Carburizing is a high-temperature surface hardening process to produce components with a hard, wear-resistant exterior and a tough, shock-absorbing core. Carburizing is the standard heat treatment for heavy-duty components in the automotive and aerospace industries, including transmission gears, drive shafts, and high-load bearings.
What is Carburizing?
Carburizing is a chemical heat treatment process in which a workpiece is heated and held in a carburizing medium to allow carbon atoms to penetrate its surface. This is a common heat treatment process for metallic materials, enabling the carburized surface of the workpiece to achieve high hardness and improved wear resistance. The carburizing process is widely used for mechanical parts in aircraft, automobiles, and tractors, such as gears, shafts, and camshafts.
Based on the different carbon-containing media, carburizing can be classified into gas carburizing, solid carburizing, and liquid carburizing.
Gas carburizing is a process where the workpiece is placed in a sealed carburizing furnace into which a gaseous carburizer (methane, ethane, etc.) or a liquid carburizer (kerosene, benzene, alcohol, acetone, etc.) is introduced. These decompose at high temperatures into active carbon atoms that penetrate the surface to obtain a high-carbon surface layer.
Solid carburizing is the earliest method, involving placing the workpiece and a solid carburizer (charcoal plus an activator) into a sealed carburizing box, which is then heated in a furnace to the carburizing temperature and held for a certain time to allow active carbon atoms to penetrate the surface.
Liquid carburizing utilizes liquid media for the process. Common liquid carburizing media include silicon carbide and “603” carburizing agents.
Gas Carburizing
Gas carburizing is currently the most widely used and fastest-growing method. It allows for a controlled atmosphere and stable product quality through the rational adjustment of process parameters.
Gas carburizing is prevalent today due to its suitability for mass production, simplified operations, and easy quality control. This method primarily uses natural gas, propane, or butane as the main agent. It offers advantages such as adjustable surface carbon concentration, easy automation of gas flow and temperature control, and simple management. Its disadvantages include expensive equipment and high costs for small batches, requiring specialized operational knowledge.
Image Credit: ScienceDirect
There are two methods of gas carburizing: the drip-feed method and the endothermic gas method. In the drip-feed method, the workpiece is placed in a sealed pit-type furnace with active carburizing media and heated to the single-phase austenite region at 900–950°C (commonly 930°C). After holding for sufficient time, active carbon atoms decomposed from the media penetrate the steel surface, resulting in high carbon at the surface while the core maintains its original composition. The carburizing atmosphere in the furnace is mainly formed by the decomposition of organic liquids like kerosene, acetone, toluene, and formaldehyde at high temperatures, consisting primarily of CO, H₂, and CH₄, with small amounts of CO₂ and H₂O.
The gas carburizing process also consists of three basic stages: decomposition, absorption, and diffusion. First, the carburizing medium decomposes at high temperatures to produce active carbon atoms:
CH₄ ⇌ 2H₂ + [C]
2CO ⇌ CO₂ + [C]
CO + H₂ ⇌ H₂O + [C]
Subsequently, active carbon atoms are absorbed by the steel surface, dissolved into high-temperature austenite, and diffuse inward to form a carburized layer of a certain depth. The depth of the layer depends mainly on the holding time. At a constant temperature, longer holding times result in thicker layers. In production, the “on-site sample check” method is often used to determine the exact discharge time.
When using a pit-type furnace at 930°C, the relationship between carburizing time and depth：
| Carburizing time/ h | 3 | 4 | 5 | 6 | 7 |
| Carburized layer depth /mm | 0.4–0.6mm | 0.6–0.8mm | 0.8–1.2mm | 1–1.4mm | 1.2–1.6mm |
Vacuum Carburizing
Vacuum carburizing involves placing the workpiece in a vacuum furnace, evacuating and heating it to purify the interior. Once the carburizing temperature is reached, a hydrocarbon (such as propane) is introduced. After a certain period, the carburizer is cut off, and the furnace is evacuated again for diffusion.
