Materials for carburising and nitrocarburising
Carburising and nitrocarburising are types of surface hardening processes that rely on diffusion to change the composition of the surface of metallic parts. The names suggest the elements involved in the diffusion process – in the carburising process, the percentage of carbon in the surface of the component is increased, and in nitrocarburising, both carbon and nitrogen are added to the surface. Both treatments are a very good way not only of hardening the surface of a component but of increasing its fatigue strength by placing the surface into a state of compression; this residual compressive stress effectively reduces the applied stress at the surface, and in cases where the stress in the component is highest at the surface, or close to it, this improves the component’s endurance limit.
These processes cannot be applied to any material though. In particular they are applied to steels, although outside of motorsport, nitrocarburising is sometimes used on cast irons. In terms of the carburising process, the materials suited to this process are low-carbon steels.
The rate of diffusion of carbon into the surface is not only a function of the carburising medium and the temperature, but also the composition of the steel in question. The difference between the ‘carbon potential’ of the carburising medium and the percentage of carbon in the steel dictates the speed at which carbon is taken up by the component; the diffusion rate is fastest at the start of the process, when the difference is greatest.
A carburising steel contains elements such as chromium, vanadium and molybdenum, which are strong carbide formers; nickel is usually also present in larger percentages but this is to aid ‘hardenability’ in thicker sections. The tempering temperature of the steels in question also places limits on the type of components that are suited to carburising. With typical tempering temperatures in the 150-180 C region, we have to ensure that carburised components are kept cool enough not to be softened during operation.
Nitrocarburising takes place after hardening and tempering, so is reserved for materials whose tempering temperatures are higher than the nitrocarburising temperature. This ensures that the component’s hardness and strength are not diminished by the nitrocarburising process. Typically the process takes place at about 480-600 C, depending on the grade of steel being treated. This is in the same region as nitriding takes place, and the materials normally used are nitriding steels.
The elements that take part in the chemical reaction to form nitrides in the surface are again chromium, molybdenum and vanadium, but aluminium is also a strong nitride former and can be found in a number of steels suitable for nitrocarburising. Aluminium-bearing steels show slightly higher surface hardness after nitrocarburising than those without, owing to the formation of aluminium nitrides. The surface hardness increases with increasing aluminium content up to 1% aluminium, after which further additions have no real effect. Tungsten is also used in steels suited to nitrocarburising for use at temperatures approaching 600 C. Nitrocarburising can be applied to typical nitriding steels, many tool steels and stainless steels. For example, many austenitic stainless steel valves are nitrocarburised.
Written by Wayne Ward