A193 B7 vs B16: Bolt Material Comparison
ASTM A193 Grade B7 and Grade B16 are the two most widely specified alloy steel stud bolt materials for pressure piping flanged joints. Both are quenched and tempered chromium-molybdenum steels, but B16 adds vanadium and has a higher tempering temperature, giving it superior stress relaxation resistance at elevated temperatures. The choice between them is driven by operating temperature.
Definition and Classification
A193 B7 is a 4140/4142 type alloy steel (1Cr-0.2Mo) quenched and tempered to achieve high tensile strength. It is the universal default stud bolt material for ASME-rated flanged joints from ambient temperature up to approximately 400°C.
A193 B16 is a 4140-type alloy steel modified with vanadium (Cr-Mo-V) and tempered at a higher temperature. The vanadium addition and higher tempering temperature provide significantly better resistance to stress relaxation at temperatures above 400°C, maintaining bolt load over the long-term service life.
When Each Grade Is Used
B7 is the standard choice for general piping services from cryogenic (with appropriate impact testing) through moderate elevated temperatures. It is the default material called out in virtually every piping specification for carbon steel and low-alloy systems.
B16 is specified when continuous operating temperatures exceed 400°C (750°F) and bolt stress relaxation would compromise joint integrity. Typical applications include main steam lines, boiler connections, hot reheat piping, and refinery heater piping where Cr-Mo alloy pipe (P11, P22, P91) is used.
Specifications Table
| Property | A193 B7 | A193 B16 |
|---|---|---|
| Steel type | AISI 4140/4142 (Cr-Mo) | Cr-Mo-V (modified 4140) |
| Chromium | 0.75-1.20% | 0.75-1.20% |
| Molybdenum | 0.15-0.25% | 0.15-0.25% |
| Vanadium | None | 0.25-0.35% |
| Heat treatment | Q+T at 593°C (1100°F) min | Q+T at 635°C (1175°F) min |
| Tensile strength (dia. <=2.5”) | 860 MPa (125 ksi) min | 860 MPa (125 ksi) min |
| Yield strength (dia. <=2.5”) | 720 MPa (105 ksi) min | 725 MPa (105 ksi) min |
| Elongation (min) | 16% | 18% |
| Reduction of area (min) | 50% | 50% |
| Hardness (max) | 321 HBW | 321 HBW |
| Max recommended temp | ~400°C (750°F) | ~480°C (900°F) |
| Stress relaxation at 450°C | Significant (loses bolt load) | Moderate (better retention) |
| Matching nut | A194 Grade 2H | A194 Grade 4 or 7 |
| Relative cost | Baseline | 10-20% premium |
Comparison: Performance at Temperature
| Temperature | A193 B7 | A193 B16 |
|---|---|---|
| Ambient to 300°C | Excellent | Over-specified |
| 300-400°C | Adequate | Preferred for critical service |
| 400-450°C | Stress relaxation begins | Adequate |
| 450-480°C | Not recommended | Adequate |
| Above 480°C | Not suitable | Marginal (consider B8 Class 1) |
Stress Relaxation
Stress relaxation is the gradual loss of bolt preload over time at elevated temperature due to creep in the bolt material. In a flanged joint, loss of bolt load means loss of gasket compression, which eventually leads to leakage. B7 bolts begin losing significant preload above approximately 370-400°C. B16’s vanadium addition and higher tempering temperature stabilize the microstructure against this degradation.
For temperatures above B16’s practical limit (~480°C), stainless steel bolting per A193 B8 Class 1 (solution annealed and strain hardened) or nickel alloy bolting may be required.
For more on flange bolting specifications, see the full guide to stud bolts for flanges.
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