What Is ASTM A193 B7?
ASTM A193 covers alloy and stainless steel bolting materials for high-temperature or high-pressure service. Grade B7 is the workhorse grade, specified in the vast majority of carbon steel piping classes across all ASME pressure classes from 150 to 2500.
Chemical Composition
B7 studs are manufactured from AISI 4140 or 4142 chromium-molybdenum steel.
| Element | Composition (%) |
|---|---|
| Carbon (C) | 0.38 - 0.48 |
| Manganese (Mn) | 0.75 - 1.00 |
| Phosphorus (P) max | 0.035 |
| Sulfur (S) max | 0.040 |
| Silicon (Si) | 0.15 - 0.35 |
| Chromium (Cr) | 0.80 - 1.10 |
| Molybdenum (Mo) | 0.15 - 0.25 |
The Cr-Mo addition provides hardenability and elevated-temperature strength. After hot forming, the bolts are quenched in oil or water and tempered at a minimum of 593 degC (1,100 degF) to develop the required combination of strength and toughness.
Mechanical Properties
Properties depend on the bolt diameter. As diameter increases, the through-thickness quenching response decreases, reducing achievable strength.
| Diameter Range | Yield Strength (min) | Tensile Strength (min) | Elongation (min) | Reduction of Area (min) | Hardness |
|---|---|---|---|---|---|
| Up to 2-1/2” (64 mm) | 105 ksi (724 MPa) | 125 ksi (862 MPa) | 16% | 50% | 235-331 HB |
| Over 2-1/2” to 4” | 95 ksi (655 MPa) | 115 ksi (793 MPa) | 16% | 50% | 235-331 HB |
| Over 4” to 7” | 75 ksi (517 MPa) | 100 ksi (690 MPa) | 18% | 50% | 235-331 HB |
Temperature Limits
| Condition | Temperature |
|---|---|
| Maximum continuous service | 450 degC (840 degF) |
| Minimum (without impact test) | -29 degC (-20 degF) |
| For lower temperatures | Use ASTM A320 L7 (impact tested) |
| For temperatures above 450 degC | Use ASTM A193 B16 (Cr-Mo-V) |
Above 450 degC, B7 loses yield strength rapidly due to tempering effects. Below -29 degC, the Charpy impact toughness of B7 may be insufficient, requiring the low-temperature grade A320 L7 instead.
B7 vs Related Grades
| Grade | Base | Key Difference |
|---|---|---|
| B7 | AISI 4140/4142 | Standard high-temp grade |
| B7M | Same as B7 | Max hardness 22 HRC for sour service (NACE MR0175) |
| B8 | SS 304 | Corrosion-resistant, lower strength |
| B16 | Cr-Mo-V | Higher-temperature service (above 450 degC) |
| L7 (A320) | Same chemistry as B7 | Impact tested for low-temperature service |
Matching Nut
B7 studs are always paired with ASTM A194 Grade 2H heavy hex nuts. The 2H nut is a carbon steel nut, quenched and tempered, with a hardness of 248-352 HB. Using a softer or mismatched nut grade undermines the joint’s load-bearing capacity.
B7 stud bolt lengths for specific flange sizes and classes are listed in the ASME B16.5 bolt chart. Proper torque application is required to achieve the target gasket stress without exceeding bolt yield.
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