growing larger compared to grain size formed in the normalizing process, similarly by cooling into the air, water, and oil media. The M23C7 islands in the grain boundary formed into a larger size after annealing or normalizing processes, and in addition, the island shape was more elongated. No significant changes were found concerning the particle size and shape of M7C6 at the grain boundary of the ASS after annealing or normalizing. Neutron diffraction patterns confirmed the Fm3m space group symmetry of ASS as obtained by the XRD method, therefore establishing ASS as an austenite phase. Results of the uniform deformation method (UDM) analysis applied upon the high-resolution powder neutron diffractometer (HRPD) intensity showed that the deformation strain was 2.3705 × 10–4. TEM results for ASS showed that the sizes of rectangular precipitates ranged from 63 × 32 nm to 84 × 42 nm with larger-sized irregulars reaching about 190 nm across. All these results showed that an ASS has been successfully synthesized and that it has a promising future to be used as a high-temperature structural material.