Industries we serve

Automotive & Parts

Screws, nuts, bolts and fasteners for cars and motorcycles — hardened and corrosion-protected to maker spec.

Automotive fasteners are specified around a clamp load that has to survive vibration, thermal cycling and a defined service life — not around how they look. That makes three things the only ones a heat-treatment and plating subcontractor is really judged on: property class, tempered core hardness, and hydrogen embrittlement control. V.S. Heat Treatment processes screws, nuts, bolts, washers and springs in production lots for car and motorcycle part makers in Thailand.

Typical specification bands in this sector

Typical specification bands in this sector
Property class	8.8 / 10.9 / 12.9 (ISO 898-1)
Hardness	28–45 HRC core after temper
Materials	SCM435 · SCM440 · S45C · SWCH10B21 · SWCH10B33
Coating system	Zn + Cr3+ (clear / blue / black) · zinc phosphate + oil
Coating thickness	5–12 µm Zn
Salt spray (NSS)	240–720 h NSS (ISO 9227 / ASTM B117)
De-embrittlement bake	≥ 320 HV or class ≥ 10.9 → ISO 4042 de-embrittlement bake
Inspection methods	ISO 6507 / ISO 6508 · ISO 3497 · ISO 15330-1

These are the bands automotive drawings typically carry. The class, hardness and coating that apply to your part come from your drawing and the customer specification, and are confirmed before the lot runs.

Standards commonly cited

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

Steel grades commonly specified

SCM435 SCM440 SWCH10B21 SWCH10B33 S45C All steel grades →

How fasteners actually fail here

1Vibration self-loosening under transverse slip

Transverse movement of the joint — the mechanism a Junker-type test reproduces — lets the mating faces slip. Thread and underhead friction fall below what holds the preload, and the nut backs off. Scatter in coating friction is the usual contributor: an uncontrolled passivate or sealer shifts the torque–tension relationship, so the assembly line's torque target no longer produces the clamp load the designer wanted. Specify a friction window measured to ISO 16047, not the words "zinc plated".

2Fatigue at the first engaged thread and the underhead fillet

Cyclic load concentrates at the root of the first engaged thread and at the head-to-shank radius. Threads rolled after heat treatment carry compressive residual stress and last considerably longer; a decarburized surface or a quench crack cuts fatigue life sharply because the crack starts where the stress already peaks. ISO 898-1 limits decarburization for exactly this reason.

3Hydrogen embrittlement on class 10.9 and 12.9

Delayed fracture hours to days after assembly, usually under the head or at the first engaged thread, with a brittle intergranular fracture face and no plastic deformation. Acid pickling and electroplating both charge hydrogen into the steel. Parts at or above 320 HV need a de-embrittlement bake started as soon as practical after plating per ISO 4042, and the risk is demonstrated by a sustained-load test to ISO 15330-1.

4Torque–tension drift between lots

Assembly plants tighten to torque or torque-plus-angle. If coating thickness, passivate type or top-coat differs from lot to lot, the same torque delivers a different preload — seen on the line as loose joints or snapped bolts with no change to the bolt itself. Lock the finish system, the thickness range and the top-coat in a drawing note; colour is not a specification.

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.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.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.Low hardness after temperingBolts and screws that read under the drawing hardness: measurement error, temper temperature, hardenability limits and quench faults, ranked by how often each is the real cause.

All defect guides →

Documentation a buyer in this sector should receive

A PPAP-style submission package is normal in this sector. The customer sets the level; the sub-tier supplies what falls inside its own process — material and heat-lot identity, the agreed process parameters, the dimensional and mechanical results, and the control plan for the outsourced operation.
Lot traceability that ties every delivered box back to the incoming heat or coil lot and to the furnace or plating run that processed it. This is what lets a containment action be bounded to a few thousand pieces instead of a month of production.
A mill / heat-lot certificate for the raw material, supplied by the fastener manufacturer. The finisher references the heat number rather than restating the chemistry.
Coating thickness results with the method and the measurement location stated — magnetic or X-ray fluorescence to ISO 3497. "8–12 µm" means nothing without saying where on the part it was measured.
A salt-spray report to ISO 9227 / ASTM B117 where the drawing calls for one, stating exposure hours, inspection interval, whether the criterion is white corrosion or red rust, and the significant surfaces.
Every report we issue carries job and lot identity, the material and agreed specification, the test location, method and instrument, the measured values against the acceptance limits, and the date, inspector and approval. We operate under ISO 9001:2015 and do not claim IATF 16949 or any carmaker-specific approval — confirm that against your own approved-supplier requirements.

What to send us for an accurate quote

1Drawing or sketch with thread size, length, head type and the property class (8.8 / 10.9 / 12.9) or the required hardness band.
2Steel grade and as-supplied condition — SCM435, SCM440, S45C, SWCH10B21 and SWCH10B33 respond differently to quench and temper.
3The finish written as a system, not a colour: zinc thickness range, Cr3+ passivate type, and whether a sealer or top-coat is applied.
4Salt-spray requirement if one applies: hours, criterion and the significant surfaces.
5Whether de-embrittlement baking is required, and any customer specification number that governs it.
6Lot size in pieces or kilograms, piece weight, packing, and which documents must travel with the delivery.

Request a quote

We reply to your quote request within 24 hours.

Recommended services

Heat Treatment Zinc Plating Cr3+Phosphate Coating Testing Laboratory

Common parts we finish

Bolts Nuts Screws Washers Springs All part types →

Questions buyers in this sector ask

Do you bake class 10.9 and 12.9 parts after zinc plating?+

Baking after plating is standard practice for parts at or above 320 HV or property class 10.9, and ISO 4042 sets out when it applies and how soon after plating it should start. Send the class, the measured hardness and any customer specification number, and we confirm the bake against your drawing before production.

Can you meet a torque–tension (friction coefficient) window?+

Friction is a property of the whole system — plating thickness, passivate, sealer or top-coat and the mating parts — so it has to be specified, never assumed. Send the friction window your drawing requires, the coating system it was validated on, and the test method. We will tell you what falls inside our process and what has to be agreed with the coating or lubricant supplier.

Are you IATF 16949 certified?+

No. We operate under ISO 9001:2015 and claim no other certification or customer-specific approval. What we do provide is lot traceability, process records and a test report per lot carrying the fields an auditor looks for. Check that against your own approved-supplier requirements before awarding the work.

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.