Industries we serve

Fastener Manufacturers — Toll Processing

Subcontract heat treatment and plating for cold-forming plants: your production lot, run to your property class, returned with the hardness and coating records your own customer asks for.

When a cold-forming plant sends work out, it is not buying a finish — it is buying conformance of its own production lot, in someone else's name. The metallurgy is not new to you; what you need from a subcontractor is a process you can put in front of your own customer, records that survive their audit, and a straight answer when a lot is marginal. V.S. Heat Treatment works as that subcontractor: we do not manufacture or sell fasteners, so your part numbers, drawings and customers are not competitive information in our hands.

Typical specification bands in this sector

Typical specification bands in this sector
Property class	bolts 4.8 / 8.8 / 10.9 / 12.9 (ISO 898-1) · nuts class 8 / 10 / 12 (ISO 898-2)
Hardness	the class band, plus surface hardness compared with core for decarburisation
Materials	SWCH10B21 · SWCH10B33 · SCM435 · SCr440 · S45C
Coating system	Zn + Cr3+ (barrel) · zinc phosphate + oil · black oxide
Coating thickness	5–12 µm Zn typical on barrel work — state the local minimum, not the average
Salt spray (NSS)	whatever your own customer wrote; we run to your number and report it
De-embrittlement bake	ISO 4042 bake at ≥ 320 HV or class ≥ 10.9, started as soon as practical after plating
Inspection methods	ISO 898-1 hardness and decarburisation · ISO 6157 surface limits · ISO 3497

This page is written for a supplier rather than an end user, so the bands below are the ones you already work to. What varies between subcontractors is not the standard — it is where hardness is measured, what the report actually says, and what happens when a lot is marginal.

Standards commonly cited

ISO 898-1 ISO 898-2 ISO 4042 ISO 15330 ISO 6507 ISO 6508 ISO 2639 ISO 3497 All standards →

Steel grades commonly specified

SWCH10B21 SWCH10B33 SCM435 SCr440 S45C All steel grades →

How fasteners actually fail here

1The lot passes here and fails at your customer

Almost every one of these arguments turns out to be a measurement disagreement rather than a metallurgical one. ISO 898-1 defines where hardness is taken on a bolt and on which prepared surface, and a reading on a head face, a shank flat and a transverse section through the threaded portion are not interchangeable numbers. Add conversion between Vickers and Rockwell scales and a lot can be rejected on paper while the steel is exactly where it should be. Agree the location, the method and the scale before the first lot, and put all three on the report.

2Decarburisation at the thread crest

Heating steel in an uncontrolled atmosphere pulls carbon out of the surface, and on a rolled thread the loss lands on the crest — the exact feature the property class is verified on. ISO 898-1 limits it precisely because a soft skin there costs strength and fatigue life that no later operation restores. Two causes look identical on a finished part and have opposite owners: an incoming wire that arrived decarburised, and furnace atmosphere or time in our process. A metallographic section, or a surface-versus-core hardness comparison, is what separates them, and it is worth running before anyone assigns cost.

3Distortion and quench cracking on the awkward geometry

Long thin screws bow, thin-walled and hollow parts crack, and heads with a sharp under-head radius crack at the radius. A forming lap or a seam from the heading operation is not created in the furnace, but it opens during quenching, which is why the argument about whose scrap it is always arrives after heat treatment. The section decides it. Send the geometry, the straightness or gauge limit and the prior forming route up front so the loading and quench practice are set for the part rather than discovered on it.

4The re-work of a rejected lot

Stripping and re-plating means a second acid exposure and a second hydrogen charge, so a part that was baked once now needs the bake cycle again from the new plating, and the strip attacks the base steel and the thread it is dissolving zinc from. Re-tempering to bring hardness down can push a part out of its class at the other end of the band. There are lots that can be re-worked and lots that should be sorted or scrapped, and the difference is worth an honest half hour before the parts move — not after.

Defect guides that matter in this sector

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.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.Quench crackingSharp, oxide-free cracks after hardening: how to tell a quench crack from a forging lap or a hydrogen crack by magnetic particle, penetrant and sectioning, and the quench and temper controls that stop them.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.Soft spots and incomplete hardeningScattered low hardness readings on hardened screws and bolts: quench wetting, basket loading, surface contamination and hardenability limits, and the mapped hardness survey that proves which one it is.

All defect guides →

Documentation a buyer in this sector should receive

Hardness per lot stated four ways at once: scale and method (Vickers to ISO 6507 or Rockwell to ISO 6508), the exact position it was taken at, the prepared surface, and the acceptance band it is judged against. Anything less is a number your customer can dispute and you cannot defend.
A decarburisation check where the class or your customer requires it — a metallographic section, or a surface-versus-core hardness comparison, stated as what it is.
For plated lots: local minimum coating thickness to ISO 3497 with the position, plus the relief-bake record and the elapsed time from plating to the start of the bake where ISO 4042 applies.
Lot identity that runs the whole way through: your wire or heat lot number, our furnace or plating run, and the box label you receive back. This is what makes a containment a few thousand pieces instead of a month.
An honest statement of scope on the report. Our lab covers hardness, coating thickness, salt spray and microstructure. Proof load, wedge tensile and other mechanical tests on finished fasteners need a tensile machine and are normally run by you or an independent test house — our report states which properties it covers and which it does not, and we hold ISO 9001:2015 only, with no accredited-laboratory status claimed.

What to send us for an accurate quote

1Your part number, the property class or nut class, and the standard your own customer imposes on it.
2Material and the condition it will arrive in — annealed or spheroidised wire, cold-worked, phosphate-and-soap coated from forming — and whether threads are rolled before or after heat treatment.
3The hardness band with the measurement position, and any decarburisation limit your customer applies.
4The coating system with local minimum thickness, the passivate and sealer, and whether a relief bake is required.
5Geometry that changes the risk: length-to-diameter ratio, wall thickness, sharp under-head radii, and any straightness or gauge limit that must hold after hardening.
6Lot size in kilograms and pieces, piece weight, packing, and which documents are needed and in whose name they are issued.

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Recommended services

Heat Treatment Zinc Plating Cr3+Phosphate Coating Testing Laboratory

Common parts we finish

Fasteners Bolts Nuts Screws Washers All part types →

Questions buyers in this sector ask

Can you run proof load or wedge tensile testing to ISO 898-1?+

No, and we would rather say so than let a report imply it. Our in-house lab runs hardness (Vickers to ISO 6507, Rockwell to ISO 6508), coating thickness, salt spray to ISO 9227 and microstructural examination. Proof load, wedge tensile and similar mechanical tests on finished fasteners need a tensile machine and are normally performed by the manufacturer or an independent test house. Our reports state exactly which properties they cover so nobody downstream assumes more than was measured.

A lot came back from our customer. Will you re-plate it?+

Sometimes, and sometimes we will advise you not to. Stripping and re-plating adds a second acid and hydrogen exposure and attacks the base steel, and a part that already had a relief bake needs another one after the new plating. Send us the rejection report, the class and the hardness, and we will tell you which of sorting, re-processing or scrapping is the honest answer for that lot rather than the one that bills the most hours.

We supply competing fastener factories. Is our work confidential here?+

We are a processor. We do not manufacture, brand or sell fasteners, so your drawings, part numbers and customer names are not commercially useful to us and are not shared. Practically, that also means our capacity is not competing with yours for the same order — the reason plants use us is that a heat-treatment or plating bottleneck should not decide how much you can head.

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.