Most bolts, screws and studs in M5 to M24 are made the same way: a coil of steel wire is formed cold, at room temperature, on fast multi-station machines. The steps below are for a typical 8.8 or 10.9 hex bolt. Smaller details change from maker to maker, but the order stays the same.
- Raw material checkWire rod coils arrive with a mill test certificate. Chemistry, diameter, surface and decarburisation are checked before use.
- AnnealingMedium-carbon and alloy steels are softened (spheroidise annealed) so they can be cold formed without cracking.
- Cleaning and coatingScale is removed and a phosphate or polymer layer is added. It works as a lubricant inside the forming dies.
- Wire drawingThe wire is pulled through dies to an exact diameter, ready for the forming machine.
- Cold headingA multi-station former cuts a blank, upsets the head, extrudes the shank and trims the hex.
- Secondary operationsPointing, drilling, slotting, grooving or fitting a washer, where the design needs it.
- Thread rollingHardened dies press the thread into the shank. No metal is cut away.
- Heat treatmentHardening and tempering give the bolt its property class: 8.8, 10.9 or 12.9.
- Surface finishZinc plating, hot-dip galvanising, zinc flake, phosphate or black oxide.
- Inspection and packingDimensions, hardness and strength are tested, parts are sorted, and each lot is labelled for traceability.
1. Raw material: wire rod
Fasteners start as hot-rolled wire rod, delivered in coils that can weigh more than a tonne each. Good makers buy from certified steel mills and check every heat before it enters production: chemical analysis, size, surface defects and the depth of any decarburised (carbon-lost) layer. The steel grade is chosen for the property class. Low-carbon steel is used for 4.6 and 4.8, and medium-carbon, boron or chromium-molybdenum steels are used for 8.8 and above. Our raw materials guide covers this in detail.
2. Annealing
Steels with more carbon or alloy are too hard to cold form as they come from the mill. They are annealed in coils, often several tonnes per furnace load: slowly heated, held and slowly cooled. Spheroidise annealing turns the hard carbides in the steel into small rounded particles, which makes the wire much softer and more ductile. Very low-carbon grades for 4.6 and 4.8 bolts often skip this step.
3. Cleaning and coating the wire
Mill scale is removed by acid pickling or by shot blasting with steel balls. The clean wire then gets a carrier coating: traditionally zinc phosphate with a soap, or a polymer coating in newer plants. This layer stops the steel from welding to the dies under the huge pressure of cold forming, and it protects the wire from rust while it waits.
4. Wire drawing
The wire is pulled through one or more tungsten-carbide dies to bring it to the exact diameter the former needs, usually within a few hundredths of a millimetre. Drawing also smooths the surface and slightly work-hardens the steel.
5. Forming the head: cold heading (or hot forging)
This is where the bolt takes shape. In a cold former the wire is fed in, sheared to length and moved through four to six stations. Each station pushes the blank a little further: upset the head, extrude the shank to the thread-rolling diameter, form the hex, trim the flash. Modern formers run at well over a hundred pieces a minute, and no material is wasted as chips.
Very large diameters, very long bolts and small batches are often hot forged instead. The end of a bar is heated by induction until it is soft, then pressed into a die to form the head. Cold heading vs hot forging explains when each is used.
6. Secondary operations
Some parts need extra work after heading: a point or dog point on the end, a cross hole or split-pin hole, a slot, a groove, or a captive washer pressed on (a "sems" screw). These are done on separate machines in batches.
7. Thread rolling
Most threads are rolled, not cut. The blank is rolled between two hardened dies that carry the thread profile, and the steel flows into shape in a single pass. Rolling keeps the grain of the steel continuous around the thread, leaves the root of the thread in compression and gives a smooth, burnished finish. All of this makes rolled threads stronger in fatigue than cut threads. Thread cutting is still used for very large sizes, special forms and small quantities.
8. Heat treatment
A bolt straight from the former is still fairly soft. For property classes 8.8, 9.8, 10.9 and 12.9, ISO 898-1 requires the steel to be quenched and tempered. Bolts travel through a continuous mesh-belt furnace with a controlled atmosphere, are heated until the steel changes structure (austenite), quenched in oil, and then tempered: reheated to a lower temperature to trade a little hardness for toughness. ISO 898-1 sets a minimum tempering temperature of 425 °C for these classes. Our heat treatment article explains why.
9. Surface finish
Plain steel rusts, so most bolts get a finish: electroplated zinc (clear, blue, yellow or black), hot-dip galvanising for outdoor steelwork, zinc flake for high-strength bolts, or phosphate and oil. The choice affects corrosion life, thread fit and, for 10.9 and 12.9 bolts, the risk of hydrogen embrittlement. See coatings and finishes.
10. Inspection, sorting and packing
Every lot is sampled and tested: dimensions with thread gauges, hardness, tensile and proof load, and coating thickness. Many factories also sort 100% of parts on camera machines to remove anything with a wrong length or a damaged head. Each box carries a lot number that links back to the steel heat, the heat-treatment batch and the test results, so a mill test certificate can be issued with the delivery. More in testing and quality checks.
How this works in India
In India the same ten steps are often split between several companies:
- Steel plants roll the wire rod. Indian integrated plants supply the common low-carbon and medium-carbon grades; boron and Cr-Mo cold-heading grades are also imported by many makers.
- Wire-drawing units anneal, coat and draw the rod to size, and sell drawn wire to fastener makers. This is very common in India's fastener clusters.
- Large manufacturers, mainly automotive suppliers around Chennai, Pune and the NCR, run the full line in-house: multi-station formers, thread rollers, continuous mesh-belt furnaces and their own plating or zinc flake lines.
- Smaller units often hot forge or turn bigger and special sizes, and send heat treatment and plating to job-workers.
Both routes can give a good bolt. The difference is control: when a part goes out to two or three job-workers, the heat-treatment batch and plating lot must still be traceable to your box. That is why it is worth asking for a test certificate and buying from a supplier who knows where each lot was made.
BIS rules. Common hex bolts, screws and nuts (IS 1363 and IS 1364), structural bolts and nuts (IS 3757, IS 6623) and transmission tower bolts (IS 12427) fall under the Fasteners Quality Control Order, so they must carry the BIS (ISI) standard mark when sold in India, subject to the order's exemptions.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.