Grades stocked: L7 · L7M · B8 Class 2 · B8M Class 2
Scope
ASTM A320 / A320M covers alloy-steel and stainless-steel bolting intended for low-temperature service in pressure vessels, valves, flanges, and fittings. The spec parallels A193 in materials but adds mandatory low-temperature Charpy V-notch impact testing to verify ductile behavior in the cold.
A320 applies when the design minimum metal temperature falls below -20°F / -29°C. Above that, A193 is typically specified.
Grades
Alloy steel grades (ferritic, heat-treated)
| Grade | Material | Impact test temperature | Typical use |
|---|---|---|---|
| L7 | AISI 4140 / 4142 Cr-Mo | -150°F (-101°C) | The default low-temp stud; pairs directly with A193 B7 above cold service |
| L7A | Carbon steel, Q&T | -150°F | Lower-cost carbon alternative |
| L7B | AISI 4140 + modifications | -150°F | Moderate alloy alternative |
| L7C | Carbon-boron alloy | -150°F | Boron-alloy alternative |
| L7M | L7 chemistry, lower hardness | -100°F (-73°C) | Sour service at low temperature (NACE MR0175) |
| L43 | AISI 4340 Ni-Cr-Mo | -150°F | Higher strength than L7; used for larger sections |
| L1 | Carbon-molybdenum | Varies | Uncommon |
Charpy impact testing on these grades must demonstrate a minimum lateral expansion of 0.015" at the specified test temperature.
Stainless grades (austenitic)
| Grade | Material | Class |
|---|---|---|
| B8 | 304 SS | Class 1 (solution annealed), Class 2 (strain hardened) |
| B8M | 316 SS | Class 1, Class 2 |
| B8T | 321 SS | Class 1, Class 2 |
| B8C | 347 SS | Class 1, Class 2 |
The austenitic stainless grades are inherently tough at low temperature and do not require Charpy testing — their FCC crystal structure doesn't exhibit the ductile-to-brittle transition that limits ferritic alloys in the cold.
Mechanical properties — Grade L7
| Diameter | Min. Tensile | Min. Yield | Min. Elong. | Min. RA | Min. Impact Energy | Max. Hardness |
|---|---|---|---|---|---|---|
| Up to 2-1/2" | 125 ksi | 105 ksi | 16% | 50% | 20 ft-lbf @ -150°F | 321 HBW |
| 2-5/8" through 4" | 115 ksi | 95 ksi | 16% | 50% | 20 ft-lbf @ -150°F | 321 HBW |
L7 mechanical properties mirror A193 B7 — the added requirement is the impact test at -150°F.
Mechanical properties — Grade L7M
| Diameter | Min. Tensile | Min. Yield | Min. Elong. | Min. RA | Min. Impact Energy | Max. Hardness |
|---|---|---|---|---|---|---|
| Up to 4" | 100 ksi | 80 ksi | 18% | 50% | 20 ft-lbf @ -100°F | 235 HBW (22 HRC) |
L7M combines the Charpy requirement with NACE MR0175 / ISO 15156 hardness limits for sour service. Every part is hardness tested.
Why low-temperature toughness matters
Carbon and low-alloy steels exhibit a ductile-to-brittle transition somewhere between ambient and cryogenic temperatures. Above the transition, the material absorbs energy through plastic deformation before fracture. Below it, fracture occurs suddenly with little warning — a catastrophic failure mode in pressurized service.
A Charpy V-notch impact test measures energy absorbed during fracture of a notched specimen at a specified temperature. The 20 ft-lbf minimum energy requirement in A320 ensures the material remains ductile at the stated test temperature — which establishes the lowest safe design service temperature for the bolting.
Recommended nuts and washers
A320 studs are paired with A194 nuts, matching the grade:
| A320 bolt | A194 nut | Washer |
|---|---|---|
| L7, L7A, L7B, L7C | Grade 4 or 7 | F436 or F844 (carbon) |
| L7M | 2HM or 7M | F436 or F844 |
| L43 | Grade 4 or 7 | F436 or F844 |
| B8 Class 1 or 2 | Grade 8, matching class | F436 or F844 (stainless) |
| B8M Class 1 or 2 | Grade 8M, matching class | F436 or F844 (stainless) |
For sour service (L7M + 2HM), hardness testing on every nut and every stud is required.
Applications
- LNG receiving and regasification terminals
- Cryogenic air separation plants
- Ethylene and propylene processing
- Natural gas liquefaction
- Industrial refrigeration
- Aerospace and aerospace ground support (liquid oxygen, nitrogen)
- Arctic pipeline service
- Deep-sea subsea equipment (where temperatures approach 32–28°F)
Service temperature guidance
- L7 — qualified to -150°F by impact test; usable somewhat lower with engineering judgment
- L7M — qualified to -100°F by impact test (the lower ceiling reflects the lower hardness)
- B8, B8M — austenitic stainless is ductile well below -320°F (liquid nitrogen); these grades are preferred for deep cryogenic service
Marking
Grade marking is required on all A320 bolts — for example, "L7," "L7M," "B8M CL2." The "L" prefix is the quick identifier distinguishing A320 from A193 on the bolt head or stud end.
Related specifications
- A193 — High-temperature companion specification
- A194 — Nut specification (pairs with A320)
- F436 — Hardened washers
- NACE MR0175 / ISO 15156 — Sour-service material requirements (governs L7M selection)
- ASME B31.3 — Process piping design code (references A320 for low-temp service)
Documentation
California Fastener A320 orders ship with mill certificates showing grade, heat number, chemistry, mechanical properties, and Charpy V-notch impact test results at the specified test temperature. L7M orders include 100% hardness test documentation and NACE MR0175 / ISO 15156 compliance statements.