Plasma carburising and plasma nitriding are thermochemical processes for increasing the surface hardness of low carbon steels. Compared with conventional carburising, they offer excellent control and repeatability, low energy consumption and fast heat treatment times. Both processes produce little distortion of components and hence minimise the need for post-treatment machining or grinding. Due to its lower process temperatures, plasma nitriding gives the least amount of distortion and is particularly suitable for surface hardening of tools, dies and precision parts.
Any steel which can be gas carburised can also be surface hardened using plasma carburisation. The principle of the process is the same and it aims to increase the carbon content at the surface of a component to allow formation of a deep and hard layer on quenching. Plain carbon steels, alloy steels and cast irons can be treated, but should have a carbon content below 0.4% and preferably below 0.25%.
The process is undertaken in a batch type vacuum furnace with an integral quenching tank (usually oil). The chamber is operated at a reduced pressure of 0.1 to 3kPa and at a temperature of 950°C or less. The components are held at a negative voltage of 400-800V whilst a hydrocarbon gas (methane or propane) - diluted with nitrogen, hydrogen and argon - is introduced into the chamber at a few litres per minute. A glow discharge plasma is produced as a layer surrounding the components. This plasma contains elemental carbon which diffuses into the components' surface. Once carburisation is complete, the gas flow and plasma are stopped and the components are held at temperature in the vacuum to allow further diffusion of the carbon into the substrate. The components are slightly cooled to about 925-850°C before being quenched in a bath of oil. Case depths of 0.2 to 2mm are achieved with a hardness of about 700HV.
The surface of the components can be cleaned in the chamber, prior to carburising, by sputtering and hydrogen reduction of any oxides. 'Sooting', which often occurs in conventional gas carburising, is avoided because the presence of carbon is restricted to the thin discharge layer. High capital cost of equipment limits the process to long production runs or short runs of specialised, high value components.
This process relies on the formation of hard nitrides in the steel surface. Elements capable of forming nitrides must be present in the steel as alloying elements. These include Al, Ti, Cr, Mo, W and V. Nitriding is particularly suitable for tools steels, which contain many of these alloying elements.
The formation of nitrides produces a very hard surface, up to 1500HV, with good fatigue resistance due to compressive stresses. Unlike carburised layers, this hardness can be retained at temperatures up to 500°C. Plasma nitriding is a similar process to plasma carburising, but with the use of a nitrogen-hydrogen gas mixture rather than a hydrocarbon to produce the glow discharge plasma. Also, no separate heating is required as the plasma provides sufficient heating of the component's surface to reach the process temperature of typically 500°C.
With plasma nitriding, carbon does not play a part in the hardening process: the surface hardness is produced by the presence of nitrides. As a consequence, a final quenching stage is not needed.
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