What is the Difference Between Carbonitriding and Nitrocarburizing?

Author by:Maijin Metal - CNC Machining Parts Manufacturer & Supplier In China

Carbonitriding and nitrocarburizing are two commonly used processes in the heat treatment of steel. While they both involve the introduction of nitrogen and carbon into the surface of the metal to improve hardness, wear resistance, and fatigue strength, there are distinct differences between the two. Understanding these differences is crucial for selecting the right process for a specific application and achieving the desired properties in the treated component. In this article, we will explore the dissimilarities between carbonitriding and nitrocarburizing, their respective benefits and drawbacks, and the applications for which each process is best suited.

Carbonitriding

Carbonitriding is a case-hardening process that involves introducing a combination of carbon and nitrogen into the surface of steel. This is typically achieved by exposing the steel to a carbon-rich atmosphere containing a small amount of ammonia gas at elevated temperatures. The carbon enrichment in the surface layer of the steel enhances its hardness and wear resistance, while the nitrogen improves the fatigue strength and corrosion resistance.

One of the key advantages of carbonitriding is its ability to produce a hardened case with a high level of toughness. This is particularly beneficial for components subjected to impact and cyclic loading, such as gears, shafts, and other transmission elements. The process can be applied to a wide range of steel types, including low-carbon and medium-carbon steels, making it versatile for various industrial applications.

Carbonitriding is also known for its ability to achieve deep case depths, which is important for components that require a substantial increase in surface hardness. By controlling the process parameters, such as time, temperature, and gas composition, it is possible to achieve case depths ranging from 0.1 to 2 millimeters, depending on the specific requirements of the application.

Another advantage of carbonitriding is its relatively low process temperature compared to other case-hardening methods, such as carburizing. This reduces the risk of distortion and cracking in the treated components, making it suitable for parts with complex geometries and tight dimensional tolerances. Additionally, the lower process temperature contributes to energy savings and improved environmental performance.

However, carbonitriding also has some limitations. For instance, it is not suitable for steels with a high percentage of alloying elements, as these materials may form unwanted compounds during the process, leading to surface quality issues and reduced effectiveness of the treatment. Additionally, the presence of nitrogen in the steel matrix can affect its machinability, which should be taken into consideration during component design and manufacturing.

Nitrocarburizing

Nitrocarburizing, also known as ferritic nitrocarburizing (FNC) or gas nitriding, is a case-hardening process that involves the diffusion of nitrogen and carbon into the surface of steel. Unlike carbonitriding, nitrocarburizing is carried out in a nitrogen-rich atmosphere, typically with the addition of a small amount of carbon-containing gas, such as methane or propane, at elevated temperatures. The nitrogen diffusion forms a compound layer on the surface of the steel, while the carbon infiltration enhances the hardness and wear resistance.

One of the primary advantages of nitrocarburizing is its ability to produce a compound layer with excellent wear resistance, corrosion resistance, and anti-galling properties. This makes it well-suited for components exposed to abrasive and sliding wear, such as machine components, injection molds, and cutting tools. The compound layer, consisting of iron nitride and iron carbonitride, has a microstructure that significantly improves the surface properties of the treated steel.

Another benefit of nitrocarburizing is its ability to achieve a shallow case depth with a high level of hardness, typically ranging from 0.1 to 0.5 millimeters. This is particularly advantageous for components that require precise dimensional control and minimal distortion, as the treatment does not affect the core properties of the steel to a significant extent.

Nitrocarburizing is also known for its ability to enhance the fatigue strength and galling resistance of steel components, making it suitable for applications where the avoidance of surface damage and premature failure is critical. The process can be applied to a wide range of steel types, including alloy steels, tool steels, and stainless steels, making it versatile for various industrial sectors.

However, nitrocarburizing also has some limitations. For example, the process can induce hydrogen embrittlement in certain steels, particularly high-strength and high-alloy materials, which can compromise their mechanical properties and performance. Additionally, the treatment may alter the dimensional tolerances of the components due to the formation of the compound layer, requiring post-treatment machining or grinding to achieve the desired fit and finish.

