The possibilities for surface hardening of titanium alloys by alloying with nitrogen are reviewed. Alloying has been carried out in the solid state by plasma nitriding and in the liquid state by laser melting in nitrogen. Microstructure, hardness profiles, and fatigue and wear properties have been examined. Wear resistance was found to be greatly improved by both techniques, although at the expense of a drop in fatigue properties. Case depths of 0.1 mm were produced by plasma nitriding, while 0.5 mm case depths and a maximum hardness of 1400 HV0.1 were achieved by laser alloying.
Recent developments in plasma heat treatment equipment and processing techniques are reviewed. This process, which is now firmly established in the UK, is being used in a wide range of applications. Case histories are presented to illustrate some of the associated advantages and potential drawbacks. Temperature uniformity within the vacuum vessel is identified as the major problem when treating mixed workloads. Advances in the field of plasma carburising have led to improvements in the design of nitriding equipment and in particular to the development of pulse power supplies and additional heating systems. Equipment incorporating these features is claimed to give improved temperature uniformity throughout the workload. Plasma carburising equipment is now available and production facilities are operational in the USA, France, Japan, and the FRG. The current status of plasma carburising is assessed, and the areas which present problems are identified.
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