In order for such engineering components as bearings and gears to withstand the high Hertzian contact stresses encountered in service, deep case hardening is necessary. Oxygen diffusion (OD) and thermal oxidation (TO) techniques have been successfully applied to the low strength Ti–6Al–4V alloy to develop a deep hardened case and a thin, hard, wear resistant surface layer. These techniques have been used to produce homogeneous, composition controlled, and highly reproducible alloyed surface layers in the Ti–6Al–4V alloy. It was the objective of the present investigation to develop and apply the novel duplex OD–TO surface treatment to the high strength titanium alloy Timet550. The higher strength of Timet550, compared with Ti–6Al–4V, would have the advantage of increasing the bending strength of a component under load. Various oxidation and diffusion treatments were carried out to optimise the OD–TO process and consequently a post-diffusion heat treatment was developed to maximise the depth and hardness of the OD case for Timet550. The surface layers were characterised using XRD, GDS, and SEM, which revealed a rutile surface with an oxygen rich case. The loadbearing capacities of duplex treated Timet550 specimens were evaluated using friction monitored scratch tests and a wheel on block Amsler wear tester. It was found that the OD, heat treated, and TO processed Timet550 specimens had the greatest loadbearing capacity.
Recently, a thermal oxidation (TO) technique has been successfully developed and applied to the titanium alloy Ti-6Al-4V. This TO technique produces a thin, hard, rutile-based, wear-resistant coating on the surface of the titanium alloy, thus significantly improving the tribological properties of the titanium alloy. In the present investigation, the same principle has been applied to the ␣ ϩ  highstrength titanium alloy Timet 550. A series of TO treatments have been carried out in air within the temperature range of 600 ЊC to 650 ЊC. This developed a rutile-based coating which greatly improved the tribological properties of Timet 550. Systematic characterization of the TO-treated surface was carried out using glow-discharge optical emission spectroscopy (GDS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution scanning electron microscopy (HR-SEM) techniques. Ball-on-disc friction testing was used to show the improvement in tribological properties for Timet 550 when TO treated. The sliding wear resistance of the TO treatment was investigated using an Amsler wear tester, utilizing a counterformal block-on-wheel configuration; the TO-treated Timet 550 was run against a carburized S156 steel with splash oil lubrication. It was found that the wear resistance of the TO-treated Timet 550 was greatly improved.
With the drive towards cost‐effective routes for the manufacture of engineering components, flow forming technologies are now under consideration for the production of structural axisymmetric geometries such as tubes and cones. This near net shape process is known to offer improvements in material utilisation when compared with traditional processes where substantial final machining is required. The microstructure, evolved as a result of the flow forming process together with subsequent heat treatments, will govern associated mechanical properties. Laboratory measurements of the structure‐property relationships of flow formed material can be problematic, mainly because of the restrictions imposed on the extraction of conventional specimen geometries since most of the finished tubular or cone structures will contain thin and curved walls. The development of a suitable specimen design and associated test technique for the measurement of fatigue crack growth rates at room and elevated temperatures is presented. Data obtained from flow formed Inconel 718 (IN 718) will be compared with specimens of the exact same geometry but machined from conventionally forged IN 718 stock. This allowed for validation of the novel flow formed test in addition to an assessment of the damage tolerance of the flow formed variant. The intimate relationship between local microstructure and fracture mechanisms will be described.
Fatigue crack propagation has been measured in flow formed Inconel 718 (IN718). Test pieces were extracted from a flow formed tubular structure in the longitudinal direction, retaining the tube curvature across their width. Crack growth rates (da/dN) were measured at 20, 300, and 400oC. For comparison, tests were repeated on specimens with an identical geometry but machined from conventionally forged IN718. Detailed metallurgy of the flow formed material is presented.
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