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Carburizing

Carburizing, also known as carburization, is a heat treatment process in which iron or steel is heated in the presence of another material (but below the metal's melting point) which liberates carbon as it decomposes. The outer surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard, while the core remains soft and tough.[1]

This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm).

Method
Carburization of steel involves a heat treatment of the metallic surface using a gaseous, liquid, solid or plasma source of carbon. Early carburization used a direct application of charcoal packed onto the metal (initially referred to as case hardening or Kolsterising), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.

Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma can penetrate into holes and tight gaps), making it very flexible in terms of component treatment.

The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix — this normally occurs at lower temperatures) or react with the host metal to form ceramic carbides (normally at higher temperatures, due to the higher mobility of the host metal's atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst imparting its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.

In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process.

A main goal when producing carbonized workpieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with fire clay.

Hardening agents
There are different types of elements or materials that can be used to perform this process, but these mainly consist of high carbon content material. A few typical hardening agents include carbon monoxide gas(CO), sodium cyanide and barium chloride, or hardwood charcoal. In gas carburizing, the CO is given off by propane or natural gas. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide(NaCN) and barium chloride(BaCl). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal。

碳化

   1、又称干馏(dry distillation)。固体燃料的热化学加工方法。将煤、木材、油页岩等在隔绝空气下加热分解为气体(煤气)、液体(焦油)和固体(焦炭)产物，焦油蒸气随煤气从焦炉逸出，可以回收利用，焦炭则由焦炉内推出。
　　2、有机化合物在隔绝空气下热分解为碳和其他产物，以及用强吸水剂(浓硫酸)将含碳、氢、氧的化合物(如糖类)脱水而成炭的作用也称碳化。碳化时氢氧原子脱去比例为2：1。
　　3、碳化同炭化，是指生物质在缺氧或贫氧条件下，以制备相应的炭材为目的的一种热解技术．其过程与生物质，木纤维，木质素的分解同步．