JIS SCr420 (JIS G4105) is Japan’s standard chromium carburizing steel, positioned between the lower-carbon SCr415 and the through-hardenable SCr430. Its international equivalents — AISI 5120 (ASTM A29/A29M) and DIN 20Cr4 (EN 10084 / 1.7027) — share virtually identical chemistry. SCr420 is widely used for small-to-medium gears, gear pins, camshaft lobes, clutch splines, and cam followers where a hard carburized case (HRC 58–64) over a tough, ductile core is required. It offers a cost-effective alternative to Ni-Cr-Mo grades (SNCM220 / AISI 8620) for section sizes up to approximately 30 mm (1.2 in).
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SCr420
- FAQ
1. International Equivalent Grades
SCr420 is defined in JIS G4105 (Chromium steels for machine structural use). The table below lists confirmed international equivalents used in global procurement and design cross-referencing.
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SCr420 | Japan | Reference |
| ASTM A29/A29M | AISI 5120 | USA | ✅ Nearest Exact |
| DIN EN 10084 | 20Cr4 / 1.7027 | Germany / EU | ✅ Nearest Exact |
| EN 10084 | 20Cr4 / 1.7027 | Europe | ✅ Nearest Exact |
Sources: JIS G4105:2015, ASTM A29/A29M, DIN EN 10084
Note on AISI 5120 Cr range: AISI 5120 specifies Cr at 0.70–0.90%, while SCr420 and DIN 20Cr4 specify 0.90–1.20%. In practice, most US mill heats of 5120 fall in the mid-to-upper portion of the 5120 range — but the lower Cr bound of 5120 is genuinely below SCr420’s minimum. For critical gear applications, verify the mill certificate Cr content when substituting 5120 for SCr420.
2. Chemical Composition
| Element | JIS SCr420 | AISI 5120 | DIN 20Cr4 (1.7027) |
|---|---|---|---|
| C | 0.18–0.23% | 0.17–0.22% | 0.17–0.22% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.60–0.85% | 0.70–0.90% | 0.60–0.90% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.025% |
| S | ≤ 0.030% | ≤ 0.040% | ≤ 0.035% |
| Cr | 0.90–1.20% | 0.70–0.90% | 0.90–1.20% |
Sources: JIS G4105:2015, ASTM A29/A29M, DIN EN 10084
The low base carbon (0.18–0.23% C) is intentional — it ensures a tough, ductile core after carburizing and quenching. The surface carbon profile is controlled by the carburizing atmosphere, not the base material chemistry. Chromium improves hardenability over plain carbon grades (compare SCr420 vs S20C), allowing oil quench to develop adequate core hardness in sections up to approximately 30 mm (1.2 in).
3. Mechanical Properties
Core properties (after carburizing + oil quench + low temper 150–200°C / 302–392°F, section ≤ 25 mm / 1.0 in)
| Property | Metric | Imperial |
|---|---|---|
| Core tensile strength | 780–980 MPa | 113–142 ksi |
| Core yield strength | ≥ 590 MPa | ≥ 85.6 ksi |
| Core elongation | ≥ 16% | ≥ 16% |
| Core Charpy impact (KV) | ≥ 69 J | ≥ 51 ft·lbf |
| Core hardness | HRC 25–38 | HRC 25–38 |
Case properties (after carburizing + quench)
| Property | Value |
|---|---|
| Surface hardness | HRC 58–64 |
| Effective case depth (to 550 HV) | 0.3–1.5 mm (0.012–0.059 in) |
Core Charpy impact for SCr420 (≥ 69 J / 51 ft·lbf) is lower than SNCM220 (≥ 88 J / 65 ft·lbf) due to the absence of nickel. For applications subject to shock loading, sudden engagement, or reverse bending, SNCM220 or SNCM415 are the correct specifications.
Sources: JIS G4105:2015, JIS G0558 (case depth measurement), ASTM A255 (hardenability)
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 | 43 W/(m·K) | 298 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C / 68–212°F) | 11.5 × 10⁻⁶ /°C | 6.4 × 10⁻⁶ /°F |
| Specific heat | ~477 J/(kg·K) | 0.114 BTU/(lb·°F) |
5. Heat Treatment Conditions
SCr420 is a carburizing grade — the heat treatment sequence is specifically designed to enrich the surface in carbon while keeping the core at low carbon for toughness. The sequence below covers the standard production route.
