What Is Bolt Grade? Marking and Strength
Quick Answer: A bolt grade is a classification that defines the mechanical properties (tensile strength, yield strength, and hardness) of a fastener. In piping, bolt grades follow ASTM specifications such as A193, A194, and A320, with each grade letter-number combination (e.g., B7, B8, L7) designating a specific alloy and heat treatment.
How Bolt Grades Work
Each ASTM bolt grade specifies the chemical composition, heat treatment, mechanical testing requirements, and allowable service conditions. The grade marking is stamped or embossed on the bolt head or nut face to enable traceability. For stud bolts, the grade is marked on one end of the threaded rod.
Mechanical Properties by Grade
| Grade | Specification | Tensile Strength (min) | Yield Strength (min) | Hardness (max) | Service Temperature |
|---|---|---|---|---|---|
| B7 | ASTM A193 | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 35 HRC | -40 to 450 degC |
| B7M | ASTM A193 | 100 ksi (690 MPa) | 80 ksi (552 MPa) | 22 HRC | Sour service (NACE) |
| B8 Cl.1 | ASTM A193 | 75 ksi (517 MPa) | 30 ksi (207 MPa) | 223 HB | Corrosive/cryogenic |
| B8 Cl.2 | ASTM A193 | 125 ksi (862 MPa) | 100 ksi (690 MPa) | 321 HB | High-strength stainless |
| B8M Cl.1 | ASTM A193 | 75 ksi (517 MPa) | 30 ksi (207 MPa) | 223 HB | Marine, chemical |
| B16 | ASTM A193 | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 35 HRC | Above 450 degC |
| L7 | ASTM A320 | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 35 HRC | Down to -100 degC |
| L7M | ASTM A320 | 100 ksi (690 MPa) | 80 ksi (552 MPa) | 22 HRC | Low-temp sour service |
Grade Markings on Bolts
Grade markings vary by specification system:
- ASTM system (piping): Grade symbol stamped on head/end (e.g., “B7”, “B8M”, “L7”). Manufacturer symbol and heat number also required.
- SAE/ISO system (structural/automotive): Radial lines or numbers on bolt head (e.g., SAE Grade 5 = 3 radial lines, Grade 8 = 6 radial lines).
- Metric ISO system: Property class number stamped on head (e.g., 8.8, 10.9, 12.9). The first number x 100 = tensile strength in MPa.
| Marking System | Example | Tensile Strength | Typical Use |
|---|---|---|---|
| ASTM A193 B7 | ”B7” stamped | 125 ksi | Piping flanges |
| SAE Grade 5 | 3 radial lines | 120 ksi | Structural |
| SAE Grade 8 | 6 radial lines | 150 ksi | High-strength structural |
| ISO 8.8 | ”8.8” stamped | 800 MPa | Metric general purpose |
| ISO 10.9 | ”10.9” stamped | 1,040 MPa | Metric high-strength |
Grade Selection Criteria
Selecting the correct bolt grade depends on three factors: operating temperature, corrosion environment, and required clamping load. ASTM A193 B7 covers approximately 90% of standard oil and gas flange bolting. Sour service applications per NACE MR0175 require the hardness-restricted B7M or L7M grades. Refer to the bolt torque charts for torque values by grade.
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