Effects of Vacuum Heating Conditions on the Brightness of Surfaces of Various Stainless Steels

Abstract

Changes in the brightness of stainless steels surfaces with vacuum heating conditions were examined by use of a die-cooling method. In the die-cooling methed, specimens were cooled in vacuum on a water cooled die of the mild steel after being vacuum heated; consequently steel surfaces were restrained from being stained with quenchants on quenching. The brightness was measured using an optical microscope equipped with a silicon photocell. (1) In SUS 304 heated for 30 min under a pressure of 5.33 Pa, the oxide film formed on heating to 900°C disappeared gradually with increasing temperature to 1050°C; consequently the brightness increased from 27% at 950°C to 100% at 1050°C. An increase in temperature above 1050°C resulted in a decrease of the brightness owing to a surface roughening. The disappearance of the oxide film was explained in terms of its reduction by the solute carbon in steels, that is, Cr₂O₃+3C=2Cr+3CO. (2) In SUS 347, it was difficult for the oxide film to be reduced because of the much lower content of the solute carbon in the steel. (3) In other stainless steels of 0∼30 wt%Ni and 12∼27 wt%Cr, the types of steels were generally classified in the order of the difficulty in the reduction of films; heat resisting cast steels > austenitic stainless steels > martensitic and ferritic stainless steels. Essentially, the oxide films were more easily reduced for the steels, in which the C activity and hence the equilibrium CO pressure is higher.