Carburizing vs Nitriding vs Carbonitriding

In the field of metal material heat treatment, surface treatment technology plays a crucial role in enhancing component performance. Carburizing, nitriding, and carburizing-nitriding are three key chemical heat treatment processes widely used across industries such as machinery manufacturing, automotive engineering, and aerospace. By altering the chemical composition and microstructure of metal surfaces, these techniques endow components with distinct performance characteristics to meet diverse operational requirements. A thorough understanding of the differences between these three processes enables manufacturers to make informed choices in practical production, ultimately achieving optimized component performance, improved product quality, and reduced manufacturing costs.

⇒Differences in process principles 

♦ Carburizing

Carburizing is a process where components are placed in a carburizing medium and heated to high temperatures (typically 900-950℃°C) to allow carbon atoms to diffuse into the surface, forming a high-carbon layer. Various methods exist: gas carburizing (using organic liquids like methanol or acetone that decompose into carbon-containing gases when injected), liquid carburizing (salt bath carburizing using carbon-containing salt mixtures), and solid carburizing (embedding components in carbon-containing solid powders). During this process, carbon atoms first diffuse from the concentrated medium to the surface, then gradually penetrate deeper layers, ultimately creating a high-carbon layer with specific depth and concentration gradients on the part's surface.

(1) Carburizing: Active carbon atoms [C] penetrate the surface layer of steel.

(2) Objective: To increase the surface C content, with a hard surface and a tough core after quenching and low-temperature tempering

(3) Carburizing methods: gas, solid and liquid carburizing

(4) Suitable steel grades:low-carbon steel and low-carbon alloy steel, such as 20, 20Cr, 20CrMnTi, etc

(5) Process: 900-950°c, 1 hour, 0.5mm; 4 hours,1mm carburized layer thickness: 0.5 to 2.5mm from the surface to the halfway point of the transition layer

(6) Heat treatment: Quenching (direct, single and double) + low-temperature tempering

(7) Organization and Performance:

• Surface layer: High-carbon tempered M+ carbides +A
• Core: Low-carbon tempered M or F, T
• Performance: High surface hardness, 58HRC to 64HRC; Good wear resistance; The core has good toughness, low hardness and high fatigue strength: surface compressive stress

 ♦ Nitriding

Nitriding, also known as nitrogen hardening, is a process where nitrogen atoms diffuse into the surface of workpieces at specific temperatures (typically 500-560℃°C) to form nitride compounds. Common methods include gas nitriding (using ammonia as a nitrogen source that releases reactive nitrogen atoms under high temperatures) and ion nitriding (utilizing ions generated by glow discharge to bombard the surface, accelerating nitrogen diffusion). Nitrogen atoms chemically react with surface metal elements (such as iron, aluminum, chromium) to form hard, wear-resistant, and highly corrosion-resistant nitride layers, including Fe₃N, Fe₄N, and CrN.

(1) Nitriding: At a certain temperature, active nitrogen atoms [N] are made to penetrate the surface of the workpiece.

(2) Objective: To enhance surface hardness, wear resistance, fatigue strength, thermal hardness and corrosion resistance, etc

(3) Process: Gas nitriding 2NH →3H+2[N] above 200℃

(4) Features: The nitriding temperature is 500 to 600°C lower

The time is 20 to 50 hours, the thickness is 0.3 to 0.5mm, and the catalyst (benzene, aniline, chloramine, etc.) is 10.3 to 3 times. Before nitriding, the steel parts must be quenched and tempered

(5) Organization

• The surface layer is white ε(FeN) or y '(FeN) phase
• In the middle is a dark black nitrogen-containing eutectoid (aty ')
• The heart part is tempered S

♦ Carbonitriding 

Carbon-nitrogen co-infiltration, a chemical heat treatment process where carbon and nitrogen simultaneously diffuse into the same infiltration layer (commonly referred to as cyanidation), can be categorized by temperature into three types: low-temperature carbon-nitrogen co-infiltration (500-600°℃, also called soft nitriding), medium-temperature carbon-nitrogen co-infiltration (700-850°℃), and high-temperature carbon-nitrogen co-infiltration (900-950°℃, similar to carburizing). Taking gas-based carbon-nitrogen co-infiltration as an example, this process typically uses carbon-nitrogen gas mixtures (such as ammonia, kerosene, or methanol) as the medium. At elevated temperatures, carbon and nitrogen atoms simultaneously diffuse toward the part's surface, forming a composite infiltration layer containing both carbides and nitrides.

⇒ Comparison of several surface heat treatments and chemical heat treatments

⇒ Differences in processing temperature and time

♦ Carburizing

Carburizing is typically performed at elevated temperatures ranging from 900°C to 950°C. These high temperatures facilitate carbon diffusion, enabling the rapid formation of a deep carburized layer. However, the intense heat also causes grain growth in components, which reduces material toughness. The processing duration depends on the part's dimensions, desired depth of the carburized layer, and type of medium used, often requiring several hours or even tens of hours. For instance, parts needing a 0.8-1.2mm carburized layer in gas-fired carburizing furnaces may require 8-12 hours of treatment.

 ♦ Nitriding 

Nitriding is typically performed at relatively low temperatures (500-560℃°C). This moderate heat prevents overheating and grain growth during processing, ensuring stable microstructure and performance in the workpiece core. However, nitriding is a slow process requiring extended treatment times—typically spanning dozens to hundreds of hours. For components demanding high surface hardness and wear resistance, gas nitriding may take 70-100 hours to achieve optimal results.

