If you’re shopping for metal parts and components, it’s important to understand the various processes used in their manufacture. Carburizing is one of the most common heat treatment methods used in industrial steel production today. It involves introducing carbon during a heating process to increase its wear resistance and strength. In this blog post, we’ll explore what carburizing is and the different types of carburization techniques that are available as well as look at how exactly these procedures are carried out. We’ll also delve into some of the reasons why carburizing has become so popular among manufacturers over recent years and some potential applications of this vital heat-treating method.
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What is Carburizing?
Carburizing is a case hardening process in which carbon diffuses into the surface layer of a steel part at a high enough temperature to change the grain structure of the steel. This modification allows the steel to absorb carbon. As a result, a wear-resistant layer is formed, making carburizing an ideal process for producing strong, safe metals.
Carburize is a heat treatment process that involves heating iron or steel in the presence of carbon-containing material, such as charcoal or carbon monoxide, to absorb carbon. The goal is to harden the metal.
The carbon content of the affected area can vary depending on the amount of time and temperature. Carbon diffusion depth is typically increased by longer carburizing times and higher temperatures.
When iron or steel is quenched, the higher carbon content on the outer surface hardens 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 used on low-carbon workpieces.
- A high-carbon gas, liquid, or solid comes into contact with the workpieces.
- It results in a hard workpiece surface; workpiece cores retain much of their toughness and ductility; and
- It can generate case hardness depths of up to 0.25 inches (6.4 mm).
In some cases, it can be used to correct undesired decarburization that occurred earlier in the manufacturing process.
Method of Carburization
Steel carburization is the heat treatment of the metallic surface with a carbon source. The surface hardness of low-carbon steel can be increased through carburization. Earlier carburization techniques used direct application of charcoal packed around the sample to be treated, whereas modern techniques use carbon-bearing gases or plasmas.
The process is primarily dependent on the composition of the ambient gas and the furnace temperature, which must be carefully controlled because heat can affect the microstructure of the remaining material. Carburization can take place in a vacuum chamber at very low pressures for applications requiring precise control over gas composition.
Plasma carburization is being used more and more to improve the surface properties of various metals, particularly stainless steel. The procedure is safe for the environment. It also provides consistent treatment of components with complex geometry, making it extremely versatile in terms of component treatment.
Carburization is accomplished through the diffusion of carbon atoms into metal surface layers. Because metals are composed of atoms tightly packed into a metallic crystalline lattice, carbon atoms diffuse into the metal’s crystal structure 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 hardened by heat treatment. Both of these mechanisms strengthen the metal’s surface, the former by forming pearlite or martensite and the latter by forming carbides. Both of these materials are tough and abrasion-resistant.
Types of Carburizing
Previously, there were three types of carburizing methods depending on the carbon source: solid carburizing, liquid carburizing, and gas carburizing. In this case, charcoal, molten salt, and carbon-containing gases such as natural gas and propane are used.
Gas Carburizing
In gas carburizing, carbon steel is heated to an austenitizing temperature in the presence of a carbon-rich atmosphere. A carrier gas, such as endothermic (“Endo”) gas, is commonly used in conjunction with hydrocarbon enrichment (natural gas or propane).
The component is kept in an oven that has a methane or propane atmosphere with a neutral carrier gas, which is usually a mixture of N2, CO, CO2, H2, and CH4. Methane (or propane) decomposes on the component surface to atomic carbon and hydrogen at the carburizing temperature, with the carbon diffusing into the surface.
The temperature is typically 925 ° C, and the carburizing times range from 2 hours for a 1 mm depth housing to a maximum of 36 hours for a 4 mm depth housing. Oil is the most common quenching medium, but water, saline, caustic soda, or polymer can also be used.
Vacuum Carburizing
This carburizing process takes place in an oxygen-free, low-pressure environment. This method employs gaseous hydrocarbons such as methane. Because the environment is oxygen-free, the carburizing temperature can be raised without fear of oxidation. The higher the temperature, the greater the carbon solubility and rate of diffusion, reducing the time required for case depth.
Liquid Carburizing
Liquid carburizing is a method of hardening steel or iron parts. The components are kept at a temperature higher than Ac1 in a molten salt that introduces carbon and nitrogen, or just carbon, into the metal. Cyanide is commonly found in liquid carburizing baths, which introduces both carbon and nitrogen into the case. Carburization of liquids or cyanides is accomplished by immersing the component in a salt bath at temperatures ranging from 845 to 955 ° C. The salt is usually a toxic cyanide-chloride-carbonate mixture. The cyanide salts introduce a trace amount of nitrogen into the surface, increasing its hardness. Although it is the quickest carburizing method, it is only appropriate for small batch sizes.
Solid Carburizing (Pack Carburizing)
Solid Carburizing is the process by 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 heat-resistant iron-nickel-chromium alloys.
The components are sealed in a box and surrounded by a carburizing medium. Typically, coke or charcoal is mixed with barium carbonate as the medium. As the CO produced dissociates into CO2 and carbon, which diffuse into the surface of the components, the process is a gas carburization process.
