ASTM A320 – Mechanical, Material & Chemical Properties
The ASTM A320 specification establishes the essential mechanical and chemical requirements for alloy and stainless steel bolting used in low-temperature applications. Centered on high-strength ferritic grades (quenched and tempered) and corrosion-resistant austenitic grades (solution annealed or strain-hardened), the standard ensures reliability for pressure-retaining components. Key technical focuses include stringent tensile property minimums and mandatory Charpy impact energy absorption levels—ranging from 20 to 40 ft-lbf—to prevent brittle failure in sub-zero service conditions.
Mechanical Properties
The mechanical requirements for ASTM A320 bolting are determined by the alloy's heat-treated condition and diameter. Ferritic grades, intended for high-strength low-temperature service, must meet stringent tensile and yield minimums through quenching and tempering. Austenitic grades offer a range of properties based on their class: Class 1 and 1A provide standard solution-treated properties, while Class 2 leverages strain hardening to achieve significantly higher strength, particularly in smaller thread sizes. Compliance is verified through tensile testing and hardness measurements—such as Brinell (HBW) or Rockwell (HRC/HRB)—to ensure structural reliability.
| Mechanical Requirements | ||||||
|---|---|---|---|---|---|---|
| Grade | Thread Size (inch & metric) | Tensile Strength, min (ksi) | Yield Strength 0.2% offset, min (ksi) | Elongation in 2 in. or 50 mm, min (%) | Reduction of Area, min (%) | Hardness, max |
| L7, L7A, L7B, L7C, L70, L71, L72, L73 | 2-1/2" or M65 and under | 125 | 105 | 16 | 50 | 321 HBW or 35 HRC |
| L43 | 4" or M100 and under | 125 | 105 | 16 | 50 | 321 HBW or 35 HRC |
| L7M | 2-1/2" or M65 and under | 100 | 80 | 18 | 50 | 235 HBW or 99 HRB |
| L1 | 1" or M25 and under | 125 | 105 | 16 | 50 | - |
| B8, B8C, B8M, B8P, B8F, B8T, B8LN, B8MLN (Class 1) | All Diameters | 75 | 30 | 30 | 50 | 223 HBW or 96 HRB |
| B8A, B8CA, B8MA, B8PA, B8FA, B8TA, B8LNA, B8MLNA (Class 1A) | All Diameters | 75 | 30 | 30 | 50 | 192 HBW or 90 HRB |
| B8, B8C, B8P, B8F, B8T (Class 2) | 3/4" or M20 and under | 125 | 100 | 12 | 35 | 321 HBW or 35 HRC |
| over 3/4" to 1" or M20 to M25 | 115 | 80 | 15 | 30 | 321 HBW or 35 HRC | |
| over 1" to 1-1/4" or M25 to M32 | 105 | 65 | 20 | 35 | 321 HBW or 35 HRC | |
| over 1-1/4" to 1-1/2" or M32 to M40 | 100 | 50 | 28 | 45 | 321 HBW or 35 HRC | |
| B8M (Class 2) | 3/4" or M20 and under | 110 | 95 | 15 | 45 | 321 HBW or 35 HRC |
| over 3/4" to 1" or M20 to M25 | 100 | 80 | 20 | 45 | 321 HBW or 35 HRC | |
| over 1" to 1-1/4" or M25 to M32 | 95 | 65 | 25 | 45 | 321 HBW or 35 HRC | |
| over 1-1/4" to 1-1/2" or M32 to M40 | 90 | 50 | 30 | 45 | 321 HBW or 35 HRC | |
Material Properties
The ASTM A320 specification defines stringent chemical composition requirements across various ferritic and austenitic steel grades to guarantee reliable performance in low-temperature service. Ferritic grades, such as L7 and L43, are alloyed with chromium and molybdenum for hardenability, while austenitic grades like B8 and B8M rely on high nickel and chromium concentrations for cryogenic stability. Specialized chemical controls are also mandated, such as the addition of boron in Grade L1 or selenium for free-machining in Grade B8F, to meet specific mechanical or manufacturing objectives.
