Carburizing is essentially the addition of carbon at the surface of low carbon steels at appropriate temperatures.
Case hardening is achieved with the quenching of the high carbon surface layer that has a good fatigue and wear resistance. This layer is applied on a tough low carbon steel nucleus. Case hardening of the carburized steels is mainly a function of carbon content. When steel carbon contents exceed a certain percentage, hardness is not affected by further carbon addition. Additional carbon at this stage is not dissolved. Steel case hardening depth of the carburized steel depends upon the carburizing time and the surface carbon intensity.
When the carburizing time is prolonged to obtain increased case depths, excessive free carbides may be formed. The micro-structural elements have an unfavorable effect on the distribution of residual stresses. Carburized steels generally have base carbon contents of approximately 0.2% while the carburized layer has carbon contents in the range of 0.8% and 1%. Most carburized steels are deoxidized by the addition of aluminum. Due to the carbon content difference in the core and the case, their case hardening capability is also different.
Methods of Carburizing
While the fundamental carburizing principle has not changed much since its inception, the carbon introduction techniques have been improving. The four carburizing processes are explained below.
Components are packed in an environment with a high carbon content such as cast iron shavings or carbon powder. The components are heated with the production of carbon monoxide, which is a reducing agent. The reduction occurs on the steel surface with the release of carbon that is diffused into the surface because of high temperatures. With the carbon absorption inside the component, the materials are hardened. The surface carbon is in the range of 0.7% to 1.3% depending on the process environment. The case depth is approximately 0.1 to 1.5 millimeters. Control of pack carburizing is difficult because uniform temperatures may not be maintained.
Gas carburizing is theoretically similar to pack carburizing aside from the supply of carbon monoxide gas to the heated furnace and the carbon decomposition. Many of the problems with pack carburizing are eliminated in this process. The carbon monoxide gas needs to be contained safely. The components are enclosed in a carbon bearing environment that is replenished continuously to maintain a high carbon content. Gas carburizing is one of the most successful and popular carburizing technique used when large quantities of parts are required.
The vacuum carburizing process involves carburizing in a low pressure environment that is oxygen-free. The atmosphere is significantly simplified although the furnace enclosure is difficult. A single component environment containing uncomplicated gaseous hydrocarbon like methane is used. Since the environment used for heating is oxygen free, the carburizing temperatures can be considerably increased without surface oxidation. The higher temperatures increase the carbon solubility and the diffusion rate. Thus, the time necessary for case depth is minimized.
The steel components are submerged in a liquefied carbon rich environment. The main component in such baths is cyanide. However, safety issues have led to baths that are non-toxic that accomplish similar results. The components are held in a molten salt that introduces carbon into the metal. Carbon is diffused inwards producing a hardened case by rapid quenching. The case produced by carbon diffusion is similar to that produced by gas carburizing. Cases formed by liquid carburizing have low nitrogen and high carbon content.