Vacuum carburizing is a type of gas carburizing performed in a vacuum, typically at higher temperatures (1030–1050°C). The vacuum has a cleaning effect on the surface, facilitating the adsorption of carbon atoms and significantly reducing the cycle time to about one-third of that required for standard gas carburizing. Moreover, vacuum furnaces do not increase maintenance costs at high temperatures.
Additionally, vacuum carburizing does not require carrier gas or carbon potential control. The carbon concentration depends on the ratio of the boost time to the diffusion time, eliminating the need for carbon potential controllers and gas generators. This process offers significant benefits for product quality and energy conservation.
Plasma Carburizing
Plasma carburizing uses plasma bombardment to assist gas carburizing. Like vacuum carburizing, it does not require gas generators or carbon potential controllers. The layer concentration can be controlled by adjusting the discharge current density. Due to plasma impact strengthening the process, the carburizing rate is much faster than general carburizing, even comparable to vacuum carburizing.
Image Credit: ScienceDirect
Steels for Carburizing
Steels used for carburizing are generally low-carbon steels and low-carbon alloy steels, with a carbon mass fraction requirement of 0.1%–0.25% to ensure sufficient strength and toughness in the core. Alloying elements such as Cr, Ni, Mn, Ti, and Mo are added to improve hardenability, refine grains, prevent overheating, and increase core toughness.
Common grades and applications:
Low-strength steels (S355, 1022, 5120, etc.): Used for low-load wear-resistant parts like friction plates, bushings, and gauges.
Medium-strength steels (20CrMn, 20CrMnTi, 20CrMnMo, etc.): Used for medium-load wear and fatigue-resistant parts like shafts, gears, and pins.
High-strength steels (12Cr2Ni4A, 18Cr2Ni4WA, 20Cr2Ni4, 30CrMnTi, etc.): Used for high-strength, heavy-load parts like high-power engine shafts and heavy-duty gears.
Heat Treatment After Carburizing
After carburizing, workpieces must undergo heat treatment to improve surface strength, hardness, and wear resistance, while refining grains and enhancing core toughness.
Direct Quenching
Direct quenching involves cooling the workpiece in the furnace (or pre-cooling) to 760–860°C after carburizing and then quenching. Pre-cooling reduces internal stress and deformation and allows high-carbon austenite to precipitate carbides, reducing retained austenite. This method increases productivity and reduces costs, but is only suitable for inherently fine-grained steels and not for cases with very high surface carbon concentrations.
Single Quenching
After slow cooling following carburizing, the workpiece is reheated above the critical temperature and quenched. If core strength requirements are high, the temperature should be slightly above A3. For parts with low load but high surface requirements, the temperature is set 30–50°C above Ac1.
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Double Quenching
Double quenching is used for parts with high mechanical requirements or those made from inherently coarse-grained steels. The first quench (above A3 + 30–50°C) improves the core structure. The second quench (above Ac1 + 30–50°C) refines the surface structure to obtain fine martensite and granular secondary carbides.
Carburized parts are tempered at 150–250°C after quenching. For non-alloy steels, a temperature of 150–180°C is typical; for alloy steels, a temperature of 160–200°C is used. After quenching and low-temperature tempering, surface hardness can reach 58–64 HRC with excellent wear resistance, while the core remains tough. Compressive stress formed at the surface also improves fatigue strength.
Performance of the Carburized Parts
The surface microstructure after quenching is primarily high-hardness martensite plus retained austenite and small amounts of carbides. The core is tough low-carbon martensite (ferrite should be avoided). Typical layer depths are 0.8–1.2mm, reaching over 2mm for deep carburizing. Surface hardness reaches 58–63 HRC, and core hardness is 30–42 HRC. Carburizing effectively increases strength, impact toughness, and wear resistance, thereby extending part life.
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