ASTM A193 / A320 High-Temperature & High-Pressure Bolting Specification (Grade B7, B8, B8M) Reference Guide
Engineering reference for ASTM A193 alloy steel and stainless steel pressure vessel flange bolting: Grade B7, B8, B8M properties, companion A194 nuts, and ASME PCC-1 bolting guidelines.
Standard Scope
Specifies alloy-steel and stainless steel bolting materials for pressure vessels, valves, flanges, and fittings for high-temperature or high-pressure service per ASTM A193 / A193M. Covers standard grades including Grade B7 (quenched & tempered AISI 4140/4142 Cr-Mo steel), Grade B16 (Cr-Mo-V steel for ultra-high temperature service), Grade B8 (AISI 304 austenitic stainless steel), and Grade B8M (AISI 316 austenitic stainless steel), detailing minimum tensile strength, yield strength, elongation, and reduction of area requirements.
Typical Applications
- •ASME Section VIII Division 1 pressure vessel girth flanges and nozzle bolting.
- •ASME B16.5 and ASME B16.47 pipe flange assemblies in petrochemical refineries and chemical plants.
- •High-temperature steam turbines, superheater headers, and boiler valve bodies.
- •Cryogenic storage tanks and marine subsea manifold bolted connections (Grades B8 and B8M).
Key Engineering Concepts
Grade B7 Chromium-Molybdenum Metallurgy
High-strength heat-treated alloy steel ($S_y = 105\text{ ksi} / 724\text{ MPa}$, $S_{ut} = 125\text{ ksi} / 862\text{ MPa}$ for diameters $\le 2-1/2\text{ in.}$); standard choice for operating temperatures up to 800 °F (427 °C).
Stainless Steel Bolting Classes
Grade B8 (304) and B8M (316) exist in Class 1 (carbide solution treated, lower strength, $S_y = 30\text{ ksi}$) and Class 2 (strain hardened, high strength, $S_y = 95\text{ ksi}$ for sizes up to 3/4 in.).
Companion Heavy Hex Nut Standards
ASTM A193 Grade B7 stud bolts are paired with ASTM A194 Grade 2H heavy hex nuts; Grade B8/B8M studs are paired with ASTM A194 Grade 8/8M stainless nuts to prevent galling and thread stripping.
8-Thread Series (8UN)
Large diameter bolts ($d > 1\text{ in.}$) in pressure vessel flanges frequently utilize 8UN thread series (8 threads per inch) rather than UNC coarse threads to optimize root stress concentration and tightening torque efficiency.
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Engineering Notes & Best Practices
- •For operating temperatures exceeding 800 °F (427 °C) and up to 1000 °F (538 °C), Grade B16 is preferred over Grade B7 due to vanadium addition which prevents high-temperature creep relaxation.
- •Thread lubricant temperature rating is critical: anti-seize paste with nickel or molybdenum disulfide ($\text{MoS}_2$) must be selected to match the flange service temperature and prevent thermal galling during disassembly.
- •Hydraulic bolt tensioners or calibrated multi-pass torque sequencing (e.g., ASME PCC-1 cross-pattern tightening) are recommended for ASME B16.5 flange sizes 2 in. and larger to achieve uniform gasket seating stress.
Official Standards Reference
Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications
American Society for Testing and Materials core bolting specification.
Standard Specification for Carbon and Alloy Steel Nuts for Bolts for High Pressure or High Temperature Service
Companion specification defining matching nut grades (Grade 2H, Grade 8, Grade 8M).
Guidelines for Pressure Boundary Bolted Flange Joint Assembly
Recommended bolting procedures, torque calculation rules, and target gasket stress limits.