Ferritic nitrocarburizing (FNC, commonly known as soft nitriding) is carried out at about 560–590 °C. The atmosphere is mainly ammonia, and CO₂, endothermic gas or other carbon-containing gases are added to form a compound layer and a thin diffusion layer mainly composed of ε phase in a short period of time, which significantly improves the wear resistance, anti-seizure and corrosion resistance. The key point of control is to manage nitriding potential and carbon activity simultaneously; post-oxidation treatment can form a dense Fe₃O₄ layer on the surface to further improve corrosion resistance.
- Temperature approx. 560–590 °C, in ferrite state
- Atmosphere: NH₃ + carbon-containing gas (CO₂, endothermic gas, etc.)
- Result: Compound layer dominated by ε phase, processing time is relatively short
- Control: dual management of nitriding potential and carbon activity
- Post-oxidation improves corrosion resistance; the sampling tube is prone to ammonium bicarbonate crystals
The difference between nitriding and nitriding
| Project | Gas nitriding | Nitrocarburizing (FNC) |
|---|---|---|
| temperature | Approximately 500–580 °C | Approximately 560–590 °C |
| atmosphere | NH₃ (+diluent gas) | NH₃ + carbonaceous gas |
| time | longer | Short (usually a few hours) |
| main goal | Diffusion layer depth, fatigue strength | Compound layer: wear-resistant, anti-seize, corrosion-resistant |
| Typical parts | Crankshaft, gear, mold | Auto parts, hydraulic parts, fasteners, general structural parts |
Principle
The addition of carbon promotes the formation and stabilization of the ε phase, allowing the compound layer to grow faster and thicker. Therefore, FNC can obtain a compound layer with good performance in a short time, suitable for mild steel and ordinary carbon steel parts.
Atmosphere composition and control
Common atmosphere combinations:
- NH₃ + CO₂；
- NH₃ + endothermic gas;
- NH₃ + N₂ + CO₂ etc.
Controls need to consider both:
- nitriding potential Kn: Determines the absorption and phase composition of nitrogen;
- carbon activity: Affects the carbon content and compound layer quality in the ε phase;
- Gas ratio and flow rate: Ensure that the ingredients are uniform throughout the furnace.
Post-oxidation
After FNC, the surface is oxidized with an oxidizing atmosphere (such as water vapor, air or N₂O) under controlled conditions to form a dense Fe₃O₄ black oxide layer, which can significantly improve salt spray resistance and have a beautiful appearance. The process temperature, time and atmosphere need to be strictly controlled to avoid the formation of loose Fe₂O₃.
Frequently asked questions
What is the difference between soft nitriding and salt bath nitriding?
Salt bath nitrocarburizing uses molten salt as the medium, which is fast and has good uniformity, but there are problems with waste salt disposal and environmental protection. Gas nitrocarburizing is cleaner and facilitates automatic control and recording.
Is quenching required after FNC?
Generally not required. It is carried out in the ferrite state, and can be directly cooled after infiltration (oil cooling, water cooling or air cooling, according to process requirements).
Can stainless steel be nitrided?
Yes, but the passivation film on the surface of stainless steel will hinder nitriding, requires special activation treatment, and will reduce its own corrosion resistance. The use scenario needs to be evaluated.