Carburization - the Most Popular Method of Case-Hardening

Carburization (also known as carburizing or carbonizing, and less commonly as cementation or acieration) is the most widely used method of case hardening steel. This article covers the basic principles, the carburizing process, the types of carburizing atmospheres, and the factors that influence the final results.

How the Case Is Hardened

Carburization is a surface thermo-chemical treatment that enriches the outer layer of low-carbon steel with carbon. The objective is to produce a hard, wear-resistant surface while retaining a tough, impact-resistant core. For this reason, the component is typically quenched after carburizing. The carburized layer becomes hard, whereas the core remains relatively soft.

Carbon is transferred from the surrounding gas into the steel through adsorption, then diffuses deeper into the metal through the process of diffusion.

Adsorption refers to the process by which free atoms from a gas or liquid settle on the surface of a solid, forming a layer that is only one atom thick.

Diffusion is a heat-activated process that distributes atoms within the metal’s lattice structure, driven by differences in concentration. For diffusion to occur, the saturating element must be soluble in the solid state of the base metal. The rate and extent of diffusion depend on temperature, time, and the concentration gradient. Several diffusion mechanisms exist; in carburizing, interstitial diffusion is key—carbon atoms move through spaces between iron atoms in the crystal lattice. Diffusion happens most easily along the surface, less readily along grain boundaries, and least efficiently within the grains themselves.

After carburizing, components typically undergo hardening followed by low-temperature tempering. The result is a surface that is extremely hard and resistant to wear and abrasion, combined with a ductile core capable of withstanding dynamic loads. Carburization is therefore commonly used for steel machine parts such as gears, splined shafts, gear shafts, camshafts, pins, rings, bushings, and rolling bearing shafts.

Types and Methods of Carburization

Several methods of carburization can be distinguished.

Pack carburizing (or box carburizing) is the oldest method of carburizing. It involves placing parts in a box filled with charcoal powder, saturated with carbonates (usually of barium and sodium), and heating it to the carburizing temperature. Carbon monoxide is generated in the box, and upon dissociation, it releases carbon particles that diffuse into the steel. This method is inexpensive and simple, but it does not yield repeatable results. The charcoal in the box is consumed during the process, which weakens the carburizing effect in an uncontrolled way. Additionally, this method is difficult to automate. Gas carburizing avoids these issues. An interesting variation is carburizing using pastes containing, for example, 50% soot, 20% barium carbonate, 20% sodium carbonate, and 10% potassium ferrocyanide, or 50% soot, 40% sodium carbonate, and 10% ferrochromium, bound with molasses. This approach allows carburized layers to be created on selected surfaces of the workpiece.

Gas carburizing (or hypercarb process) is currently the most popular method of carburizing. The earliest form of gas carburizing involved placing parts in a furnace chamber with kerosene, benzene, or pyrobenzol. At the carburizing temperature, the liquid would evaporate and decompose into gaseous components. However, the resulting atmosphere often caused an excessively high carbon concentration in the surface layer and offered limited control. An advancement of this method involves introducing two liquids with different carburizing properties into the furnace. By changing their proportions, the process can be precisely regulated.

Carburizing in molten salts (or liquid carburizing) is carried out by immersing the parts in molten salts— mixtures of carbonates, chlorides, or cyanides of alkali metals—at 830–850°C. Typical mixtures contain:

• 47.5% sodium carbonate, 47.5% potassium chloride, 5% calcium carbide, or
• 60% sodium carbonate, 5% barium carbonate, 20% sodium chloride, 15% silicon carbide.

Carburizing in fluidized beds – a neutral powder (sand or alumina) is kept in suspension by a hot saturating gas flowing upward through the bed, giving the powder liquid-like properties. The steel is put into the fluid-like powder and carburized by the gas.

Vacuum carburizing is performed at reduced pressure in an atmosphere of methane, propane, and other gases. These gases decompose, releasing atomic carbon, which then penetrates the steel. Vacuum carburizing ensures better carbon adsorption from the atmosphere, reduced gas consumption, and shorter processing time.

