The characterisation of white-etching layers formed on engineering steels

Abstract

Metal surfaces, and particularly steels, may be modified by processes which include; plastic-deformation, chemical changes and heating. These layers are often characterised by high hardness and a pronounced resistance to chemical etching. This latter characteristic giving rise to the generically descriptive term "White Layer", which is often applied to such features. Processes which may result in the formation of "White Layer" can be broadly separated into three groups; thermal, chemical and mechanical. In practice, "White Layers" observed on materials removed from service environments have generally experienced a combination of these processes. In this work white-etching layers formed on engineering steels have been characterised, using a variety of electron beam and X-ray analytical techniques, to establish the chemical nature and structural properties of "white layer" material. Specimens drawn from such diverse service applications as; high-pressure tank gun barrels, digger-teeth from gravel extraction plant and adiabatically-sheared armour steel have been compared with samples produced in the laboratory by conventional pin-on-disc wear testing apparatus, specialised machining techniques and laser surface-hardening heat-treatment. The presence of a hard white-etching layer on each of the samples was first established using metallographic examination by light microscopy and microhardness testing to confirm the etch-resistance and high hardness of the white-layer. The chemical composition of the whiteetching layer was then compared with the bulk steel composition using scanning electron microscopy and conventional microprobe analysis ( Z > 11 ), and no significant differences were observed. Light element microprobe analysis and SIMS showed an increase in H, C, N, 0 in the white-etching layers formed within gun barrels, and on abusively turned steel. The levels detected were not considered to be significant in terms of white-layer formation but may well have an influence on strain aging and embrittlement phenomena. A technique for the preparation of cross-sectional thin foils was developed which allowed the structure of the white-etching layers to be compared with the underlying matrix by TEM and electron diffraction. The results of this study showed clear similarities between white-etching layers formed by wear and abusive machining and those formed by adiabatic shear. From these results it is concluded that " white layer " is a fine grain martensite which forms on steel as a result of thermo-mechanical transformation.