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Nitriding
Nitriding is a surface hardening process that diffuses nitrogen into the surface of a steel workpiece at temperatures typically between 450°C – 600°C, below the lower critical temperature (Ac1). Unlike conventional quenching, nitriding produces an extremely hard surface layer (HV 700 – 1200) without requiring rapid cooling, resulting in minimal distortion. The process enhances wear resistance, fatigue strength, and corrosion resistance while maintaining the toughness of the core material.
Process Overview
- Pre-Treatment: Ensure the workpiece has been properly quenched and tempered to achieve the desired core properties
- Surface Preparation: Clean the surface thoroughly to remove any decarburized layers, oxide scales, or contaminants
- Ammonia Dissociation: Place parts in a sealed furnace with ammonia gas (NH₃); the ammonia dissociates at temperature to release active nitrogen atoms
- Nitrogen Diffusion: Active nitrogen atoms diffuse into the surface, forming iron nitrides (Fe₄N, Fe₁₆N₂) and alloy nitrides depending on the steel composition
- Controlled Cooling: Cool slowly in the furnace under a protective atmosphere to minimize distortion
Benefits
- Exceptional Surface Hardness: Achieves Vickers hardness of HV 700 – 1200, far exceeding conventional hardened steel
- Minimal Distortion: Process temperature below critical point means no phase transformation, resulting in minimal dimensional change
- Preserved Core Toughness: Low process temperature maintains the underlying material's toughness and ductility
- Improved Fatigue Resistance: Surface compressive stresses significantly enhance fatigue life under cyclic loading
- Corrosion Resistance: The nitride layer provides good resistance to water, steam, and certain chemicals
Technical Specifications
| Parameter | Specification |
| Process Temperature | 450°C – 600°C |
| Typical Temperature | 500°C – 530°C |
| Case Depth Range | 0.1mm – 0.7mm |
| Typical Case Depth | 0.25mm – 0.50mm |
| Surface Hardness | HV 700 – 1200 (≈ HRC 60 – 70) |
| Surface Hardness Retention | Up to 500°C |
| Process Duration | 10 – 100 hours |
| Typical Duration | 24 – 72 hours |
| Surface Roughness Change | Ra +0.2 – +0.5 μm increase |
Compatible Materials
✔ Nitriding Steels: AISI 4140, 4340, 4150, 5130, 6118, 8630
✔ Tool Steels: H13, P20, A2, D2, S7
✔ Stainless Steel: 400 series (precipitation hardening)
✔ Mold Steels: NAK80, 718HH, STAVAX
✔ Pre-hardened Steels: 30 – 45 HRC pre-hardened grades
Typical Applications
- Engine components: camshafts, crankshafts, valve parts
- Transmission parts: gears, shafts, synchro hubs
- Precision molds and plastic injection molds
- Hydraulic cylinders, pump housings, and valve components
- Turbine blades and compressor components
Comparison
| Feature | Nitriding | Carburizing | Induction Hardening |
| Surface Hardness | HV 700 – 1200 | HRC 58 – 62 | HRC 55 – 65 |
| Case Depth | 0.1mm – 0.7mm | 0.5mm – 2.0mm | 1mm – 10mm |
| Process Temperature | 450°C – 600°C | 900°C – 950°C | 850°C – 950°C |
| Distortion | Minimal | Moderate to high | Moderate |
| Core Toughness | Fully maintained | Soft and tough | Heat-affected |
| Fatigue Improvement | +30% – +80% | +40% – +100% | +30% – +60% |
Design Considerations
- Steel Selection Matters: Use dedicated nitriding steels with alloying elements (Cr, Mo, V, Al) that form stable nitrides; plain carbon steels are not suitable
- Pre-Hardening Required: Parts should be pre-hardened to the desired core strength before nitriding; the process is not for soft parts
- Masking Necessary Areas: Any areas that must remain soft (such as threads or bearing seats) should be masked or machined oversize before treatment
- Surface Cleanliness: Oil, grease, or decarburized layers must be removed; they inhibit nitrogen absorption and cause soft spots
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