Review on a new austenitic 57Fe15Cr25Ni stainless steel at temperature of 850C for 30 minutes followed by water quenching treatments

Abstract

Special steel demand is indispensable in nuclear field, especially as a construction material for structure of high temperature reactor. An austenite 57Fe15Cr25Ni stainless steel has been synthesized by using casting technique at temperature more than 1250°C in electromagnetic inductive-thermal furnace. The steel comprised of (%wt) 57%Fe, 15%Cr, 25%Ni, 0.34%C and less than 0.1% of other elements: titanium, phosphor, copper, niobium and sulphur. It is due to promoted for a high pressure-temperature operating system, the material must have strength, creep and corrosion resistances. To increase the strength of the material, heat treatment is needed in certain cooling media, i.e. water. As suspected to that of temperature selection of 850°C, that ferritic phase contributes to the strength of steel. The hardness of material increases up to 18% after being cooled rapidly into water medium. Even in the diffraction pattern, the ferritic phases do not grow clearly due to probably small amount of fraction. X-ray diffraction pattern just shows that ascast with an fcc’s lattice parameter of about 3.69 Å. Meanwhile, the water quenched sample has a lattice parameter which is slightly shorter than that ascast¢s lattice parameter, i.e. 3.59Å. The peak shift of the (111) plane in the profile diffraction, is shown approximately 0.079 degrees between ascast and water quenched samples, indicates the presence of residual strain that threatens the structural stability of the material. The ascast microstructure shows that austenite phase grains look very massive and illustrate irregular structures, with an average grain size of about 6-8 µm, showing coarse grains that are very different from the water quenched microstructure shown by fine grains and slightly porous. On the other hand, the viscosity (h) of quenching media has a very significant role in grain boundary formation (sensitization), because the rate of temperature drop is strongly influenced by heat diffusion from high to low temperature spaces at the surface of material. Moreover, there were no new phase formation and fatal scales in the corrosion appearance of the steel surface at 850°C after more than 15 hours of oxidation tests. Therefore, at temperatures 850°C, the steel is proven to withstand of its high temperature environmental corrosion.