What is Carburizing?
Carburizing is a case hardening process in which carbon diffuses into the surface layer of a steel part at a temperature high enough to change the steel grain structure. This change enables the steel to absorb carbon. The result is a wear-resistant layer that makes carburizing an ideal process in the production of strong, safe metals.
Carburizing is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder.
Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion.
When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.
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).
In some cases, it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process.
Method of Carburization
Carburization of steel involves heat treatment of the metallic surface using a source of carbon. Carburization can be used to increase the surface hardness of low carbon steel. Early carburization used a direct application of charcoal packed around the sample to be treated, but modern techniques use carbon-bearing gases or plasmas.
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 remainder 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 to improve the surface characteristics of various metals, notably stainless steel. The process is environmentally friendly. It also provides an even treatment of components with complex geometry, making it very flexible in terms of component treatment.
The process of carburization works via the diffusion of carbon atoms into the surface layers of metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the carbon atoms diffuse into the crystal structure of the metal and either remain in solution or react with elements in the host metal to form carbides.
If the carbon remains in a solid solution, the steel is then heat-treated to harden it. Both of these mechanisms strengthen the surface of the metal, the former by forming pearlite or martensite, and the latter via the formation of carbides. Both of these materials are hard and resist abrasion.
Gas carburizing is normally carried out at a temperature within the range of 900 to 950 °C.
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.
Types of Carburizing
In the past, depending on the carbon source, there were three types of carburizing methods: solid carburizing, liquid carburizing, and gas carburizing. Charcoal, molten salt, and carbon-containing gases such as natural gas and propane are used accordingly.
There are three types of carburization that are commonly used:
- Gas carburization
- Carburizing liquids
- Solid carburization
All three processes are based on the transformation of austenite into martensite during quenching. The increase in carbon content at the surface must be high enough to result in a martensitic layer with sufficient hardness, typically 700 HV, to provide a wear-resistant surface.
The required carbon content on the surface after diffusion is usually 0.8 to 1.0% C. These processes can be carried out on a variety of carbon steels, alloy steels, and cast irons, in which the carbon content in the mass is a maximum of 0.4% and usually less than 0.25%. Incorrect heat treatment can lead to oxidation or decarburization.
Although it is a relatively slow process, carburizing can be used as a continuous process and is suitable for high volume surface hardening.
1. Gas Carburizing
In gas carburizing, involves heating carbon steel to austenitizing temperature in the presence of a carbon-rich atmosphere. It is common to use a carrier gas, such as endothermic (“Endo”) gas along with hydrocarbon enrichment (natural gas or propane).
The component is held in an oven that contains an atmosphere of methane or propane with a neutral carrier gas, usually a mixture of N2, CO, CO2, H2, and CH4. At the carburizing temperature, methane (or propane) decomposes on the component surface to atomic carbon and hydrogen, with the carbon diffusing into the surface.
The temperature is typically 925 ° C and the carburizing times range from 2 hours for a housing with a depth of 1 mm to a maximum of 36 hours for a housing with a depth of 4 mm. The quenching medium is usually oil, but it can be water, saline, caustic soda, or polymer.
2. Vacuum carburizing
This carburizing process involves a low-pressure, oxygen-free environment. This process uses gaseous hydrocarbons such as methane. Since the environment is free of oxygen, the carburizing temperature can be increased without worrying about oxidation. The higher the temperature, the higher the carbon solubility and diffusion rate, which minimizes the time required for case depth
3. Liquid Carburizing
Liquid carburizing is a process used for case hardening steel or iron parts. The parts are held at a temperature above Ac1 in a molten salt that introduces carbon and nitrogen, or carbon alone, into the metal. Most liquid carburizing baths contain cyanide, which introduces both carbon and nitrogen into the case.
Liquids or cyanides are carburized by placing the component in a salt bath at a temperature of 845 to 955 ° C. The salt is usually a cyanide-chloride-carbonate mixture and is highly toxic. The cyanide salts introduce a small amount of nitrogen into the surface, which further improves its hardness. Although it is the fastest carburizing process, it is only suitable for small batch sizes.
4. Solid Carburizing (Pack Carburizing)
In solid or pack carburizing is a process in which carbon monoxide derived from a solid compound decomposes at the metal surface into nascent carbon and carbon dioxide. Carburizing containers are made of carbon steel, aluminum-coated carbon steel, or iron-nickel-chromium heat-resisting alloys.
The components are surrounded by a carburizing medium and placed in a sealed box. The medium is usually coke or charcoal mixed with barium carbonate. The process is really a process of gas carburization as the CO produced dissociates into CO2 and carbon which diffuse into the surface of the components.
Temperatures are typically 790 to 845 ° C for times of 2 to 36 hours. Packing carburizing is the least sophisticated carburizing process and therefore remains a widely used method.
Carbonitriding is carried out on a similar section of steel, although the carbon content by mass can be 0.4-0.5%. The method is particularly suitable for hardening the surface of components that require a hardened core, such as B. gears and shafts.
Carbonitriding is a modification of gas carburizing in which ammonia is added to the methane or propane and is the source of nitrogen.