A320 bolts can be specified in several different grades. This website summarizes the similarities and differences between many of the grades. For more specific information, please refer to the actual ASTM A320 specification which can be purchased directly from ASTM International. Additional grades of A320 bolts exist but are not used frequently enough to list on this site.
Grade L7
This is the most common A320 bolt grade. A320 Grade L7 bolts and threaded studs are manufactured from alloy steel with chromium, manganese, and molybdenum. The bolts undergo a heat treating process to reach their required mechanical properties. L7 threaded studs and short, headed bolts are readily available in the marketplace. These bolts are typically bought and sold in a plain, bare metal finish. L7 bolts are available through 2-1/2” in diameter. A320 Grade L7 bolts larger than 1/2″ diameter require a Charpy impact test.
Grade B8
ASTM A320 Grade B8 Class 1 bolts and threaded studs are manufactured from AISI 304 stainless steel. These bolts are heated and then water-quenched through carbide solution treatment, or solution annealing, which ensures the bolts are corrosion resistant. A320 Grade B8 Class 2 bolts are annealed and strain hardened to achieve increased strength characteristics. These bolts do not require an impact test.
Grade B8M
ASTM A320 Grade 8M Class 1 fasteners are similar to Grade B8 but instead are manufactured from AISI 316 stainless steel as opposed to AISI 304 stainless steel. Although more expensive, type 316 stainless offers superior corrosion resistance compared to type 304 stainless. Grade B8M Class 1 fasteners require carbide solution treatment, while Class 2 fasteners require an additional strain hardening operation. A320 Grade B8M bolts also do not require an impact test.
Grade L43
ASTM A320 Grade L43 bolts and threaded studs are manufactured from AISI 4340 material and are also quenched and tempered. L43 bolts can be as large as 4” in diameter, whereas Grade L7 does not exceed 2-1/2” diameter. L43 strength and chemical properties are virtually identical to L7 but may perform more favorably with regard to Charpy impact testing.
Grade L7M
ASTM A320 Grade L7M bolts and threaded studs are also manufactured from quenched and tempered alloy steel. The mechanical properties and requirements are lower compared to L7 bolts. L7M bolts are required to have 100% hardness testing. The final heat-treatment must be performed after machining or rolling of the threads and any type of cutting. This grade is typically used in sour gas applications.
| 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: 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 | |
| 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 |
| 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% |