Introduction
The Carbon content in the steel determines whether it can be directly hardened. If the Carbon content is low (less than 0.25% for example) then an alternate means exists to increase the Carbon content of the surface. The part then can be heat-treated by either quenching in liquid or cooling in still air depending on the properties desired. Note that this method will only allow hardening on the surface, but not in the core, because the high carbon content is only on the surface. This is sometimes very desirable because it allows for a hard surface with good wear properties (as on gear teeth), but has a tough core that will perform well under impact loading.
Carburizing
Carburizing is a process of adding Carbon to the surface. This is done by exposing the part to a Carbon rich atmosphere at an elevated temperature and allows diffusion to transfer the Carbon atoms into steel. This diffusion will work only if the steel has low carbon content, because diffusion works on the differential of concentration principle. If, for example the steel had high carbon content to begin with, and is heated in a carbon free furnace, such as air, the carbon will tend to diffuse out of the steel resulting in Decarburization.
Pack Carburizing: Parts are packed in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC (1652 ºF). At this temperature CO gas is produced which is a strong reducing agent. The reduction reaction occurs on the surface of the steel releasing Carbon, which is then diffused into the surface due to the high temperature. When enough Carbon is absorbed inside the part (based on experience and theoretical calculations based on diffusion theory), the parts are removed and can be subject to the normal hardening methods.
The Carbon on the surface is 0.7% to 1.2% depending on process conditions. The hardness achieved is 60 - 65 RC. The depth of the case ranges from about 0.1 mm (0.004 in) upto 1.5 mm (0.060 in). Some of the problems with pack carburizing is that the process is difficult to control as far as temperature uniformity is concerned, and the heating is inefficient.
Gas Carburizing: Gas Carburizing is conceptually the same as pack carburizing, except that Carbon Monoxide (CO) gas is supplied to a heated furnace and the reduction reaction of deposition of carbon takes place on the surface of the part. This processes overcomes most of the problems of pack carburizing. The temperature diffusion is as good as it can be with a furnace. The only concern is to safely contain the CO gas. A variation of gas carburizing is when alcohol is dripped into the furnace and it volatilizes readily to provide the reducing reaction for the deposition of the carbon.
Liquid Carburizing: The steel parts are immersed in a molten carbon rich bath. In the past, such baths have cyanide (CN) as the main component. However, safety concerns have led to non-toxic baths that achieve the same result.
Nitriding
Nitriding is a process of diffusing Nitrogen into the surface of steel. The Nitrogen forms Nitrides with elements such as Aluminum, Chromium, Molybdenum, and Vanadium. The parts are heat-treated and tempered before nitriding. The parts are then cleaned and heated in a furnace in an atmosphere of dissociated Ammonia (containing N and H) for 10 to 40 hours at 500-625 ºC (932 - 1157 ºF). Nitrogen diffuses into the steel and forms nitride alloys, and goes to a depth of upto 0.65 mm (0.025 in). The case is very hard and distortion is low. No further heat treatment is required; in fact, further heat treatment can crack the hard case. Since the case is thin, surface grinding is not recommended. This can restrict the use of nitriding to surfaces that require a very smooth finish.
Carbonitriding
Carbonitriding process is most suitable for low carbon and low carbon alloy steels. In this process, both Carbon and Nitrogen are diffused into the surface. The parts are heated in an atmosphere of hydrocarbon (such as methane or propane) mixed with Ammonia (NH3). The process is a mix of Carburizing and Nitriding.
Carburizing involves high temperatures (around 900 ºC, 1652 ºF) and Nitriding involves much lower temperatures (around 600 ºC, 1112 ºF). Carbonitriding is done at temperatures of 760 - 870 ºC (1400 - 1598 ºF), which is higher than the transformation temperatures of steel that is the region of the face-centered Austenite.
It is then quenched in a natural gas (Oxygen free) atmosphere. This quench is less drastic than water or oil-thus less distortion. However this process is not suitable for high precision parts due to the distortions that are inherent. The hardness achieved is similar to carburizing (60 - 65 RC) but not as high as Nitriding (70 RC). The case depth is from 0.1 to 0.75 mm (0.004 to 0.030 in). The case is rich in Nitrides as well as Martensite. Tempering is necessary to reduce the brittleness.