Carburizing: Types, Principle, Applications

Carburizing, also known as Case Hardening, is a heat treatment process that improves the surface properties of steel components. By diffusing carbon into the surface, carburizing creates a hardened case while maintaining the toughness and strength of the core. This article explores the benefits, process, and applications of carburizing in various industries.

Understanding Carburizing

Carburizing is a heat treatment technique used primarily on low carbon steel parts after machining. It is also applied to high alloy steel bearings, gears, and other components. The process involves increasing the strength and wear resistance of the steel by introducing carbon into its surface.

The process of carburizing involves heating iron or steel to a high temperature, causing a change in the grain structure of the metal. This change allows the steel to absorb carbon, resulting in the formation of a wear-resistant layer. Carburizing is widely used in the production of strong and safe metals.

During carburizing, the metal is heated in the presence of a carbon-bearing material like charcoal or carbon monoxide. The goal is to increase the hardness of the metal. The carbon content in the affected area can vary depending on the duration and temperature of the process. Generally, longer carburizing times and higher temperatures lead to greater carbon diffusion and deeper carbon penetration.

After the carburizing process, the iron or steel is rapidly cooled through quenching. This causes the outer surface, which has a higher carbon content, to become hard as it transforms from austenite to martensite. Meanwhile, the core of the metal remains soft and tough, maintaining a ferritic and/or pearlite microstructure.

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Key characteristics of carburizing include:

  1. Its application to low-carbon workpieces,
  2. The use of high-carbon gases, liquids, or solids in contact with the workpieces,
  3. The production of a hard surface while retaining toughness and ductility in the core, and
  4. The ability to achieve case hardness depths of up to 0.25 inches (6.4 mm).

Types Of Carburizing

Carburizing is a heat treatment process used to increase the carbon content on the surface of a metal object, typically steel, to improve its hardness and wear resistance. There are several types of carburizing techniques commonly used:

  1. Gas Carburizing: This is the most widely used method of carburizing. It involves heating the metal object in a furnace with a carbon-rich gas atmosphere, such as methane or propane. The carbon atoms from the gas diffuse into the surface of the metal, forming a high-carbon layer.
  2. Liquid Carburizing: Also known as cyaniding, this method involves immersing the metal object in a liquid bath containing a carbon-rich compound, such as sodium cyanide. The metal is heated in the bath, allowing carbon atoms to diffuse into its surface.
  3. Pack Carburizing: In this method, the metal object is packed in a container with a carbon-rich material, such as charcoal or bone charcoal. The container is then heated to a high temperature, causing carbon atoms to diffuse into the metal surface.
  4. Vacuum Carburizing: This technique involves placing the metal object in a vacuum chamber and introducing a carbon-rich gas. The chamber is then heated to a high temperature, allowing carbon atoms to diffuse into the metal surface. Vacuum carburizing offers precise control over the carburizing process and is often used for high-quality, complex-shaped parts.
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These processes rely on the conversion of austenite to martensite during the cooling process. To achieve a durable surface that is resistant to wear, it is necessary to have a significant increase in carbon content at the surface. Typically, the martensitic layer should have a hardness of around 700 HV.

The desired carbon content on the surface, after the diffusion process, is typically between 0.8% and 1.0% C. These procedures can be applied to various types of carbon steels, alloy steels, and cast irons, as long as the carbon content in the overall material is no more than 0.4% and usually less than 0.25%. If the heat treatment is not done correctly, it can result in oxidation or loss of carbon from the surface (decarburization).

Benefits of Carburizing

Carburizing offers several advantages, including:

  • Enhanced wear resistance: The hardened case formed during carburizing provides excellent resistance against wear, extending the lifespan of the component.
  • Retained toughness: Despite the hardened surface, the core retains its toughness and strength, ensuring the component can withstand heavy loads and impact.
  • Cost-effective: Carburizing allows the use of lower-cost materials that can be easily machined or formed before heat treatment, making it an economical choice for producing complex-shaped parts.
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Carburizing Process

The carburizing process involves heating the components in a controlled environment, either in a pit furnace or a sealed atmosphere furnace. Carburizing gases are introduced at a specific temperature to facilitate carbon diffusion. Gas carburizing offers precise control over temperature and carbon potential, ensuring the desired case depth.

Time/Temperature Control

Carburizing is a time/temperature process. The duration of the carburizing atmosphere exposure determines the depth of the case. By adjusting the carbon potential of the gas, excess carbon in the surface layer can be avoided, allowing for optimal diffusion.

Quenching and Tempering

After carburizing, the workpiece is either slow cooled for subsequent quench hardening or directly quenched into oil. The choice of quenching method depends on achieving the desired properties while minimizing dimensional changes. Hot oil quenching is suitable for minimizing distortion, while press quenching is used for maintaining dimensional tolerances in bearing races.

Cryogenic Processing

In certain cases, the carburized product may undergo tempering followed by cryogenic processing. Cryogenic treatment converts retained austenite to martensite, further enhancing the hardness and stability of the component. Subsequent retempering ensures the desired balance of hardness and toughness.

Conclusion

Carburizing is a valuable heat treatment process that improves the wear resistance and strength of steel components. By creating a hardened case while preserving the core’s toughness, carburizing allows for the production of intricate and complex-shaped parts using cost-effective materials. Gas carburizing offers precise control over the process, ensuring optimal results.