Stud bolt A320 Grade L7 vs. A320 Grade L7M
What is the difference between A320 Grade L7 vs. A320 Grade L7M：
What is A320 grade L7 material?
ASTM A320 L7 Stud bolt
ASTM A320 specification covers alloy steel bolting materials for pressure vessels, valves, flanges, and fittings for low-temperature service. Each alloy shall conform to the prescribed chemical composition requirements. The material, as represented by the tension specimens, shall conform to the requirements as to tensile properties such as tensile strength, yield strength, elongation, and hardness. The material shall meet the prescribed impact energy absorption requirements and the recommended test temperature. According to ASTM A320 mechanical tests shall be conducted on the material, namely: impact testing, tension testing, and hardness testing.
Inside ASTM A320 several grades are covered, including both ferritic and austenitic steels designated L7, B8, etc. Selection will depend on design, service conditions, mechanical properties, and low-temperature characteristics
The values stated in either inch-pound units or SI units are to be regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system are not exact equivalents; therefore, each system must be used independently of the other. Combining values from the two systems may result in nonconformance with the specification. Like the ASTM A193 specification, unless otherwise specified, the 8UN thread series is specified on fastener larger than 1 ” in diameter.
ASTM A320, Grade L7 – This grade is intended for low-temperature service down to minus 150ºF and has a minimum Charpy impact value of 15 ft-lb at this temperature. Sizes 2-1/2 in. and under.
What is A320 grade L7M material?
ASTM A320 Grade L7M is a standard material specification for alloy steel bolting materials for pressure vessels, valves, flanges, and fittings for low temperature services. ASTM A320 Grade L7M is heat treated chromium molybdenum alloy steel (AISI 4140/AISI 4140H) with notch toughness properties.
| Grade |
Yield, ksi, min |
Tensile, ksi, min |
Tempering temp, F, min |
Hardness, max |
| L7 |
105 |
125 |
1100 |
321 HB/35 HRC |
| L7M |
80 |
100 |
1150 |
235 HB/99 HRB |
Grade L7 bolts are significantly higher in strength than Grade L7m bolts. ASTM A320 L7M bolts are required to have 100% hardness testing. The final heat-treatment must be performed after machining or rolling of the threads and any type of cutting. This grade is typically used in sour gas applications.
Both A320 Grade L7 and A320 Grade L7M classes use the same chemical composition (C 0.4 – Cr 1.0 – Mo 0.25). The difference is that L7M is required to be tempered at 620 [°]C minimum, which reduces the mechanical properties, but improves toughness and resistance to embrittlement. L7M can meet NACE requirements for use in sour service.