Industries we serve

Steel Structure & Fabrication

Anchor rods, structural bolting, purlin and roof-sheet screws — finished with the assembly, the tightening method and the fabricator's execution standard in view.

A structural connection is not a bolt; it is an assembly. Bolt, nut and washer are tightened together by a defined method to a defined preload, and everything about the finish is judged by whether that preload still arrives. That is the difference between this page and general construction hardware: here the coating is part of a tightening procedure and part of a certification held by the fabricator, and changing the coating after the fact can quietly invalidate both. V.S. Heat Treatment hardens and plates the small structural hardware — bolts, nuts, washers, anchor bolt sets, purlin and roof-sheet screws — in production lots.

Typical specification bands in this sector

Typical specification bands in this sector
Property class	8.8 / 10.9 bolts · class 8 / 10 nuts · anchor rods to the project grade
Hardness	22–32 HRC (8.8) · 32–39 HRC (10.9)
Materials	SS400 / Q235 anchor rod · S45C · SCM435 · SWCH10B33
Coating system	Zn + Cr3+ + sealer · zinc phosphate under a painted assembly
Coating thickness	8–25 µm Zn (ASTM B633 SC3–SC4)
Salt spray (NSS)	240–720 h NSS
De-embrittlement bake	class 10.9 → ISO 4042 bake; some structural specifications forbid electroplating this class
Inspection methods	ISO 3497 local minimum · ISO 6508 · ISO 9227 · thread gauge after coating

A structural drawing usually mixes components that are inspected under different rules — a certified assembly, a plain nut and a roof screw are three different conversations. Say which is which and the bands below stop being a general table.

Standards commonly cited

ISO 898-1 ISO 898-2 ISO 4042 ASTM B633 ISO 9227 ISO 6508 ISO 3497 All standards →

Steel grades commonly specified

S45C SCM435 SWCH10B33 SWCH10B21 All steel grades →

How fasteners actually fail here

1A certified bolting assembly re-finished after the fact

High-strength structural bolting is normally supplied as a matched set — bolt, nut and washer manufactured, lubricated and tested together, with a declared tightening behaviour. Stripping and re-coating that set changes the friction it was tested with, which is the property the certification is about. If the project uses certified assemblies, the finish belongs to the order for those assemblies, not to a later plating decision. Send us the loose components and the specification, and we will say plainly which of the two situations you are in.

2The preload the tightening method assumed never arrives

Turn-of-nut, calibrated wrench and direct-tension-indicating washers all convert something a site crew can control into a clamp force nobody can see. Two of the three depend directly on friction under the nut and in the threads, and friction depends on the coating, the passivate, any sealer, any lubricant and whether the nut was re-used. A tightening procedure that names a torque without naming the finish it was calibrated on is incomplete, and the failure it produces is a joint that looks perfectly tight.

3Electroplating a class the project may already forbid

The highest-strength structural fasteners sit where hydrogen embrittlement is a genuine risk, and structural specifications frequently restrict or prohibit electroplating those grades outright rather than relying on a relief bake. Where electroplating IS permitted, ISO 4042 governs the bake and it has to start as soon as practical after plating. Read what the project actually says before the parts are sent — this is a rule about the specification, not about our process capability, and no bake cycle overrides a clause that says do not plate.

4Roof and purlin screws: the fastener is a hole in the envelope

A self-drilling screw through metal roof sheet is both a structural fixing and a penetration through the waterproof layer. Corrosion at the drill point, at the swarf it leaves behind or at the sheet cut edge produces a rust streak down a new roof within a season, and the sheet's own metallic coating and the fastener's finish have to be compatible rather than merely both present. The drill point is also case-hardened, which puts these small screws into the hardness range where the embrittlement question has to be asked even though nobody thinks of a roof screw as a high-strength fastener.

