SNB7 (JIS G4107) and ASTM A193 Gr.B7 are the Japanese and American specifications for Cr-Mo alloy stud bolts used in high-pressure flanges, pressure vessels, and elevated-temperature piping. On paper they look identical — in the heat treatment certificate, one number separates them.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SNB7 / A193 B7
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4107 | SNB7 | Japan | Reference |
| ASTM A193/A193M | Gr.B7 | USA / International | ✅ Nearest Exact — essentially identical Cr-Mo composition and property requirements |
| EN ISO 898-1 / EN 10269 | 42CrMo4 / 1.7225 (bolt quality) | Europe | ⚠️ Nearest Equivalent |
| ISO 898-1 | Class 10.9 | International | ⚠️ Nearest Equivalent (by strength class, not composition) |
2. Chemical Composition
| Element | JIS SNB7 | ASTM A193 B7 | EN 42CrMo4 |
|---|---|---|---|
| C | 0.38–0.43% | 0.37–0.49% | 0.38–0.45% |
| Si | 0.15–0.35% | 0.15–0.35% | ≤ 0.40% |
| Mn | 0.65–1.10% | 0.75–1.00% | 0.60–0.90% |
| P | ≤ 0.035% | ≤ 0.035% | ≤ 0.025% |
| S | ≤ 0.040% | ≤ 0.040% | ≤ 0.035% |
| Cr | 0.75–1.20% | 0.75–1.20% | 0.90–1.20% |
| Mo | 0.20–0.30% | 0.15–0.25% | 0.15–0.30% |
Sources: JIS G4107:2014, ASTM A193/A193M, EN ISO 898-1
3. Mechanical Properties
Size-Dependent Tensile and Yield Requirements
| Nominal Diameter | SNB7 Tensile | SNB7 Yield | A193 B7 Tensile | A193 B7 Yield |
|---|---|---|---|---|
| ≤ 65 mm (≤ 2.5 in) | ≥ 860 MPa (≥ 125 ksi) | ≥ 720 MPa (≥ 105 ksi) | ≥ 860 MPa (≥ 125 ksi) | ≥ 724 MPa (≥ 105 ksi) |
| 65–100 mm (2.5–4.0 in) | ≥ 790 MPa (≥ 115 ksi) | ≥ 655 MPa (≥ 95 ksi) | ≥ 793 MPa (≥ 115 ksi) | ≥ 655 MPa (≥ 95 ksi) |
| 100–180 mm (4.0–7.0 in) | ≥ 690 MPa (≥ 100 ksi) | ≥ 515 MPa (≥ 75 ksi) | ≥ 690 MPa (≥ 100 ksi) | ≥ 517 MPa (≥ 75 ksi) |
Additional Property Requirements (Both SNB7 and A193 B7)
| Property | Requirement |
|---|---|
| Charpy impact (longitudinal, room temp) | ≥ 54 J (≥ 40 ft·lbf) |
| Hardness | HRC 26–36 |
| Elongation (≤ 65 mm) | ≥ 16% |
| Reduction of area (≤ 65 mm) | ≥ 50% |
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 | 42 W/(m·K) | 291 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C) | 12.0 × 10⁻⁶ /°C | 6.7 × 10⁻⁶ /°F |
| Specific heat | ~477 J/(kg·K) | 0.114 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Austenitize + quench | 840–900°C (1544–1652°F) | Oil or water quench | Full hardening of Cr-Mo alloy |
| Temper (SNB7) | ≥ 540°C (≥ 1004°F) | Air or furnace cool | Achieve HRC 26–36; relieve quench stress |
| Temper (A193 B7) | ≥ 593°C (≥ 1100°F) | Air or furnace cool | Higher minimum ensures sufficient toughness for ASME service |
Service temperature limit: Approximately 430°C (806°F) for continuous bolt load-bearing service. Above this temperature, stress relaxation becomes significant and bolt load is gradually lost, leading to flange leakage.
6. Machinability
- Machinability approximately 60% relative to AISI 1212 in annealed condition.
- In the quenched and tempered condition (HRC 26–36), conventional thread cutting is difficult; grinding and thread rolling are preferred for final dimensions.
- Thread rolling (cold forming) is preferred over cut threading for stud bolts — it preserves surface integrity, induces compressive residual stress in the thread root, and improves fatigue life.
