White Layers on Surface of Ground EN24 Steel: 1Microstructure,Composition,Internal Stress,and Corrosion Properties

The microstructure, composition, internal stress, and corrosion properties of white layers, typically less than 2·5 μm thick, formed on EN24 steel by centreless grinding, have been investigated using microscopy, electron probe analysis, X-ray diffraction, profilometry, and simple immersion tests. No accumulation or depletion of chromium, nickel, molybdenum, phosphorus, carbon, nitrogen, or oxygen occurred over a depth of 100 j1m. The hoop and axial residual stresses at the surface were compressive and about 200 and 600 MN m^-2 respectively. The hoop stress changed from compressive to tensile at 2 j1m depth and reached a maximum value of about 850 MN m^-2 at a depth of 15 μm. The white layer has an ultrafine microstructure, and the mechanism of formation is consistent with a process of intense thermomechanical plastic deformation. Localised corrosion occurs in solutions containing chloride where the white layer is thin, but as corrosion proceeds it spreads and becomes more general.