A320 L7 vs A193 B7: Cold Service Bolting
ASTM A193 B7 and ASTM A320 L7 are alloy steel stud bolt specifications with essentially the same chemical composition and mechanical properties. The critical difference is that A320 L7 requires mandatory Charpy V-notch impact testing at low temperature, making it the required bolt material for cryogenic and cold-service flanged joints. A193 B7 has no mandatory impact testing and is limited to standard and elevated-temperature service.
Quick Answer
Use A193 B7 for standard service (ambient to ~400°C). Use A320 L7 for low-temperature service down to -101°C. The two grades are metallurgically identical; A320 L7 simply adds the impact test guarantee that design codes require for bolting at sub-zero temperatures.
Detailed Comparison
Chemical Composition
Both grades use the same base alloy (AISI 4140/4142 type: 1Cr-0.2Mo). The A320 specification imposes additional heat treatment requirements (quench and temper with specific tempering temperatures) to achieve the grain refinement needed for low-temperature toughness.
Impact Testing
A193 B7 has no mandatory impact testing requirement. The bolts may or may not have adequate toughness at low temperatures depending on the specific heat treatment and steel quality.
A320 L7 mandates Charpy V-notch testing at -101°C (-150°F) with minimum absorbed energy requirements. This guarantees that the bolts will behave in a ductile manner and resist brittle fracture at cryogenic temperatures.
Specifications Table
| Property | A193 B7 | A320 L7 |
|---|---|---|
| Specification | ASTM A193 | ASTM A320 |
| Service type | Standard / elevated temp | Low temperature / cryogenic |
| Alloy type | AISI 4140/4142 (Cr-Mo) | AISI 4140/4142 (Cr-Mo) |
| Chromium | 0.75-1.20% | 0.75-1.20% |
| Molybdenum | 0.15-0.25% | 0.15-0.25% |
| Heat treatment | Q+T | Q+T (specific temper for toughness) |
| Tensile strength (dia. <=2.5”) | 860 MPa (125 ksi) min | 860 MPa (125 ksi) min |
| Yield strength (dia. <=2.5”) | 720 MPa (105 ksi) min | 720 MPa (105 ksi) min |
| Elongation (min) | 16% | 16% |
| Reduction of area (min) | 50% | 50% |
| Hardness (max) | 321 HBW | 321 HBW |
| Impact test | Not mandatory | Mandatory at -101°C |
| Impact energy (min) | - | 20 J (15 ft-lbs) average |
| Min service temp | -29°C (-20°F) per code | -101°C (-150°F) |
| Max service temp | ~400°C (750°F) | ~400°C (750°F) |
| Matching nut | A194 Grade 2H | A194 Grade 4 or 7 |
| Relative cost | Baseline | 15-30% premium |
When to Specify Each Grade
| Application | Recommended Grade |
|---|---|
| General refinery piping (ambient) | A193 B7 |
| Steam systems | A193 B7 |
| Standard petrochemical service | A193 B7 |
| Arctic installations (-29 to -46°C) | A320 L7 |
| LNG terminal piping | A320 L7 |
| Cold box / air separation units | A320 L7 |
| Cryogenic nitrogen/oxygen service | A320 L7 |
| Depressurization scenarios (auto-refrigeration) | A320 L7 |
System Matching
When A320 L7 bolts are specified, the entire flanged joint must be rated for low-temperature service:
- Pipe: ASTM A333 Grade 6 or Grade 3 (instead of A106)
- Fittings: ASTM A420 WPL6 (instead of A234 WPB)
- Flanges: ASTM A350 LF2 (instead of A105)
- Gaskets: Verified for low-temperature service
- Nuts: A194 Grade 4 or 7 (impact tested)
This ensures all components in the pressure boundary have verified toughness at the design temperature. For more on stud bolt specifications, see the dedicated guide.
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