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Carburizing
Carburizing is a surface hardening process that increases the carbon content on the surface of low-carbon steel parts (typically < 0.25% C) while leaving the core relatively soft and tough. The workpiece is heated in a carbon-rich atmosphere (gas, liquid, or solid) at 900°C – 950°C, allowing carbon atoms to diffuse into the surface. After carburizing, the part is quenched and tempered to produce a hard, wear-resistant case (HRC 58 – 62) over a ductile core.
Process Overview
- Pre-Cleaning: Remove oils, scale, and contaminants from the workpiece surface
- Loading: Place parts in the furnace chamber with appropriate fixtures to ensure uniform gas flow
- Austenitizing + Carbon Diffusion: Heat to 900°C – 950°C in a carbon-rich atmosphere (endothermic gas, methane, or propane); hold for calculated time to achieve target case depth
- Carbon Potential Control: Monitor and control carbon potential (typically 0.80% – 1.20% C at surface) using oxygen probes and dew point sensors
- Quenching: Directly quench in oil or gas to transform the carburized layer to martensite
- Tempering: Low-temperature tempering (150°C – 200°C) to relieve quench stresses and reduce brittleness
Benefits
✔ High Surface Hardness: Achieves consistent hardness of HRC 58 – 62 on the surface for excellent wear resistance
✔ Deep Case Options: Case depths from 0.5mm to 2.0mm allow tailoring for different load conditions
✔ Tough Core: Low-carbon core remains soft and ductile, providing impact resistance and toughness
✔ Excellent Fatigue Resistance: Surface compressive stresses and high hardness dramatically improve fatigue life
✔ Cost-Effective for High Volume: Gas carburizing is highly automated and suitable for high-volume production
Technical Specifications
| Parameter | Specification |
| Process Temperature | 900°C – 950°C |
| Carbon Potential Range | 0.80% – 1.20% C |
| Case Depth Range | 0.5mm – 2.0mm |
| Typical Case Depth | 0.8mm – 1.5mm |
| Surface Hardness After Quench | HRC 58 – 62 |
| Core Hardness | HRC 15 – 25 (soft and tough) |
| Soaking Time (per 0.1mm depth) | ~1 – 1.5 hours |
| Quench Medium | Oil / Gas |
Compatible Materials
- ✔ Low-Carbon Steel (≤ 0.25% C, AISI 1018, 1020, 1025)
- ✔ Low-Alloy Steel (AISI 4118, 4320, 8620, 8720, 9310)
- ✔ Carburizing-Grade Steels (20Cr, 20CrMo, 20CrMnTi, 15CrNi3, 12CrNi2)
- ✔ Case-Hardening Steels (SAE 8620, 4320, 3310)
Typical Applications
- Automotive transmission gears and differential pinions
- Heavy-duty truck and off-highway equipment gears
- Aerospace bearings, fasteners, and structural components
- Agricultural machinery gears and power take-off components
- Industrial machinery: sprockets, worms, and cam followers
Comparison
| Feature | Carburizing | Carbonitriding | Nitriding |
| Surface Hardness | HRC 58 – 62 | HRC 55 – 62 | HV 700 – 1200 |
| Case Depth | 0.5mm – 2.0mm | 0.2mm – 1.5mm | 0.1mm – 0.7mm |
| Core Properties | Soft and tough | Moderate | High strength |
| Process Temperature | 900°C – 950°C | 820°C – 900°C | 450°C – 600°C |
| Distortion | Moderate to high | Moderate | Minimal |
| Cost | Moderate | Lower | Higher |
Design Considerations
- Minimum Case Depth Rule: Design case depth at least 10% of the fillet radius to avoid case crushing under bending loads
- Avoid Heavy Sections: Thick sections cool non-uniformly, causing soft spots or excessive distortion; use stepped designs if deep cases are needed
- Grinding Allowance: Carburized parts distort slightly; specify a grinding allowance of 0.1mm – 0.3mm per surface
- Core Hardness Limitation: For best results, specify core hardness and carbon content compatible with the required case depth; consult with your heat treater
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