cementation
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
Case hardening is a surface treatment that includes a carbon enrichment phase (0.6 to 0.9%) of the surface of a steel containing initially little of this element (about 0.2%) and a hardening phase by quenching.
The treatment temperature is generally between 900 and 1050°C. The holding time varies between 1 hour and several days.
The carburizing medium can be made up of:
A solid such as charcoal or sodium or barium acetate which is only used for the treatment of very large parts.
A mixture of gases at atmospheric pressure (gas cementation) or at reduced pressure (low pressure cementation)
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 steels used are for example 16MnCr5 or 18CrMo4 steel with usually oil quenching. Nickel-containing steels such as 18CrNiMo7-6 are compatible with gas pressure quenching.
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 does not change much, which makes it possible to maintain an interesting impact resistance;
The residual surface stresses are in compression between -100 and -250MPa which increases the resistance to mechanical fatigue;
The targeted treatment depth varies between 0.4 and 2.5mm in common cases;
The surface fatigue strength is greatly increased because the stressed surface area has a high yield point;
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.
Applications:
Gears, pinions and drive shafts ;
Bearing cages ;
General purpose mechanical parts that are abrasion resistant and can withstand impact.