Common grades: B21 · B22 · B23 · B24
Scope
ASTM A540 covers alloy steel bolting materials for special applications where high strength, tight section-specific mechanical control, and elevated-temperature service requirements exceed what A193 can deliver. The spec is structured around four alloy chemistries (B21, B22, B23, B24), each available in multiple classes that define strength level and section size.
A540 is a specialized, low-volume specification. Most fastener distributors don't stock it routinely. Orders are typically project-specific, tied to power-generation or large pressure-equipment work.
Grades and classes
Grade B21 — Cr-Mo steel, similar to 4140 but tighter
| Class | Min. tensile | Min. yield | Max. section |
|---|---|---|---|
| 1 | 155 ksi | 135 ksi | 2" |
| 2 | 145 ksi | 120 ksi | 4" |
| 3 | 135 ksi | 105 ksi | 7" |
| 4 | 125 ksi | 105 ksi | 9-1/2" |
| 5 | 120 ksi | 100 ksi | 12" |
Grade B22 — higher nickel than B21
| Class | Min. tensile | Min. yield | Max. section |
|---|---|---|---|
| 1 | 165 ksi | 150 ksi | 3" |
| 2 | 155 ksi | 135 ksi | 5" |
| 3 | 145 ksi | 120 ksi | 8" |
| 4 | 140 ksi | 115 ksi | 12" |
Grade B23 — highest nickel and vanadium
| Class | Min. tensile | Min. yield | Max. section |
|---|---|---|---|
| 1 | 180 ksi | 165 ksi | 2" |
| 2 | 170 ksi | 155 ksi | 4" |
| 3 | 160 ksi | 140 ksi | 7" |
| 4 | 150 ksi | 125 ksi | 10" |
| 5 | 145 ksi | 120 ksi | 12" |
Grade B24 — similar chemistry to B23 with different heat treatment path
Class structure mirrors B23 with adjusted properties per the spec tables.
The class number in A540 indicates the combination of strength and maximum-qualified cross-section — lower class numbers are stronger but only certified in smaller section sizes. Higher class numbers are lower strength but qualified for larger through-hardened sections.
What makes A540 different from A193
Both A540 and A193 Grade B7 / B16 are alloy-steel bolting specs. The distinctions:
- Tighter class-by-section controls. A540 explicitly qualifies each class for a maximum section size that can reliably be through-hardened. A193 B7 permits the same grade designation across a wider size range with acknowledged strength drop in larger sections.
- Higher achievable strength levels. A540 Grade B23 Class 1 delivers 180 ksi tensile — significantly above A193 B7's 125 ksi maximum.
- Stricter NDT requirements. A540 parts are typically ultrasonically tested through the full section to verify freedom from internal defects — a requirement driven by the critical service the material sees.
- Full mechanical testing on product-form specimens. Testing is often performed on separately-heat-treated test coupons representing the full section thickness, not just surface properties.
Chemistry overview
A540 grades are all nickel-chromium-molybdenum-vanadium alloys with tight compositional control. Specific chemistry varies by grade:
| Element | B21 (nominal) | B22 (nominal) | B23 (nominal) | B24 (nominal) |
|---|---|---|---|---|
| Carbon | 0.36–0.44 | 0.36–0.44 | 0.37–0.44 | 0.37–0.44 |
| Manganese | 0.70–0.90 | 0.45–0.70 | 0.60–0.95 | 0.60–0.95 |
| Nickel | 0.55–0.90 | 1.30–1.75 | 1.55–2.00 | 1.60–2.25 |
| Chromium | 0.80–1.15 | 0.80–1.15 | 0.65–0.95 | 0.70–0.95 |
| Molybdenum | 0.15–0.25 | 0.15–0.25 | 0.25–0.45 | 0.25–0.50 |
| Vanadium | — | 0.25–0.35 | 0.25–0.35 | 0.25–0.35 |
These chemistries are chosen to provide deep hardenability — the ability to develop specified strength through large cross sections — plus stable elevated-temperature service up to around 900°F.
Applications
Power generation
- Steam turbine rotor bolting — the archetypal A540 application; large studs holding rotor sections together
- Generator field bolting — high-strength studs in generator shaft assemblies
- Turbine casing bolting on very large frames where A193 B16 is inadequate
- Main steam stop valve fasteners
Heavy industrial
- Large-diameter pressure vessel bolting
- Mining and heavy-equipment shaft assembly hardware
- Specialized process-equipment fasteners
Aerospace (limited)
- Some ground-support equipment using large-diameter high-strength bolting
Service considerations
- Section size governs strength class. Ordering a Grade B22 Class 1 in a 5" section doesn't work — the spec doesn't qualify that combination. Always confirm class-to-section compatibility before specification.
- NDT requirements add cost and lead time. Ultrasonic inspection, surface crack detection, and sometimes radiography are typical. Budget and schedule accordingly.
- Traceability is rigorous. Heat number, ingot or slab origin, forging records, heat-treat records, and test coupon results are all documented.
- Repairability is limited. A540 is not a spec that supports field rework; components are made right in manufacturing or rejected.
Related specifications
- A193 B7, B16 — Lower-intensity alternatives for less-critical pressure service
- A453 — High-temperature superalloy alternative for elevated-temperature service
- AMS specifications — Aerospace alternatives when aerospace quality requirements apply
- A194 Grade 4 or 7 — Typical nut pairing for A540
- ASME BPVC Section III — Reference code for nuclear pressure vessel bolting where A540 may apply
Documentation
California Fastener A540 orders are project-specific and come with complete mill certification: heat number, complete chemistry, heat-treatment records, mechanical property test results on test coupons representing the section thickness, ultrasonic inspection reports, and surface inspection documentation. Third-party testing certification is available when customer requirements dictate.