Scope
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.
A320 Grades
L7 Alloy steel | L43 Alloy steel | B8 Class 1 Stainless steel | B8M Class 1 Stainless steel | B8 Class 2 Stainless steel | B8M Class 2 Stainless steel |
AISI 4140/4142 Quenched and tempered | AISI 4340 Quenched and tempered | AISI 304, carbide solution treated | AISI 316, carbide solution treated | AISI 304, carbide solution treated, strain hardened | AISI 316, carbide solution treated, strain hardened |
A320 Mechanical Properties
Grade | Size | 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 |
B8 Class 1 | All | 75 | 30 | N/A | 30 | 50 |
B8M Class 1 | All | 75 | 30 | N/A | 30 | 50 |
B8 Class 2 | 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 | |
B8M Class 2 | 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 |
A320 Chemical Properties
Element | L7 (AISI 4140) | L43 (AISI 4340) | B8 (AISI 304) | B8M (AISI 316) |
Carbon | 0.37 – 0.49% | 0.38 – 0.43% | 0.08% max | 0.08% max |
Manganese | 0.65 – 1.10% | 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 | 0.75 – 1.20% | 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% |
A320 Recommended Hardware
Grade | Nuts | Washers |
L7 | A194 Grade 4 or 7 | F436 |
L43 | A194 Grade 4 or 7 | F436 |
B8 Class 1 | A194 Grade 8 | SS304 |
B8M Class 1 | A194 Grade 8M | SS316 |
B8 Class 2 | A194 Grade 8 | SS304 |
B8M Class 2 | A194 Grade 8M | SS316 |
Strain hardened nuts available as a supplementary requirement | ||