Common finishes: Plain / black oxide · Zinc plated
Scope
ASTM A574 covers alloy steel socket head cap screws in inch-series diameters from #0 (0.060") through 4" nominal. Sizes are dimensionally controlled by ASME B18.3. The spec defines alloy chemistry, heat treatment, mechanical properties, and hardness requirements.
Metric socket-head cap screws are covered by ASTM A574M and ISO 4762.
Mechanical properties
A574 establishes a single high-strength class for SHCS:
| Size range | Min. Tensile | Min. Yield | Min. Elong. | Min. RA | Hardness |
|---|---|---|---|---|---|
| Up to 1/2" | 180 ksi | 160 ksi | 10% | 35% | 39–45 HRC |
| Over 1/2" through 1" | 170 ksi | 150 ksi | 10% | 35% | 37–45 HRC |
| Over 1" through 1-1/2" | 160 ksi | 140 ksi | 10% | 35% | 33–39 HRC |
| Over 1-1/2" through 4" | 140 ksi | 115 ksi | 10% | 35% | 26–36 HRC |
These strength levels exceed SAE Grade 8 (150 ksi) for the common sizes up to 1", which is why A574 SHCS is frequently selected over hex-head bolts when strength per unit of installed footprint matters.
Chemistry and heat treatment
Alloy steel with at least one of: chromium, nickel, molybdenum, or vanadium. Typical production uses AISI 4037, 4137, 4140, 4340, or 8740 alloys.
- Carbon: 0.30–0.48%
- Alloying to develop hardenability through section
- Quenched and tempered to the specified hardness range
- Minimum tempering temperature: 650°F (conservative; higher for larger sections)
Head style and drive
A574 heads are cylindrical with a flat top and a hex-socket drive:
- Standard (full-height) SHCS — the default head style
- Low-head SHCS — reduced head height for tight clearance applications (covered by A574 dimensionally per ASME B18.3)
- Flat-head SHCS — countersunk with 82° or 100° angle (for flush-mount applications)
- Button-head SHCS — rounded low-profile head
Warning on low-head, flat-head, and button-head versions: these reduced-head styles have lower tensile and shear ratings than the standard cylindrical-head SHCS at the same nominal size. The wrench recess is smaller, head-to-shank transition is shorter, and achievable pretension is lower. Always reference the specific manufacturer's ratings — don't assume the full A574 mechanicals apply.
Finishes
SHCS are commonly supplied in:
- Plain black oxide — the traditional finish; oil-coated for short-term corrosion protection, not suitable for long-term outdoor exposure
- Plain uncoated — typically for indoor / controlled environments
- Zinc plated per F1941 — for general corrosion protection
- Cadmium plated per QQ-P-416 — on legacy defense and aerospace drawings (phased out on new work)
- Nickel plated — decorative or moderate corrosion service
- Zinc flake coatings (F1136, F2833) — engineered corrosion protection
Hot-dip galvanizing is not used on A574 — the coating is too thick for precision SHCS threads and the temperature exposure can temper the steel.
Installation — tapped holes vs through holes
SHCS are used two ways:
- Threaded into a tapped hole (most common). No nut required. Design thread engagement typically 1.0–1.5 × nominal diameter in steel, 1.5–2.0× in aluminum, 2.0–2.5× in cast iron.
- Through-hole with a nut on the back side. Less common but used where back-side access exists. Pair with A563 DH or A194 2H heavy hex nuts.
In tapped-hole installations, the tapped thread strength must exceed the bolt tensile strength — otherwise the threads strip before the bolt breaks. Tap chart references apply.
Applications
- Semiconductor manufacturing equipment (Applied Materials, Lam Research, KLA)
- Automation and robotic systems
- Machine-tool building (CNC machines, grinders, presses)
- Aerospace ground support equipment and tooling
- Precision instruments and optical equipment
- Die and mold assemblies
- Pneumatic and hydraulic equipment
- Die and mold components where standard hex heads won't fit
Common field issues
- Counterbore depth wrong. The head must sit flush or slightly below surface. Too shallow — head protrudes and interferes; too deep — the shank is held in the counterbore and doesn't engage threads fully.
- Allen key worn or wrong size. Rounds out the socket, especially on smaller sizes. Always use a new, properly-sized hex bit; for high-torque applications use a torque wrench with a hex socket rather than an L-key.
- Galled threads. Alloy-steel SHCS galls less than stainless, but high-torque installation into aluminum or brass tapped holes can still seize. Anti-seize compound helps.
- Reduced-head SHCS assumed full strength. See warning above.
Related specifications
- ASME B18.3 — Dimensional standard for socket-head cap screws (the geometry spec)
- A574M / ISO 4762 — Metric socket-head cap screws
- A307, A449 — Hex-head alternatives (lower strength, larger head footprint)
- F835 — Alloy steel socket button and flat countersunk head cap screws
- F837 — Stainless steel socket head cap screws (the stainless SHCS spec)
- F912 — Alloy steel socket set screws (the set-screw counterpart)
Documentation
California Fastener A574 orders ship with mill certificates showing heat number, chemistry, mechanical properties (tensile, yield, hardness), and finish. Certifications supporting ITAR, RoHS, REACH, and DFARS compliance are available on request for semiconductor and defense customer programs.