Formation mechanism of white layer on a martensitic stainless steel and its effect on corrosion resistance

Abstract

1Cr17Ni2 martensitic stainless steel has good corrosion resistance and mechanical properties, and thus it is widely used in corrosion-resistant and load-bearing parts. After the machining process, commercial 1Cr17Ni2 steel components are usually solution treated and tempered under vacuum condition to achieve higher strength as well as dimensional accuracy. However, severe premature corrosion of 1Cr17Ni2 steel components is always correlated with a white layer formed during the heat-treatment process on the surface. In this study, the 1Cr17Ni2 alloy samples were solution treated at 950 °C for 90 min by practical commercial vacuum furnace with the pressure of 13.3–26.6 Pa, followed by furnace cooling instead of oil quenching. The microstructure and local elemental distribution of the white layer on the surface of a 1Cr17Ni2 steel were characterized in detail by scanning electron microscopy, backscattered electron diffraction, scanning transmission electron microscopy and three-dimensional atomic probe. The sample surface transformed to a ∼15 µm thick white layer of austenite phase with fine particles rich in Cr and C atoms, which is different from the inner matrix containing a mixture of martensite and δ-ferrite. Elemental distribution showed a depletion of Cr and enrichment of C atoms in white layer compared with the matrix. This could expand the austenitic region, resulting in retained austenitic microstructure on the surface. In addition, since Cr elements were consumed locally, the corrosion resistance of the steel with white layer decreased significantly, as proved by salt spray test and intergranular corrosion resistance test. These microstructure observations and the elements distribution were interpreted consistently by carburizing from the contaminated commercial vacuum furnace.