SNCM415 is JIS G4105’s high-Ni carburizing grade, engineered for heavy-duty gears and shafts demanding both deep case and high core toughness. With Ni at 1.60–2.00% — more than double that of SNCM220 — it delivers significantly better hardenability and low-temperature impact resistance for sections up to 80 mm (3.1 in). This guide covers international equivalents, composition, carburized case and core properties, heat treatment parameters, and selection criteria for manufacturing engineers.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SNCM415
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SNCM415 | Japan | Reference |
| ASTM A29/A29M | AISI 4320 | USA | ✅ Nearest Exact (carburizing intent) |
| ASTM A29/A29M | AISI 4340 | USA | ⚠️ Nearest Equivalent (same Ni level; through-hardening grade) |
| DIN EN 10084 | 18CrNiMo7-6 / 1.6587 | Germany / Europe | ⚠️ Nearest Equivalent |
| EN 10084 | 18CrNiMo7-6 / 1.6587 | Europe | ⚠️ Nearest Equivalent |
AISI 4340 appears as a nearest equivalent by Ni content, but it is a through-hardening grade with C 0.38–0.43%. SNCM415 is a carburizing grade with C 0.12–0.18%. They are used for fundamentally different applications. Applying 4340 heat treatment data to SNCM415 will not produce the expected results. AISI 4320 (C 0.17–0.22%) is the correct carburizing-intent match.
2. Chemical Composition
| Element | JIS SNCM415 | AISI 4320 | DIN 18CrNiMo7-6 |
|---|---|---|---|
| C | 0.12–0.18% | 0.17–0.22% | 0.15–0.21% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.40–0.70% | 0.45–0.65% | 0.50–0.90% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.025% |
| S | ≤ 0.030% | ≤ 0.040% | ≤ 0.035% |
| Ni | 1.60–2.00% | 1.65–2.00% | 1.40–1.70% |
| Cr | 0.40–0.65% | 0.40–0.60% | 1.50–1.80% |
| Mo | 0.15–0.30% | 0.20–0.30% | 0.25–0.35% |
Sources: JIS G4105:2015, ASTM A29/A29M, DIN EN 10084
Note that DIN 18CrNiMo7-6 has significantly higher Cr (1.50–1.80%) than SNCM415 (0.40–0.65%), which improves hardenability but shifts the alloy balance. For large-section gears in European supply chains, 18CrNiMo7-6 is the most commonly available high-Ni carburizing grade and is widely used as the global substitute.
3. Mechanical Properties
3-1. Core Properties (After Carburizing + Quench + Low Temper, Section ≤ 30 mm / 1.2 in)
| Property | Metric | Imperial |
|---|---|---|
| Core tensile strength | 930–1130 MPa | 135–164 ksi |
| Core yield strength (0.2%) | ≥ 740 MPa | ≥ 107 ksi |
| Core elongation | ≥ 14% | ≥ 14% |
| Core impact (Charpy) | ≥ 108 J | ≥ 80 ft·lbf |
| Core hardness | HRC 32–42 | HRC 32–42 |
3-2. Case Properties (After Carburizing + Quench + Low Temper)
| Property | Value |
|---|---|
| Surface hardness | HRC 58–64 |
| Effective case depth (to 550 HV) | 0.8–2.5 mm (0.031–0.098 in) |
3-3. SNCM415 vs SNCM220 — Core Impact Comparison
| Property | SNCM220 | SNCM415 |
|---|---|---|
| Ni content | 0.40–0.70% | 1.60–2.00% |
| Core Charpy impact | ≥ 88 J (≥ 65 ft·lbf) | ≥ 108 J (≥ 80 ft·lbf) |
| Max effective section | ~50 mm (2 in) | ~80 mm (3.1 in) |
| Typical application | Automotive gears, module 4–7 | Heavy machinery, large-module gears |
4. Physical Properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s modulus | 205 GPa | 29,700 ksi |
| Thermal conductivity | 38 W/(m·K) | 264 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C / 68–212°F) | 11.3 × 10⁻⁶ /°C | 6.3 × 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 | 800–840°C (1472–1544°F) | Oil quench | Harden case; preserve core toughness |
| Low tempering | 150–200°C (302–392°F) | Air cool | Reduce residual stress; retain HRC 58–64 |
| Core re-hardening (optional) | 830–860°C (1526–1580°F) | Oil quench | Optimize core grain size independently |
SNCM415 allows effective case depths up to 2.5 mm (0.098 in), but excessive case depth relative to the tooth module leads to case spalling under bending fatigue. As a guideline, target case depth at 15–20% of the tooth module. For module 8 gears, this means 1.2–1.6 mm (0.047–0.063 in) effective case depth. Exceeding this ratio without design review is a common cause of premature gear failure.
