ASTM A540 Grades B21, B22, B23, B24 Bolting (Pressure Vessel and Nuclear)
The pressure-vessel and nuclear bolting specification: four alloy steels in five strength classes from 120 to 165 ksi, with Charpy impact requirements and tight chemistry — the material behind SA-540 B23 Class 2 studs on reactor and compressor closures.
What A540 B21–B24 is.
ASTM A540 covers alloy steel bolting material for special applications: pressure-vessel closures, nuclear components and rotating machinery where a large stud must combine high strength, guaranteed toughness and controlled hardness. Grade B21 is a chromium-molybdenum-vanadium steel, B22 is 4142H, B23 is E4340H, B24 is a 4340 modification and B24V adds vanadium to it; each is supplied in Classes 1 through 5, from 165 ksi (Class 1) down to 120 ksi (Class 5), with Charpy V-notch requirements at +10 °F on many class and size combinations of B22, B23 and B24 (B21 is tested for information only).
We machine A540 studs, bolts and nuts from certified bar, heat treat and test to the class ordered, with Charpy testing and full-size hardness on every lot. SA-540 B23 Class 2 is a regular order for compressor and heat-exchanger closures.
Chemical composition.
| Element | B21 (Cr-Mo-V) | B22 (4142H) | B23 (E4340H) | B24 (4340 Mod) |
|---|---|---|---|---|
| Carbon | 0.36–0.44 | 0.39–0.46 | 0.37–0.44 | 0.37–0.44 |
| Manganese | 0.45–0.70 | 0.65–1.10 | 0.60–0.95 | 0.70–0.90 |
| Phosphorus, max | 0.025 | 0.025 | 0.025 | 0.025 |
| Sulfur, max | 0.025 | 0.025 | 0.025 | 0.025 |
| Silicon | 0.15–0.35 | 0.15–0.35 | 0.15–0.35 | 0.15–0.35 |
| Chromium | 0.80–1.15 | 0.75–1.20 | 0.65–0.95 | 0.70–0.95 |
| Nickel | — | — | 1.55–2.00 | 1.65–2.00 |
| Molybdenum | 0.50–0.65 | 0.15–0.25 | 0.20–0.30 | 0.30–0.40 |
| Vanadium | 0.25–0.35 | — | — | — |
Values per ASTM A540/A540M-15 Table 1 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
Mechanical properties.
| Class and diameter (in.) | Tensile strength, min (ksi) | Yield strength 0.2% offset, min (ksi) | Elongation in 2 in., min (%) | Reduction of area, min (%) | Surface hardness (HBW) · Charpy |
|---|---|---|---|---|---|
| Class 5 — to 6 | 120 | 105 | 15 | 50 | 248–311 · 35 ft·lbf avg |
| Class 4 — to 3 | 135 | 120 | 13 | 45 | 269–341 · 35 ft·lbf avg |
| Class 3 — to 3 | 145 | 130 | 12 | 40 | 293–363 · 30 ft·lbf avg |
| Class 2 — to 3 | 155 | 140 | 11 | 40 | 311–388 · information only |
| Class 1 — to 3 | 165 | 150 | 10 | 35 | 321–415 · information only |
Values per ASTM A540/A540M-15 Table 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. Grades B21, B22 and B24 carry the same class strengths with their own size limits and Charpy notes; see the specification for the full table.
Specifications that apply.
How A540 B21–B24 ships from Houston.
- Hex and heavy hex bolts and cap screws
- Studs and threaded rod
- Nuts and washers to match
- Machined specials to drawing
A540 B21–B24 fasteners we make.
Every item is manufactured to your drawing or standard in A540 B21–B24, imperial or metric, with an EN 10204 3.1 certificate.
What is ASTM A540?
A540 is the bolting specification for jobs where a stud failure is not an option. It uses four proven alloy steels, restricts their chemistry more tightly than the bar specifications do, and defines five strength classes for each so that the designer can trade strength against toughness. Most classes carry a Charpy V-notch minimum at +10 °F, and every lot is hardness tested on the surface. The result is a stud that is certified for strength, toughness and hardness together, which is what nuclear and pressure-vessel codes require.
Reading an A540 call-out
- Grade letter and number pick the steel: B21 Cr-Mo-V, B22 4142H, B23 E4340H, B24 4340 Mod, B24V 4340V Mod.
- Class picks the strength: Class 1 is 165 ksi, Class 5 is 120 ksi, with size limits that shrink as the class rises.
- SA-540 is the ASME adoption; the values are the same.
- Nuts are normally the same grade and class, tempered slightly softer; A194 Grade 4 or 7 is used where the order allows.
Testing, certification and marking
- Tensile, hardness and Charpy V-notch per heat, size and tempering charge
- Grade and class marked on every stud where size allows
- EN 10204 3.1 material test report; 3.2 witness available
- Magnetic particle and ultrasonic inspection on request
Recent A540 B21–B24 jobs.
ASTM A193 Grade B7
The workhorse alloy-steel bolting grade for flanges, valves and pressure equipment — 125 ksi tensile, quenched and tempered 4140.
ASTM A322 bar · ASTM A434 Q&T barAISI 4340 Alloy Steel (UNS G43400)
The deep-hardening high-strength steel — 4340 reaches 155 ksi and beyond in large sections — for crankshafts, drilling tools, aircraft parts and A320 L43 bolting.
ASTM A322 / A434 alloy steel bar4142 Alloy Steel (UNS G41420)
4140's higher-carbon sibling: the same chromium-molybdenum steel with 0.40 to 0.45 percent carbon for a little more hardenability and strength in large sections — quenched and tempered to A434 Class BB, BC or BD.