B7 vs B16 stud bolts: what's the difference, and when do you need B16?
Both are ASTM A193 quenched-and-tempered chromium-molybdenum alloy steels with the same minimum strength up to 2-1/2": 125 ksi tensile, 105 ksi yield and 321 HBW maximum hardness.
B16 adds 0.25–0.35% vanadium, carries three times the molybdenum and is tempered at 1200 °F instead of 1100 °F, so it resists creep and relaxation at high temperature. B7 is the general-purpose flange stud; B16 is chosen for hot service such as steam turbines, high-pressure steam flanges and hot valve bonnets.
Side by side
| B7 | B16 | |
|---|---|---|
| Steel | Cr-Mo (AISI 4140, 4142, 4145) | Cr-Mo-V (UNS K14072) |
| Chromium | 0.80–1.10% | 0.80–1.15% |
| Molybdenum | 0.15–0.25% | 0.50–0.65% |
| Vanadium | — | 0.25–0.35% |
| Minimum tempering temperature | 1100 °F (593 °C) | 1200 °F (650 °C) |
| Tensile / yield, min, 2-1/2" and under | 125 / 105 ksi | 125 / 105 ksi |
| Tensile / yield, min, over 2-1/2" to 4" | 115 / 95 ksi | 110 / 95 ksi |
| Tensile / yield, min, over 4" | 100 / 75 ksi (to 7") | 100 / 85 ksi (to 8") |
| Hardness, max | 321 HBW / 35 HRC | 321 HBW / 35 HRC |
| Usual nut | ASTM A194 2H | ASTM A194 Grade 4 or 7 |
Values per ASTM A193/A193M-24, Tables 1 and 2. Confirm the edition on your purchase order.
Why B16 holds up when it's hot
A bolted joint stays tight only while the stud keeps its stretch. At high temperature, steel slowly creeps and the clamp load relaxes. The vanadium and extra molybdenum in B16 form fine, stable carbides that resist that creep, and the higher tempering temperature means the steel is not being further softened by the heat it sees in service.
That is why B16 is the stud on steam turbine casings and horizontal joints, superheater and reheat steam flanges, and hot valve bonnets. For hotter service still, A453 Grade 660 or nickel alloys such as Inconel 718 take over.
Which one does your joint need?
The grade comes from the design: the piping or pressure vessel code (ASME B31.3, B31.1, Section VIII) and the joint's design temperature, set by your engineer. A supplier shouldn't pick it for you. If the drawing or line class says B16, B7 is not a substitute.
Going the other way, B16 meets the same minimum strength as B7 up to 2-1/2", but a change of grade still needs the engineer's approval, and the nut grade changes with it.
Telling them apart
A193 studs carry the grade symbol (for example "B7" or "B16") and the maker's mark on the ends. Nuts are marked with their own grade ("2H", "7"). If the marking is missing or worn, positive material identification (PMI) separates them by the vanadium and molybdenum content.
Also asked.
Can I use B16 in place of B7?
At room temperature B16 meets the same minimum strength as B7 up to 2-1/2", so it is often accepted as an upgrade, but the substitution needs your engineer's approval and the matching nut grade.
Can I use B7 in place of B16?
Not for the hot service B16 is specified for. B7 relaxes faster at high temperature; only the engineer responsible for the joint can approve a change.
What nuts go with B16 studs?
ASTM A194 Grade 4 or Grade 7 heavy hex nuts are the usual companions; your line class or drawing will say which.
Is B16 harder to get than B7?
B7 bar is far more common. Ask for B16 early on a shutdown job so material can be confirmed before the outage starts.