Defect guides that matter in this sector

Hydrogen embrittlement — delayed fracture after platingWhy class 10.9 and 12.9 fasteners crack hours or days after zinc plating, how the preloading test to ISO 15330-1 confirms it, and the ISO 4042 baking rules — ≥ 390 HV, start within 4 hours — that prevent it.Early red rust on plated fastenersRed rust well before the specified salt-spray hours: thin coating in thread roots and recesses, damaged passivation, handling damage and test-plan mismatches, with the checks that find the real cause.Thread gauge failure after platingWhy plated screws stop accepting the GO gauge: coating thickness lands in the thread allowance. How ISO 4042, ISO 965-1 and ISO 1502 handle it, and what to specify before production.Decarburization — surface carbon loss after heat treatmentSoft skin on hardened screws and bolts: how partial and complete decarburization fail ISO 898-1, how to measure it by metallographic section or microhardness traverse, and the furnace controls that prevent it.

All defect guides →

Documentation a buyer in this sector should receive

Which component was actually sent, and whether it belongs to a certified matched assembly. If it does, expect us to say the finish decision belongs upstream rather than take the work.
Coating thickness reported as a local minimum with the method and the position, quoted against the classification the project names — an ASTM B633 service condition or an ISO 2081 designation.
The nut fit record. Where nuts were tapped oversize to allow for the coating, it must be written down: a site crew that expects standard thread fit will read a free-running nut as a defect, and a crew that does not expect it will force a nut that binds.
The relief-bake record for class 10.9 work where the specification permits electroplating at all, including the elapsed time between plating and the start of the bake.
Salt-spray results to ISO 9227 / ASTM B117 where the project names hours, with criterion, interval and significant surfaces, plus lot traceability from the delivered box back to the plating run.
A clear statement of whose certification is whose. EN 1090 factory production control, welding qualifications and the fabrication execution class belong to the fabricator; we hold ISO 9001:2015 and nothing else, and our records support the fabricator's file rather than replace it.

What to send us for an accurate quote

1The project or execution standard cited, and what it says about coating high-strength components.
2Which component it is — anchor rod, structural bolt, nut, washer, purlin or roof-sheet screw — and whether the parts form a matched assembly.
3Property class or grade, and whether the parts have already been heat treated or arrive as-formed.
4The tightening method the erector will use, and whether a friction or torque-coefficient requirement applies to the finish.
5Coating classification or thickness range, plus who owns the thread-fit decision — if the nuts come from a different supplier than the bolts, the oversize tapping has to be agreed between them before either part is coated.
6Dimensions and lot quantity, including the length of any anchor rods, plus packing and the documents required at delivery.

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Recommended services

Zinc Plating Cr3+Heat Treatment Testing Laboratory

Common parts we finish

Anchor Bolts Bolts Nuts Washers Screws All part types →

Questions buyers in this sector ask

Can you zinc plate class 10.9 structural bolts?+

Technically the process exists and ISO 4042 covers the relief bake that has to follow. The first question is not ours though: many structural specifications restrict or prohibit electroplating on the highest-strength grades because of hydrogen embrittlement, so read the project document before the parts leave your yard. Send us that clause together with the class and measured hardness and we will tell you whether the work is allowed, not just whether it is possible.

Are you EN 1090 or AISC certified?+

No. Those belong to the steel fabricator and the erector, and no plating or heat-treatment subcontractor can hold them on your behalf. We operate under ISO 9001:2015 and supply per-lot process records, hardness and coating results and traceability that go into your file. If your client is asking for an execution class, that question has to be answered by the fabrication contract, not by a finishing report.

Can you handle long anchor rods?+

Our line is built for small fasteners processed in bulk lots — screws, bolts, nuts, washers, pins and springs — and a long bent anchor rod may simply not fit the way we load and process work. Send the length, diameter and quantity and we will give you a straight yes or no rather than accept the order and discover it on the shop floor. Anchor nuts, washers and the shorter anchor hardware are routine.

The bands on this page are typical industry values, not a guarantee for any specific part. The governing drawing, customer specification and standard always take precedence, and every value is confirmed against your part before production. V.S. Heat Treatment operates under ISO 9001:2015; no other certification or sector approval is claimed.