Carbonitriding
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
The carbonitriding process involves enriching the surface with carbon (between 0.7 and 0.9% by mass) and nitrogen (between 0.15 and 0.3% by mass) through diffusion in the austenitic region (825 to 900°C), followed by quenching and, if necessary, tempering, in order to achieve high surface hardness (58 to 64 HRC) with a decreasing gradient over a given depth.
The achievable depths range from 0.05 to 1 mm; beyond 0.6 mm, the addition of nitrogen is no longer beneficial.
Characterization:
Surface Carbon and Nitrogen Content
Surface hardness (HRC, HRA, HV)
Conventional hard layer depth (CHD) according to the NF EN ISO 18203 standard.
Microstructure requirements
Residual Strain Rates
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.
Benefits
Wear resistance
Improving mechanical and surface fatigue resistance
Carbonitriding steels
Carbonitriding treatment is applicable to case-hardening steels, steels such as 27MnCr5 or 25CrMo4, and structural steels such as 34CrMo4, depending on the dimensions of the workpiece being treated, in order to account for hardenability and the required strength in the surface layer and 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.
Processes
Carbonitriding is almost exclusively performed using the gas phase method. The liquid-phase process is developing only marginally.
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.
The combined presence of carbon and nitrogen results in higher compressive stresses than those achieved through carburizing at the same hardened depth.
Applications
The most important applications are in the field of mechanical parts for the automotive industry:
Gears, pinions and shafts for automotive transmissions
Fasteners
General purpose mechanical parts resistant to abrasion
This treatment is particularly well-suited for hardening stamped parts made from low-carbon steel sheet, most often using grades not originally designed for this application. It is therefore important to verify the suitability of these grades for the treatment (this involves, in particular, monitoring the grain size and the residual tempering aluminum content).
Limitations
Internal oxidation after gas carbonitriding reduces the mechanical fatigue resistance. Reworking by grinding or machining is then necessary to optimize the properties. This internal oxidation does not occur during low pressure carbonitriding treatments;
The deformations and the enlargement of the grain after treatment are less important than after case-hardening because of the shorter holding time at high temperature;
The residual austenite content after quenching is higher than after carburizing (between 15 and 50%). The stability of the austenite is improved by the addition of nitrogen, but it can be partly re-transformed by the action of a temperature rise above 200°C, plastic deformation or by being held at negative temperature.
Process Parameters
Under controlled or gaseous conditions, the resulting atmosphere is endothermic:
By cracking an alkane in a generator with the addition of methane or propane to maintain the carbon potential,
By injecting a mixture of methanol and nitrogen directly into the furnace, with the addition of methane or propane to maintain the carbon potential,
Nitrogen is introduced by injecting ammonia gas directly into the furnace. The typical composition of the atmosphere is 20 to 25% CO, 20 to 40% N₂, 40 to 55% H₂ (traces of CO₂, O₂, and H₂O). The ammonia injection rate is typically 1 to 5% of the total gas volume (lower values are suitable for alloy steels). The carbon potential of the atmosphere is measured and controlled by analyzing the gas concentrations (%CO, CO₂, H₂O, O₂). The reaction kinetics are fairly rapid thanks to cycle optimization.
Quenching is most often performed directly in oil, in a mixture of water and polymers, or under pressurized gas (up to 20 bar). Nitrogen dissolved in the carbonitrided layer significantly increases hardenability, allowing for mild quenching in hot oil (80 to 160°C) or in a salt bath (150 to 180°C), thereby minimizing distortion (up to 950°C).
In the case of gas quenching, it is possible to adjust the cooling rate to approximate the conditions of step cooling.