ASTM A320

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.

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

 

Mechanical Properties

GradeSizeTensile,
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

 

Chemical Properties

ElementL7 (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%

 

Recommended Nuts and Washers

GradeNutsWashers
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

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