SCM420 is the most widely used carburizing grade in JIS G4105’s Cr-Mo steel series. With 0.18–0.23% carbon, it delivers higher core strength than SCM415 while retaining HRC 58–64 case hardness after carburizing. This page covers international equivalents (AISI, DIN, EN), chemical composition, mechanical and physical properties, heat treatment, machinability, weldability, and application guidance for manufacturing engineers.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SCM420
- FAQ
1. International Equivalent Grades
No single ASTM or DIN grade matches SCM420’s combination of Cr 0.90–1.20% and Mo 0.15–0.30%. DIN 20MnCr5 is the most common European substitute but lacks molybdenum. AISI 4120 has significantly lower chromium (0.40–0.60%). Always verify composition and section-specific hardenability when substituting.
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SCM420 | Japan | Reference |
| ASTM A29/A29M | AISI 4120 | North America | ⚠️ Nearest Equivalent (Cr 0.40–0.60% vs SCM420’s 0.90–1.20% — significantly lower Cr) |
| DIN EN 10084 | 20MnCr5 / 1.7147 | Europe | ⚠️ Nearest Equivalent (no Mo, higher Mn — commonly used as substitute in European practice) |
| EN 10084 | 20MnCrMo5 / approx. 1.7325 | Europe | ⚠️ Nearest Equivalent |
2. Chemical Composition
| Element | JIS SCM420 | AISI 4120 | DIN 20MnCr5 |
|---|---|---|---|
| C | 0.18–0.23% | 0.18–0.23% | 0.17–0.22% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.60–0.90% | 0.70–0.90% | 1.10–1.40% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.035% |
| S | ≤ 0.030% | ≤ 0.040% | ≤ 0.035% |
| Cr | 0.90–1.20% | 0.40–0.60% | 1.00–1.30% |
| Mo | 0.15–0.30% | 0.20–0.30% | — (none) |
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 | 930–1130 MPa | 135–164 ksi |
| Core yield point | ≥ 690 MPa | ≥ 100 ksi |
| Core elongation | ≥ 15% | ≥ 15% |
| Core reduction of area | ≥ 50% | ≥ 50% |
| Core hardness | HRC 32–42 | HRC 32–42 |
3-2. Case Properties After Carburizing + Quench
| Property | Value |
|---|---|
| Surface hardness | HRC 58–64 |
| Effective case depth (550 HV limit) | 0.5–2.0 mm (0.020–0.079 in) |
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 | 41 W/(m·K) | 284 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 | 830–860°C (1526–1580°F) | Oil quench | Harden case |
| Low tempering | 150–200°C (302–392°F) | Air cool | Retain case hardness, relieve quench stress |
| Core re-hardening | 850–880°C (1562–1616°F) | Oil quench | Maximize core strength |
6. Machinability
- Rating: approximately 65% vs AISI 1212 baseline (100%)
- Harder to machine than SCM415 due to slightly higher carbon content
- Carbide tooling required for all production passes
- Anneal before heavy rough machining to reduce tool wear
- After carburizing and hardening, grinding only — no cutting operations
7. Weldability
- Rating: Fair. Carbon equivalent (Ceq) ≈ 0.40–0.53
- Preheat to 100–150°C (212–302°F) required for most section sizes
- Use low-hydrogen welding consumables
- Post-weld heat treatment strongly recommended
- Do not weld after carburizing — this is a fundamental design rule for all carburizing grades
8. Common Mistakes
DIN 20MnCr5 achieves similar case hardness but lower core hardness in sections greater than 25 mm (1 in) because it lacks molybdenum. For thin sections less than 20 mm (0.8 in), the substitute is generally acceptable. For sections greater than 25 mm, verify core hardness requirements against 20MnCr5 Jominy data before approving the substitution.
The H suffix denotes a guaranteed hardenability band (Jominy end-quench range). Standard SCM420 has no guaranteed Jominy profile — lot-to-lot hardenability variation is possible. If a specific Jominy end-quench profile is required for consistency across production lots (particularly for automated heat treatment lines), specify SCM420H explicitly on the drawing and purchase order.
9. When to Choose SCM420
- ✅ Heavily loaded transmission gears where core tensile strength greater than 900 MPa (130 ksi) is required
- ✅ Differential and final drive gear sets in automotive applications
- ✅ Bearing cages and races for medium-duty applications
- ✅ Any part where Mo-enhanced hardenability is needed for consistent case/core properties across production lots
- ❌ Small, lightly loaded gears where SCM415 is sufficient and more economical
- ❌ Applications requiring welding — select SCM415 or a lower-Ceq grade and weld before carburizing
10. Frequently Asked Questions
Q: Why doesn’t ASTM have a direct SCM420 equivalent?
SCM420 combines Cr 0.90–1.20% and Mo 0.15–0.30% — a composition range that ASTM distributes across multiple grades. AISI 4120 has lower Cr (0.40–0.60%), AISI 4320 adds Ni, and AISI 8620 has lower Cr and Mo. The JIS SCM series reflects a Cr-Mo alloy design philosophy that does not map directly onto ASTM’s historical compositional groupings. When sourcing globally, specify the composition range directly rather than the JIS grade name.
Q: What effective case depth is standard for automotive gears?
Typically 0.6–1.2 mm (0.024–0.047 in) effective case depth measured to 550 HV for module 3–5 automotive gears. Larger module gears (greater than module 5) may require 1.0–2.0 mm. Always specify case depth relative to the tooth module and application contact stress — do not use a single fixed value across all gear sizes.
Q: Is SCM420 available outside Japan?
SCM420 as a JIS-certified grade is primarily a Japanese-market specification. European mills supply 20MnCr5 or 18CrMo4 as substitutes. North American mills supply AISI 4120, 4118, or 8620 for carburizing applications. When designing for global manufacturing, specify the chemical composition range directly on the drawing and note acceptable international equivalents explicitly.
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