An In-Situ Study on Surface Coloring Mechanism of a 12cr Ferrite/Martensite Stainless Steel During Vacuum Heat Treatment

Abstract

Color changes during oxidation of a 12Cr ferrite/martensite stainless steel were investigated in-situ by using a high vacuum (5×10-3 Pa) heating stage. The color of the oxide film changed from initial metallic silver to brown, then consecutively to purple, liberty blue, blue-gray and finally to azure blue during the 90 min heat treatment at 690 ℃ in high vacuum. In contrast to the above changes, during oxidation at 1130 ℃, the color changes from metallic silver to brown, then to various blue and finally changed back to metallic silver. The color as a result thin-film interference mainly depended on the oxide film thickness. The significant reduction of the thickness of the HV1130 sample is owing to the volatilization of oxide, which is a result of the reaction of the already formed Cr2O3 with residual oxygen to generate CrO3 gas, which evaporated from the surface at 1130 ℃. Ferrite grains have a much lower oxidation kinetics than that of martensite.

Keywords: Stainless steelVacuum heat treatmentIn-situ oxidationColoring mechanismX-ray photoelectron spectroscopy