While the metal flange and the gasket often take center stage in discussions of bolted joints, the integrity of the entire connection hinges absolutely on the performance of its bolting. The nuts and bolts are the unsung partners, providing the crucial clamping force that compresses the gasket and secures the joint. Their reliability is dictated by their metallurgy, which must be carefully selected to match the flange’s material and the system’s operating conditions.
Bolts for industrial flanges are not just generic fasteners; they are highly engineered components made from specific alloy steels, designed to withstand high tensile stresses, often at extreme temperatures, and resist various forms of degradation.
Key Metallurgical Properties for Bolting:
- High Tensile Strength and Yield Strength:
- Bolts must be strong enough to be tightened to a high preload without yielding (permanently stretching). This preload is what compresses the gasket.
- Quenching and Tempering: Many high-strength bolts undergo this heat treatment. Quenching (rapid cooling) hardens the steel, and tempering (reheating to a specific temperature) then improves its toughness and ductility while maintaining strength.
- Creep Resistance:
- For high-temperature applications, bolts must resist creep, which is the gradual stretching or deformation under sustained stress at elevated temperatures. If bolts creep, they lose their preload, leading to gasket relaxation and leaks. Alloying elements like chromium and molybdenum enhance creep resistance.
- Stress Relaxation Resistance:
- Similar to creep, but specifically refers to the reduction of stress in a material over time at constant strain (i.e., the bolt maintains its length but loses its clamping force). This is critical for maintaining gasket compression.
- Toughness (Impact Resistance):
- Bolts, especially for low-temperature or dynamic applications, must possess good toughness to resist brittle fracture under sudden impacts or stress concentrations. Impact testing (e.g., Charpy V-notch) is performed at the lowest anticipated operating temperature.
- Corrosion Resistance:
- External Corrosion: Exposed bolts are vulnerable to atmospheric corrosion, which can weaken the bolt and make removal difficult. Coatings (e.g., hot-dip galvanizing, fluoropolymers) or inherently corrosion-resistant materials (e.g., stainless steel, super duplex stainless steel, nickel alloys) are used.
- Stress Corrosion Cracking (SCC): A combination of tensile stress, a susceptible material, and a specific corrosive environment (e.g., chlorides for stainless steel) can lead to cracking. Bolt materials must be chosen to resist SCC in their service environment.
- Hydrogen Embrittlement: In sour gas service (containing H2S) or environments where hydrogen can be generated, hydrogen can diffuse into susceptible high-strength steels, causing brittle fracture. NACE MR0175/ISO 15156 standards specify material and hardness limits to mitigate this.
Common Bolting Materials and Their Applications (ASTM Standards):
- ASTM A193 Grade B7: This is the most common carbon steel bolt material for general high-temperature service up to 450°C (850°F). It is quenched and tempered chromium-molybdenum steel. Nuts are typically A194 Grade 2H.
- ASTM A193 Grade B8/B8M: Austenitic stainless steel bolts (304/316 equivalent) for corrosion resistance. B8M (316) offers superior resistance to pitting and crevice corrosion. Used in corrosive environments or where specific material compatibility is needed.
- ASTM A320 Grade L7: Low-alloy steel (similar to B7) but specifically tested for low-temperature service down to -101°C (-150°F), ensuring good toughness at cryogenic temperatures. Nuts are typically A194 Grade 7.
- ASTM A193 Grade B16: A chromium-molybdenum-vanadium alloy steel for very high-temperature and high-pressure service, offering excellent creep resistance.
- Specialty Alloys: For extremely aggressive environments (e.g., sour service, very high chlorides), bolting made from super duplex stainless steels (e.g., UNS S32750) or nickel alloys (e.g., Alloy 625, Alloy 718) may be required.
The metallurgy of bolting is a complex and critical field. The careful selection, manufacturing, and installation of bolts are as important as the flange and gasket themselves. By providing the unwavering clamping force that is the very essence of a bolted joint, high-quality bolting ensures the long-term integrity and safety of industrial piping systems worldwide.