Comparison of Properties

While carbonitriding and nitrocarburizing both aim to improve the surface properties of steel through the introduction of carbon and nitrogen, there are notable differences in the characteristics of the treated surfaces and the resulting properties. Understanding these distinctions is crucial for selecting the most suitable process for a specific application and ensuring that the desired performance requirements are met.

In carbonitriding, the diffusion of carbon and nitrogen results in the formation of a hardened case with a complex microstructure, including a diffusion zone enriched with carbon and nitrogen and a transition zone with varying concentrations of these elements. This structure contributes to the improved toughness, wear resistance, and fatigue strength of the treated components, making it well-suited for applications subjected to impact and cyclic loading.

On the other hand, nitrocarburizing forms a compound layer on the surface of the steel, consisting primarily of iron nitride and iron carbonitride, with a diffusion zone beneath it. This compound layer provides excellent wear resistance, corrosion resistance, and anti-galling properties, making it suitable for components exposed to abrasive and sliding wear.

The case depth achieved through carbonitriding is typically deeper compared to nitrocarburizing, ranging from 0.1 to 2 millimeters, depending on the process parameters and steel composition. This is beneficial for components that require a substantial increase in surface hardness and wear resistance, as well as improved fatigue strength.

In contrast, nitrocarburized components typically have a shallower case depth, ranging from 0.1 to 0.5 millimeters, while maintaining a high level of hardness and wear resistance. This is advantageous for applications requiring precise dimensional control and minimal distortion, as well as enhanced fatigue strength and galling resistance.

Process Comparison and Selection

When selecting between carbonitriding and nitrocarburizing for a specific application, several factors should be considered to ensure that the chosen process aligns with the desired properties and performance requirements of the treated components.

Firstly, the type of steel and its alloying elements play a crucial role in determining the suitability of the process. Carbonitriding is well-suited for low-carbon and medium-carbon steels, while nitrocarburizing can be applied to a wider range of steel types, including alloy steels, tool steels, and stainless steels. If the steel composition includes a high percentage of alloying elements, nitrocarburizing may be preferred to avoid surface quality issues and maintain the effectiveness of the treatment.

Secondly, the required case depth and the specific surface properties needed for the application should be taken into consideration. Carbonitriding is capable of achieving deeper case depths, making it suitable for components that require a substantial increase in surface hardness and wear resistance, as well as enhanced fatigue strength. Nitrocarburizing, on the other hand, produces a shallower case depth with excellent wear resistance, corrosion resistance, and anti-galling properties, making it suitable for components subjected to abrasive and sliding wear.

Additionally, the process temperature and the risk of distortion and cracking should be evaluated when selecting between carbonitriding and nitrocarburizing. Carbonitriding operates at a lower process temperature compared to nitrocarburizing, reducing the risk of undesirable dimensional changes in the treated components. This makes it suitable for parts with complex geometries and tight dimensional tolerances, as well as energy-efficient and environmentally-friendly.

In contrast, nitrocarburizing may pose a higher risk of hydrogen embrittlement and dimensional changes, requiring careful consideration of the steel type and post-treatment machining or grinding to achieve the desired fit and finish. However, the process excels in producing a compound layer with excellent surface properties, making it valuable for components exposed to abrasive and sliding wear.

Conclusion

In conclusion, carbonitriding and nitrocarburizing are two distinct processes used in the heat treatment of steel to improve the surface properties of components, such as hardness, wear resistance, and fatigue strength. While both processes involve the introduction of carbon and nitrogen into the surface of the steel, they differ in their case depth, surface characteristics, and suitability for specific steel types and applications.

Carbonitriding is known for its ability to produce a deep, toughened case with enhanced wear resistance and fatigue strength, making it suitable for components subjected to impact and cyclic loading. On the other hand, nitrocarburizing forms a compound layer with excellent wear resistance, corrosion resistance, and anti-galling properties, making it well-suited for components exposed to abrasive and sliding wear.

When selecting between carbonitriding and nitrocarburizing for a specific application, factors such as the type of steel, required case depth, surface properties, process temperature, and dimensional stability should be carefully evaluated to ensure that the chosen process aligns with the desired performance requirements of the treated components. By understanding the differences and capabilities of these processes, it is possible to make informed decisions and achieve the desired surface properties for various industrial applications.