| Chemical Requirements (Composition, %) | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Grade | UNS Designation | Carbon, (%) | Manganese, (%) | Phosphorus, (%) | Sulfur, (%) | Silicon, (%) | Chromium, (%) | Nickel, (%) | Molybdenum, (%) | Vanadium, (%) | Aluminum, (%) | Columbium + Tantalum, (%) | Titanium, (%) | Nitrogen, (%) | Boron/Selenium, (%) |
| L7, L7M, L70 | - | 0.38 - 0.48 | 0.75 - 1.00 | 0.035 (max) | 0.040 (max) | 0.15 - 0.35 | 0.80 - 1.10 | - | 0.15 - 0.25 | - | - | - | - | - | - |
| L7A, L71 | - | 0.35 - 0.40 | 0.70 - 0.90 | 0.035 (max) | 0.040 (max) | 0.15 - 0.35 | - | - | 0.20 - 0.30 | - | - | - | - | - | - |
| L7B, L72 | - | 0.35 - 0.40 | 0.60 - 0.90 | 0.035 (max) | 0.040 (max) | 0.15 - 0.35 | 0.70 - 0.90 | - | 0.15 - 0.25 | - | - | - | - | - | - |
| L7C, L73 | - | 0.38 - 0.43 | 0.70 - 0.90 | 0.035 (max) | 0.040 (max) | 0.15 - 0.35 | 0.40 - 0.60 | 1.65 - 2.00 | 0.20 - 0.30 | - | - | - | - | - | - |
| L43 | - | 0.38 - 0.43 | 0.60 - 0.85 | 0.035 (max) | 0.040 (max) | 0.15 - 0.35 | 0.70 - 0.90 | 1.65 - 2.00 | 0.20 - 0.30 | - | - | - | - | - | - |
| L1 | - | 0.17 - 0.24 | 0.70 - 1.40 | 0.035 (max) | 0.050 (max) | 0.15 - 0.30 | - | - | - | - | - | - | - | - | 0.001 - 0.003 (Boron) |
| B8, B8A | S30400 (304) | 0.08 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 18.0 - 20.0 | 8.0 - 11.0 | - | - | - | - | - | - | - |
| B8C, B8CA | S34700 (347) | 0.08 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 17.0 - 19.0 | 9.0 - 12.0 | - | - | - | 10 x Carbon content (min) - 1.10 (max) | - | - | - |
| B8T, B8TA | S32100 (321) | 0.08 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 17.0 - 19.0 | 9.0 - 12.0 | - | - | - | - | 5 x Carbon content (min) | - | - |
| B8P, B8PA | S30500 | 0.08 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 17.0 - 19.0 | 10.5 - 13.0 | - | - | - | - | - | - | - |
| B8F, B8FA | S30300 (303) / S30323 (303Se) | 0.15 (max) | 2.00 (max) | 0.20 (max) | 0.15 (min) or 0.06 (max) | 1.00 (max) | 17.0 - 19.0 | 8.0 - 10.0 | 0.60 (max) | - | - | - | - | - | 0.35 (min) Selenium (if S30323) |
| B8M, B8MA | S31600 (316) | 0.08 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 16.0 - 18.0 | 10.0 - 14.0 | 2.00 - 3.00 | - | - | - | - | - | - |
| B8LN, B8LNA | S30453 | 0.030 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 18.0 - 20.0 | 8.0 - 10.5 | - | - | - | - | - | 0.10 - 0.16 | - |
| B8MLN, B8MLNA | S31653 | 0.030 (max) | 2.00 (max) | 0.045 (max) | 0.030 (max) | 1.00 (max) | 16.0 - 18.0 | 10.0 - 14.0 | 2.00 - 3.00 | - | - | - | - | 0.10 - 0.16 | - |