Small machine fasteners — screws, nuts and bolts — hardened in volume for wear resistance and spec hardness.
Machinery fasteners are torqued to a specification, run for a defined maintenance interval, and are expected to come apart again. Failure here is shock load and seizure rather than weather: a bolt that snapped instead of yielding, a pin that ovalised and let a linkage develop backlash, or a thread that galled and destroyed the housing the first time the machine was serviced. Serviceability is a specification, and it usually points at phosphate with oil rather than a decorative finish.
| Property class | 8.8 / 10.9 / 12.9 |
|---|---|
| Hardness | 28–45 HRC fasteners · 55–62 HRC pins & bushes |
| Materials | SCM435 · SCM440 · SUJ2 · SK85 · SKS3 · SUP9 |
| Coating system | zinc phosphate + oil · Zn + Cr3+ |
| Coating thickness | phosphate by coating mass or approved sample · Zn 5–12 µm |
| Salt spray (NSS) | 96–240 h NSS |
| De-embrittlement bake | ≥ 320 HV or class ≥ 10.9 → ISO 4042 bake |
| Inspection methods | ISO 6508 / ISO 6507 · ISO 2639 case depth · ISO 6157-1 |
Machinery drawings mix two populations: fasteners specified to a property class, and pins, bushes and springs specified to a hardness band. Say which one your part is — the two are quoted and inspected differently.
Machine joints see sudden load reversals from clutches, presses, drives and jams. A part hardened too far for its section fractures at the first real shock; a part tempered too soft yields and the joint quietly loses preload. The temper band is therefore a design decision, not a shop-floor one, and it has to be written as a band with a measurement location rather than as a single target number.
At the maintenance interval the fastener has to come out. Dry threads under high contact stress cold-weld and gall, and the joint is destroyed on removal — often taking the tapped hole with it. Zinc phosphate with oil is the traditional answer because the crystalline coating holds oil in its structure and keeps the sliding surfaces separated; the coating is chosen for that function, not for its colour.
Pins in linkages and bushes at pivots see high contact stress and small oscillating movements. Through-hardened bearing and tool steels such as SUJ2, SK85 and SKS3 at 55–62 HRC hold their form; a soft pin ovalises, the linkage develops backlash, and the backlash then damages the mating parts far more expensively than the pin cost.
Disc springs, compression springs and lock washers exist to hold preload. Spring steel that is under- or over-tempered takes a permanent set, the preload falls away, and the joint loosens without anything visibly breaking. SUP9 and SK85 are tempered to a defined band precisely to control relaxation, which is why a spring drawing states a band and not a minimum.
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They are chosen by function, not by colour. Zinc phosphate is the usual paint base and general oil-retaining coating; a manganese-containing phosphate is chosen where run-in and sliding wear matter. Both are controlled by coating mass or an approved sample and by the oil or sealer applied afterwards, so send the function and the drawing note.
Bearing and tool steels such as SUJ2, SK85 and SKS3 reach that range by through-hardening with a low temper, and we run them on small parts in production lots. Send the grade, the section, the hardness band and the measurement location. On a long slender pin the limiting factor is usually the distortion tolerance, not the hardness.
Distortion is driven by section change, prior cold work, how parts are loaded and how they are quenched. The controls are a stress relief before hardening where the prior process justifies it, consistent loading and quench practice, and a defined gauge check after tempering. Give us the straightness or gauge limit up front so it is a process requirement rather than a sorting problem.
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.
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