Overview
JIS SCM440 is a chromium-molybdenum alloy steel widely used for high-strength structural components such as shafts, gears, bolts, and dies. Its closest international equivalent is ASTM/AISI 4140, making it straightforward to substitute in global procurement. SCM440 delivers excellent hardenability, fatigue strength, and toughness after quench-and-temper treatment, making it the go-to alloy steel for demanding mechanical applications worldwide.
Quick Comparison Box
| Standard | Designation | Region | Match Type |
|---|---|---|---|
| JIS G4105 | SCM440 | Japan | — |
| ASTM A29 / AISI | 4140 | USA | Nearest Equivalent |
| ISO 683-2 | 42CrMo4 | International | Nearest Equivalent |
| DIN 17200 | 42CrMo4 | Germany | Nearest Equivalent |
| EN 10083-3 | 42CrMo4 (1.7225) | Europe | Nearest Equivalent |
Chemical Composition
| Element | JIS SCM440 (JIS G4105) |
ASTM 4140 (ASTM A29) |
EN 42CrMo4 (EN 10083-3) |
|---|---|---|---|
| C (%) | 0.38–0.43 | 0.38–0.43 | 0.38–0.45 |
| Si (%) | 0.15–0.35 | 0.15–0.35 | ≤ 0.40 |
| Mn (%) | 0.60–0.85 | 0.75–1.00 | 0.60–0.90 |
| P (%) | ≤ 0.030 | ≤ 0.035 | ≤ 0.025 |
| S (%) | ≤ 0.030 | ≤ 0.040 | ≤ 0.035 |
| Cr (%) | 0.90–1.20 | 0.80–1.10 | 0.90–1.20 |
| Mo (%) | 0.15–0.30 | 0.15–0.25 | 0.15–0.30 |
Sources: JIS G4105:2015, ASTM A29/A29M, EN 10083-3:2006
Mechanical Properties
Values below are for quenched-and-tempered (Q&T) condition, 25 mm (1 in) diameter bar.
| Property | JIS SCM440 (Q&T) |
ASTM 4140 (Q&T) |
EN 42CrMo4 (Q&T) |
|---|---|---|---|
| Tensile Strength | ≥ 980 MPa (142 ksi) | ≥ 965 MPa (140 ksi) | 900–1100 MPa (130–160 ksi) |
| Yield Point / 0.2% Proof Stress | ≥ 835 MPa (121 ksi) | ≥ 655 MPa (95 ksi) | ≥ 650 MPa (94 ksi) |
| Elongation (A) | ≥ 12% | ≥ 17% | ≥ 11% |
| Reduction of Area (Z) | ≥ 45% | ≥ 46% | ≥ 45% |
| Hardness (as Q&T) | 28–34 HRC / 269–331 HB | 18–22 HRC / 197–235 HB (typical) | 26–34 HRC / 250–330 HB |
| Charpy Impact (V-notch) | ≥ 78 J (58 ft·lbf) | ≥ 54 J (40 ft·lbf) (typical) | ≥ 35 J (26 ft·lbf) |
Sources: JIS G4105:2015 Table 3, ASTM A29/A29M, EN 10083-3:2006 Table 5
Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ (0.284 lb/in³) |
| Elastic Modulus | 205 GPa (29,700 ksi) |
| Thermal Conductivity (20°C / 68°F) | 42.6 W/(m·K) (24.6 BTU/(hr·ft·°F)) |
| Thermal Expansion (20–200°C / 68–392°F) | 12.3 × 10⁻⁶ /°C (6.8 × 10⁻⁶ /°F) |
| Specific Heat Capacity | 460 J/(kg·K) (0.11 BTU/(lb·°F)) |
| Electrical Resistivity | ~0.22 μΩ·m |
Heat Treatment Conditions
| Process | Temperature | Cooling Method | Purpose |
|---|---|---|---|
| Annealing | 830–880°C (1526–1616°F) | Slow furnace cool | Soften for machining; HB ≤ 229 |
| Normalizing | 850–900°C (1562–1652°F) | Air cool | Refine grain structure |
| Austenitizing (Quench) | 830–880°C (1526–1616°F) | Oil quench (water for thin sections) | Achieve martensite; HRC 55–60 max |
| Tempering (High-strength) | 530–620°C (986–1148°F) | Air cool or water cool | Target 28–34 HRC, ≥ 980 MPa TS |
| Tempering (Medium-strength) | 620–680°C (1148–1256°F) | Air cool | Target 22–28 HRC, tougher profile |
| Stress Relief | 550–650°C (1022–1202°F) | Slow air cool | After heavy machining or welding |
| Induction / Flame Hardening | Surface layer only | Water quench | Surface HRC 50–60, tough core |
Practical Advice
Machinability
- In annealed condition (HB 170–229): machinability index approximately 65–70% relative to AISI 1212 free-cutting steel.
- In Q&T condition (HB 269–331): expect approximately 40–50% machinability index — use coated carbide inserts and flood coolant.
- Recommended surface speed (carbide): 120–180 m/min (394–591 ft/min) annealed; 80–120 m/min (262–394 ft/min) hardened.
