Stud bolt ASTM A193 B7 VS ASTM A320 L7
ASTM A193 B7 and ASTM A320 L7 stud bolts are both chromium-molybdenum alloy steel fasteners with similar mechanical properties. The key difference is that L7 is specifically designed for low-temperature service and requires impact testing, while B7 is intended for medium to high-temperature applications.
What is ASTM A193 B7?
ASTM A193 B7 is a heat-treated chromium-molybdenum (Cr-Mo) alloy steel used for high-strength bolting in:
- Medium to high-temperature service
- Pressure vessels, flanges, and valves
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).Grade B7 is a heat-treated chromium-molybdenum (Cr-Mo) alloy steel with minimum tensile strength requirement of 125 ksi, yield 105 ksi, and maximum hardness of 35 HRC
Heat Treatment: ASTM A193 B7 bolts undergo a specialized heat treatment process, typically quenching and tempering, to enhance their mechanical properties and performance. This heat treatment results in improved hardness, toughness, and resistance to fatigue and stress corrosion cracking.
ASTM A193 B7 bolts are widely used in various industrial sectors, including:
- Oil and Gas: Offshore drilling rigs, refineries, pipelines, and petrochemical plants.
- Power Generation: Turbines, boilers, heat exchangers, and nuclear reactors.
- Construction: Structural steel framing, bridges, and high-rise buildings.
What is ASTM A320 L7?
ASTM A320 L7 is a low-temperature alloy steel fastener with similar composition to B7 but designed for:
- Cryogenic and sub-zero service
- Applications requiring impact toughness
A320 L7 has the same chemical and physical properties as B7, with additional Charpy V Notch tests taken at -101°C (-150°F) for low temperature applications. ASTM A320 L7 has the same chemical and physical properties as B7, with additional Charpy V Notch tests taken at -101°C (-150°F) for low temperature applications.
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.
|
Property class |
Chemical composition |
|||||||||
|
C. |
Mn. |
Si. |
P. |
S. |
Cr. |
Ni. |
Mo. |
V. |
Ti. |
|
|
L7 |
0.38—0.48 |
0.75—1.00 |
0.15—0.35 |
0.035 max. |
0.040 max. |
0.80—1.10 |
– |
0.15-0.25 |
– |
– |
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 |
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% |
Key Differences Between B7 and L7
| Feature | ASTM A193 B7 | ASTM A320 L7 |
|---|---|---|
| Application | High / medium temperature | Low temperature |
| Impact Testing | Not required | Mandatory |
| Test Temperature | N/A | -101°C (-150°F) |
| Mechanical Properties | High strength | Same as B7 |
| Main Advantage | Heat resistance | Low-temperature toughness |
When to Use B7 vs L7
Use B7 when:
- Operating at medium or high temperature
- No low-temperature impact requirement
Use L7 when:
- Operating at sub-zero temperatures
- Impact resistance is required
- Cryogenic service conditions
What to learn more about the Stud bolt ASTM A320 L7 VS ASTM A320 L7M?
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