Aug 2025
ASTM A320
The ASTM A320 specification, first introduced in 1948, is designed for low-temperature service applications. It covers alloy steel and stainless steel bolting materials, including bolts, screws, studs, and stud bolts, which are commonly used in pressure vessels, valves, flanges, and fittings. These fasteners are manufactured through rolling, forging, or strain hardening to ensure durability and strength in cold and cryogenic conditions. For fasteners larger than one inch in diameter, the 8UN thread series is generally specified unless otherwise stated. While ASTM A320 includes several grades, the most widely used ones are highlighted for practical use in industries that demand safety and reliability.
Common ASTM A320 Grades (Material & Heat Treatment)
| Grade |
Base Material (AISI) |
Heat Treatment / Notes |
| L7 (Alloy Steel) |
4140 / 4142 |
Quenched & tempered |
| L43 (Alloy Steel) |
4340 |
Quenched & tempered |
| B8 Class 1 (Stainless) |
304 |
Carbide solution treated |
| B8M Class 1 (Stainless) |
316 |
Carbide solution treated |
| B8 Class 2 (Stainless) |
304 |
Carbide solution treated, strain hardened |
| B8M Class 2 (Stainless) |
316 |
Carbide solution treated, strain hardened |
A320 Mechanical properties
| Grade |
Diameter |
Tensile, ksi, min. |
Yield, ksi, min. |
Charpy Impact 20-ft-lbf @ temp |
Elong, %, min. |
RA, %, min. |
| L7 |
Up to 2-1/2"
|
125 |
105 |
-150°F |
16 |
50 |
| L43 |
Up to 4 |
125 |
105 |
-150°F |
16 |
50 |
| Class 1: B8, 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 |
| 1-1/8" - 1-1/4" | 105 |
65 |
N/A |
20 |
35 |
| 1-3/8" - 1-1/2" | 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 |
| 1-1/8" - 1-1/4" | 95 |
65 |
N/A |
25 |
45 |
| 1-3/8"- 1-1/2" | 90 |
50 |
N/A |
30 |
45 |
A320 Checmical 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% |
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 |