- Chip control is good in Q&T condition; gummy chips are common in annealed condition.
- Compared to S45C: SCM440 machines slightly harder but produces better surface finish after heat treatment due to fine martensite structure.
Welding Tips
- Preheat required: Always preheat to 150–250°C (302–482°F) before welding to prevent hydrogen-induced cracking (HIC).
- Use low-hydrogen electrodes (E7018 or equivalent) or MIG/MAG with ER70S-6 wire.
- Post-weld heat treatment (PWHT) at 550–600°C (1022–1112°F) is strongly recommended to restore toughness in the HAZ.
- Avoid welding in the hardened condition if possible — weld in annealed or normalized state, then heat treat the assembly.
- Carbon equivalent (CE) of SCM440 ≈ 0.75–0.85 — this is a high-CE steel; weldability is limited compared to mild steel.
Common Mistakes
While very similar, ASTM 4140 specifies a minimum yield of 655 MPa at standard tempering, while JIS SCM440 Q&T grade requires ≥ 835 MPa. Using 4140 without re-specifying the tempering condition may result in under-strength parts in demanding JIS-designed assemblies.
Engineers familiar with structural steel (SS400, A36) sometimes skip preheat when welding SCM440, leading to cold cracks in the HAZ within hours or days of welding. Always preheat, especially in cold shop environments.
As-rolled SCM440 has inconsistent hardness and residual stress. Always specify Q&T + stress relief before finish grinding for precision shaft applications.
EN 42CrMo4 +QT (quenched and tempered to +QT900 or +QT1100 condition) is comparable to SCM440 Q&T. However, EN 42CrMo4 +A (annealed) or +N (normalized) supplied in Europe is NOT equivalent to SCM440 Q&T. Confirm the delivery condition on the mill certificate.
When to Choose SCM440
- Choose SCM440 over S45C when: tensile strength above 900 MPa is required, cross-section exceeds 50 mm (1.97 in), or cyclic fatigue loading is a design driver.
- Choose SCM440 over SNCM439 (4340 equivalent) when: cost is a concern and ultra-high toughness (impact > 100 J) is not mandatory.
- Choose SCM440 for: gears, shafts, connecting rods, high-strength bolts (10.9 grade), hydraulic cylinder rods, die holders, and jig components.
- Avoid SCM440 when: corrosion resistance is needed (use SUS or coated alternatives), very large sections (> 150 mm / 5.91 in) need through-hardening (use SNCM439), or temperatures exceed 400°C (752°F) continuously (consider hot-work tool steels).
FAQ
Q: Is JIS SCM440 the same as ASTM 4140?
SCM440 and ASTM 4140 are very close in composition and properties, but they are not identical. The key difference is that JIS SCM440 in Q&T condition specifies a minimum yield strength of 835 MPa (121 ksi), while standard ASTM 4140 Q&T (at typical tempering temperatures) has a minimum yield of 655 MPa (95 ksi). When specifying 4140 as a substitute for SCM440, always add a minimum yield or hardness requirement to your purchase order.
Q: What is the hardness of SCM440 after quench and temper?
After oil quenching from 850–880°C (1562–1616°F) and tempering at 530–620°C (986–1148°F), SCM440 typically achieves 28–34 HRC (269–331 HB). The as-quenched hardness before tempering can reach HRC 55–60. Annealed (pre-machining) condition is HB 179–229.
Q: Can SCM440 be welded?
Yes, but with precautions. SCM440 has a high carbon equivalent (CE ≈ 0.75–0.85), so preheat to 150–250°C (302–482°F) is mandatory to prevent hydrogen-induced cracking. Use low-hydrogen consumables (E7018 equivalent) and perform post-weld heat treatment (PWHT) at 550–600°C (1022–1112°F) to restore HAZ toughness. Welding in the hardened condition without these steps frequently causes delayed cold cracking.
Q: What is the European equivalent of SCM440?
The nearest European equivalent is EN 42CrMo4 (material number 1.7225, specified in EN 10083-3). The DIN designation is also 42CrMo4. ISO 683-2 uses the same designation 42CrMo4. These are nearest equivalents, not exact matches — minor differences in Mn range and P/S limits exist. Always compare mill certificates before direct substitution.
Q: What is SCM440 used for?
SCM440 is primarily used for high-strength mechanical components requiring good hardenability and fatigue resistance: gear shafts, transmission gears, connecting rods, high-strength bolts and studs (Grade 10.9), hydraulic cylinder rods, crankshafts, tooling jigs, and die holder blocks. It is one of the most widely used alloy steels in Japanese manufacturing and its global equivalents (4140, 42CrMo4) have the same dominant position worldwide.
- Chromium-molybdenum alloy steel; nearest equivalent: ASTM 4140, EN 42CrMo4 (1.7225)
- Q&T tensile strength ≥ 980 MPa (142 ksi); yield ≥ 835 MPa (121 ksi)
- Excellent hardenability: oil-quenchable up to ~100 mm (3.94 in) diameter
- Always preheat before welding; post-weld heat treatment strongly recommended
- Specify delivery condition carefully when sourcing overseas equivalents
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