Originally approved in 1948, the ASTM A320 specification covers alloy steel and stainless steel bolting materials for low temperature service. This standard covers rolled, forged, or strain hardened bars, bolts, screws, studs, and stud bolts used for pressure vessels, valves, flanges, and fittings. Like the ASTM A193 specification, unless otherwise specified, the 8UN thread series is specified on fastener larger than 1” in diameter.
Below is a basic summary of a few of the common grades within the ASTM A320 specification. A number of other less common grades of ASTM A320 exist, but not covered in the description below.
ASTM A320 Grades
| Grade | Description |
|---|---|
| L7 Alloy steel | AISI 4140/4142 Quenched and tempered |
| L43 Alloy steel | AISI 4340 Quenched and tempered |
| B8 Class 1 Stainless steel | AISI 304, carbide solution treated |
| B8M Class 1 Stainless steel | AISI 316, carbide solution treated |
| B8 Class 2 Stainless steel | AISI 304, carbide solution treated, strain hardened |
| B8M Class 2 Stainless steel | AISI 316, carbide solution treated, strain hardened |
ASTM A320 Mechanical Properties
| Grade | Diameter | Tensile, ksi, min | Yield, ksi, min | Charpy Impact 20-ft-lbf @ temp | Elong, %, min | RA, %, min |
|---|---|---|---|---|---|---|
| L7 | Up to 21⁄2 | 125 | 105 | -150° F | 16 | 50 |
| L43 | Up to 4 | 125 | 105 | -150° F | 16 | 50 |
| Class 1: BC, B8C, B8M, B8P, B8F, B8T, B8LN, B8MLN | All | 75 | 30 | N/A | 30 | 50 |
| Class 1A: B8A, B8CA, B8MA, B8PA, B8FA, B8TA, B8LNA, B8MLNA | All | 75 | 30 | N/A | 30 | 50 |
| Class 2: B8, B8C, B8P, B8F, B8T | Up to 3⁄4 | 125 | 100 | N/A | 12 | 35 |
| 7⁄8 - 1 | 115 | 80 | N/A | 15 | 35 | |
| 11⁄8 - 11⁄4 | 105 | 65 | N/A | 20 | 35 | |
| 13⁄8 - 11⁄2 | 100 | 50 | N/A | 28 | 45 | |
| Class 2: B8M | Up to 3⁄4 | 110 | 95 | N/A | 15 | 45 |
| 7⁄8 - 1 | 100 | 80 | N/A | 20 | 45 | |
| 11⁄8 - 11⁄4 | 95 | 65 | N/A | 25 | 45 | |
| 13⁄8 - 11⁄2 | 90 | 50 | N/A | 30 | 45 |
ASTM A320 Chemical Properties
| Element | L7 (AISI 4140) | L43 (AISI 4340) | B8 (AISI 304) | B8M (AISI 316) |
|---|---|---|---|---|
| Carbon | 0.38 - 0.48% | 0.38 - 0.43% | 0.08% max | 0.08% max |
| Manganese | 0.75 - 1.00% | 0.75 - 1.00% | 2.00% max | 2.00% max |
| Phosphorus, max | 0.035% | 0.035% | 0.045% | 0.045% |
| Sulfur, max | 0.040% | 0.040% | 0.030% | 0.030% |
| Silicon | 0.15 - 0.35% | 0.15 - 0.35% | 1.00% max | 1.00% max |
| Chromium | 0.80 - 1.10% | 0.70 - 0.90% | 18.0 - 20.0% | 16.0 - 18.0% |
| Nickel | 1.65 - 2.00% | 8.0 - 11.0% | 10.0 - 14.0% | |
| Molybdenum | 0.15 - 0.25% | 0.20 - 0.30% | 2.00 - 3.00% |
ASTM A320 Recommended Hardware
| Grade | Nuts | Washers | Grade |
|---|---|---|---|
| L7 | A194 Grade 4 or 7 | F436 | L7 |
| L43 | A194 Grade 4 or 7 | F436 | L43 |
| B8 Class 1 | A194 Grade 8 | SS304 | B8 Class 1 |
| B8M Class 1 | A194 Grade 8M | SS316 | B8M Class 1 |
| B8 Class 2 | A194 Grade 8 | SS304 | B8 Class 2 |
| B8M Class 2 | A194 Grade 8M | SS316 | B8M Class 2 |