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Some processing details about carburising

The carburising process has found very wide use in motorsport as well as the wider engineering industry. It provides a hard surface on a tough core, and puts the surface of the component into a state of residual compressive stress, which we know have a positive effect on component life. The number of applications for carburising is growing, owing to the adoption by many companies of low-pressure carburising (also referred to as vacuum carburising), and we covered this subject briefly in a previous article on carburising.

Low-pressure carburising minimises the distorting effect of carburising, which has been one of the main drawbacks with the process. In order to control the properties of the carburised material, it is necessary to quench the component, and it is at this stage where traditional processes can introduce distortion, especially where liquids such as oil are used for the quenching. It is very hard to achieve an even rate of heat removal when an extremely hot component is plunged into cool oil.

One solution is to quench using a gas. Even though the carburising process is low pressure, the pressure of the gas quench can easily be 10 bar or more. In order to improve surface finish, inert gases are used and, in contrast to traditional oil-quenched carburising processes, the parts emerging from a vacuum carburising process followed by an inert gas quench remain bright and shiny in appearance. Typically nitrogen is used as a quench gas, but helium is also finding some use as it offers higher rates of heat removal owing to its lower introduction temperature and high specific heat capacity. Argon is also occasionally used.

If we consider where our carburised part fits in the engine, we might find that we need to fasten it to another component, perhaps even using threaded holes; we may also need to have thin areas of material. The danger with threaded holes is not limited to distortion but also the fact that the whole thread might be completely carburised, rendering it very brittle and prone to failure. In the same way, very thin sections of material might be hardened completely through.

Fortunately though we can indicate to the carburising supplier which areas of the component are to be treated, those that must not and those that are optional, by supplying them with a drawing. The part of the drawing that deals with heat treatments and surface treatments is often the key to success in component design, so it is important to ensure any general notes, instructions on drawing views or attempts to specify which areas are not to be treated are specific and clear.

The areas that are not to be carburised can be mechanically masked, but are most often plated or painted to prevent a carbon-rich case being formed. Copper plating is an effective method of preventing carburising; it’s a process known as ‘stopping off’. Also, copper-bearing paints can be applied either by spraying, brushing or dipping, and a number of the more modern stop-off paints are water-based and so water soluble, so can simply be washed off after use.

Written by Wayne Ward

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