Overview
JIS SNCM439 is a nickel-chromium-molybdenum alloy steel standardized under JIS G4053, offering one of the highest hardenability levels among JIS structural alloy steels. It is the preferred choice for large-section shafts, gears, bolts, and dies where through-hardening is required across cross-sections that standard Cr-Mo grades (SCM440) cannot achieve. Its nearest international equivalent is AISI/SAE 4340, one of the most widely specified high-strength alloy steels in North America and Europe.
The combination of Ni (1.60–2.00%), Cr (0.60–1.00%), and Mo (0.15–0.30%) in SNCM439 delivers exceptional hardenability, high tensile strength after heat treatment, and excellent toughness at elevated strength levels. This makes it suitable for demanding applications including aircraft landing gear components, heavy-duty transmission shafts, and pressure vessel fasteners.
Quick Comparison: SNCM439 vs. Global Equivalents
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4053 | SNCM439 | Japan | Reference |
| SAE/AISI | 4340 | USA | Nearest Equivalent |
| EN 10083-3 | 36CrNiMo4 (1.6511) | Europe | Nearest Equivalent |
| DIN | 36CrNiMo4 | Germany | Nearest Equivalent |
| ISO 683-1 | 36CrNiMo4 | International | Nearest Equivalent |
SNCM439 and AISI 4340 are nearest equivalents with overlapping but non-identical composition ranges — particularly for Ni (SNCM439: 1.60–2.00% vs. 4340: 1.65–2.00%) and Cr (SNCM439: 0.60–1.00% vs. 4340: 0.70–0.90%). Verify the mill certificate before substituting in safety-critical applications.
Chemical Composition
| Element | SNCM439 % (JIS G4053) | AISI 4340 % | EN 36CrNiMo4 % |
|---|---|---|---|
| C | 0.36–0.43 | 0.38–0.43 | 0.32–0.40 |
| Si | 0.15–0.35 | 0.15–0.35 | 0.10–0.40 |
| Mn | 0.60–0.90 | 0.60–0.80 | 0.50–0.80 |
| Ni | 1.60–2.00 | 1.65–2.00 | 0.90–1.20 |
| Cr | 0.60–1.00 | 0.70–0.90 | 0.90–1.20 |
| Mo | 0.15–0.30 | 0.20–0.30 | 0.15–0.25 |
| P | ≤0.030 | ≤0.035 | ≤0.025 |
| S | ≤0.030 | ≤0.040 | ≤0.035 |
Source: JIS G4053, SAE J404, EN 10083-3
Key difference vs. EN 36CrNiMo4: EN 36CrNiMo4 has significantly lower Ni (0.90–1.20%) compared to SNCM439 (1.60–2.00%). For large cross-sections requiring deep hardenability, SNCM439 / AISI 4340 are the superior choice.
Mechanical Properties
Values for quenched and tempered condition per JIS G4053. Properties vary with section size and tempering temperature.
| Property | ≤25 mm dia. | 25–50 mm | Imperial (≤25 mm) |
|---|---|---|---|
| Tensile Strength | ≥1,030 MPa | ≥980 MPa | ≥149 ksi |
| Yield Strength (0.2%) | ≥835 MPa | ≥785 MPa | ≥121 ksi |
| Elongation | ≥16% | ≥15% | ≥16% |
| Reduction of Area | ≥45% | ≥45% | ≥45% |
| Charpy Impact (20°C) | ≥78 J | ≥63 J | ≥57 ft·lbf |
| Hardness (typical) | 30–38 HRC | 28–36 HRC | — |
Higher strength option: By tempering at lower temperatures (400–450°C / 752–842°F), SNCM439 can achieve tensile strengths above 1,400 MPa (203 ksi) with hardness up to 45 HRC — used in high-strength bolts and aerospace components.
Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Thermal Conductivity | ~42 W/(m·K) | ~292 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–200°C) | ~12.3 × 10⁻⁶ /°C | ~6.8 × 10⁻⁶ /°F |
| Young’s Modulus | ~205 GPa | ~29,700 ksi |
Heat Treatment Conditions
| Process | Temperature | Cooling |
|---|---|---|
| Normalizing | 850–900°C (1562–1652°F) | Air cool |
| Austenitizing (Quench) | 820–870°C (1508–1598°F) | Oil quench |
| Tempering (standard) | 550–650°C (1022–1202°F) | Air or water cool |
| Tempering (high-strength) | 400–500°C (752–932°F) | Air cool |
| Annealing | 830–880°C (1526–1616°F) | Furnace cool ≤15°C/hr |
SNCM439 (like all Ni-Cr-Mo steels) is susceptible to temper embrittlement when slowly cooled through the 375–575°C (707–1067°F) range. After tempering above 500°C (932°F), cool rapidly (water quench or fast air cool) to avoid embrittlement. This is especially important for large sections.
