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ASTM A193 B7 vs A320 L7 Stud Bolts

ASTM A193 B7 is a Bolting specification for medium-high temperature applications. It is a heat treated Chromium Molybdenum steel and is considered for applications up to 450° Celsius (840° Fahrenheit).

ASTM A320 L7 has the same chemical and physical properties as B7, with additional Charpy V Notch tests taken at -101° Celsius (-150° Fahrenheit) for low temperature applications.

Steel is manufactured via Electric Arc Furnace, continuously cast followed by heat treatment and Bright conversion. It is typically used for the manufacture of Bolts, Fasteners, Studs and Studbolts in the Petrochemical industry. Available Drawn or Turned depending on the size in both Metric and Imperial sizes in standard and Thread Rolling Diameters. Typical stock size range 8mm to 4in Diameter in B7/L7 and 110mm to 130mm Diameter B7 only.

Mechanical Properties B7/L7

Grade Size Tensile ksi, min Yield, ksi, min Elong, %, min RA % min
B7 Up to 2-1/2 125 105 16 50
L7 Up to 2-1/2 125 105 16 50
Bolt

Chemical Composition B7/L7

Element B7 (AISI 4140) L7 (AISI 4140)
Carbon 0.37 - 0.49% 0.37 - 0.49%
Manganese 0.65 - 1.10% 0.65 - 1.10%
Phosphorus, max 0.035% 0.035%
Sulfur, max 0.040% 0.040%
Silicon 0.15 - 0.35% 0.15 - 0.35%
Chromium 0.75 - 1.20% 0.75 - 1.20%
Nickel
Molybdenum 0.15 - 0.25% 0.15 - 0.25%

Alloy Steel Stud Bolting Materials

ASTM A193

The ASTM A193 is heavily utilized in petroleum and chemical construction applications. The ASTM standard covers alloy steel and stainless steel bolting materials for high temperature or high pressure service. This specification includes fasteners intended for use in pressure vessels, valves, flanges, and fittings. Although, this material is often available in national coarse (UNC) thread pitches, if being used in traditional applications, threads are specified 8 threads per inch (tpi) for diameters above one inch.

ASTM 193, Grade B7
Heat treated chromium-molybdenum steel recommended for medium high temperature service. (Liquid quench -50° to 900° F, Air quench -40° to 900° F)

ASTM A193, Grade B7M
Similar to B7 studs except that the minimum yield and tensile strength requirements are reduced and the hardness controlled to 235 Brinell maximum. Recommended for use in corrosive environments. (-50° to 900° F)

ASTM A193, Grade B16
A heat treated chromium-molybdenum-vanadium steel recommended for high pressure, high temperature service. (-50° to 1100° F)

ASTM A320, Grade L7
This grade is intended for low temperature service down to minus 150° F and has a minimum Charpy impact value of 20 ft. lbs. at this temperature. (-50° to 1100° F)

ASTM A320, Grade L7M
Similar to L7 studs except that the minimum yield and tensile strength requirements are reduced and the hardness controlled to 235 Brinell maximum. This stud is recommended for use in low temperature corrosive environments. (-50° to 1100° F)

ASTM A320, Grade L43
This grade is intended for low temperature service down to minus 150° F and has a minimum Charpy impact value of 20 ft. lbs. at this temperature. Available in sizes up to a 4 inch diameter. (-150° to 1100° F)

ASTM A193, Grade B8
These Chromium-Nickel (AISI 304) austenitic steel studs are recommended for use in corrosive environments. (-325° to 1500° F)

ASTM A193, Grade B8M
These chromium-nickel molybdenum (AISI 316) austenitic steel studs are recommended for use in corrosive environments. (-325° to 1500° F)

Carbon and Alloy Steel Nuts

ASTM A194

The ASTM A194 specification covers carbon, alloy and stainless steel nuts intended for use in high-pressure and/or high-temperature service. Unless otherwise specified, the American National Standard Heavy Hex Series (ANSI B 18.2.2) shall be used. Nuts up to and including 1in nominal size shall be UNC Series Class 2B fit. Nuts over 1in nominal size shall be either UNC Series Class 2B fit or 8 UN Series Class 2B fit. High strength ASTM A194 grade 2H nuts are common in the marketplace.

ASTM A194 Nuts

ASTM A194 Grade 2
Suitable for use in high temperatures and high pressure conditions.

ASTM A194 Grade 2H
Suitable for use in high temperatures and high pressure conditions. Quenched & tempered.

ASTM A194 Grade 2HM
Similar to 2H nuts except this grade is recommended for use in corrosive environments.

ASTM A194 Grade 4
Heat treated molybdenum steel nuts suitable for severe temperature and pressure conditions.

ASTM A194 Grade 7
Heat treated chrome-molybdenum steel nuts suitable for extreme temperature and pressure conditions. Suitable for sub-zero service conditions and have minimum Charpy impact values in accordance with ASTM specifications.

ASTM A194 Grade 7L
Heat treated chrome-molybdenum steel nuts suitable for extreme temperature and pressure conditions. Suitable for sub-zero service conditions and have minimum Charpy impact values in accordance with ASTM specifications.

ASTM A194 Grade 7M
Similar to grade L7 nuts except this grade is recommended for use in corrosive environments.

ASTM A194 Grade 8/8M
Stainless steel nuts recommended for use in corrosive environments.

Remark(s) of the Author...

A194 gr. 2H vs A194 gr. 7 Nuts

Mostly ASTM A193 Grade B7 bolts are selected along with ASTM A194 Grade 2H nuts. However there is no ASTM or other standard where it recommends using A194 Gr. 2H nuts with this bolt.

The reason that A194 grade 2H nuts are used more often with A193 grade B7 bolts is simply cost. Both A194 2H and A194 grade 7 nuts have the same minimum mechanical requirements, they differ only in chemistry.

The industry has chosen 2H nuts to be the primary nut for B7 bolts. The result is that, in addition to be a less costly nut to manufacture, they are much more widely mass produced, leading to economies of scale.

That said, there is no reason that a specific engineer cannot require grade 7 nuts on a particular job if he feels that they will perform better in that application. The cost will be higher and the lead time may be slower due to availability, but the A194 grade 7 nuts would be acceptable for use with A193 B7 bolts or studs.

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