Carbonitriding involves adding a small amount of ammonia gas to the carburizing atmosphere to simultaneously infiltrate carbon and nitrogen into the steel surface at a lower temperature (approximately 820–880 °C). Nitrogen can improve the hardenability of the surface layer and reduce the critical cooling rate of quenching. It is suitable for thin-layer low-carbon steel parts that require small deformation. The key point of control is the coordination of the carbon potential and ammonia ratio: too much nitrogen will lead to excess retained austenite and black microstructure.
- Temperature lower than carburizing, typically around 820–880 °C
- Add a small amount of NH₃ to the carburizing atmosphere to provide nitrogen
- Nitrogen improves the hardenability of the surface layer, and a milder quenching medium can be used
- Risks: Excessive retained austenite, black structure, holes
- Applicable: Thin layers, small parts, mild steel, fasteners and stamping parts
Principle
Add a certain proportion of ammonia gas to an endothermic or nitrogen-methanol atmosphere. Ammonia decomposes at high temperatures to produce active nitrogen atoms, which penetrate into the austenite surface together with carbon. Carbonitriding is often used to obtain a shallower hardened layer due to lower temperatures and slower diffusion.
The difference from carburizing
| Project | Carburizing | Carbonitriding |
|---|---|---|
| temperature | Approximately 900–950 °C | Approximately 820–880 °C |
| Infiltrating elements | carbon | carbon + nitrogen |
| layer depth | Deeper | Shallow |
| deformation | relatively large | smaller |
| Hardenability | Depends on steel grade | Nitrogen improves surface hardenability |
| Typical parts | Gears, shafts | Pinions, fasteners, stampings, chain conditions |
The role of nitrogen
- Improve hardenability: Nitrogen expands the austenite zone and reduces the critical cooling rate. Low carbon steel can also obtain a good hardened layer in oil;
- Improve tempering resistance and wear resistance；
- Reduce martensite transition temperature (Ms): Increased retained austenite. An appropriate amount of retained austenite can improve contact fatigue performance, while an excessive amount can reduce hardness and dimensional stability.
Key process parameters
- temperature: The lower the temperature, the more nitrogen is absorbed and the slower the diffusion of carbon;
- Carbon potential: Set according to the target surface carbon, usually slightly lower than pure carburizing;
- Ammonia gas flow: Generally accounts for a small proportion of the total gas volume, and needs to be adjusted according to the furnace type and target surface nitrogen content;
- time: Determine the layer depth;
- Quenching:Select oil quenching or other media according to the parts.
Control difficulties
- Ammonia will affect oxygen probe readings and carbon potential calculations, and the controller should consider the presence of ammonia;
- Ammonia, CO₂, and H₂O may form ammonium carbonate crystals in low-temperature pipelines and block the sampling pipe;
- The residual ammonia content in the furnace is difficult to measure directly with conventional carbon potential sensors.
Common defects
| Defect | Reason | Countermeasures |
|---|---|---|
| Too much retained austenite | Nitrogen and carbon are too high | Reduce ammonia content and carbon potential, or increase cold treatment |
| Black microstructure (internal oxidation/holes) | Too much nitrogen and oxidizing atmosphere | Control the amount of ammonia and improve the atmosphere |
| Low surface hardness | Insufficient layer depth or decarburization | Check temperature, time and carbon potential |
| Uneven hardness | Poor atmosphere circulation and too dense furnace loading | Improve furnace charging and circulation |
Frequently asked questions
Can carbonitriding be done at a deep level?
The time can be extended, but due to low temperature and slow diffusion, the efficiency is not as good as carburizing. Carburizing is generally used for deep requirements.
How much ammonia is appropriate to add?
There is no uniform ratio, it needs to be determined through experiments based on furnace type, temperature, target surface nitrogen and retained austenite requirements, and accurately controlled with a flow meter.
Are carbonitriding and nitrocarburizing the same thing?
no. Carbonitriding is carried out in the austenitic state, dominated by carbon, and then quenched; nitrocarburizing (soft nitriding) is carried out in the ferrite state (approximately 560–590 °C), dominated by nitrogen, and generally not quenched.