SNCM220 is JIS G4105’s Ni-Cr-Mo carburizing grade — the Japanese equivalent of AISI 8620, the world’s most widely used case-hardening alloy steel. The Ni addition (0.40–0.70%) improves core toughness compared to Cr-Mo grades such as SCM415 and SCM420, making SNCM220 the preferred choice for heavily loaded gears and shafts requiring impact-resistant cores. This guide covers international equivalents, composition, carburized case and core properties, heat treatment, and selection guidance for manufacturing engineers crossing JIS and ASTM supply chains.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SNCM220
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SNCM220 | Japan | Reference |
| ASTM A29/A29M (SAE) | AISI 8620 | USA | ✅ Nearest Exact |
| DIN EN 10084 | 21NiCrMo2 / 1.6523 | Germany / Europe | ⚠️ Nearest Equivalent |
| EN 10084 | 21NiCrMo2 / 1.6523 | Europe | ⚠️ Nearest Equivalent |
AISI 8620 and SNCM220 are effectively the same material. Compositions overlap completely and mechanical property ranges are identical — no conversion factor is required when crossing between JIS and ASTM/SAE data sheets. DIN 21NiCrMo2 is a near equivalent, but specifies Mo 0.08–0.15% vs SNCM220’s 0.15–0.30%, which reduces hardenability slightly for larger sections.
2. Chemical Composition
| Element | JIS SNCM220 | AISI 8620 | DIN 21NiCrMo2 |
|---|---|---|---|
| C | 0.17–0.23% | 0.18–0.23% | 0.17–0.23% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.60–0.90% | 0.70–0.90% | 0.60–0.90% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.035% |
| S | ≤ 0.030% | ≤ 0.040% | ≤ 0.035% |
| Ni | 0.40–0.70% | 0.40–0.70% | 0.40–0.70% |
| Cr | 0.40–0.65% | 0.40–0.60% | 0.35–0.65% |
| Mo | 0.15–0.30% | 0.15–0.25% | 0.08–0.15% |
Sources: JIS G4105:2015, ASTM A29/A29M, DIN EN 10084
3. Mechanical Properties
3-1. Core Properties (After Carburizing + Quench + Low Temper, Section ≤ 25 mm / 1 in)
| Property | Metric | Imperial |
|---|---|---|
| Core tensile strength | 830–1030 MPa | 120–149 ksi |
| Core yield strength (0.2%) | ≥ 635 MPa | ≥ 92 ksi |
| Core elongation | ≥ 16% | ≥ 16% |
| Core impact (Charpy) | ≥ 88 J | ≥ 65 ft·lbf |
| Core hardness | HRC 30–40 | HRC 30–40 |
3-2. Case Properties (After Carburizing + Quench + Low Temper)
| Property | Value |
|---|---|
| Surface hardness | HRC 58–64 |
| Effective case depth (to 550 HV) | 0.5–2.0 mm (0.020–0.079 in) |
Note on carburizing grades: Mechanical properties for SNCM220 are measured on the core (away from the carburized surface). Surface hardness is determined by the carburized carbon profile, not the base composition. Core toughness — represented by Charpy impact — is where SNCM220 distinguishes itself from SCM420 and other Cr-Mo grades.
4. Physical Properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s modulus | 206 GPa | 29,900 ksi |
| Thermal conductivity | 40 W/(m·K) | 277 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C / 68–212°F) | 11.2 × 10⁻⁶ /°C | 6.2 × 10⁻⁶ /°F |
| Specific heat | ~477 J/(kg·K) | 0.114 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 850–900°C (1562–1652°F) | Air cool | Grain refinement before carburizing |
| Carburizing | 900–950°C (1652–1742°F) | Oil quench | Carbon enrichment of surface layer |
| Case hardening (direct quench) | 820–860°C (1508–1580°F) | Oil quench | Harden case and core simultaneously |
| Low tempering | 150–200°C (302–392°F) | Air cool | Reduce residual stress; maintain HRC 58–64 |
| Core re-hardening (optional) | 840–870°C (1544–1598°F) | Oil quench | Optimize core grain size independently |
Effective case depth (to 550 HV) must be matched to the application. For automotive ring gears (module 4–7), target 0.8–1.4 mm (0.031–0.055 in). Exceeding 2.0 mm case depth without adjusting the tooth module risks case brittleness and spalling under bending fatigue. Always specify minimum and maximum case depth on engineering drawings.
6. Machinability
- Machinability approximately 65% relative to AISI 1212 free-machining steel — similar to SCM420.
