Differences Between Carburizing, Nitriding, And Carbonitriding

1. Introduction

In the field of metal material heat treatment, surface treatment technologies play a crucial role in enhancing the performance of components. As three important chemical heat treatment processes, carburizing, nitriding, and carbonitriding are widely applied in numerous industries such as machinery manufacturing, automotive industry, and aerospace. By modifying the chemical composition and microstructure of the metal surface, they endow components with distinct performance characteristics to meet the requirements of various complex working conditions. A thorough understanding of the differences among these three processes is conducive to their rational selection and application in actual production, thereby optimizing component performance, improving product quality, and reducing production costs.

2. Differences in Process Principles

2.1 Carburizing

Carburizing is a process where components are placed in a carburizing medium, and at a high temperature (typically 900 - 950°C), carbon atoms diffuse into the surface of the components to form a high - carbon surface layer. There are various types of carburizing media. Common ones include gas carburizing (e.g., drop - feed gas carburizing, which uses organic liquids such as methanol and acetone dripped into the furnace to decompose and generate carbon - containing gases), liquid carburizing (salt bath carburizing, which employs a mixture of carbon - containing salts as the medium), and solid carburizing (where components are embedded in carbon - containing solid powders). During the carburizing process, carbon atoms diffuse from the carburizing medium with a high carbon concentration to the surface of the components and then further into the interior of the components. Eventually, a high - carbon layer with a certain depth and carbon concentration gradient is formed on the component surface.

Objective: Increase the carbon content on the surface. After quenching and low - temperature tempering, the surface becomes hard while the core remains tough.
Suitable Steel Grades: Low - carbon steels and low - carbon alloy steels, such as 20, 20Cr, and 20CrMnTi.
Process Parameters: At 900 - 950°C, a carburizing layer thickness of 0.5mm can be achieved in 1 hour, and 1mm in 4 hours. The carburizing layer thickness is measured from the surface to the mid - point of the transition layer, ranging from 0.5 to 2.5mm.
Heat Treatment After Carburizing: Quenching (direct quenching, single quenching, and double quenching) followed by low - temperature tempering.
Microstructure and Properties:

Surface Layer: High - carbon tempered martensite (M) + carbides + retained austenite (A').
Core: Low - carbon tempered martensite (M) or ferrite (F) and pearlite (T).
Properties: The surface has high hardness, ranging from 58HRC to 64HRC, and good wear resistance. The core has good toughness, relatively low hardness, and high fatigue strength, along with surface compressive stress.

2.2 Nitriding

Nitriding, also known as nitridation, is a process in which nitrogen atoms diffuse into the surface of components at a certain temperature (generally 500 - 560°C) to form nitrogen - containing compounds. Common nitriding methods include gas nitriding (using ammonia as the nitrogen source, which decomposes to produce active nitrogen atoms at high temperatures) and ion nitriding (utilizing ion bombardment on the component surface generated by glow discharge to accelerate the diffusion of nitrogen atoms). Nitrogen atoms react chemically with metal elements (such as iron, aluminum, and chromium) on the component surface to form nitride layers with high hardness, good wear resistance, and strong corrosion resistance, such as Fe₃N, Fe₄N, and CrN.

Objective: Improve surface hardness, wear resistance, fatigue strength, hot hardness, and corrosion resistance.
Process Reaction (Gas Nitriding): 2NH₃ → 3H₂ + 2[N] (above 200°C). Catalysts (such as benzene, aniline, and ammonium chloride) can increase the reaction rate by 0.3 - 3 times.
Characteristics:

The nitriding temperature is low, ranging from 500 to 600°C.
The treatment time is long, usually 20 - 50 hours, and the thickness of the nitrided layer is 0.3 - 0.5mm.
The steel components must undergo quenching and tempering treatment before nitriding.

Microstructure:

Surface Layer: White ε (Fe₂N) or γ' (Fe₄N) phase.
Middle Layer: Dark - colored nitrogen - containing eutectoid (α + γ').
Core: Tempered sorbite (S).

2.3 Carbonitriding

Carbonitriding is a chemical heat treatment process that involves the simultaneous diffusion of carbon and nitrogen into the same diffusion layer, also known as cyaniding. Based on the treatment temperature, it can be divided into low - temperature carbonitriding (500 - 600°C, also called soft nitriding), medium - temperature carbonitriding (700 - 850°C), and high - temperature carbonitriding (900 - 950°C, similar to the carburizing temperature). Taking gas carbonitriding as an example, a gas mixture containing carbon and nitrogen (such as ammonia, kerosene, and methanol) is usually used as the medium. At high temperatures, carbon and nitrogen atoms simultaneously diffuse into the component surface to form a composite diffusion layer containing carbides and nitrides.
The following table provides a comparison of several surface heat treatment and chemical heat treatment processes:

Heat Treatment Method Treatment Process Production Cycle Surface Layer Depth (mm) Hardness (HRC) Wear Resistance Fatigue Strength Corrosion Resistance Deformation After Heat Treatment Application Examples
Surface Quenching Surface heating quenching + low - temperature tempering Very short, a few seconds to several minutes 0.5 - 7.0 55 - 58 Good Good General Small Machine tool gears, crankshafts
Carburizing Carburizing + quenching + low - temperature tempering Long, approximately 3 - 9 hours 0.5 - 2.0 60 - 65 (1000HV - 1100HV) Good Better General Large Claw - type clutches, automotive gears, brake cams
Nitriding Quenching and tempering + nitriding Very long, approximately 20 - 50 hours 0.3 - 0.5 65 - 70 Best Best Best Minimal Oil pump gears, lead screws, precision machine tool spindles
Carbonitriding Carbonitriding + quenching + low - temperature tempering Short, approximately 1 - 2 hours 0.2 - 0.5 58 - 63 Good Good Better Small -

3. Differences in Treatment Temperature and Time
3.1 Carburizing

The carburizing temperature is relatively high, generally ranging from 900 to 950°C. The high temperature is beneficial for the diffusion of carbon atoms, enabling the rapid formation of a relatively thick carburizing layer. However, high temperatures can also cause grain growth in the components, reducing the toughness of the material. The treatment time depends on the size of the components, the required carburizing layer depth, and the type of carburizing medium, usually taking several hours or even dozens of hours. For example, for components requiring a carburizing layer depth of 0.8 - 1.2mm, the treatment in a gas carburizing furnace may take 8 - 12 hours.