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 850–900°C (1562–1652°F) | Air cool | Grain refinement before machining |
| Carburizing | 900–950°C (1652–1742°F) | Oil quench | Surface carbon enrichment to 0.7–0.9% C |
| Case hardening (direct quench from carburize) | 820–860°C (1508–1580°F) | Oil quench | Simultaneous case and core hardening |
| Low tempering | 150–200°C (302–392°F) | Air cool | Stress relief; maintain HRC 58–64 case |
| Optional: core re-hardening | 840–870°C (1544–1598°F) | Oil quench | Optimize core properties separately from case |
Carburizing case depth control
Effective case depth (to 550 HV) is controlled by carburizing temperature and soak time. Typical parameters for gas carburizing in an endothermic atmosphere:
| Target case depth | Approx. soak time at 920°C / 1688°F |
|---|---|
| 0.3–0.5 mm (0.012–0.020 in) | 1.5–3 hours |
| 0.5–0.8 mm (0.020–0.031 in) | 3–5 hours |
| 0.8–1.2 mm (0.031–0.047 in) | 5–9 hours |
| 1.2–1.5 mm (0.047–0.059 in) | 9–14 hours |
Actual soak time depends on furnace carbon potential, load density, and part geometry. Always verify case depth on witness bars or sacrificial parts cut from each batch.
6. Machinability
SCr420 machinability is approximately 60% relative to AISI 1212. The low base carbon (0.18–0.23% C) makes SCr420 relatively easy to machine in the normalized condition — easier than S45C or S50C.
Key machining rules for carburizing grades
- Machine all features — including threads, holes, keyways, and undercuts — to finished dimensions before carburizing. Do not leave machining allowance for post-carburize cutting of case-hardened surfaces.
- Surfaces that must not be case-hardened (e.g., bore diameters, thread roots) should be copper-plated or masked with anti-carburizing paste before furnace loading.
- All finish dimensions are achieved by cylindrical grinding or surface grinding after heat treatment. Grinding allowance (typically 0.1–0.2 mm / 0.004–0.008 in per side) must be factored into pre-carburize dimensions.
- Carbide tooling (P20–P30) is recommended for production turning. HSS is acceptable for small-batch drilling and tapping before carburizing.
7. Weldability
SCr420 weldability is restricted. Carbon equivalent Ceq ≈ 0.38–0.52. Welding is feasible before carburizing but should be avoided after carburizing — the high-carbon case in the HAZ is extremely susceptible to cold cracking, and welding destroys the case properties in the weld zone.
Welding a carburized SCr420 part introduces localized re-austenitizing and quenching cycles in the HAZ that produce untempered martensite in the high-carbon case layer. This results in micro-cracking and potential delamination of the case. Any weld repair required after carburizing must be approached as a scrap-and-replace decision unless the welded zone can be re-carburized and re-hardened in its entirety.
If welding before carburizing is required: preheat to 100°C (212°F) minimum for sections thicker than 15 mm (0.6 in). Apply post-weld stress relief before carburizing if distortion is a concern in close-tolerance gear blanks.
8. Common Mistakes
Without nickel or molybdenum, SCr420’s oil-quench hardenability is limited to approximately 30–35 mm (1.2–1.4 in) effective section for core hardness above HRC 30. For sections in the 35–60 mm (1.4–2.4 in) range, the core will remain insufficiently hard after oil quench to support the Hertzian contact stresses and bending fatigue loads typical of heavily loaded gears. This is not a conservative estimate — it reflects the physics of carbon diffusion and the Jominy hardenability band of SCr420/AISI 5120. For sections above 35 mm (1.4 in), upgrade to SNCM220 (AISI 8620) or SCM420, which provide adequate core hardness through the section with oil quench due to their Ni and/or Mo content.
