SCM435 and SCM440 are JIS G4105 chromium-molybdenum steels that differ by a single variable: carbon content. SCM435 runs 0.33–0.38% C; SCM440 runs 0.38–0.43% C. They are the Japanese equivalents of AISI 4135 and AISI 4140 respectively — together forming the most widely used engineering steel family in the world. This comparison gives engineers the specific data and decision criteria to choose correctly between them, with quantified property differences and application-specific guidance.
- Grade Overview & Key Differences
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties Comparison
- Physical Properties
- Heat Treatment — Differences Between the Two Grades
- Machinability & Weldability Comparison
- Common Mistakes
- Decision Guide: When to Choose SCM435 vs SCM440
- FAQ
1. Grade Overview & Key Differences
The only compositional difference between SCM435 and SCM440 is carbon. All other alloying elements — Si, Mn, P, S, Cr, Mo — are specified at identical ranges in JIS G4105. This means hardenability curves (Jominy end-quench), microstructure after austenitizing, and weld procedure requirements are nearly identical. The meaningful engineering difference is as-tempered hardness and strength for a given temper temperature.
| Grade | C Range | ASTM Equivalent | Typical Q+T Tensile (550°C / 1022°F temper) |
|---|---|---|---|
| SCM435 | 0.33–0.38% | AISI 4135 | ~900–1000 MPa (131–145 ksi) |
| SCM440 | 0.38–0.43% | AISI 4140 | ~1000–1100 MPa (145–160 ksi) |
The practical decision rule: If your design requires tensile strength in the 800–1000 MPa (116–145 ksi) range, SCM435 tempered at a higher temperature delivers better toughness than SCM440 tempered to the same tensile. If your design requires 1000–1200 MPa (145–174 ksi), SCM440 is the correct grade. If procurement from global service centers is a priority, SCM440 / AISI 4140 / 42CrMo4 is the dominant stock grade worldwide.
2. International Equivalent Grades
| Grade | JIS (G4105) | ASTM / SAE | DIN / EN | BS |
|---|---|---|---|---|
| SCM435 | SCM435 | AISI 4135 | 34CrMo4 / 1.7220 | 708M37 |
| SCM440 | SCM440 | AISI 4140 | 42CrMo4 / 1.7225 | 708M40 |
42CrMo4 / AISI 4140 / SCM440 is by far the more prevalent grade in global markets. North American and European service centers hold it in bar, flat bar, and plate from 10 mm (0.39 in) to over 300 mm (11.8 in) diameter. SCM435 / 34CrMo4 / AISI 4135 is available but not routinely stocked at all size ranges. For mixed JIS/EN supply chains or multi-source procurement, specifying SCM440/4140 provides substantially better material availability.
Sources: JIS G4105:2016, EN 10083-1:2006, SAE J404
3. Chemical Composition
| Element | SCM435 | SCM440 |
|---|---|---|
| C | 0.33–0.38% | 0.38–0.43% |
| Si | 0.15–0.35% | 0.15–0.35% |
| Mn | 0.60–0.85% | 0.60–0.85% |
| P | ≤ 0.030% | ≤ 0.030% |
| S | ≤ 0.030% | ≤ 0.030% |
| Cr | 0.90–1.20% | 0.90–1.20% |
| Mo | 0.15–0.30% | 0.15–0.30% |
Source: JIS G4105:2016 Table 1
4. Mechanical Properties Comparison
Normalized Condition
| Property | SCM435 | SCM440 |
|---|---|---|
| Tensile strength | ~800 MPa (116 ksi) | ~880 MPa (128 ksi) |
| Yield strength | ~620 MPa (90 ksi) | ~690 MPa (100 ksi) |
| Hardness | ~235–255 HB | ~255–280 HB |
Quenched and Tempered at 500°C (932°F)
| Property | SCM435 | SCM440 |
|---|---|---|
| Tensile strength | ~1000–1100 MPa (145–160 ksi) | ~1100–1200 MPa (160–174 ksi) |
| Yield strength | ~850–950 MPa (123–138 ksi) | ~950–1050 MPa (138–152 ksi) |
| Hardness | HRC 30–38 | HRC 33–42 |
| Charpy impact | Higher (better toughness) | Slightly lower at same tensile |
Quenched and Tempered at 550°C (1022°F)
| Property | SCM435 | SCM440 |
|---|---|---|
| Tensile strength | ~900–1000 MPa (131–145 ksi) | ~1000–1100 MPa (145–160 ksi) |
| Yield strength | ~750–870 MPa (109–126 ksi) | ~850–950 MPa (123–138 ksi) |
| Hardness | HRC 28–34 | HRC 30–38 |
Sources: JIS G4105:2016 (mechanical property guidance); typical values based on 25 mm / 1.0 in reference diameter, oil quenched
5. Physical Properties
Physical properties are identical for both grades. The minor carbon difference has no measurable effect on density, modulus, or thermal characteristics.