Plasma carburizing (ion carburizing) involves heating the steel in a vacuum furnace while simultaneously applying a high DC voltage between the workpiece (the cathode) and the anode. This creates a glow discharge and plasma, which activates the formation of carbon ions. The ions are accelerated in the electric field near the part and bombard its surface, significantly enhancing carbon penetration into the metal. This method provides high efficiency, precise control of layer thickness and structure.

Despite the development of modern carburizing methods, for economic and practical reasons, gas carburizing remains the most commonly used type. Therefore, the rest of the article will focus primarily on this method.

Structure, Layers, and Properties of Carburized Components

In steel after carburizing, the following layers can be distinguished:

• Case or carburized layer containing:
  • surface layer,
  • transitional layer,
• Core.

The chemical composition and heat treatment after carburizing allow for different microstructures within these layers.

Carburization followed by slow cool – the surface layer has a pearlitic structure, and carbides may form in the near-surface zone due to precipitation during cooling. The transitional layer is a mixture of pearlite and ferrite, with the amount of pearlite decreasing and ferrite increasing along with the depth. The core contains both ferrite and pearlite, but with a higher ferrite content. Even with the same surface layer thickness, the transitional layer thickness may vary. Alloy steels after carburizing and slow cooling may have the same structure as described above, but the carburized layer may also contain bainite or a mixture of bainite and martensite, while the core may contain bainite and ferrite. However, since components are usually quenched after carburizing rather than slowly cooled, the structures formed after slow cooling are of small practical importance.

Carburization followed by quench– The surface carburized layer has a hard martensitic structure. In alloy steels, there may be unwanted soft retained austenite as well as carbides. The amount of retained austenite and carbides depends on the alloy's chemical composition, the carbon concentration in the layer, and the quenching method. The transitional zone may contain both bainite and martensite, or only bainite (in alloy steels with large cross-sections, sometimes bainite with ferrite), with martensite content decreasing with depth. The core has a bainitic structure. The direct, single quench, produces fine grain, while quenching with undercooling results in coarse-grained structures.

Case-hardening steel grades

Case-hardening steel grades include carbon and alloy steels with low carbon content ranging from 0.10–0.25%. Case-hardening steel grades are classified into several main groups based on alloying elements. The chemical composition significantly influences the required properties of both the core and the case.

Carbon steels have low hardenability, and thus low core properties even with small cross-sections. The case is hard and wear-resistant, but the core has limited resistance to dynamic loads. The main advantage is low cost. Carbon steels are used for less critical components. Examples: 1.1121, 1.1141, 1.1151.

Chromium steels contain 0.7 to 1.0% chromium and are slightly more hardenable than carbon steels, but are still generally intended for lightly loaded parts. Further increasing chromium content would improve hardenability but also raise the risk of carburization-related issues like excessive carburization and internal oxidation. Advantages of chromium steels include low cost and low retained austenite content in the carburized layer (high hardness). Example grades include 17Cr3 and 20Cr4.

Chromium-manganese steels have better hardenability than chromium steels because manganese reduces the critical cooling rate. Therefore, chromium-manganese steels are suitable for larger cross-section, moderately loaded parts. However, manganese promotes overcarburization and internal oxidation and lowers the martensitic transformation temperature, increasing the amount of retained austenite in the carburized layer, which reduces wear resistance. Example grades include 16MnCr5, 20TiMnCr12.

Chromium-manganese-molybdenum steels contain less manganese and more molybdenum, providing similar hardenability to chromium-manganese steels but with reduced internal oxidation and less retained austenite in the carburized layer. An example grade is 18CrMo4.

Chromium-nickel steels contain 1.4 to 3.2% nickel, which significantly increases case-hardenability compared to non-nickel steels and improves core ductility. These steels are suitable for heavily loaded components with large cross-sections. Especially high-performance grades contain increased levels of chromium, as nickel mitigates the negative effects of higher chromium content. However, these steels have high levels of retained austenite in the surface layer, reducing hardness and wear resistance. Additionally, nickel is rather expensive. Example grades include 17CrNi6-6 / 1.5918, 16NiCrS4 / 1.5715, 18CrNi8 / 1.5920, and 18NiCr5-4 / 1.5810.