- For large-diameter studs (> 50 mm / 2.0 in), rough machine in annealed condition, then quench and temper, then finish grind.
7. Weldability
- Rating: Not recommended — stud bolts are not designed as welded components.
- Carbon equivalent (Ceq) is approximately 0.65–0.80, requiring substantial preheat if welding is ever attempted.
- High preheat temperature: 200–300°C (392–572°F) minimum for any repair welding.
- In practice, SNB7 / A193 B7 studs are machined components; welding to flanges or vessels is not part of the intended design. Any welding on installed studs should be reviewed by a pressure vessel engineer.
8. Common Mistakes
The minimum temper temperature for SNB7 is 540°C (1004°F), while A193 B7 requires 593°C (1100°F). A stud bolt manufactured and heat-treated to JIS SNB7 may be tempered at 545–570°C — fully compliant with JIS, but out of compliance with ASTM A193 B7. On an ASME-coded pressure vessel or piping system in North America, the inspector will check the mill certificate’s recorded temper temperature, and a value below 593°C (1100°F) will result in rejection. Always request and verify the heat treatment record, not just the composition certificate.
A193 B7 is designed and rated for elevated-temperature service, but it is not rated above approximately 430°C (806°F) for sustained bolt loading. Above this temperature, Cr-Mo steel undergoes significant stress relaxation (creep), causing bolt preload to drop and flanges to leak. For service above 430°C (806°F), the correct specification is ASTM A193 B16 (Cr-Mo-V alloy, rated to ~540°C / 1004°F) or, for very high temperatures or corrosive environments, B8/B8M class (austenitic stainless). Specifying B7 for steam lines above 430°C is a well-documented source of flange leaks during hot restarts.
9. When to Choose SNB7 / A193 B7
✅ Choose SNB7 / A193 B7 when:
- ✅ High-pressure flange bolting in refineries, chemical plants, and power stations up to 430°C (806°F)
- ✅ ASME B16.5 / B16.47 flange assemblies requiring certified bolt material documentation
- ✅ Japanese domestic procurement specifications (SNB7) or ASME/ANSI projects (A193 B7)
- ✅ Stud bolts for pressure vessel heads and nozzle flanges requiring HRC 26–36 with documented Charpy
- ✅ Standard process plant bolting for carbon steel and low-alloy flanges
❌ Avoid SNB7 / A193 B7 when:
- ❌ Cryogenic service — specify ASTM A320 L7 (same alloy, additional low-temperature Charpy requirement)
- ❌ Stainless flanges or corrosive environments — use B8/B8M class (Type 304/316 austenitic)
- ❌ Sustained service temperatures above 430°C (806°F) — upgrade to A193 B16 (Cr-Mo-V)
10. FAQ
Q: Are SNB7 and ASTM A193 B7 interchangeable on ASME-coded equipment?
Not automatically. For ASME-coded pressure equipment in North America, ASTM A193 B7 certification is the standard requirement. JIS G4107 SNB7 documentation is not automatically accepted — the engineer of record or authorized inspector must formally review whether a JIS certificate is acceptable under the applicable ASME code edition and the owner’s quality plan. In Japan, JIS G4107 SNB7 is the standard specification for the same bolt service. Where international projects require cross-certification, dual-certified material (tested to both JIS and ASTM requirements) is available from major mills.
Q: What nuts should be paired with B7 studs?
ASTM A194 Gr.2H heavy hex nuts are the standard pairing for A193 B7 studs in ASME piping and pressure vessel applications. Grade 2H nuts are quenched and tempered carbon or alloy steel with hardness and proof load matched to the B7 stud class. For JIS SNB7 applications, JIS B1181 Gr.6H nuts at equivalent tensile strength are used. The nut grade must be matched to the bolt tensile strength class — mismatching nut and bolt grades can result in thread stripping under preload.
Q: Why does the yield requirement decrease for larger diameter studs?
Larger diameter studs have lower surface-to-volume ratios, which result in slower quench cooling rates at the centerline. This slower rate reduces through-hardenability, meaning the core of a large stud cannot achieve the same hardness as a small-diameter stud from the same oil or water quench. The specification steps down the minimum yield requirement in proportion to this hardenability limitation, ensuring that the requirement is physically achievable across the full certified diameter range. This is why both SNB7 and A193 B7 list three diameter-dependent property tiers.
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