| 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) | 12.3 × 10⁻⁶ /°C | 6.8 × 10⁻⁶ /°F |
| Specific heat | ~477 J/(kg·K) | 0.114 BTU/(lb·°F) |
6. Heat Treatment — Differences Between the Two Grades
Austenitizing temperature and quench medium are identical. The practical differences appear in the temper temperature required to achieve a given strength level, and in the maximum achievable surface hardness for induction hardening applications.
| Parameter | SCM435 | SCM440 |
|---|---|---|
| Austenitizing (oil quench) | 830–870°C (1526–1598°F) | 830–870°C (1526–1598°F) |
| Temper temp to reach ~900 MPa (131 ksi) | ~575–625°C (1067–1157°F) | ~600–650°C (1112–1202°F) |
| Temper temp to reach ~1100 MPa (160 ksi) | ~480–510°C (896–950°F) | ~500–530°C (932–986°F) |
| Max hardness (as-quenched) | HRC 55–57 | HRC 57–60 |
| Hardness at 400°C (752°F) temper | HRC 40–46 | HRC 44–50 |
Induction Hardening
For induction-hardened shafts and journals, SCM440 achieves 2–3 HRC higher surface hardness than SCM435 under identical induction process parameters. If the surface hardness target is HRC 52–56, SCM440 is the reliable choice. SCM435 may fall short of this target with the same induction setup, requiring process adjustment or a harder temper on the core — which may reduce toughness.
Both SCM435 and SCM440 are susceptible to temper embrittlement when tempered in the 300–370°C (572–698°F) range. This range produces lower toughness than both lower and higher tempering temperatures. Avoid tempering in this zone for impact-critical applications. The standard practice is to temper above 400°C (752°F) and cool rapidly through the embrittlement range after holding.
7. Machinability & Weldability Comparison
Machinability
Both grades machine at approximately 65% relative to AISI 1212. SCM435 machines marginally more easily due to its lower carbon content, but the practical difference in production operations is negligible. Tool life, surface finish, and cutting parameters are effectively identical for both grades in the normalized or annealed condition.
Weldability
Carbon equivalent for both grades falls in the 0.65–0.85 range (IIW formula), placing both firmly in the “preheat required” category for structural welding. The following requirements apply to both grades equally:
| Parameter | SCM435 & SCM440 (identical) |
|---|---|
| Preheat (t ≤ 25 mm / 1.0 in) | 100–150°C (212–302°F) |
| Preheat (t > 25 mm / 1.0 in) | 150–200°C (302–392°F) |
| Interpass temperature (max) | 250°C (482°F) |
| PWHT (stress relief) | 600–650°C (1112–1202°F), 1 hr/25 mm (1 in) |
| Recommended filler (SMAW) | E8018-B2 (1Cr-0.5Mo) |
8. Common Mistakes
Most engineers default to SCM440 because it is more widely recognized and stocked — even when the design only requires 800–900 MPa (116–131 ksi) tensile. When a component is designed for 900 MPa, SCM435 tempered at a higher temperature (575–625°C / 1067–1157°F) achieves that tensile with better toughness and slightly better elongation compared to SCM440 tempered to the same 900 MPa level (which would require a lower temper temperature). Over-specifying the grade does not reduce machining difficulty — it increases it slightly and reduces the toughness margin at the design tensile.