Chromium-nickel-molybdenum steels have slightly lower hardenability than chromium-nickel steels and thus lower load resistance—but still significantly better than nickel-free steels. Molybdenum/manganese additives reduce the amount of retained austenite, increasing hardness and wear resistance of the carburized layer. Example grades include 18NiCrMo7 or 18H2N4WA.

Most advances case-hardening steel grades are the chromium-molybdenum and chromium-nickel-molybdenum ones.

Temperature and Duration of Gas Carburizing

The temperature for gas carburizing is 850–950°C, most commonly 900–920°C. At these temperatures, the solubility of carbon in austenite is sufficiently high to achieve a surface carbon concentration of 0.7–1.0% without carbide precipitation. Both increasing the temperature and prolonging the carburizing time result in a thicker carburized layer. However, carburizing time has the most impact during the initial hours of the process, and its influence diminishes with each passing hour. Therefore, in practice, thicker case is typically obtained by using higher temperatures. For example, achieving a carburized layer 1.5 mm thick at 900°C requires 10 hours of carburizing, while at 950°C, it takes only 4 hours—a time savings of 6 hours. For a 2.5 mm thick layer, the time savings would amount to 8 hours in this case.

In general, carburizing above 950°C is not recommended due to grain growth. However, in the case of alloy steels with large cross-sections requiring thick carburized layers, increasing the temperature can shorten the process by more than half and may be justified. Alloying elements that limit grain growth include, for example, 0.02–0.04% Al and 0.01% N. Chromium and molybdenum, which are added to enhance hardenability, also inhibit grain growth during case-hardening. At temperatures above 1000°C, grain growth can be controlled by adding 0.045% Nb, 0.01% Ti, at least 0.017% N, and more than 0.03% Al.

Carburizing Atmospheres in Gas Carburizing

The main carburizing gases are carbon monoxide (CO) and hydrocarbons C_nH_2n+2 or C_nH_2n. Under high temperature and the catalytic action of iron, these gases dissociate, releasing free carbon atoms:

2 CO → CO₂ + C

C_nH_2n+2 → (2n+2)H + nC

C_nH_2n → 2nH + nC

The free carbon atoms are highly chemically active and are absorbed by the steel. However, if the steel surface cannot absorb the released carbon quickly enough, free carbon is deposited on the surface in the form of soot, which impedes the further carburizing process. Hence, controlling the gas composition and other process parameters is crucial.

Four gas carburizing atmospheres are most commonly used in industry—two derived from fuel gases (enriched endothermic) and two from liquid organic compounds via decomposition or conversion:

• Enriched endothermic atmosphere from natural gas (0.4% CO₂; 20.8% CO; 0.5% CH₄; 41.8% H₂; 36.5% N₂),
• Enriched endothermic atmosphere from propane (0.5% CO₂; 24.0% CO; 0.5% CH₄; 32.2% H₂; 42.8% N₂),
• From methanol decomposition with ethyl acetate (0.48% CO₂; 32.4% CO; 0.9% CH₄; 65.9% H₂; 0% N₂),
• From acetone conversion with water (0.8% CO₂; 34.2% CO; 1.7% CH₄; 62.9% H₂; 0% N₂).

Endothermic atmospheres are cheaper, but those derived from organic compounds have a higher carbon potential.

Carburizing with Constant and Variable Carbon Potential of the Carburizing Atmosphere

A successful carburizing process requires maintaining an appropriate carbon potential in the carburizing atmosphere. The carbon potential of the carburizing atmosphere is its ability to supply carbon to the surface of the steel. An atmosphere with too low a carbon potential will not carburize the steel effectively and may even decarburize it. An atmosphere with too high a carbon potential will result in soot formation, which hinders further carburizing. As carburizing progresses, the carbon potential of the atmosphere changes—it initially drops sharply, because partially carburized steel becomes less capable of absorbing carbon, but may later increase if previously released soot in cooler parts of the furnace is reabsorbed by the atmosphere.