 ♦ Carbonitriding 

The low-temperature carbon-nitrogen co-enrichment process operates at 500-600°℃ with a relatively short treatment duration of 1-3 hours, enabling rapid formation of a hardened, wear-resistant, and anti-adhesive coating layer on the part surface. The medium-temperature carbon-nitrogen co-enrichment process reaches 700-850°℃, with treatment durations typically ranging from 2 to 6 hours depending on component specifications. While the high-temperature carbon-nitrogen co-enrichment process shares similar temperatures and treatment durations with carburizing, it uniquely achieves simultaneous penetration of carbon and nitrogen.

⇒ Comparison of microstructure and performance characteristics

♦ Carburizing

Carburized components develop a high-carbon martensitic structure on their surfaces, delivering exceptional hardness (HV550-750) and wear resistance. The carburized layer depth is tailored to operational requirements, typically ranging from 0.5 to 2.5mm. Their core structure consists of low-carbon martensite or a combination of ferrite and pearlite, providing robust toughness and strength to withstand significant impact loads. These components are ideal for applications involving alternating stress, friction, and wear, such as gears and shaft components.

Carbides in the carburized layer of gears Level: Level 2 Metallographic structure: Carbides are distributed in fine granular form Corrosive agent: 4% nitric acid alcohol solution	Carbides in the carburized layer of gears Level: Level 8 Metallographic structure: Carbides form a coarse network Corrosive agent: 4% sulfuric acid alcohol solution	Martensite and retained austenite in the carburized layer of gears Level: Level 2 Corrosive agent: 4% nitric acid alcohol solution

 ♦ Nitriding 

The nitrided layer primarily consists of ε phase (Fe₃N), γ' phase (Fe₄N), and compound layers, exhibiting exceptional hardness and wear resistance with surface hardness reaching HV1000-1200. Additionally, it demonstrates excellent corrosion resistance and anti-galling properties, enabling operation in harsh environments. The nitrided layer is relatively thin, typically ranging from 0.1 to 0.6 mm. Nitrided components are ideal for applications requiring high precision, superior wear resistance, and corrosion resistance, such as molds and precision shaft components.

Material: 38CrMoAlA Process: gas nitriding after tempering (520℃ keep 20h,560℃ keep 34h, slow cooling) Erosion method: 4% nitric acid alcohol solution erosion	Material: Cr12 Process: gas nitriding after quenching and tempering Erosion method: 4% nitric acid alcohol solution erosion	Material: QT450-10 Process: Ion nitriding Erosion method: 4% nitric acid alcohol solution erosion

 ♦ Carbonitriding 

After low-temperature carbon-nitrogen co-enrichment, the surface of parts forms a composite structure containing nitrogen carbides and ε phases. This results in high hardness (HV800-1000), excellent wear resistance, anti-galling properties, and moderate corrosion resistance. Medium-temperature carbon-nitrogen co-enrichment produces thicker infiltration layers with structures containing both carbides and nitrides, offering intermediate hardness and wear performance between carburizing and nitriding. High-temperature carbon-nitrogen co-enrichment maintains similar infiltration layer structures to carburizing but incorporates nitrogen, significantly enhancing surface hardness and wear resistance. These components are ideal for applications requiring both high hardness and wear resistance, as well as good toughness and anti-galling properties, such as piston pins and valves in automotive engines.

Material: 18CrMnTi Name: Carbon ammonia co-epitaxial structure Treatment process: air cooling after co-etching Metallurgical structure ： Corrosive agent: 4% nitric acid +4% bitter acid alcohol solution	Material: 18Cr2Ni4WA Name: Baicheng nitrogen co-epitaxial structure Treatment process: isothermal quenching after co-etching Metallurgical structure: Corrosive agent: 4% nitric acid + 4% picric acid alcoholic solution

⇒ Application domain differences

♦ Carburizing

Carburizing is extensively used in automotive manufacturing, where it enhances surface hardness and wear resistance of components like gears and transmission shafts, thereby extending their service life. In the construction machinery sector, carburized parts are employed in heavy-load-bearing and friction-prone areas such as excavator gears and drive shafts. Additionally, this process is applied in the production of components for bicycles, motorcycles, and other mobility vehicles.

 ♦ Nitriding 

 Nitriding is a widely used process in mold manufacturing, particularly for plastic molds and die-casting dies. It enhances surface hardness, wear resistance, and corrosion resistance while reducing wear and failure, thereby improving product quality and production efficiency. In precision machinery manufacturing, nitrided components are employed for high-precision shafts and sleeve parts, ensuring dimensional accuracy and structural stability. Furthermore, this treatment is extensively applied in aerospace applications where components require exceptionally high surface performance standards.

♦ Carbonitriding 

The carbon-nitrogen co-enrichment process is widely used in automotive engine component manufacturing, particularly for piston pins and valves. This treatment significantly enhances wear resistance, anti-locking properties, and fatigue strength, making parts capable of withstanding the extreme heat and high-speed operation required in engines. In the machine tool manufacturing sector, carbon-nitrogen co-enrichment is employed to improve surface performance of components such as machine tool guideways and gears, thereby enhancing both processing precision and service durability.

⇒ CONCLUSIONS

Carburizing, nitriding, and carburitic nitriding represent three distinct chemical heat treatment processes with distinct characteristics in terms of operational principles, processing parameters (temperature and duration), microstructural features of the hardened layers, and application scenarios. Carburizing is particularly suitable for components requiring high surface hardness combined with moderate core toughness. Nitriding focuses on enhancing surface hardness, wear resistance, and corrosion resistance, making it ideal for precision-engineered parts operating in harsh environments. Carburitic nitriding integrates key advantages from both methods, addressing applications demanding comprehensive surface performance. In practical manufacturing, engineers should strategically select appropriate chemical heat treatment processes based on operational conditions, performance requirements, and cost-effectiveness to achieve optimal results.