Zeron 100 hex cap screws are super duplex stainless steel hex-headed fasteners (UNS S32760 / Werkstoff 1.4501) machined or hot-headed to the ASME B18.2.1 finished hex cap screw pattern, the same nominal dimensions as a hex bolt, but tighter tolerance class on the under-head bearing face, head height, and shank straightness. The "cap screw" designation indicates a precision-machined bearing surface designed to seat flat against the joint face under preload, important when the joint relies on a bolt-clamping plate-stack rather than a free flange-and-nut assembly.
Precision bearing-face fit · PREN ≥ 40 · Hardness ≤ 28 HRC · SA / FG / FLT / AFP supply states
Use hex cap screws in tapped-hole connections, valve cover bolting, pump bearing housings, equipment frame fasteners on offshore topside skids, and any joint where the bolt installs into a tapped blind-end or through-hole and bears directly against a finished plate. Where a free-end nut is on the back side, a hex bolt (B18.2.1 or B18.2.6 heavy) is the lower-cost choice; where the bolt seats against a precision finish, a cap screw is the engineering-correct call.
Dimensionally identical at the basic-size level (same head AF, head height, shank diameter for a given thread). The distinction is in tolerance class: a hex cap screw has tighter under-head bearing-face squareness, lower head-height variation, and tighter shank concentricity than a hex bolt. Functionally that matters when the bolt seats against a precision-machined surface and the joint relies on uniform bearing pressure to retain preload. A hex bolt can be substituted into many applications cheaper, but for tapped-hole connections in valve covers, pump bearing housings, and equipment frames, the cap-screw spec is the engineering-correct call.
| Standard | Coverage |
|---|---|
| ASME B18.2.1 | Square and hex bolts and screws, finished hex cap screw pattern, head height, AF, bearing-face squareness |
| ISO 4014 / 4017 | Metric hex cap screws (partial / full thread) |
| ASTM A1082 | Stainless stud, bolt, screw, threaded rod, mechanical envelope |
| ASTM A479 / A276 | Source bar specs |
| ASME B1.1 / ISO 965 | Thread tolerance class 2A / 6g |
| NACE MR0175 | Sour-service hardness ≤ 28 HRC |
| NORSOK M-650 / M-630 MDS 055 | QTR'd manufacturer for North Sea |
| API 6A PSL 3 | Wellhead and tree equipment qualification |
| Series | Diameter | Length range | Head spec |
|---|---|---|---|
| UNC (coarse) | 1/4 to 4 inch | 1/2 to 24 inch | ASME B18.2.1 hex cap screw |
| UNF (fine) | 1/4 to 1-1/2 inch | 1/2 to 18 inch | ASME B18.2.1 hex cap screw |
| Metric coarse | M6 to M64 | 15 to 400 mm | ISO 4014 / 4017 |
| Metric fine | M8 to M48 | 20 to 300 mm | ISO 8765 / 8676 |
| Property | SA | FG | FLT |
|---|---|---|---|
| 0.2% Yield | 620-720 MPa | 700-800 MPa | 620-720 MPa |
| UTS | 800-870 MPa | 870-940 MPa | 800-870 MPa |
| Hardness body | 22-27 HRC | 24-28 HRC | 22-27 HRC |
| Charpy V-notch -46°C | 60-110 J | 50-90 J | 80-130 J |
| Ferrite | 40-52 % | 40-52 % | 38-48 % |
| CPT (G48-E) | ≥ 50°C | ≥ 50°C | ≥ 50°C |
| Service | Recommended supply state |
|---|---|
| Valve cover / bonnet bolting (tapped blind-end) | FG (NACE-controlled for sour) |
| Pump bearing housing closure screws | SA |
| Compressor casing access-cover screws | FG |
| Heat exchanger bonnet bolting (against precision flange face) | SA or FG depending on load class |
| Subsea instrument-housing fasteners | FLT |
| FGD scrubber inspection-port fasteners | SA + AFP coating |
| Repeated-makeup precision-fit equipment fasteners | AFP-coated SA or FG |
For non-critical applications, yes, both fit the same B18.2.1 size envelope. For tapped-hole bonnet closures, valve covers, and bearing-housing screws where bearing-face flatness matters, the hex cap screw class is the right specification. Cost difference is small (~5-10%); install correctness is much more important.
Standard 2A (inch, ASME B1.1) or 6g (metric, ISO 965), the loose-fit class. Super duplex galls when interference threads are torqued; class 2A/6g provides clearance to prevent metal-to-metal lockup. For repeated-makeup specify AFP (coated) supply state, coating drops μ from ~0.30 to ~0.10.
Stock UNC sizes (1/2", 5/8", 3/4", 1") + metric (M16, M20, M24) ship 5-7 days with 3.1 MTC, 10-14 days with witnessed 3.2 MTC. NORSOK M-650 QTR'd lots 14-21 days. Specials machined from solution-annealed bar in 5 working days.
Send the spec to info@torqbolt.com or +91-22-66157017 / WhatsApp.
