Industries we serve

EV & Battery Assembly Fasteners

Screws, bolts and washers for battery packs, busbar joints and charger enclosures — hexavalent-free finishes, embrittlement control on the high classes, and a cleanliness limit stated with its method.

Electric-vehicle hardware moves the finishing problem in three specific directions. The restricted-substance clause is not negotiable and it applies to the whole finish system, sealer included. A large share of the fasteners end up inside an enclosure that is closed for the life of the vehicle, where a loose particle is an electrical event rather than a cosmetic one. And the joints that carry structural or crash load are exactly the high-preload, high-hardness joints where hydrogen embrittlement lives. V.S. Heat Treatment hardens and plates the small fasteners that go into those assemblies, in production lots, for part makers in Thailand.

Typical specification bands in this sector

Typical specification bands in this sector
Property class	8.8 / 10.9 / 12.9 · joints tapped into aluminium are often held at 8.8
Hardness	28–45 HRC core after temper
Materials	SCM435 · SCM440 · SWCH10B21 · SUS304
Coating system	Zn + Cr3+ , hexavalent-free · Zn-flake where the customer spec names it
Coating thickness	5–12 µm Zn with the UPPER limit stated — pack clearances are tight
Salt spray (NSS)	240–720 h NSS (ISO 9227 / ASTM B117)
De-embrittlement bake	≥ 320 HV or class ≥ 10.9 → ISO 4042 bake, demonstrated by ISO 15330-1
Inspection methods	ISO 3497 · ISO 6507 · extraction-based cleanliness where the customer specifies it

EV drawings often arrive as a general automotive coating note with a restricted-substance clause added on top, and the two can contradict each other. Send both and we will flag the conflict before the lot runs rather than after.

Standards commonly cited

ISO 898-1 ISO 4042 ISO 15330 ASTM F1941 ISO 9227 ISO 6507 ISO 3497 All standards →

Steel grades commonly specified

SCM435 SCM440 SWCH10B21 SUS304 All steel grades →

How fasteners actually fail here

1Delayed fracture in a joint nobody will ever open

Acid cleaning and electroplating charge hydrogen into steel, and a part at or above roughly 320 HV held at high preload can fracture hours or days later with a brittle, intergranular face and no yielding beforehand. What makes this worse in a pack is access: the joint is torqued once during assembly, sealed, and never inspected again, so a failure surfaces in the field as an enclosure or module fault. ISO 4042 sets out when relief baking applies and that it should start as soon as practical after plating; ISO 15330-1 is the sustained-load test that demonstrates the risk was actually controlled.

2Zinc on a surface that was supposed to conduct

A busbar joint is an electrical connection first. Zinc and its passivate are chosen to corrode sacrificially, which is the opposite of what a low-resistance contact face needs, and contact resistance that rises heats the joint, which raises resistance further. Contact surfaces on conductors are normally tin, nickel or silver plated — processes we do not run. If a drawing shows a conductive contact face on a part sent to us, it has to be masked or the part has to be finished by a supplier who does that plating.

3Process residue inside a sealed high-voltage enclosure

Barrel plating, blasting and phosphating all leave residue in recesses, blind holes and thread roots. Inside a high-voltage assembly, a conductive particle that works loose is a creepage and tracking risk across insulation, not a cosmetic complaint, and the pack cannot be re-cleaned once it is sealed. Cleanliness is only inspectable when it is written as a method plus a limit — an extraction with a residue weight, or a particle count with size classes. A drawing note that says "clean" cannot be passed or failed.

4The aluminium thread is the weakest member

Pack housings, cooling plates and enclosures are frequently aluminium, sometimes with steel inserts, and the fastener is stronger than what it threads into. Torque then has to be set to protect the aluminium rather than to exploit the bolt, and the friction the coating delivers dominates how much of that torque becomes clamp load. A coating thicker than the drawing intended also eats the thread clearance in a thin-walled insert, which is why the thickness specification needs an upper limit and not just a minimum.

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.Blistering of the zinc coatingBubbles and lifted coating on plated screws: trapped hydrogen, contaminated substrate, laps and seams, and pre-treatment faults — plus the adhesion tests and sections that identify which one.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.Distortion after heat treatment — bend, ovality, pitch driftBent screws, out-of-round nuts and threads that stop gauging after hardening: released residual stress, uneven quenching and loading faults, with the before-and-after measurements that prove it.

All defect guides →

Documentation a buyer in this sector should receive

A written statement of the finish system actually applied — plating, passivate family and sealer — against the restricted-substance specification number your customer imposes, so it can be carried into the material-data declaration your customer collects.
The de-embrittlement record: the class or measured hardness that triggered it, that a relief bake was applied, and the elapsed time between plating and the start of the bake. Baking is time-sensitive and a record without the elapsed time is only half a record.
Coating thickness to ISO 3497 with the method and position, reported against BOTH limits — pack hardware fails as often for being too thick as for being too thin.
Where the customer specifies cleanliness, the result with the extraction method and the acceptance limit it was measured against. Where the customer does not specify it, we will not invent a number.
Lot traceability tying every delivered box to the heat-treatment or plating run, so a containment action stays bounded.
Every report carries job and lot identity, material and agreed specification, position, method and instrument, measured values against the limits, and date, inspector and approval. We operate under ISO 9001:2015 and claim no IATF 16949 registration and no carmaker or battery-maker approval — verify that against your own approved-supplier requirements.

What to send us for an accurate quote

1Property class or hardness band, and whether the joint is inside a sealed pack or on the outside of the vehicle.
2The mating thread: steel, a steel insert, or tapped aluminium — it changes the torque strategy and the acceptable coating thickness.
3The finish written as a system with a minimum AND a maximum thickness, the passivate, and any sealer.
4The restricted-substance specification number your customer imposes, and who has to sign the declaration.
5Any surface that must stay unplated or masked — busbar contact faces, sealing faces, earth points.
6Cleanliness method and limit if one applies, plus lot size, piece weight, packing and the documents required at delivery.

Request a quote

We reply to your quote request within 24 hours.

Recommended services

Zinc Plating Cr3+Heat Treatment Testing Laboratory

Common parts we finish

Bolts Screws Washers Nuts Pins All part types →

Questions buyers in this sector ask

The drawing says the finish must be hexavalent-chromium free. What do you actually apply?+

We use trivalent (Cr3+) passivates, which is the chemistry chosen precisely to avoid hexavalent chromium. Colour proves nothing on its own — clear, blue and black are all available from either family — so send the substance specification you have to satisfy and we will confirm in writing what the line uses, including the sealer, since the sealer is part of the system your declaration has to cover.

Can you plate busbar and high-voltage contact hardware?+

Not the contact faces. Zinc is a sacrificial corrosion coating and is the wrong surface for a low-resistance electrical joint; tin, nickel and silver plating are the usual choices there and we do not run them. We can finish the structural fasteners around a busbar, and if a part has both functions the conductive face has to be masked and the masking agreed before the lot runs.

The pack is sealed and dry. Do class 10.9 bolts still need baking?+

Yes. The hydrogen that causes delayed fracture is introduced during pickling and plating, before the part ever reaches your assembly line, so a dry service environment does not remove the risk that the process created. ISO 4042 sets the trigger by hardness and class rather than by application, and ISO 15330-1 is how it is demonstrated. Send the class, the measured hardness and any customer hydrogen specification and we confirm the bake against your drawing.

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.