3.2 Nitriding

The nitriding temperature is relatively low, typically in the range of 500 - 560°C. The low temperature can prevent overheating and grain growth of the components during the treatment process, maintaining the stability of the microstructure and properties of the component core. However, the nitriding rate is slow, and the treatment time is long, generally taking dozens of hours or even hundreds of hours. For instance, for some components requiring high surface hardness and wear resistance, the gas nitriding treatment time may reach 70 - 100 hours.

3.3 Carbonitriding

The temperature of low - temperature carbonitriding is 500 - 600°C, and the treatment time is relatively short, generally about 1 - 3 hours, which can quickly form a diffusion layer with high hardness, good wear resistance, and anti - seizing property on the component surface. The temperature of medium - temperature carbonitriding is 700 - 850°C, and the treatment time varies according to the component requirements, usually 2 - 6 hours. The temperature of high - temperature carbonitriding is similar to that of carburizing, and the treatment time is also comparable to that of carburizing, but it can achieve the effect of simultaneous diffusion of carbon and nitrogen.

4. Comparison of Diffusion Layer Microstructure and Performance Characteristics
4.1 Carburizing

After carburizing, a high - carbon martensite structure is formed on the component surface, which has relatively high hardness and wear resistance, with a hardness of HV550 - 750. The depth of the carburizing layer varies according to the working requirements of the components, generally ranging from 0.5 to 2.5mm. The core structure is usually low - carbon martensite or ferrite plus pearlite, which has good toughness and strength and can withstand large impact loads. Carburized components are suitable for working conditions subject to alternating stress, friction, and wear, such as gears and shaft - like components.

4.2 Nitriding

The nitrided layer is mainly composed of ε phase (Fe₃N), γ' phase (Fe₄N), and a compound layer, which has extremely high hardness and wear resistance, with a surface hardness of HV1000 - 1200. Meanwhile, the nitrided layer also has good corrosion resistance and anti - seizing property, enabling it to work in harsh environments. The depth of the nitrided layer is relatively thin, generally ranging from 0.1 to 0.6mm. Nitrided components are suitable for working conditions requiring high precision, high wear resistance, and corrosion resistance, such as molds and precision shaft - like components.

4.3 Carbonitriding

After low - temperature carbonitriding, a composite structure of nitrogen - containing carbides and ε phase is formed on the component surface, which has relatively high hardness (HV800 - 1000), good wear resistance, and anti - seizing property, as well as a certain degree of corrosion resistance. Medium - temperature carbonitriding can obtain a relatively thick diffusion layer, and the structure contains carbides and nitrides, with hardness and wear resistance between those of carburizing and nitriding. The microstructure of the diffusion layer after high - temperature carbonitriding is similar to that of carburizing, but it contains a certain amount of nitrogen, which improves the surface hardness and wear resistance. Carbonitrided components are suitable for working conditions that require a certain degree of hardness, wear resistance, as well as good toughness and anti - seizing property, such as piston pins and valves of automotive engines.

5. Differences in Application Fields
5.1 Carburizing

The carburizing process is widely used in the automotive manufacturing industry, such as automotive gears and transmission shafts. Through carburizing treatment, the surface hardness and wear resistance of the components are improved, and their service life is prolonged. In the field of construction machinery, carburized components are used in parts that bear heavy loads and friction, such as gears and transmission shafts of excavators. In addition, it is also applied in the manufacturing of components for transportation tools such as bicycles and motorcycles.

5.2 Nitriding

The nitriding process is often used in mold manufacturing, such as plastic molds and die - casting molds. It can improve the surface hardness, wear resistance, and corrosion resistance of the molds, reduce mold wear and failure, and improve product quality and production efficiency. In the field of precision machinery, nitrided components are used in high - precision shaft - like and sleeve - like components to ensure the dimensional accuracy and stability of the components. At the same time, in the aerospace field, some components with extremely high surface performance requirements also adopt nitriding treatment.

5.3 Carbonitriding

The carbonitriding process is widely applied in the manufacturing of automotive engine components, such as piston pins and valves. It can improve the wear resistance, anti - seizing property, and fatigue strength of the components, enabling them to adapt to the high - temperature and high - speed working environment of the engine. In the machine tool manufacturing industry, carbonitriding treatment is used to improve the surface performance of machine tool guideways, gears, and other components, thereby enhancing the machining accuracy and service life of the machine tools.

6. Conclusion

As three different chemical heat treatment processes, carburizing, nitriding, and carbonitriding have obvious differences in process principles, treatment temperature and time, diffusion layer microstructure and performance characteristics, and application fields. Carburizing is suitable for components that require high surface hardness and a certain degree of core toughness; nitriding focuses on improving the surface hardness, wear resistance, and corrosion resistance of components and is suitable for components used in high - precision and harsh environments; carbonitriding combines some advantages of carburizing and nitriding and is suitable for working conditions that have comprehensive requirements for the surface performance of components. In actual production, a suitable chemical heat treatment process should be rationally selected based on factors such as the working conditions of the components, performance requirements, and economic costs to achieve the best application effect.