ASTM A320 Gr. L7M STUD BOLT for Low Temperature service
ASTM A320 is the specification covers alloy steel bolting materials for pressure vessels, valves, flanges, and fittings for low-temperature service
Grade L7M is a heat-treated chromium- molybdenum alloy steel with minimum tensile requirement under ASTM A320
ASTM A320 L7M is manufactured from AISI 4140 round bar and is supplied in a quenched and tempered condition.
ASTM A320 L7M Chemical composition
| Element | Composition, % A |
|---|---|
| Chromium Molybdenum B | |
| Carbon, max | 0.38 – 0.48 |
| Manganese | 0.75 – 1.00 |
| Phosphorus, max | 0.035 |
| Sulfur, max | 0.040 |
| Silicon | 0.15 – 0.35 |
| Chromium | 0.80 – 1.10 |
| Molybdenum | 0.15 – 0.25 |
ASTM A320 Impact Test Requirements (Notch Toughness)
| Size of Specimen, mm | Test Temperature | Minimum impact value required for average of each set of three specimens, ft-lbf [J] | Minimum impact value permitted for one specimen only of a set, ft-lbf [J] | |
|---|---|---|---|---|
| °F | °C | |||
| 10 X 10 | -100 | -73 | 20 [27] | 15 [20] |
| 10 X 7.5 | 16 [22] | 12 [16] | ||
Nuts Hardness Requirement for A320 L7M Stud bolt
A194 Grade 7M nuts at a hardness not exceeding 235 HBW (or equivalent) shall be used with Grade L7M bolts, studs, and stud bolts. All nut impact tests shall be made on test specimens taken from the bar or plate from the heat of steel used for manufacturing the nuts, and heat treated with the nut blanks.
A320 L7M Stud bolt heat Treatment
For L7M bolting, the final heat treatment, which may be the tempering or stress-relieving operation conducted at 1150 °F [620 °C] minimum, shall be done after machining or rolling of the threads and any type of cutting
A320 L7M Stud bolt Hardness requirement
The maximum hardness of Grade L7M shall be 235 HBW or 99 HRB (conversion in accordance with Table Number 2B of Test Methods and Definitions A370). Minimum hardness shall not be less than 200 HBW or 93 HRB. Conformance to this hardness shall be ensured by testing each bolt or stud by Brinell or Rockwell B methods in accordance with 6.3.1
Surface Treatments
ASTM A320 L7M Stud bolt specification
| Property | ASTM A320 L7M Requirement |
| Material | Alloy Steel (AISI 4140/4142) |
| Tensile Strength | ≥ 100 ksi (690 MPa) |
| Yield Strength | ≥ 80 ksi (550 MPa) |
| Hardness | ≤ 235 HBW or ≤ 99 HRB |
| Impact Toughness | Charpy V-notch at -73°C |
| Heat Treatment | Quenched & Tempered |
| Corrosion Resistance | Suitable for sour service (NACE MR0175/ISO 15156) |
| Thread Type | UNC (Coarse) or UNF (Fine) Metric thread |
Difference Between ASTM A320 L7 vs. L7M
| Feature | L7 | L7M |
|---|---|---|
| Hardness | ≤ 35 HRC (≥ 248 HBW) | ≤ 235 HBW (≤ 99 HRB) |
| Application | General low-temp use | Sour gas (H₂S) service |
| Impact Test | Required at -101°C | Required at -73°C |
| Toughness | Moderate | Higher toughness |
| Corrosion Resistance | Standard | Better (NACE MR0175 compliant) |
L7M is specifically designed for environments with H₂S (sour gas), where lower hardness improves crack resistance.
Application of ASTM A320 L7M Stud Bolts
LNG Plants & Cryogenic Storage (-73°C / -150°F applications)
Petrochemical & Refineries (Piping, flanges, and equipment)
Offshore & Subsea Structures (Sour gas environments)
Power Plants & Boilers (Cold service bolting)
