SNCM625 is one of the highest-hardenability alloy steels in JIS G4053, developed for large-section shafts, heavy-duty gears, rolls, and structural components where through-section hardness uniformity is critical. With Ni at 2.50–3.50% and Mo at 0.25–0.40%, it delivers superior core toughness and deep hardenability for sections exceeding 150 mm (6 in) — territory where SCM440, SCr440, and even SNCM439 begin to lose their effectiveness. This guide covers international equivalents, composition, mechanical and physical properties, heat treatment, and selection criteria for manufacturing engineers working with large forgings and heavy industrial components.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SNCM625
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4053 | SNCM625 | Japan | Reference |
| DIN EN 10083-3 | 34CrNiMo6 / 1.6582 | Germany / Europe | ⚠️ Nearest Equivalent |
| EN 10083-3 | 34CrNiMo6 / 1.6582 | Europe | ⚠️ Nearest Equivalent |
| ASTM A29/A29M | AISI 4340 | USA | ⚠️ Nearest Equivalent |
No single international grade is a direct compositional match for SNCM625. The JIS grade was developed for Japanese heavy industry with a higher Ni ceiling (up to 3.50%) than most Western equivalents. DIN 34CrNiMo6 and AISI 4340 share the same design intent — ultra-high-strength through-hardening for large sections — but with lower Ni and different composition windows. For global sourcing, DIN 34CrNiMo6 (EN 1.6582) is the most widely available equivalent and is the standard choice in European heavy industry.
2. Chemical Composition
| Element | JIS SNCM625 | DIN 34CrNiMo6 | AISI 4340 |
|---|---|---|---|
| C | 0.20–0.27% | 0.30–0.38% | 0.38–0.43% |
| Si | 0.15–0.35% | ≤ 0.40% | 0.15–0.35% |
| Mn | 0.60–0.90% | 0.50–0.80% | 0.60–0.80% |
| P | ≤ 0.030% | ≤ 0.025% | ≤ 0.035% |
| S | ≤ 0.030% | ≤ 0.035% | ≤ 0.040% |
| Ni | 2.50–3.50% | 1.30–1.70% | 1.65–2.00% |
| Cr | 0.60–1.00% | 1.30–1.70% | 0.70–0.90% |
| Mo | 0.25–0.40% | 0.20–0.30% | 0.20–0.30% |
Sources: JIS G4053:2016, DIN EN 10083-3, ASTM A29/A29M
The key differentiator is Ni: SNCM625 (2.50–3.50%) vs DIN 34CrNiMo6 (1.30–1.70%) vs AISI 4340 (1.65–2.00%). SNCM625 compensates with lower C (0.20–0.27%) for better toughness at equivalent strength, relying on high Ni for hardenability rather than the higher Cr of 34CrNiMo6.
3. Mechanical Properties
3-1. Quench and Temper — Typical Values for Section 100–200 mm (4–8 in)
| Temper Temp | Tensile | Yield | Elongation | Charpy Impact |
|---|---|---|---|---|
| 450°C (842°F) | ~1080–1200 MPa (157–174 ksi) | ≥ 900 MPa (≥ 131 ksi) | ≥ 13% | ≥ 54 J (≥ 40 ft·lbf) |
| 550°C (1022°F) | ~900–1050 MPa (131–152 ksi) | ≥ 780 MPa (≥ 113 ksi) | ≥ 15% | ≥ 81 J (≥ 60 ft·lbf) |
| 630°C (1166°F) | ~780–900 MPa (113–131 ksi) | ≥ 650 MPa (≥ 94 ksi) | ≥ 17% | ≥ 108 J (≥ 80 ft·lbf) |
3-2. Grade Comparison — Large Section Performance
| Grade | Ni % | Max Effective Section | Core Charpy (at 550°C temper) |
|---|---|---|---|
| SCM440 | — | ~40 mm (1.6 in) | ~54 J (~40 ft·lbf) |
| SNCM439 | 1.60–2.00% | ~100 mm (4 in) | ~70 J (~52 ft·lbf) |
| SNCM625 | 2.50–3.50% | ~200 mm (8 in) | ≥ 81 J (≥ 60 ft·lbf) |
| DIN 34CrNiMo6 | 1.30–1.70% | ~150 mm (6 in) | ~70 J (~52 ft·lbf) |
4. Physical Properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s modulus | 205 GPa | 29,700 ksi |
| Thermal conductivity | 36 W/(m·K) | 250 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C / 68–212°F) | 11.4 × 10⁻⁶ /°C | 6.3 × 10⁻⁶ /°F |
| Specific heat | ~477 J/(kg·K) | 0.114 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Normalizing | 830–870°C (1526–1598°F) | Air cool | Homogenize forging microstructure |
| Annealing | 800–850°C (1472–1562°F) | Furnace cool | Soften for rough machining |
| Hardening (oil quench) | 820–860°C (1508–1580°F) | Oil quench | Full martensite transformation |
| Tempering | 450–650°C (842–1202°F) | Air cool | Final strength and toughness balance |
For sections exceeding 150 mm (6 in), soaking time at austenitizing temperature must be sufficient to achieve full through-section temperature. A common guideline is 1 hour per 25 mm (1 in) of section thickness minimum. Insufficient soaking produces incomplete austenitization at the core, leading to mixed martensite-bainite microstructure and unpredictable mechanical properties. Verify core temperature with thermocouples or simulation for critical forgings.
