What is Carburizing?
Carburizing also referred to as Case Hardening, is a heat treatment process that produces a surface that is resistant to wear, while maintaining the toughness and strength of the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components.
Carburizing is a thermochemical process in which carbon diffuses into the surface of low carbon steels to increase the carbon content to a sufficient level so that the surface reacts to the heat treatment and creates a hard, wear-resistant layer.
Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core. This treatment is applied to low carbon steels after machining.
Strong and very hard-surface parts of intricate and complex shapes can be made of relatively lower cost materials that are readily machined or formed prior to heat treatment.
Most carburizing is done by heating components in either a pit furnace, or sealed atmosphere furnace, and introducing carburizing gases at temperature. Gas carburizing allows for accurate control of both the process temperature and carburizing atmosphere (carbon potential).
Carburizing is a time/temperature process; the carburizing atmosphere is introduced into the furnace for the required time to ensure the correct depth of case. The carbon potential of the gas can be lowered to permit diffusion, avoiding excess carbon in the surface layer.
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.
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.
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
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.
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.
Carburizing, also referred to as Case Hardening, is a heat treatment process that produces a surface which is resistant to wear, while maintaining toughness and strength of the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components.
Carburizing is a thermochemical process in which carbon is diffused into the surface of low carbon steels to increase the carbon content to sufficient levels so that the surface will respond to heat treatment and produce a hard, wear-resistant layer.
Historically, there are three types of carburizing methods depending on the carbon source: solid carburizing, liquid carburizing, and gas carburizing. Charcoal, molten salt, and carbon-bearing gases, such as natural gas and propane, are used correspondingly.
Types of Carburizing
1. Pack Carburizing
2. Gas Carburizing
3. Vacuum Carburizing
4. Liquid Carburizing