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Everything You Need to Know About Carburizing and Carbonitriding

3:08 Min read time
FEBRUARY 12, 2025
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Surface hardening of metal components is often necessary to ensure their performance and longevity. Carburizing and carbonitriding are two processes often used to increase the surface hardness of metals. This blog will provide what you need to know about these processes to assist you in choosing the heat treatment that is most appropriate for your product application.

What is Carburizing?

The hardness a steel can achieve is dependent on the carbon content – the higher the carbon content, the harder the metal. One way to increase the carbon content on the surface, and therefore surface hardness, is through carburization.

Carburizing is a heat treatment process that diffuses carbon into the surface layer of a steel part, typically low carbon and alloy steels. This increases the surface hardness while maintaining a ductile core.

The carburizing process requires heating the steel in a furnace with a controlled carbon load. The depth of carbon penetration is determined by the temperature, carbon potential in the atmosphere, metal’s chemical composition, and duration of high-temperature exposure. The process usually includes a quench and temper phase to achieve the desired hardness.

Types of Carburizing

Several methods of carburizing may be used. They differ in how the carbon load is added.

  • Liquid Carburizing — The steel is heated in molten salt, generally barium cyanide, which introduces carbon into the process.
  • Gas Carburizing— The steel is heated in a gas containing a controlled carbon content. This method allows for precise control of the carbon level.
  • Pack Carburizing— The steel is sealed with a carbonaceous material in a gas-tight container and heated.

Of these, gas carburizing is the most commonly used method because of its precision and homogeneous depth control. It is used in both atmosphere and low-pressure vacuum carburizing.

What is Carbonitriding?

Carbonitriding is a modification of the gas carburizing process, where in addition to carbon, nitrogen is added to the furnace atmosphere in the form of ammonia gas. It is performed at a slightly lower temperature than carburizing. Like with carburization, a phase change in the steel’s crystal structure allows the carbon and nitrogen atoms to diffuse into the part. The addition of nitrogen further enhances the surface hardness and wear resistance of the steel.

Benefits and Limitations of Carburizing and Carbonitriding

Both carburizing and carbonitriding improve steel parts by increasing hardness, wear resistance, and fatigue strength. These processes provide a cost-effective way to enhance low-carbon steels while allowing for customization in hardness and case depth based on application needs. However, they each have distinct advantages and drawbacks. Understanding these can help engineers select the appropriate process for a given application.

Carburizing Benefits

  • Deep Case Hardening – Produces a hardened surface layer up to 5 mm deep, making it suitable for parts exposed to heavy wear and impact.
  • Strong Core Toughness – The process allows for a tough, ductile core with a hard outer shell, improving fatigue resistance.
  • Versatile Application – Can be used on medium- and low-carbon steels to create wear-resistant surfaces.
  • Good Fatigue Resistance – The gradual carbon diffusion and heat treatment improve resistance to cracking under cyclic loads.
  • Customizable Hardness – By adjusting temperature, time, and carbon potential, hardness and case depth can be controlled.

Limitations of Carburizing

  • Longer Cycle Time – The process can take hours, especially for deeper case depths, leading to slower production rates.
  • Risk of Distortion – The high heat and quenching process may cause warping, requiring post-process machining or grinding.
  • Limited Suitability for Thin or Small Parts – Due to deeper case penetration, carburizing is not ideal for small, delicate components.

Carbonitriding Benefits

  • Higher Surface Hardness (Up to 70 HRC) – The addition of nitrogen creates a harder surface compared to carburizing, improving wear resistance.
  • Lower Process Temperature (775–900°C) – Reduces energy consumption and minimizes distortion risks compared to carburizing.
  • Shorter Processing Time – The case hardening layer develops faster, improving production efficiency for high-volume manufacturing.
  • Improved Wear and Corrosion Resistance – The nitrogen-enriched layer enhances resistance to surface wear and mild corrosion.
  • Better Performance on Low-Carbon Steels – Carbonitriding is effective for improving the hardness of inexpensive low-carbon steels.

Limitations of Carbonitriding:

  • Shallow Case Depth (0.1–1 mm) – The hardened layer is thinner than carburized parts, making it unsuitable for high-impact applications.  
  • Slightly Brittle Surface – The nitrogen-enhanced layer is extremely hard but can be more brittle, which increases the risk of chipping under heavy loads.
  • Not as Strong for Fatigue Resistance – The thinner case depth and fine-grained microstructure do not provide the same fatigue resistance as carburizing.
  • Limited to Smaller Parts – Typically used for small precision components, fasteners, and sheet metal parts rather than large structural elements.

What to Include in Heat Treating Specifications for Carburizing and Carbonitriding

When working with a heat treater, it’s essential to provide clear specifications to ensure the final part meets performance requirements. While experienced heat treaters will have expertise in process parameters, they rely on precise specifications to achieve the desired hardness, case depth, and mechanical properties. Specifications should include material grade/composition and prior processing, case depth requirement, surface hardness requirements, core hardness (if applicable), dimensional tolerances, areas requiring masking, and inspection and testing requirements.

Choosing the Right Partner

When looking for a carburizing or carbonitriding partner, choose one with proven experience and expertise in these processes. Selecting the right carburizing partner can significantly impact the performance and durability of your parts. Ensure that your chosen partner can meet your specific needs and deliver high-quality results.

Bluewater Thermal has the expertise to help you select the most effective process and the experience to deliver high-quality products.  Contact us for a quote or to learn more about our processes.

Tell us how we can help

Bluewater Thermal Solutions specializes in all types of heat treating processes including brazing, nitriding, boronizing, carbonitriding & carburizing, and more. We offer a wide array of heat treating and boronizing processes at 11 different locations. To find out how we can serve your induction hardening needs, call us today at 877.990.0050, or request a quote on our website for services.

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