6. Machinability
- Machinability approximately 45–50% relative to AISI 1212 free-machining steel — lower than most Cr-Mo grades due to the high Ni content and alloy complexity.
- Anneal before all roughing machining. Rough machine in the normalized condition; do not attempt heavy cuts in the Q+T condition above HRC 30.
- Carbide tooling (P30–P40 grade) required for production turning; coated carbide or CBN for hard turning above HRC 40 if grinding is not feasible.
- Finish grind after Q+T to correct distortion from heat treatment and achieve final tolerances.
- Thread milling preferred over tapping in the hardened condition.
7. Weldability
- Weldability: Severely restricted. The combination of high Ni, Cr, and Mo produces a high carbon equivalent and significant HAZ cold cracking risk.
- Preheat temperature: Minimum 200°C (392°F) if welding is unavoidable; maintain throughout.
- Post-weld: Full stress relief at 600–650°C (1112–1202°F) immediately after welding, before any cooling to room temperature.
- Design guidance: SNCM625 is not intended for welded fabrication. If a weld joint exists in the design, redesign to eliminate it or substitute a lower-alloy weldable grade for the welded zone.
8. Common Mistakes
DIN 34CrNiMo6 Ni content (1.30–1.70%) is less than half of SNCM625’s typical 3.0% Ni. For sections below 100 mm (4 in), the difference in core hardness and toughness is acceptable and 34CrNiMo6 is a reasonable substitute. For sections above 150 mm (6 in), the Ni deficit becomes significant — core hardness may fall 5–8 HRC below SNCM625 values after the same quench cycle, and Charpy core impact can be 20–30% lower. Always confirm section size when evaluating this substitution.
SNCM625’s high alloy content — particularly the 2.50–3.50% Ni — commands a significant price premium over SCM440 or SNCM439. For sections below 50 mm (2 in), SCM440 or SNCM439 achieves equivalent mechanical properties after Q+T at substantially lower material cost. Specifying SNCM625 for small shafts or bolts adds cost without metallurgical benefit. Reserve SNCM625 for the heavy-section applications it was designed for — typically section ≥ 100 mm (4 in) in demanding industrial environments.
9. When to Choose SNCM625
- ✅ Large shafts, rolls, and rotors with section > 150 mm (6 in) requiring deep, uniform hardness after Q+T
- ✅ Heavy-duty mining, energy generation, and marine propulsion equipment where core Charpy > 80 J is required at full section
- ✅ Applications where DIN 34CrNiMo6 or AISI 4340 are insufficient due to section size or toughness requirements
- ✅ JIS G4053 compliance in Japanese heavy industry and energy sector supply chains
- ✅ Large forgings where through-section hardness uniformity is a design requirement
- ❌ Small or medium sections (< 80 mm / 3.1 in) — SNCM439 or SCM440 is sufficient and significantly less expensive
- ❌ Welded fabricated structures
- ❌ Carburizing applications — use SNCM415 or 18CrNiMo7-6 for carburizing
10. FAQ
Q: What is the difference between SNCM625 and SNCM439?
The primary differentiator is Ni content: SNCM625 (2.50–3.50%) vs SNCM439 (1.60–2.00%). The additional Ni in SNCM625 provides markedly better hardenability and low-temperature core impact toughness for sections above 150 mm (6 in). For medium sections of 50–100 mm (2–4 in), SNCM439 achieves comparable properties at lower alloy cost and is the preferred choice. SNCM625 is reserved for the heaviest sections in demanding applications — large industrial rolls, propulsion shafts, and heavy mining equipment — where through-hardness uniformity across the full section is a non-negotiable requirement.
Q: Is AISI 4340 a valid substitute for SNCM625?
It depends on section size and toughness requirements. For sections at or below 100 mm (4 in), AISI 4340 often achieves comparable mechanical properties and is more globally available, making it a practical substitute. For sections above 150 mm (6 in), SNCM625’s higher Ni provides meaningfully better core hardness and Charpy impact — the substitution may not meet design requirements. Additionally, AISI 4340 has higher carbon (0.38–0.43% vs SNCM625’s 0.20–0.27%), which raises the preheat requirement for any incidental welding and changes the tempering response curve. Review both composition and section requirements before approving the substitution.
Q: Where is SNCM625 primarily available for procurement?
SNCM625 is a JIS-specific designation, primarily available through Japanese steel mills including JFE Steel, Nippon Steel, and Aichi Steel, typically in the form of large round bars and forgings. For global sourcing outside Japan, DIN 34CrNiMo6 (EN 1.6582) is the most widely available equivalent and is produced by major European mills (Ovako, Saarstahl, Böhler). North American sourcing typically specifies AISI 4340 as the closest available grade, with the understanding that the Ni level is lower and large-section performance should be verified.
Q: Can SNCM625 be used for carburizing?
No. SNCM625 has C 0.20–0.27%, which is above the practical upper limit for carburizing grades (typically ≤ 0.22% for effective case-to-core gradient). Carburizing SNCM625 would produce a poorly defined case boundary and a core that is too hard for the toughness benefit that carburizing is meant to provide. For large-section carburized components requiring high Ni, use SNCM415 (C 0.12–0.18%) or DIN 18CrNiMo7-6 (C 0.15–0.21%).
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