- Ni addition improves core toughness but does not significantly alter cutting behavior compared to the SCM series.
- Carbide tooling (P20–P30 grade) recommended for production turning; HSS acceptable for small-batch drilling and tapping.
- All heavy machining should be completed before carburizing. Light finishing and grinding are performed after heat treatment.
- Work hardening tendency is low due to the low base carbon content (0.17–0.23%).
7. Weldability
- Carbon equivalent: Ceq ≈ 0.38–0.52 (IIW formula). Lower Cr than SCM420 makes SNCM220 slightly more weldable than Cr-Mo grades.
- Preheat temperature: 100°C (212°F) minimum for t > 20 mm (0.79 in).
- Recommended process: SMAW with low-hydrogen electrodes (E7018), GMAW with ER70S-2.
- Critical rule: Always weld before carburizing. Welding a carburized surface introduces severe HAZ hardness gradients and cracking risk.
- Post-weld: Stress relief before carburizing if distortion control is critical.
8. Common Mistakes
SNCM220 and AISI 8620 are essentially the same material. Engineers sourcing in Japan from a North American design that used 8620 test data sometimes apply a conversion factor or request re-testing, adding cost and delay. If your design data cites 8620 tensile, yield, impact, or hardness values, those numbers apply directly to SNCM220 material. No separate qualification testing is required for equivalent applications.
DIN 21NiCrMo2 specifies Mo 0.08–0.15% vs SNCM220’s 0.15–0.30%. At the DIN lower Mo bound, hardenability in sections larger than 30 mm (1.2 in) will be measurably reduced. Core hardness after oil quench may fall 3–5 HRC below SNCM220 values for the same section. For automotive gears with section ≤ 25 mm this difference is minor. For larger agricultural or industrial gears, verify the Mo content on mill certificates when substituting 21NiCrMo2.
9. When to Choose SNCM220
- ✅ Automotive transmission and differential gears requiring impact-resistant carburized cores
- ✅ Parts manufactured in North American or European supply chains where AISI 8620 or DIN 21NiCrMo2 is standard stock
- ✅ Carburized gears where SCM415 or SCM420 core toughness is insufficient for shock-load conditions
- ✅ Ring gears, pinions, and CV joint components in passenger and light commercial vehicles
- ✅ Cross-standard procurement — SNCM220 and 8620 mill data is interchangeable
- ❌ Large sections > 50 mm (2 in) — insufficient Ni for deep hardenability; use SNCM415 or SNCM439
- ❌ Through-hardening applications — use a higher-carbon grade (SCr440, SCM440)
- ❌ Heavy-duty large-module gears requiring core Charpy > 100 J — use SNCM415
10. FAQ
Q: Is SNCM220 the same as AISI 8620?
Yes, for all practical engineering purposes. The compositions overlap completely and the mechanical property ranges are identical. If you have AISI 8620 material test data, it applies directly to SNCM220. The grade designation differs between JIS (Japan) and ASTM/SAE (USA) standards, but the metallurgy, heat treatment response, and carburizing behavior are the same. No conversion factor or re-qualification is needed when crossing between the two designations.
Q: Why choose SNCM220 over SCM420 for gears?
The Ni addition in SNCM220 (0.40–0.70%) improves low-temperature impact toughness and fatigue resistance in the carburized core. For gears subject to shock loading, sudden load reversals, or operation below 0°C (32°F), SNCM220 typically shows 20–40% better Charpy core impact values than SCM420 in equivalent heat treatment conditions. For monotonic loading applications without significant impact, SCM420 with its higher Cr provides better hardenability for sections above 30 mm (1.2 in), making it more suitable for larger automotive structural parts.
Q: What case depth is standard for automotive ring gears made from SNCM220?
Typically 0.8–1.4 mm (0.031–0.055 in) effective case depth (to 550 HV) for module 4–7 ring gears. Larger module gears (module 8–12) may require case depths up to 2.0 mm (0.079 in) to ensure adequate bending fatigue resistance at the tooth root. The Ni-enriched core of SNCM220 supports deeper cases without excessive brittleness risk, a practical advantage over plain Cr-Mo carburizing grades for large-module applications.
Q: Can SNCM220 be through-hardened instead of carburized?
Technically yes, but not advantageous. At 0.17–0.23% C, through-hardening SNCM220 produces relatively low core hardness (approximately HRC 25–35 depending on section) without a hard case. The alloy cost of SNCM220 is not justified for through-hardening; use SCr430 or SCM430 instead. SNCM220 is specifically designed to develop a hard carburized case (HRC 58–64) over a tough Ni-strengthened core.
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