SCM418 is the intermediate carburizing grade in JIS G4105’s Cr-Mo steel series, bridging the gap between SCM415 (0.13–0.18% C) and SCM420 (0.18–0.23% C). With 0.16–0.21% carbon, it provides higher core strength than SCM415 without the full alloy cost of SCM420. DIN 18CrMo4 (1.7243) is the closest international equivalent. This page covers equivalent grades, composition, mechanical and physical properties, heat treatment, machinability, weldability, and application selection guidance.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SCM418
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SCM418 | Japan | Reference |
| ASTM A29/A29M | AISI 4118 | North America | ⚠️ Nearest Equivalent (Cr 0.40–0.60% vs JIS 0.90–1.20% — significantly lower Cr) |
| DIN EN 10084 | 18CrMo4 / 1.7243 | Europe (Germany) | ✅ Nearest Exact (C 0.15–0.21%, Cr 0.90–1.20%, Mo 0.15–0.25% — very close match) |
| EN 10084 | 18CrMo4 / 1.7243 | Europe | ✅ Nearest Exact |
2. Chemical Composition
| Element | JIS SCM418 | AISI 4118 | DIN 18CrMo4 |
|---|---|---|---|
| C | 0.16–0.21% | 0.18–0.23% | 0.15–0.21% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.60–0.90% | 0.70–0.90% | 0.50–0.80% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.035% |
| S | ≤ 0.030% | ≤ 0.040% | ≤ 0.035% |
| Cr | 0.90–1.20% | 0.40–0.60% | 0.90–1.20% |
| Mo | 0.15–0.30% | 0.08–0.15% | 0.15–0.25% |
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 | Value (Metric) | Value (Imperial) |
|---|---|---|
| Core tensile strength | 835–1030 MPa | 121–149 ksi |
| Core yield point | ≥ 635 MPa | ≥ 92 ksi |
| Core elongation | ≥ 16% | ≥ 16% |
| Core hardness | HRC 30–40 | HRC 30–40 |
3-2. Case Properties After Carburizing + Quench
| Property | Value |
|---|---|
| Surface hardness | HRC 58–64 |
| Effective case depth (550 HV limit) | 0.5–1.8 mm (0.020–0.071 in) |
SCM418 in context: Core tensile strength is approximately 50–100 MPa higher than SCM415 and approximately 100 MPa lower than SCM420 at equivalent section size. When your core stress requirement falls between the other two grades, SCM418 avoids over-specifying (SCM420) or under-specifying (SCM415).
4. Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s modulus | 206 GPa | 29,900 ksi |
| Thermal conductivity | 42 W/(m·K) | 291 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 | Pre-machine grain refinement |
| Carburizing | 900–950°C (1652–1742°F) | Oil quench | Surface carbon enrichment |
| Case hardening | 820–860°C (1508–1580°F) | Oil quench | Harden case and core |
| Low tempering | 150–200°C (302–392°F) | Air cool | Stress relief, retain case hardness |
6. Machinability
- Rating: approximately 68% vs AISI 1212 baseline (100%) — intermediate between SCM415 (~70%) and SCM420 (~65%)
- Anneal before heavy rough machining to reduce tool wear
- Carbide tooling recommended for all production passes
- After carburizing and hardening, grinding only — no cutting operations
7. Weldability
- Rating: Fair. Carbon equivalent (Ceq) ≈ 0.38–0.51
- Preheat to 100°C (212°F) for sections greater than 20 mm (0.8 in)
- Use low-hydrogen welding consumables
- Post-weld stress relief recommended
- Always weld before carburizing — welding a carburized surface is not acceptable practice
8. Common Mistakes
Many engineers default to SCM415 or SCM420 without evaluating SCM418. When a core tensile requirement of approximately 800–950 MPa (116–138 ksi) is needed, SCM418 provides this without the higher alloy cost of SCM420 or the risk of under-specifying with SCM415. Review the actual core stress calculation before defaulting to the next higher grade.
DIN 18CrMo4 is the closest international match, but its Mo range (0.15–0.25%) has a slightly lower ceiling than JIS SCM418 (0.15–0.30%). In practice this difference is unlikely to affect performance, but for critical applications always verify the actual Mo content on the mill certificate rather than assuming the DIN and JIS grades are fully interchangeable.
9. When to Choose SCM418
- ✅ Medium-duty gears where SCM415 core strength is insufficient but SCM420 is over-specified
- ✅ Parts where DIN 18CrMo4 sourcing is available in European supply chains
- ✅ Bridging applications between SCM415 and SCM420 in gear and shaft design
- ✅ Cost-optimized designs where full SCM420 specification is not justified by load analysis
- ❌ Heavy-duty transmission gears under high contact stress — specify SCM420 or SNCM420
- ❌ Large sections greater than 50 mm (2 in) — hardenability may be insufficient without Ni addition
10. Frequently Asked Questions
Q: How does SCM418 differ from SCM415 and SCM420?
Carbon content is the defining variable: SCM415 (0.13–0.18%), SCM418 (0.16–0.21%), SCM420 (0.18–0.23%). Each 3-point carbon step raises achievable core strength after carburizing and Q+T by approximately 50–100 MPa (7–15 ksi). SCM418 core tensile is approximately 50–100 MPa higher than SCM415 and approximately 100 MPa lower than SCM420. Choose SCM418 when your core stress requirement falls between the other two grades and a full SCM420 specification is not justified.
Q: Is AISI 4118 a valid substitute for SCM418?
Not precisely. AISI 4118 has Cr 0.40–0.60% vs SCM418’s 0.90–1.20% — a major hardenability difference. For thin sections less than 15 mm (0.6 in), both grades achieve similar case and core results because section size limits hardenability expression regardless of alloy content. For sections greater than 20 mm (0.8 in), SCM418 maintains better core hardness due to higher Cr and Mo. Specify DIN 18CrMo4 as the preferred international equivalent for most applications outside Japan.
Q: When was SCM418 introduced and why?
SCM418 (along with SCM421, SCM822, and other intermediate grades) was added to JIS G4105 to give designers more precise control over hardenability and core properties without requiring custom heat specifications. The 3-point carbon increments in the SCM carburizing series allow engineers to match material to application load requirements without defaulting to the nearest higher grade and accepting unnecessary cost or over-engineering.
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