ASTM A320

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 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

A320 Mechanical Properties

Grade Diameter Tensile, ksi, min Yield, ksi, min Charpy Impact 20-ft-lbf @ temp Elong, %, min RA, %, min
L7 Up to 212 125 105 -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 34 125 100 N/A 12 35
78 - 1 115 80 N/A 15 35
118 - 114 105 65 N/A 20 35
138 - 112 100 50 N/A 28 45
B8M Class 2 Up to 34 110 95 N/A 15 45
78 - 1 100 80 N/A 20 45
118 - 114 95 65 N/A 25 45
138 - 112 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