What Is ASTM A320?
ASTM A320 is the standard specification for alloy-steel and stainless steel bolting materials intended for low-temperature service. It covers stud bolts, bolts, screws, and studs that must maintain adequate toughness (impact resistance) at temperatures below the minimum service limit of standard ASTM A193 bolting (-29 degC / -20 degF).
| Feature | Specification |
|---|---|
| Standard | ASTM A320/A320M |
| Title | Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service |
| Key requirement | Charpy V-notch impact testing at the specified service temperature |
| Temperature range | Down to -196 degC (-320 degF) depending on grade |
| Matching nuts | ASTM A194 Grade 4 or 7 (for L7); Grade 8 (for B8) |
| Thread standard | ASME B1.1 (Unified coarse, 8-thread series) |
ASTM A320 Grades
| Grade | Base Material | Min Service Temp | Key Difference vs A193 |
|---|---|---|---|
| L7 | AISI 4140/4142 (Cr-Mo) | -100 degC (-148 degF) | Impact tested; same chemistry as A193 B7 |
| L7A | AISI 4037 (Mo) | -100 degC (-148 degF) | Molybdenum steel alternative |
| L7B | AISI 4137 (Cr-Mo) | -100 degC (-148 degF) | Chromium-molybdenum variant |
| L7M | AISI 4140/4142 | -100 degC (-148 degF) | Restricted hardness (22 HRC max) for NACE sour + cold service |
| L43 | Ni-Cr-Mo steel | -100 degC (-148 degF) | Higher nickel content for improved toughness |
| B8 (Class 1) | AISI 304 (18Cr-8Ni) | -196 degC (-320 degF) | Austenitic SS; cryogenic capable |
| B8M (Class 1) | AISI 316 (16Cr-12Ni-2Mo) | -196 degC (-320 degF) | Austenitic SS with molybdenum |
| B8T (Class 1) | AISI 321 (Ti-stabilized) | -196 degC (-320 degF) | Titanium-stabilized austenitic |
Mechanical Properties
| Grade | Tensile Strength (min) | Yield Strength (min) | Elongation (min) | Impact Energy (min) |
|---|---|---|---|---|
| L7 (up to 2.5”) | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 16% | 20 ft-lbs (27 J) at test temp |
| L7M (up to 2.5”) | 100 ksi (690 MPa) | 80 ksi (552 MPa) | 18% | 20 ft-lbs (27 J) at test temp |
| L43 (up to 4”) | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 16% | 25 ft-lbs (34 J) at test temp |
| B8 Class 1 | 75 ksi (517 MPa) | 30 ksi (207 MPa) | 30% | Not required (austenitic) |
The Charpy V-notch impact test is the defining requirement of A320. Each heat must demonstrate adequate energy absorption at the specified minimum service temperature. Austenitic grades (B8, B8M, B8T) are exempt from impact testing because the FCC crystal structure maintains ductility at all temperatures down to absolute zero.
Impact Test Requirements
| Grade | Test Temperature | Min Impact Energy | Specimen Size |
|---|---|---|---|
| L7 | At or below the specified service temp | 20 ft-lbs (27 J) | Full-size (10 x 10 mm) |
| L7M | At or below the specified service temp | 20 ft-lbs (27 J) | Full-size |
| L43 | At or below the specified service temp | 25 ft-lbs (34 J) | Full-size |
| B8, B8M, B8T | Not required | N/A | N/A |
The purchaser must specify the service temperature on the order. The manufacturer performs impact tests at that temperature (or lower) and reports results on the MTC 3.1.
When to Specify A320
- LNG (liquefied natural gas) facilities operating at -162 degC
- Ethylene plants with process temperatures down to -104 degC
- Cryogenic storage tanks and piping
- Cold climate installations where ambient temperatures fall below -29 degC
- Any service below the A193 B7 minimum of -29 degC (-20 degF)
A320 stud bolts pair with A194 Grade 4 or 7 nuts (for L7 grades) and require controlled bolt torque per the ASME bolt chart.
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