Practical Advice
Machinability
In annealed condition, SNCM439 machines at approximately 55–60% of AISI 1212 free-machining steel — slightly lower than SCM440 due to higher Ni content. Carbide tooling is recommended. Use lower cutting speeds than for plain carbon steels and ensure adequate coolant flow. In the quenched and tempered condition (30–35 HRC), hard turning with CBN tooling is viable for finishing operations.
Heat Treatment Guide
Austenitize at 820–870°C (1508–1598°F) with 20–40 minutes hold depending on section size. Oil quench immediately. The high hardenability of SNCM439 means through-hardening is achievable in sections up to approximately 100 mm (3.94 in) diameter with oil quenching. Temper within 2 hours of quenching to prevent quench cracking. For standard structural applications, temper at 550–650°C (1022–1202°F) for optimum strength-toughness balance.
Welding
SNCM439 has a carbon equivalent of approximately 0.75–0.85, making it highly susceptible to cold cracking without preheat. Minimum preheat temperature: 200–300°C (392–572°F) depending on section thickness and hydrogen content of the process. Use low-hydrogen electrodes (E-LH) or MIG/TIG processes. Post-weld stress relief at 600–650°C (1112–1202°F) is strongly recommended for structural welds.
Common Mistakes
While SNCM439 and AISI 4340 are nearest equivalents, the Cr lower limit differs (SNCM439: 0.60% vs. 4340: 0.70%). For hardenability-critical large-section parts, verify the actual Cr content from the mill certificate. Heats near the SNCM439 lower Cr limit may show marginally less hardenability than standard 4340.
Slowly cooling SNCM439 through the 375–575°C (707–1067°F) temper embrittlement range causes intergranular embrittlement, dramatically reducing impact toughness. Always cool rapidly after tempering above 500°C (932°F). This mistake is common when furnace cooling is used for convenience.
SNCM439 costs significantly more than SCM440 due to Ni content. For cross-sections under 50 mm (1.97 in) where SCM440 can achieve sufficient through-hardening, the Ni-Cr-Mo grade is over-specified. Reserve SNCM439 for sections where SCM440 shows inadequate core hardness.
When to Choose SNCM439
- Large-section shafts and gears (diameter >50 mm / 1.97 in) requiring through-hardening
- High-strength structural fasteners (tensile strength >1,200 MPa / 174 ksi)
- Applications requiring combined high strength and impact toughness at low temperatures
- When SCM440 shows insufficient core hardness in service
- Aerospace and defense components requiring AISI 4340 equivalent material
FAQ
Q: Is SNCM439 the same as AISI 4340?
A: They are nearest equivalents. Composition ranges overlap closely but are not identical — particularly the Cr lower limit (0.60% for SNCM439 vs. 0.70% for 4340) and the carbon range. For most structural applications they are interchangeable, but verify the mill certificate for safety-critical applications.
Q: What tensile strength can SNCM439 achieve?
A: After quench and temper at 550–650°C (1022–1202°F), SNCM439 achieves ≥1,030 MPa (≥149 ksi) for sections ≤25 mm. By tempering at lower temperatures (400–450°C / 752–842°F), tensile strength above 1,400 MPa (203 ksi) is achievable with hardness up to 45 HRC.
Q: Can SNCM439 be welded?
A: Yes, but with mandatory preheat (200–300°C / 392–572°F), low-hydrogen process, and post-weld stress relief. Without preheat, cold cracking in the heat-affected zone is virtually certain due to the high carbon equivalent (~0.80).
Q: How does SNCM439 compare to SCM440?
A: SNCM439 offers significantly higher hardenability than SCM440, making it the choice for large cross-sections. For sections under ~50 mm (1.97 in), SCM440 typically provides sufficient through-hardening at lower material cost. Choose SNCM439 when SCM440 cannot achieve the required core hardness.
Q: What is the maximum section size for through-hardening SNCM439?
A: With oil quenching, through-hardening (achieving martensitic core) is reliably achievable up to approximately 100 mm (3.94 in) diameter. For larger sections, consider air-hardening grades or discuss achievable hardness profiles with the heat treater before finalizing the design.
Summary
- JIS SNCM439 = Ni-Cr-Mo high-hardenability alloy steel, JIS G4053
- Nearest equivalents: AISI 4340, EN 36CrNiMo4 (1.6511)
- Tensile strength ≥1,030 MPa (≥149 ksi) after Q&T at 550–650°C
- Through-hardenable up to ~100 mm (3.94 in) diameter with oil quench
- Best for: large-section shafts, gears, high-strength fasteners, aerospace components
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