Principle of carburizing

A chemical heat treatment process in which a steel workpiece is heated to high temperatures in a carbon-containing medium to increase the carbon content in its surface layer. The material of carburized workpieces is generally low-carbon steel or low-carbon alloy steel (with a carbon content of less than 0.25%). After carburizing, the chemical composition of the steel surface can approach that of high-carbon steel. After carburizing, the workpiece must also be quenched in order to achieve high surface hardness, high wear resistance, and fatigue strength, while preserving the strength and toughness of the low-carbon steel core after quenching, so that the workpiece can withstand impact loads. The carburizing process is widely used for mechanical parts in aircraft, automobiles, and tractors, such as gears, shafts, and camshafts. Like other chemical heat treatments, carburizing also involves 3 basic processes. ①Decomposition: The decomposition of the carburizing medium produces active carbon atoms. ②Adsorption: Once the active carbon atoms are absorbed by the surface of the steel component, they dissolve into the austenite in the surface layer, increasing the carbon content in that austenite. ③Diffusion: An increase in the carbon content on the surface creates a concentration difference with that in the core, causing carbon to diffuse from the surface toward the interior. The diffusion rate of carbon in steel mainly depends on temperature, and is also related to the concentration difference between the element being diffused and its surroundings within the workpiece, as well as the content of alloying elements in the steel.

Carburizing and hardening is one of the surface hardening methods, belonging to the category of chemical surface hardening. Carburizing first creates primary carbon on the surface of the steel, which is then allowed to penetrate into the surface layer of the steel and gradually spread inward. The carbon in primary ecosystems is obtained from the decomposition of gases such as CO or CH4. CO can originate from gases containing CO, or be produced within the carburizing vessel as a result of reactions with solid carburizing agents, or it can come from salt baths containing cyanides. When the primary carbon diffuses from the surface of steel into its interior, the temperature of the steel must be raised to the range of the austenitizing temperature, so as to facilitate the diffusion of this primary carbon. This is because austenite can dissolve a larger amount of ’C’, whereas ferrite has very limited dissolving capacity; therefore, the carburization temperature must be above the Ac3 temperature. so that carburization can take place. Combined with various heat treatment methods, this results in the formation of a high-carbon hardened core and a low-carbon, low-hardness layer in the steel. This treatment gives the material a hard and wear-resistant surface, as well as a tough and impact-resistant core.

Carburizing refers to the process of allowing carbon atoms to penetrate into the surface layer of steel. It also enables the workpiece made of low-carbon steel to have a surface layer similar to that of high-carbon steel; through quenching and low-temperature tempering, this surface layer acquires high hardness and wear resistance, while the core part of the workpiece retains the toughness and ductility of low-carbon steel.