A193 B7 vs B8
ASTM A193 Grade B7 is a chromium-molybdenum alloy steel bolt, and Grade B8 is an austenitic stainless steel (Type 304) bolt. B7 provides high strength for carbon steel piping systems in moderate-to-high temperature service. B8 provides corrosion resistance for stainless steel piping, chemical plants, and marine environments, but at significantly lower mechanical strength.
When to Use Each Grade
B7 is the default for carbon steel flanges (ASTM A105, A350 LF2). B8 is specified when the piping system uses stainless steel flanges (ASTM A182 F304, F316) and the service requires corrosion resistance in the bolting as well.
- Use B7 for: carbon steel process piping, high-pressure/high-temperature hydrocarbon service, steam systems, and all general-purpose applications
- Use B8 for: stainless steel piping systems, chemical plants handling corrosive fluids, offshore/marine environments, and cryogenic service
- Use B8M (Type 316) for: chloride-containing environments, seawater, and higher corrosion resistance than B8
Specifications and Composition
| Property | A193 B7 | A193 B8 (Class 1) | A193 B8M (Class 1) |
|---|---|---|---|
| Base material | AISI 4140/4142 (Cr-Mo) | AISI 304 (18Cr-8Ni) | AISI 316 (16Cr-12Ni-2Mo) |
| Carbon (C) | 0.38-0.48% | 0.08% max | 0.08% max |
| Chromium (Cr) | 0.80-1.10% | 18.0-20.0% | 16.0-18.0% |
| Nickel (Ni) | - | 8.0-10.5% | 10.0-14.0% |
| Molybdenum (Mo) | 0.15-0.25% | - | 2.0-3.0% |
| Heat treatment | Quenched & tempered | Carbide solution treated | Carbide solution treated |
| Magnetic | Yes | No (austenitic) | No (austenitic) |
Mechanical Properties Comparison
| Property | A193 B7 (up to 2.5”) | A193 B8 Class 1 | A193 B8 Class 2 |
|---|---|---|---|
| Yield strength (min) | 105 ksi (724 MPa) | 30 ksi (207 MPa) | 80 ksi (551 MPa) |
| Tensile strength (min) | 125 ksi (862 MPa) | 75 ksi (517 MPa) | 100 ksi (690 MPa) |
| Elongation (min) | 16% | 30% | 12% |
| Hardness | 235-331 HB | 223 HB max | - |
| Matching nut | A194 2H | A194 8 | A194 8 |
B8 Class 1 is carbide solution treated (annealed) and has relatively low strength. B8 Class 2 is strain-hardened (cold worked) to achieve higher strength, but availability is more limited and cost is higher.
Temperature Range
| Grade | Min Service Temp | Max Service Temp |
|---|---|---|
| B7 | -29 degC (-20 degF) | 450 degC (840 degF) |
| B8 Class 1 | -254 degC (-425 degF) | 540 degC (1,000 degF) |
| B8M Class 1 | -254 degC (-425 degF) | 540 degC (1,000 degF) |
| B7M (NACE) | -29 degC (-20 degF) | 450 degC (840 degF) |
B8 and B8M have a major advantage in cryogenic service. Their austenitic structure maintains toughness at temperatures where B7 becomes brittle. For low-temperature carbon steel applications (down to -100 degC), use A320 L7 instead of B7.
Cost Comparison
B8 and B8M stud bolts cost 3-5 times more than B7 for equivalent sizes. The nickel and chromium content of austenitic stainless drives the price premium. On large EPC projects, using B7 where corrosion resistance is not required produces significant cost savings on bolt procurement without compromising safety.
| Size (diameter x length) | B7 Approx Cost | B8 Approx Cost | B8M Approx Cost |
|---|---|---|---|
| 5/8” x 3-3/4” | $ | $$$ | $$$$ |
| 7/8” x 5-1/2” | $ | $$$ | $$$$ |
| 1-1/4” x 7-1/2” | $$ | $$$$ | $$$$$ |
Pair B7 or B8 studs with the correct gasket type (SWG, RTJ, or soft) and apply torque per the ASME bolt chart.
Leave a Comment
Have a question or feedback? Send us a message.