6. Machinability
- Machinability approximately 55–60% relative to AISI 1212 free-machining steel — slightly lower than SNCM220 due to higher Ni content.
- Higher Ni increases work hardening tendency; maintain consistent tool engagement to avoid rubbing.
- Carbide tooling (P25–P35 grade) recommended for production turning. Coated carbide or CBN for finishing after hardening.
- Complete all heavy roughing and semi-finishing before carburizing. Finish grind after Q+T to remove distortion and meet tolerances.
- Drill and tap in the normalized or annealed condition only.
7. Weldability
- Carbon equivalent: Restricted. High Ni combined with Mo raises cold cracking risk in the HAZ significantly.
- Preheat temperature: 125–175°C (257–347°F) minimum; maintain throughout the weld operation.
- Critical rule: Always weld before carburizing. Welding after carburizing is not permitted — the high-carbon case will crack in the HAZ without exception.
- Post-weld: Stress relief at 600°C (1112°F) before carburizing if weld joints are present.
- Design guidance: Avoid weld joints in SNCM415 components wherever possible. Redesign to bolted or press-fit assembly if welding was specified.
8. Common Mistakes
Both grades contain approximately 1.8% Ni, leading to frequent cross-referencing errors. However, SNCM415 has C 0.12–0.18% and is designed to develop a carburized case, while AISI 4340 has C 0.38–0.43% and is a through-hardening structural grade. Applying 4340 quench-and-temper data to SNCM415 will produce a soft, uncarburized part with poor surface hardness. Always verify the application — carburizing vs through-hardening — before selecting the grade or the heat treatment procedure.
SNCM415’s high hardenability allows effective case depths up to 2.5 mm (0.098 in), but this capability is not a license to maximize case depth. For large-module gears, case depth exceeding 20% of the tooth module creates a brittle surface layer that spalls under bending fatigue at the tooth root — even though the surface hardness appears correct at HRC 58–64. Match case depth specification to 15–20% of the tooth module and document this on the heat treatment drawing.
9. When to Choose SNCM415
- ✅ Large-module gears (module ≥ 6) in heavy machinery, mining equipment, and industrial drives
- ✅ Differential ring gears and planetary gear sets in heavy trucks and off-highway vehicles
- ✅ Sections up to 80 mm (3.1 in) where SNCM220 hardenability is insufficient for core hardness targets
- ✅ Applications requiring core Charpy impact > 100 J under shock or cyclic loading
- ✅ JIS G4105 compliance requirements in Japanese heavy industry supply chains
- ❌ Small gears (module < 4) where SNCM220 or SCM420 is metallurgically sufficient and less expensive
- ❌ Through-hardening applications — use SNCM439 or SNCM447 for those
- ❌ Welded fabricated assemblies
10. FAQ
Q: Is AISI 4320 available in North America as a substitute for SNCM415?
Yes, AISI 4320 is available from specialty alloy mills in North America, but it is not routinely stocked at service centers the way 8620 or 4340 is. Confirm material availability and lead time before specifying 4320 for production. For European procurement, DIN 18CrNiMo7-6 (EN 1.6587) is the most widely available high-Ni carburizing grade globally and is the preferred substitute in European supply chains. Always obtain mill certificates and verify Ni, Cr, and Mo content when substituting between standards.
Q: Why does SNCM415 have lower carbon than SNCM220?
SNCM415’s lower C (0.12–0.18% vs SNCM220’s 0.17–0.23%) is intentional. The lower base carbon ensures a sharper case-to-core carbon gradient after carburizing — producing a tougher, lower-carbon core while maintaining the target surface carbon of ~0.8% in the carburized layer. The high Ni content (1.60–2.00%) compensates by providing the hardenability that lower carbon alone cannot achieve. This design philosophy is why SNCM415 outperforms SNCM220 in core impact toughness for equivalent-section carburized parts.
Q: What is the maximum section diameter for effective carburizing with SNCM415?
Effective carburizing — achieving HRC ≥ 30 in the core after oil quench — is practical up to approximately 80 mm (3.1 in) diameter for SNCM415. For sections between 80–120 mm (3.1–4.7 in), core hardness consistency degrades and SNCM625 or DIN 18CrNiMo7-6 should be evaluated. Beyond 120 mm, upgrading to a grade with higher Cr for added hardenability is strongly recommended. Confirm core hardness requirements with simulation or test bars before committing to production heat treatment of large sections.
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