Do you make curved bolts to a specific radius?

Yes. Curved segment bolts are produced to the drawing; the bending radius is matched to the duct geometry of your segment. Please include the radius, chord length and overall length on the drawing.

Why does the same torque value give different results on different coatings?

Because the torque coefficient depends on the friction of the surface treatment. A zinc-flake coating is lubricious while a hot-dip galvanized surface is rougher, so the same torque produces different preload. Confirm the torque value against the coating actually supplied.

Are the nuts and washers supplied as a set?

They can be. Supplying bolt, nut and washer from the same production batch keeps the torque coefficient consistent, which is recommended when the contractor has to verify torque on site.

Can bolts be reused after they have been tightened?

Reuse is not recommended for bolts that have been loaded or over-torqued. Yielding or coating damage makes the preload inconsistent between pieces.

Surface Treatment for Tunnel and High-Strength Fasteners

The coating is what survives after the bolt is buried. For tunnel segment bolts and high-strength fasteners, the right coating protects the thread, shank and head for the design life — and avoids introducing hydrogen embrittlement during processing.

Our manufacturing partner in Yongnian runs three hot-dip galvanizing lines, a Dacromet (zinc-flake) line, two sherardizing lines, heat treatment and an anti-alkali coating line in one production chain. The coating system is selected by environment, grade and project specification, and confirmed before production.

Coating options

Hot-dip galvanizing

Parts are degreased, pickled, fluxed, dried, dipped in molten zinc at about 450–500 °C, cooled, passivated and inspected. A metallurgically bonded zinc layer gives long-term barrier and cathodic protection — suitable for wet tunnels, water transfer and outdoor structures.

Standard: ISO 1461 / GB/T 13912
Typical use: segment bolts, anchor bolts, embedded plates, U-bolts
Note: internal threads may require tapping after coating; thickness and appearance depend on part size and thread.

Dacromet / zinc-flake coating

A water-based zinc-aluminium flake dispersion is applied by dip-spin and cured at about 300 °C. The process avoids acid pickling and electroplating, so the risk of hydrogen embrittlement is much lower — important for grade 10.9 and 12.9 fasteners.

Characteristics: thin, uniform, good salt-spray performance, no hydrogen embrittlement from coating
Typical use: high-strength segment bolts, connection sets, spring and disc parts

Sherardizing (powder zinc)

Parts are heated with zinc powder at about 400 °C, forming a zinc-iron diffusion layer. The coating is metallurgically bonded, uniform on threads and edges, and suitable for fasteners that need good abrasion resistance.

Multi-element composite and anti-alkali coatings

For alkaline groundwater, high-chloride ground or project-specific specifications, a multi-element composite or anti-alkali coating can be specified. We discuss the coating system against the salt-spray and service-life requirements.

Coating comparison

Coating	Thickness	Salt spray	Hydrogen embrittlement risk	Best for
Hot-dip galvanizing	Thick, 45–85 µm typical	Good	Low (no electroplating)	Wet tunnels, outdoor, long life
Dacromet / zinc-flake	Thin, 5–12 µm	Very good for thickness	Low	High-strength bolts, threads
Sherardizing	Medium	Good	Low	Fasteners, complex shapes
Electroplating (white zinc)	Thin	Moderate	Higher — baking required	Indoor / dry conditions

Actual values depend on part geometry, thread and process parameters; test reports are issued per batch or per project.

Testing and records

Salt-spray test to GB/T 10125 / ISO 9227
Coating thickness measurement (magnetic gauge)
Adhesion and appearance inspection
Material certificates (EN 10204 3.1) and third-party inspection on request
Hydrogen embrittlement testing to ISO 15330 for high-strength grades where specified

What to tell us

Base material and grade (e.g. 45#, 40Cr, 35CrMoA, 42CrMo; 8.8, 10.9, 12.9)
Coating system required (hot-dip, Dacromet, sherardized, composite)
Salt-spray duration and standard
Thread and tolerance after coating
Packing and marking requirements

We confirm the coating route, thickness target, testing and lead time before quoting.

Send your drawing and coating requirement →