AISI 8620 is a low-carbon nickel-chromium-molybdenum alloy steel specifically designed for case-hardening applications. It develops a very hard, wear-resistant surface through carburizing or carbonitriding while maintaining a tough, ductile, and shock-resistant core.
This combination makes 8620 one of the most widely used steels for gears, shafts, and power-transmission components where fatigue strength, impact resistance, and long service life are essential.
Chemical Composition
| Element | % | Function |
|---|---|---|
| Carbon (C) | 0.18 – 0.23 | Enables case hardening |
| Nickel (Ni) | 0.40 – 0.70 | Core toughness & fatigue resistance |
| Chromium (Cr) | 0.40 – 0.60 | Hardenability & wear resistance |
| Molybdenum (Mo) | 0.15 – 0.25 | Strength & resistance to softening |
| Manganese (Mn) | 0.70 – 0.90 | Strength & deoxidation |
| Silicon (Si) | 0.15 – 0.35 | Improves strength |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Machinability |
| Iron (Fe) | Balance | Base metal |
Microstructure & Metallurgy
• Annealed: Ferrite + pearlite (excellent machinability)
• After carburizing:
– Surface: High-carbon martensite (very hard)
– Core: Tempered martensite or bainite (tough & ductile)
• Fine-grained core structure ensures high impact and fatigue resistance
Mechanical Properties
Core Properties (After Case Hardening)
| Property | Typical Range |
|---|---|
| Core Tensile Strength | 620 – 850 MPa |
| Core Yield Strength | 415 – 650 MPa |
| Core Hardness | 20 – 32 HRC |
| Elongation | 15 – 25% |
| Impact Toughness | Very high |
Surface Properties (After Carburizing)
| Property | Typical Value |
|---|---|
| Surface Hardness | 58 – 62 HRC |
| Case Depth | 0.8 – 2.0 mm |
| Wear Resistance | Excellent |
| Fatigue Life | Very high |
Standards & Equivalents
| Standard | Equivalent |
|---|---|
| AISI / SAE | 8620 |
| ASTM | A29 / A322 |
| EN | 1.6523 |
| DIN | 20NiCrMo2-2 |
| JIS | SNCM220 |
| BS | 805M20 |
Applications
Automotive & Power Transmission: Gears, pinions, camshafts, axles
Industrial Machinery: Gearboxes, sprockets, couplings
Oil & Gas / Mining: Wear-resistant rotating parts, shafts
Agricultural & Construction: Drive gears, load-bearing components
8620 vs 4140 vs 4340
| Property | 8620 | 4140 | 4340 |
|---|---|---|---|
| Primary Use | Case-hardening | Through-hardening | Ultra-high strength |
| Core Toughness | Excellent | Good | Excellent |
| Surface Hardness | Very High | High | High |
| Cost | Moderate | Moderate | High |
Advantages
✔ Excellent wear resistance after carburizing
✔ Tough, crack-resistant core
✔ Long fatigue life
✔ Cost-effective for gears and shafts
✔ Widely accepted global standard
Limitations
⚠ Not suitable for through-hardening
⚠ Requires controlled carburizing process
⚠ Limited corrosion resistance (coating recommended)
Why Choose AISI 8620
AISI 8620 is a benchmark steel for case-hardened mechanical components. Its combination of extreme surface hardness and a tough, fatigue-resistant core ensures reliability, safety, and long service life in gears, shafts, and heavily loaded power-transmission systems.