It is possible to aim for either a constant or variable carbon potential in the carburizing atmosphere. In both cases, the atmosphere is regulated over time.

Carburizing with a constant carbon potential – in this method, the atmosphere flow is controlled to maintain a relatively steady carbon potential. A typical sequence might be:

• Hour 1 – high flow rate, soot forms in the cooler parts of the furnace
• Hour 2 – low flow rate, soot is reabsorbed by the atmosphere
• From hour 3 – increased flow again, but not as high as at the start; the soot reserve has been depleted, so flow is increased to maintain a constant carbon potential

In carburizing with a constant carbon potential, the potential is usually 0.8–1.0% C. Variable potential carburizing reaches higher values.

Carburizing with a variable carbon potential is carried out in two phases. In the first phase, the atmosphere has the highest possible carbon potential that allows the desired layer thickness to be achieved in the shortest time. However, this atmosphere causes excessive carbon concentration in the layer. Therefore, in the second phase, an atmosphere with a lower carbon potential is used—one that matches the optimal surface carbon concentration for the layer.

There is also a method of carburizing with a set carbon potential atmosphere without controlling the flow. However, the repeatability of results is unsatisfactory, so this method should be limited to low-responsibility production cases.

Case-Hardening Step by Step

The gas carburizing process consists of the following steps:

1. Furnace cleaning – involves purging the furnace of air and reducing oxides formed during heating. Skipping this step may lead to oxidation instead of carburization at the start of the process.
2. Charge preparation – includes arranging the parts in the charge, cleaning their surfaces, and, in some cases, masking specific areas to prevent carburization.
3. Heating – the heating time depends on the size and mass of the charge. During this phase, the carburizing atmosphere is not yet introduced to avoid soot formation at lower temperatures.
4. Carburizing – the carburizing atmosphere is introduced into the furnace, and the process is carried out using either a constant or variable carbon potential, as described earlier.
5. Cooling / Quenching – the parts are cooled either gradually or rapidly, depending on the desired properties. This step is discussed in more detail below.

Quenching After Carburizing

There are several quenching methods used after carburizing. The choice depends on technological constraints and the required properties of the carburized layer and core.

Direct quench involves rapidly cooling (quenching) the charge directly from the carburizing temperature. This is the simplest, fastest, and most cost-effective method. It also preserves the surface carbon concentration and reduces the risk of distortion due to fewer heating and cooling cycles. However, quenching from high temperature promotes grain growth and degrades mechanical properties, as described in this article. It also increases retained austenite in alloy steels with high surface carbon content. This method is suitable for parts with low performance requirements and works best with fine-grain steels.

Quenching with cooling involves reducing the temperature of the charge from the carburizing range (850–950°C) to the optimal hardening range (780–850°C), followed by immersion in a cooling bath. The requirement to cool the entire furnace lengthens the process. However, this method helps prevent grain growth and offers better protection against distortion. Selecting the correct quenching temperature is critical. In some steels, for instance, at 790°C, the core austenite may transform into ferrite, which does not convert to bainite during quenching, leading to inferior mechanical properties.

Quenching with pearlitic transformation involves cooling the charge after carburizing to 600–650°C, holding it at that temperature until the pearlitic transformation is complete, then reheating to 780–850°C and quenching after temperature equalization. This approach enables the formation of a fine-needle martensitic structure in coarse-grained steels.

Single hardening involves slow cooling of the charge after carburizing, followed by reheating to 780–850°C and subsequent quenching. This method entails time loss and additional energy consumption due to reheating. Moreover, slow cooling may cause carbide precipitation in the surface layer and ferrite formation in the core. Single hardening is used when machining is required between carburizing and quenching, or when direct quenching is not feasible.

Double hardening combines direct and single quenching—first quenching from the carburizing temperature, then reheating to 780–850°C and quenching again. This method is more effective than single quenching, as it eliminates the risk of carbide or ferrite formation. However, it poses a higher risk of distortion due to the repeated quenching process.