AISI 5120 specifies Cr at 0.70–0.90%, while SCr420 specifies Cr at 0.90–1.20%. This is not a rounding difference — heats at the lower Cr bound of the 5120 range (0.70–0.75% Cr) will have measurably lower hardenability than SCr420 heats at the lower bound of their range (0.90% Cr). For critical automotive transmission gears where the design assumed SCr420 hardenability, substituting 5120 at the low Cr end can result in insufficient core hardness in sections approaching 25–30 mm (1.0–1.2 in). When procuring AISI 5120 as an SCr420 substitute for critical gears, specify a minimum Cr of 0.90% on the purchase order and verify the mill certificate accordingly.
9. When to Choose SCr420
- ✅ Small-to-medium gears (module 1–5) with section ≤ 30 mm (1.2 in)
- ✅ Gear pins, wrist pins, clutch splines, and cam followers
- ✅ Camshaft lobes and tappets for engines and machinery
- ✅ Applications where AISI 5120 or DIN 20Cr4 is the design standard
- ✅ Cost-sensitive designs where Ni-Cr-Mo grades are over-specified for the section size
- ❌ Sections > 35 mm (1.4 in) requiring core HRC > 30 — specify SCM420 or SNCM220
- ❌ Heavy shock-loading gears requiring Charpy impact > 88 J (65 ft·lbf) — specify SNCM220 or SNCM415
- ❌ Through-hardening applications — specify SCr430 or SCr440
- ❌ Induction hardening — base C is too low (0.18–0.23%); specify SCr430 or S45C
10. FAQ
What is the difference between SCr420 and SCr415?
The difference is carbon content: SCr415 (0.13–0.18% C) versus SCr420 (0.18–0.23% C). Both are carburizing grades where the case hardness and surface carbon profile are controlled by the carburizing process rather than the base carbon. However, the higher base carbon in SCr420 provides marginally better core strength and slightly improved hardenability after quenching — the core responds more readily to oil quench. SCr415 is used when maximum core ductility is required, or in very thin-section parts where even SCr420’s core hardenability is more than needed. For the majority of carburizing applications, SCr420 is the preferred choice over SCr415.
Is SCr420 equivalent to 16MnCr5?
No — these are different alloy systems and should not be treated as equivalent. DIN 16MnCr5 (EN 10084) uses elevated manganese (1.00–1.30%) and chromium (0.80–1.10%) as the primary hardenability additions, while SCr420 uses chromium alone (0.90–1.20%) with standard manganese (0.60–0.85%). 16MnCr5 is extremely widely used in European automotive production and is a close equivalent to AISI 5115 — not to SCr420 or AISI 5120. For European procurement to a JIS SCr420 design, the correct equivalent is DIN 20Cr4 (1.7027), not 16MnCr5. Confusing these two European grades is a common sourcing error.
Can SCr420 replace SNCM220 (AISI 8620) for automotive transmission gears?
For small-to-medium section gears with section ≤ 25 mm (1.0 in), SCr420 can produce equivalent case properties: HRC 58–64 surface hardness and similar effective case depth. The relevant limitation is core impact resistance: SCr420 typically achieves ≥ 69 J (51 ft·lbf) Charpy KV vs SNCM220’s ≥ 88 J (65 ft·lbf). For gears subject to shock loading, sudden clutch engagement, reverse loading, or operating at low temperatures, SNCM220 is the correct specification — its nickel content provides the additional impact resistance the application requires. For monotonic-load gears in cost-sensitive designs at sections ≤ 25 mm where shock loading is not a design condition, SCr420 is a viable and commercially sensible alternative to SNCM220.
What surface carbon should be targeted during carburizing of SCr420?
The standard target surface carbon for SCr420 is 0.75–0.90% C after carburizing. Surface carbon above 0.90% risks forming a continuous grain-boundary carbide network (carbide netting), which severely reduces bending fatigue strength by providing initiation sites for inter-granular fracture. Surface carbon below 0.75% results in insufficient surface hardness and reduced wear life. The carbon potential of the furnace atmosphere is typically controlled to 0.80–0.85% C for general gear production. For high-performance gears with demanding fatigue life requirements, verify surface carbon by microprobe or hardness traverse on witness specimens from each batch.
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