SCM435’s international equivalent is 34CrMo4 / AISI 4135 — not 4140. AISI 4140 property data (from supplier datasheets, ASM handbooks, or Matweb) is approximately 50–100 MPa (7–15 ksi) higher than the equivalent heat treatment on SCM435. If a design allowable was calculated using 4140 mechanical data, and the procurement specification or mill cert shows SCM435 (or 34CrMo4 / 4135), the as-delivered properties will be lower than assumed. Always confirm heat treatment achieves the required tensile and hardness on the actual grade procured — do not rely on the grade name alone to guarantee properties.
9. Decision Guide: When to Choose SCM435 vs SCM440
Choose SCM435 when:
- ✅ Required tensile strength is 800–1000 MPa (116–145 ksi) — SCM435 at a higher temper temperature provides better toughness at this level
- ✅ The design is based on AISI 4135 or DIN 34CrMo4 reference data
- ✅ Toughness at a given strength level is a priority — lower C yields better Charpy impact at the same tensile target
- ✅ The component requires maximum elongation and ductility after Q+T (e.g., connecting rods, torsion bars)
Choose SCM440 when:
- ✅ Required tensile strength is 1000–1200 MPa (145–174 ksi) — SCM440 is purpose-designed for this tier
- ✅ Surface hardness via induction hardening ≥ HRC 50 is required
- ✅ The design is based on AISI 4140 or DIN 42CrMo4 data — by far the more common global reference
- ✅ Procurement from global service centers — SCM440 / 4140 / 42CrMo4 is the dominant stocked grade worldwide
- ✅ Applications: high-torque drive shafts, spindles, bolts above M30 class 10.9, gear shafts, crankshafts
Neither grade when:
- ❌ Section diameter > 100 mm (4.0 in) requiring uniform core hardness — add Ni; specify SNCM439 or SNCM630
- ❌ Carburizing case hardening — C is too high for carburizing; use SCM415 or SCM420
- ❌ Surface hardness > HRC 60 required — use tool steel (SKD11, SKD61) or case-hardening grades
- ❌ Severe corrosion environment — these are uncoated alloy steels; consider stainless or surface coating
10. FAQ
Is AISI 4140 the same as SCM440?
Yes, for all practical engineering purposes. Composition ranges overlap completely between JIS G4105 SCM440 and SAE/ASTM 4140. Heat treatment response — austenitizing temperature, quench medium, temper curves — is equivalent. Mechanical property data from AISI 4140 datasheets (tensile, yield, hardness at specific Q+T conditions) applies directly to SCM440. The only difference is the designation system: JIS G4105 for SCM440, SAE J404 / ASTM A322 for 4140.
Which grade is more common globally?
SCM440 / AISI 4140 / 42CrMo4 is significantly more common. North American and European service centers stock it in bar, round, flat bar, and plate across essentially every diameter and thickness range. SCM435 / 4135 / 34CrMo4 is available but not routinely held at all sizes. If rapid procurement from multiple global vendors is a project requirement, SCM440/4140 provides substantially shorter lead times and lower minimum order quantities.
Can SCM435 and SCM440 be welded together?
Yes. The compositions are nearly identical and Ceq values are in the same range. Use E8018-B2 (1Cr-0.5Mo) filler metal, preheat to 150°C (302°F) minimum for t > 25 mm (1.0 in), and apply post-weld stress relief at 600–650°C (1112–1202°F) for 1 hour per 25 mm (1.0 in) of thickness. There is no metallurgical incompatibility between the grades in a dissimilar weld joint. The weld procedure qualification (WPS/PQR) can cover both grades under the same P-number grouping (P-4 per ASME IX).
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