The characterisation of white-etching layers formed on engineering steels

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.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

Failure analysis of a steel motorcycle kickstand

Copyright @ 2009 Springer US.A fractured steel motorcycle kickstand was metallurgically investigated using a range of failure analysis tools [visual examination, energy dispersive X-ray (EDX) analysis, electron microprobe analysis (EPMA), scanning electron microscopy (SEM), fractography, optical microscopy, hardness testing and non-destructive testing (NDT)]. The steel kickstand’s composition, its microstructure, electron fractographs, and mechanical test results have been critically interpreted. Some evidence of wear damage, in the failed kickstand, was observed. The microstructural and fractographic analyses showed pre-existing micro-cracks which were believed to have grown to result in ductile failure followed by acceleration of corrosion. Recommendations have been made to avoid the failure of the motorcycle kickstand

CHARACTERISTICS OF CASEHARDENED SURFACES OF PLAIN LOW CARBON STEEL FOR TRIBOLOGICAL PROPERTIES USING AN ATOMIC FORCE MICROSCOPE

This study is aimed at evaluating mechanical properties (such as modulus and hardness) of nitro- carbonised mild steel surfaces as a function of surface topography using nanoscale testing techniques. A consistent metallographic surface preparation was carried out on varied process parameters of nitro-carbonised mild steel specimens. It was found that surface topography varied as a function of carbonized process parameters. Furthermore, nanoscale tests show that the topography of the surfaces significantly influences the mechanical properties of the surface. Thus, it is concluded that nanoscale testing techniques offer good candidacy for the study of tribological properties where lubrication and spatial distances of hardness are important