ASTM A193 Grade B16 is a specially heat-treated chromium-molybdenum-vanadium alloy steel designed for high-temperature bolting applications up to 590°C. With superior strength retention and stress relaxation resistance compared to B7, it’s the critical fastener material for supercritical power plants and high-temperature petrochemical equipment.
Core Definition & Material
Governing Standard: It falls under the ASTM A193 standard (Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications).
Material Type: Alloy Steel, specifically a Chromium-Molybdenum-Vanadium steel.
Common Approximate Grade: Its chemical composition is closest to Type AISI 422 stainless steel (though strictly speaking, A193 B16 has its own independent composition and heat treatment requirements and is not completely identical).
Key Characteristic: It undergoes a “double tempering” or “secondary tempering” heat treatment process, which allows it to retain very high strength and excellent resistance to relaxation (creep) at elevated temperatures.
Primary Chemical Composition
- Chromium (Cr): 11.0 – 13.5% (Provides high-temperature strength, oxidation, and corrosion resistance)
- Molybdenum (Mo): 0.50 – 1.00% (Significantly increases high-temperature strength and creep resistance)
- Vanadium (V): 0.20 – 0.30% (Refines grain structure, enhances high-temperature strength and toughness)
- Carbon (C): 0.15 – 0.21% (Ensures strength)
- Silicon (Si), Manganese (Mn), etc.: Within specified ranges.
Performance & Advantages
The design goal of B16 is very clear: To provide reliable connection strength in the higher temperature range where B7 bolt strength begins to decline significantly.
Exceptionally High Elevated-Temperature Strength:
- The recommended maximum service temperature for B7 bolts is about ~480°C.
- The recommended maximum service temperature for B16 bolts can reach about ~590°C.
- In this temperature range (480°C – 590°C), the strength of B16 is far superior to that of B7, making it the preferred alloy steel bolting material for this segment.
Excellent Stress Relaxation Resistance: Under prolonged exposure to high temperature and stress, materials undergo creep, leading to preload loss. The special heat treatment of B16 gives it superior resistance to relaxation, maintaining flange seal integrity for extended periods.
Mechanical Property Requirements
- Tensile Strength: 115 ksi (795 MPa) Minimum (Note: This strength value is slightly lower than B7’s 125 ksi, but its strength retention rate at high temperatures is far superior to B7’s)
- Yield Strength: 95 ksi (655 MPa) Minimum
- Hardness: Typically controlled within the range of HB 248-321 or HRC 24-35.
Application Scenarios
B16 is specifically used in applications where the temperature is too high for B7 to be suitable, but the use of expensive high-temperature stainless steels or nickel-based alloys is not yet required.
- Power Plants: Connections for high-temperature steam line flanges, valves, and turbine casings in supercritical and ultra-supercritical thermal power generation units.
- Petrochemical Industry: Connections for certain high-temperature reactors, reformers, and heat exchangers.
- Any critical equipment operating between 500°C and 600°C that requires high-strength bolted connections.
Comparison Summary with B7 and B8
| Characteristic | A193 B7 | A193 B16 | A193 B8 (304/316) |
| Material Type | Cr-Mo Alloy Steel | Cr-Mo-V Alloy Steel | Austenitic Stainless Steel |
| Max Service Temp | ~480°C | ~590°C | ~815°C (Oxidation resistance) |
| Core Advantage | High strength at RT/moderate temp, Cost-effective | Excellent strength & relaxation resistance at medium-high temps | Good corrosion & oxidation resistance |
| Strength Level | 125 ksi (highest at RT) | 115 ksi (RT), High strength retention at temp | 75-100 ksi |
| Cost | Medium | High (Higher than B7) | High (Depends on type) |
| Selection Principle | High-pressure connections where temp ≤ 480°C | High-strength connections where 480°C < temp ≤ 590°C | Corrosive environments or very high oxidation temp required |
Important Considerations
Not a General-Purpose Material: B16 is a specialty material designed specifically for high temperatures. It should not be used for low or room temperature applications due to its higher cost and potentially lower room temperature toughness compared to B7.
Matching Nuts: Must be used with compatible high-temperature nuts, typically ASTM A194 Grade 7.
Strict Code Compliance: Usage must strictly follow design codes such as the ASME Boiler and Pressure Vessel Code to confirm its allowable stress values at the intended temperature.
Conclusion
ASTM A193 B16 is a “high-temperature specialist” in joining technology. It fills a critical performance gap between B7 alloy steel and advanced stainless steels/nickel-based alloys, serving as a core fastener material in industries like high-temperature, high-pressure power generation.
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