Carbonitriding
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle
Carbonitriding is a surface treatment derived from carburizing which includes a carbon and nitrogen enrichment phase of the surface and a hardening phase by quenching. The interest of the presence of nitrogen lies in the increase of the treatment kinetics for shallow treatments. Nitrogen also stabilizes the residual austenite.
The treatment temperature is generally between 850 and 950°C. The holding time varies between 1 and 10 hours.
The carbonitriding medium can be a mixture of gases at atmospheric pressure (gaseous carbonitriding) or at reduced pressure (low pressure carbonitriding)
Hardening is carried out in oil, in a mixture of water and polymers or under pressurized gas. A stress relieving annealing is generally performed after quenching to limit the risk of brittleness.
The steels used are for example 27MnCr5, 27CrMo4 or 23MnCrMo5 for automotive applications. Structural steels such as 34CrMo4 are sometimes carbonitrided.
Typical treatment features
The surface hardness is between 58 and 65 HRC which increases the resistance to wear by abrasion;
The hardness of the core depends on the choice of the treated material;
The residual surface stresses are in compression, between -150 and -450MPa which increases the resistance to mechanical fatigue. This value can exceed -700MPa after shot peening;
The targeted treatment depth varies between 0.1 and 0.5mm in common cases. In the case of automotive transmission parts, depths greater than 0.7mm are sometimes sought;
The surface fatigue strength is greatly increased because the stressed surface area has a high yield point;
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.
Applications
Gears, pinions and shafts for automotive transmissions
Fasteners
General purpose mechanical parts resistant to abrasion