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prothermo@prothermo.hu Carburizing
The carburizing atmosphere is generated directly inside the furnace. Carburizing takes place in dissociated methanol (CH₃OH) and nitrogen (N₂). The actual carburizing is done by CO, which deposits diffusible carbon on the workpiece surface. During process control, propane (C₃H₈) is added to the furnace for enrichment when needed, and air is admitted to reduce the carbon potential.
Carburizing is carried out in a gas-tight furnace from which the gases (CO, CO₂, H₂, N₂, H₂O, CH₄) leave to a burner where, with sufficient excess air, the combustible components are burned. The gases vented to the atmosphere (at least 6 m above ground level) contain only water vapour, nitrogen and carbon dioxide.
The target furnace atmosphere is 20 % CO + 40 % H₂ + 40 % N₂. The carburizing effect of the atmosphere is set on the basis of the so-called carbon potential. Carbon potential can be measured and controlled with an oxygen probe or a lambda probe.
Advantages of gas carburizing:
- The process is very well controllable; with computerised control the pre-designed carbon curve can be guaranteed.
- It is suitable for mass production; the size of the parts to be carburized is not limited.
Nitro-carburizing
Nitro-carburizing is performed at 750–880 °C, where the carburizing effect dominates. The nitro-carburized case contains 0.5–0.8 % carbon and 0.4–3.0 % nitrogen. The same gases are used as in gas carburizing, with additional ammonia. A 1–3-hour treatment at 780 °C produces a case 0.1–0.4 mm thick.
Carbonitriding
Carbonitriding is performed at 540–580 °C, where the nitriding process becomes more effective. It is most often used to extend the service life of tool steels. Carbonitriding can be performed partly in a suitable mixture of the gas components above, or in a 1:1 mixture of ammonia and carbon dioxide with about 50 % ammonia dissociation. The hold at the usual 570 °C is 1–3 hours.
Advantages of nitro-carburizing and carbonitriding:
- Short cycle time; an economical process.
- The brittle, thin nitrided case is supported by a thicker diffusion layer.
- The process temperature is lower than that of carburizing.
- Nitrogen lowers the critical cooling rate Vcrit, so slower cooling (in a gas stream) is sufficient for the martensitic transformation, with less distortion.
- Nitrogen also makes the martensite harder and more wear-resistant.
Nitriding
Gas-phase nitriding is performed in ammonia fed into a well-sealed furnace chamber. Ammonia dissociates above 400 °C. Atomic nitrogen converts to molecular nitrogen within a few seconds. Part of the nitrogen formed during dissociation adheres to the steel surface and diffuses into the workpiece. The extent of nitriding depends on how much ammonia dissociates per unit time — this is expressed by the dissociation rate.
Dissociation rate: the amount of ammonia decomposed in a unit time relative to the amount of ammonia introduced, in percent. Considerable hydrogen forms during ammonia dissociation, which is explosive within a wide range when mixed with air. Therefore, the sealed furnace must be thoroughly purged with an inert gas — e.g. nitrogen — before starting and after ending gas dosing.
Advantages of nitriding:
- The nitrided case is harder than martensite and significantly more wear-resistant.
- The nitrided case retains its hardness and wear resistance up to about 500 °C.
- Low-temperature nitriding causes practically no distortion or shrinkage of the workpiece.
- The fine-grained, spheroidite microstructure is preserved after nitriding.
- The nitrided case has slight corrosion resistance.