Carbonitriding is a widely used thermochemical treatment in the metallurgical industry, designed to enhance the surface properties of steels, making them more resistant to wear and corrosion.
The carbonitriding process involves enriching the metal's surface with carbon and nitrogen, elements that, at high temperatures, diffuse into the material’s structure, altering its physical and mechanical characteristics.
Thanks to the benefits it offers in terms of surface hardness and component durability, carbonitriding has become a popular choice in sectors such as automotive, aerospace, and tool manufacturing. But how exactly does the process work? What advantages does carbonitriding offer compared to other heat treatments?
In this post, we will delve into what carbonitriding is, how it is performed, and why it represents an effective solution for improving the performance of mechanical components.
What Exactly Is "Carbonitriding"?
Carbonitriding is a heat treatment that involves the diffusion of carbon and nitrogen into the surface layers of steel. This process takes place at high temperatures, generally between 700 and 900 °C, in a controlled environment where a gas mixture containing carbon (usually methane or carbon monoxide) and nitrogen (typically in the form of ammonia) is introduced.
The primary goal of carbonitriding is to increase the surface hardness of steel without compromising the toughness and ductility of its internal structure, thereby creating a surface that is more resistant to wear and corrosion.
The carbonitriding process was developed as an evolution of other heat treatment techniques, such as carburizing and nitriding, to meet the growing industrial demand for materials that offer both surface hardness and structural strength. The first experiments in carbonitriding date back to the first half of the 20th century, a period during which the interactions between carbon and nitrogen and their effects on metallic structures began to be better understood.
Over the years, thanks to advancements in furnace technology and atmospheric control systems, carbonitriding has become increasingly efficient, enabling large-scale application in highly specialized industries.
How Does Carbonitriding Work?
Below are the chemical principles and main stages of the carbonitriding process.
Chemical Principles of the Process
Carbonitriding is based on chemical diffusion principles: when steel is exposed to an atmosphere rich in carbon and nitrogen, these elements begin to diffuse into the metal’s surface layers. This phenomenon occurs at high temperatures, typically between 700 and 900 °C, in a controlled environment where gases like ammonia (for nitrogen) and methane or carbon monoxide (for carbon) are used to ensure uniform absorption of these elements.
During carbonitriding, carbon and nitrogen combine with the iron atoms in the crystalline structure of steel, resulting in a harder and more resilient surface.
The increase in hardness is due to the formation of interstitial compounds and nitrocarburides, which reduce surface deformability and enhance wear resistance. The amount of carbon and nitrogen absorbed can be adjusted by modifying process parameters, such as temperature and atmospheric composition, to meet the specific requirements of different components.
Stages of the Carbonitriding Process
The carbonitriding process is divided into several stages, each critically important to ensure the quality of the treatment and the final characteristics of the component. Here are the main stages:
- Material Preparation: Before carbonitriding, the part is meticulously cleaned to remove any traces of oxide, oil, or impurities that could compromise the uniformity of the treatment. This step ensures that carbon and nitrogen can penetrate the metal surface evenly.
- Controlled-Atmosphere Heat Treatment: The part is placed in a special furnace, where it is exposed to an atmosphere rich in carbon- and nitrogen-containing gases. The temperature is raised to between 700 and 900 °C, depending on the type of steel and the desired treatment characteristics. During the heat treatment, carbon and nitrogen diffuse into the metal’s surface, creating a new, more resistant surface layer.
- Cooling Phase: At the end of the heat treatment, the component is rapidly cooled through quenching, or in some cases, cooled more slowly to achieve a specific microstructure. Rapid cooling helps to preserve the achieved surface hardness.
Suitable Materials for Carbonitriding
Carbonitriding is particularly suited for low- and medium-carbon steels, which benefit most from the surface enrichment of carbon and nitrogen. Common materials subjected to carbonitriding include carbon steels and alloy steels, especially those containing elements such as manganese, chromium and nickel, which further enhance the material's responsiveness to the treatment and optimize the results.
Low-carbon steels (with less than 0.3% carbon) are ideal for this process, as they retain good core toughness while achieving high surface hardness. Alloy steels, such as chromium-molybdenum steels, are especially well-suited for carbonitriding because they allow for a thicker, more resistant surface layer, making them ideal for components that must withstand high loads and wear resistance.
Carbonitriding is not recommended for high-carbon steels or for quenched and tempered steels, as these may not respond well to the treatment or may exhibit excessive brittleness. Some types of cast iron can also undergo carbonitriding, although the process requires specific adjustments to achieve optimal results.
Benefits of Carbonitriding
The primary benefits of the carbonitriding process include:
- Increased Surface Hardness: The process creates a harder, more wear-resistant surface, ideal for components that must endure high friction and stress. This provides high surface hardness without affecting the core toughness.
- Improved Corrosion Resistance: The combination of carbon and nitrogen forms a protective barrier on the metal surface, reducing susceptibility to corrosion. This is advantageous for components exposed to chemicals, moisture, or harsh environmental conditions.
- Extended Durability of Mechanical Components: The surface hardness and corrosion resistance allow components to withstand repetitive stresses and continuous friction. This reduces the need for frequent replacements and maintenance.
- Enhanced Dimensional Stability: Carbonitriding improves the dimensional stability of components, reducing deformation during operation and maintaining precision tolerances over time. This is crucial in fields such as precision mechanical engineering, where component dimensions must remain stable even under high stress.
- Versatility for Various Industrial Applications: The carbonitriding process is applicable across diverse industries, including automotive, aerospace, and tool manufacturing.
- Process Efficiency: Compared to other heat treatments, carbonitriding can be performed at lower temperatures and with faster processing cycles, reducing energy consumption and operating costs.
Difference between Carbonitriding, Cementation and Nitriding
Carbonitriding, carburizing and nitriding are among the most widely used methods in industry to increase the strength and durability of steel components. Let's see below the differences between various processes.
Carbonitriding vs. Carburizing
Carbonitriding and carburizing are similar processes, as both involve introducing carbon into the steel surface to increase surface hardness. However, they have several key differences:
- Elements Introduced: carbonitriding adds both carbon and nitrogen to the metal surface, while carburizing introduces only carbon.
- Process Temperature: carbonitriding occurs at slightly lower temperatures, typically between 700 and 900 °C, whereas carburizing requires higher temperatures, reaching up to around 950 °C.
- Ideal Applications: carbonitriding is often preferred for smaller components subject to high wear, such as gears and screws. Carburizing, on the other hand, is commonly used for parts requiring a greater depth of surface hardening, such as shafts and bearings.
Carbonitriding vs. Nitriding
Nitriding is another thermochemical treatment that introduces only nitrogen into the metal surface, without adding carbon. Here are the main differences compared to carbonitriding:
- Elements Introduced: While carbonitriding introduces both carbon and nitrogen, nitriding only adds nitrogen. This gives nitriding a superior corrosion resistance but lacks the surface hardness increase typical of carbonitriding.
- Process Temperature and Duration: Nitriding occurs at lower temperatures, typically between 500 and 600 °C, and is a longer process, often lasting several hours.
- Surface Structure: Nitriding produces a very thin, hard surface layer, ideal for components that need to resist surface wear but do not require deep hardening. Carbonitriding, with its combination of carbon and nitrogen, creates a harder and slightly deeper surface layer.