Cementation
Note: This document reflects industrial experience. The information is provided without any warranty, express or implied, as to its accuracy.
Reviewed by A3TS on July 29, 2026
Principle
Carburizing of steels involves enriching the surface with carbon (between 0.7 and 0.9% by mass) through diffusion in the austenitic region (900 to 1050°C, depending on the process), followed by quenching and, if necessary, tempering, in order to achieve a high surface hardness (58 to 64 HRC) with a decreasing gradient over a given depth.
The depths that can be achieved are between 0.1 and 6 mm. The most common ones are between 0.3 and 3 mm.
Characterization:
Surface carbon content
Surface hardness (HRC, HRA, HV)
Conventional hard layer depth (CHD) according to the NF EN ISO 18203 standard.
Microstructure requirements
Residual stress rate in compression
Benefits
Wear resistance
Resilient
Improving mechanical and surface fatigue resistance
Applications of Case Hardening of Steels
Gears, pinions and drive shafts ;
Bearing cages ;
General purpose mechanical parts that are abrasion resistant and can withstand impact.
Case-hardening steels
Steels are selected for their low carbon content, limited to 0.3% in accordance with the NF EN ISO 683-3 standard (16MnCr5, 18CrMo4, 17NiCrMo6–4, 18CrNiMo7-6, …) and based on the dimensions of the workpiece to be treated, in order to account for hardenability and the required strength in the surface layer and throughout the core.
The choice of depth is determined by the specific needs:
For fatigue strength based on stress distribution: location of shear stresses in components subjected to surface fatigue (cams, gear teeth), importance of stress concentration.
For wear resistance: depending on the acceptable rate of functional wear.
Low pressure carburizing processes have been applied to parts with fine holes that need to be carburized (e.g. diesel engine injectors).
The metallographic requirements are an important component in the service life, they relate to the possible presence of defects (internal oxidation type) in the absence of grinding and the residual austenite rate which can have an effect on the fatigue life, the impact resistance and the dimensional stability in time.
A cryogenic treatment can be carried out after treatment, precisely to transform the austenite. It must be followed by a stress relieving tempering.
Pre-stress shot peening can transform the residual austenite present on the surface into very fine, low-fragility martensite. This increases compressive stresses and surface fatigue resistance.
Hardening is done in oil, in a mixture of water and polymers or under pressurized gas (up to 20 bars). A stress relieving annealing is generally performed after quenching to limit the risk of brittleness.
Limitations
Internal oxidation after gas carburizing reduces the resistance to mechanical fatigue. Reworking by grinding or machining is then necessary to optimize the properties. This internal oxidation does not occur during low pressure carburizing treatments;
The higher the temperature and the longer the cycle time, the greater the deformation after treatment. Deformations can be reduced by gas quenching. Depending on the required geometrical accuracy, deformations are eliminated by grinding or machining;
A coarsening of the grain, detrimental to the impact resistance, can occur when high temperatures are held for a long time;
The presence of residual austenite after quenching (between 10 and 25%) must be controlled because it is not stable and can be re-transformed under the action of a temperature rise above 200°C, plastic deformation or maintenance at negative temperature. A cryogenic treatment can reduce the residual austenite content.
Process Parameter
Implementation TECHNOLOGIES (there may be several, or at least variations)
DIFFERENT PROCESSES:
Solid or in a container:This process usesactivated charcoal-based granules and is now used only for small-scale applications or for carburizing parts in air furnaces. Quenching cannot be performed immediately; the parts are cooled and then reheated for quenching. The process is relatively slow; carbon content must be checked on a sample or on the part itself. Case hardening temperature ranges from 900 to 950°C.
Liquid or salt bath process: useof salts based on alkali chlorides (NaCl, KCl) with the addition of sodium cyanide (NaCN) (approximately 10%); cyanide-free formulations are also available. The reaction is rapid, and controlling the cyanide concentration allows for precise control of the carbon potential. Quenching is performed directly in oil or in salt baths (after “rinsing” in neutral salts). Case-hardening temperature ranges from 875 to 930°C. This process is rarely used due to waste treatment constraints.
In a controlled or gaseous atmosphere: theresulting atmosphereis endothermic:
by cracking an alkane in a generator, with methane or propane added to the furnace to maintain the carbon potential,
by injection of a methanol-nitrogen mixture directly into the furnace with addition of methane or propane in the furnace to maintain the carbon potential,
or by injection of an alkane and air mixture.
The typical composition of the atmosphere is 20 to 25% CO, 20 to 40% N2, 40 to 55% H2, plus trace amounts of CO2, O2, and H2O. The carbon potential of the atmosphere is measured and controlled through gas analysis: %CO, CO₂, H₂O, O₂. The reaction kinetics are fairly rapid thanks to cycle optimization. Quenching is most often performed directly in oil. The carburizing temperature in the atmosphere ranges from 900 to 975°C.
In a fluidized bed:fluidization is achieved using a gas similar to those found in gaseous atmospheres. The reaction kinetics are fast, but the process is not very easy to use.
Under low pressure: Afterheating in a vacuum furnace, a sequential pressure in the range of 5 to 20 mbar is established by injecting a hydrocarbon—such as propane, ethylene, or acetylene—and the carburizing sequences are followed by diffusion sequences under vacuum. The reaction kinetics are very rapid. The process allows for high carburizing temperatures, which accelerate the reaction kinetics. The main advantages are the absence of surface oxidation and good penetration into fine pores. This process is often combined with gas quenching (5 to 20 bar of nitrogen or helium). Process control is based on a gas-metal reaction model that assumes carbon saturation of the surface during the carburization and diffusion phases. Carburizing temperature ranges from 900 to 1050°C.
Plasma-assisted or ion carburizing: carburizing is performed in a propane plasma. The advantages are comparable to those of low-pressure carburizing, with the added benefit of being able to easily create uncarburized areas using metal shields. This process has not been widely adopted due to the significant maintenance requirements of the equipment, and it has faced competition from low-pressure carburizing.
Regardless of the process used, carburizing is followed by either immediate or delayed quenching after machining. The pre-quenching temperatures and quenching medium are selected to induce compressive stress in the carbon-enriched layer.