Industrial Fasteners

The cheapest category per unit and the most expensive per failure. A bolt carries a head marking that says 8.8; whether the steel behind that marking was actually quenched and tempered is a separate question, and the only way to answer it is to test.

What this category covers

Threaded fasteners and anchors:

  • Bolts and screws — hex head, socket head, flange, carriage, machine screws
  • Nuts and washers — hex nuts, nyloc, flange nuts, flat and spring washers
  • Self-tapping and self-drilling — for sheet metal, timber and drywall
  • Anchors — wedge anchors, sleeve anchors, chemical anchors, drop-in
  • Special — U-bolts, eye bolts, threaded rod, non-standard per drawing

The parameters that actually matter

Property class, and what it implies. Under ISO 898-1, class 8.8 means 800 MPa minimum tensile and 640 MPa yield, which requires a medium-carbon steel that has been quenched and tempered. Class 10.9 means 1040 / 940 MPa and requires an alloy steel such as 35CrMo or 40Cr. Buying “8.8” without a material certificate means buying a marking.

Hardness, as the cheap proxy for class. A hardness reading in the 250–320 HV band is consistent with 8.8; 320–380 HV with 10.9. It does not prove tensile strength, but it catches the common failure — a low-carbon bolt that was never heat treated — in seconds, on the factory floor.

Decarburisation depth. Heat treatment in an uncontrolled atmosphere burns carbon out of the surface layer, and that layer is the thread. Above class 8.8, decarburisation on the thread flank is limited to about 0.015mm. It is invisible and it is one of the reasons a bolt strips at the specified torque.

Thread tolerance and how it was formed. Rolled threads are stronger than cut threads because the grain flows around the profile instead of being interrupted. Tolerance is 6g external, 6H internal, verified with go/no-go gauges.

Coating, by standard. Electroplated zinc to ISO 4042 at 5 or 8 microns, hot-dip galvanised to ISO 10684 at 45 microns and above, zinc-flake to ISO 10683. Measure at the thread root, where electroplating is thinnest.

Hydrogen embrittlement control. Anything at class 10.9 or above that has been electroplated needs a de-embrittlement bake. This is a process control question, not a product question: ask for the bake temperature, the duration, and the interval between plating and baking.

Where it comes from

China is the largest producer of threaded fasteners in the world, and output is concentrated in a small number of clusters where wire rod supply, cold heading, heat treatment and plating sit in the same industrial zone. That clustering is why a container of mixed standard parts can be assembled in a week, and also why quality varies enormously inside the same cluster — the equipment is shared, the process discipline is not.

Cold heading machines and heat treatment lines are capital, but the difference between a good and a bad plant in this category is almost always process control: furnace atmosphere, quench timing, plating chemistry, bake discipline. None of that shows in a sample photo.

Common sourcing failures

The marking is not the material. The most common and the most expensive failure. A tensile test on a sample from the lot resolves it; a certificate from the factory does not, unless the certificate is tied to the heat number on the parts.

Hydrogen embrittlement. Bolts that pass every dimensional check and snap in service a day later. The failure is brittle and sudden, and it shows up in high-strength plated parts where the bake was skipped or delayed.

Coating measured at the crest. Electroplating is thicker at the thread crest and thinner at the root. Thickness measured at the crest passes; the root, which is what corrodes, does not.

Mixed heat numbers in one carton. Cheap for the plant, impossible to trace for you. If a claim arises, there is no way to isolate the affected lot.

Cut threads presented as rolled. Visually similar until you look at the thread root under magnification, or until the bolt fails at 60% of rated load.

What we need to quote

  • Product type, standard (DIN, ISO, ANSI, JIS) and drawing if non-standard
  • Property class, and whether the application is structural
  • Material requirement, if the class is not sufficient on its own
  • Coating standard and thickness, and the environment the part will see
  • Thread tolerance and whether gauging is required on the lot
  • First order quantity, annual estimate, and the sizes involved
Questions

Frequently asked

No. The marking is applied by the manufacturer, and applying 8.8 to a low-carbon bolt that was never quenched and tempered is trivially easy. The property class is a specification under ISO 898-1 covering tensile strength, yield strength, elongation and hardness - a genuine 8.8 bolt reaches 800 MPa tensile and 640 MPa yield with hardness in the 250 to 320 HV range. Verifying it means a tensile test plus a hardness check on a sample from the actual lot, not a photograph of the head.
Acid pickling and electroplating generate hydrogen that diffuses into the steel. In high-strength fasteners the trapped hydrogen causes delayed brittle fracture - the bolt installs correctly and fails hours or days later, with no warning and no visible deformation. Above roughly 320 HV, which is class 10.9 and above, the standard requires a de-embrittlement bake: typically 200 to 230 degrees Celsius for at least four hours, started within four hours of plating. A supplier who cannot state the bake temperature and the time between plating and baking has not controlled the risk.
By standard, not by feel. Electroplated zinc under ISO 4042 is normally Fe/Zn 5 or Fe/Zn 8, meaning 5 or 8 microns minimum. Hot-dip galvanised under ISO 10684 runs 45 microns and above on M10 and larger, with the coating thickness varying across the thread. On electroplated parts the coating is thicker at the thread crest and thinner in the root, so measure at the root. For outdoor structural work, hot-dip or a zinc-flake coating under ISO 10683 is the usual specification.
Either the thread was rolled with a worn die, or it was cut rather than rolled, or the coating is too thick in the root. Thread tolerance for bolts is 6g and for nuts 6H, checked with a go/no-go gauge. A shipment that will not run onto a nut by hand will jam a production line. Ask for the gauge result on the lot, and check a sample with your own gauges - it takes a minute and it is the cheapest defect to catch before shipping.
You can, but keep them physically separated and labelled, because the failure modes and the installation torque are different. Mixed lots are where a 4.8 bolt ends up where a 10.9 was specified. If a factory quotes one price for a mixed order without stating class by line item, the quotation is not usable for anything structural.

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