Checks at every stage
- Incoming steel: mill certificate checked, chemistry confirmed (often by spectrometer), diameter and surface checked.
- In process: head height, across-flats size, length and thread checked at the machine at regular intervals; first-off and last-off parts kept for each run.
- After heat treatment: hardness checked on every batch; strength tests on samples.
- Final: lot sampled and tested; optional 100% optical sorting; labels and lot numbers applied.
Mechanical tests in ISO 898-1
| Test | What it proves |
|---|---|
| Tensile test | The full bolt is pulled to failure. Shows minimum tensile strength; it should break in the thread or shank, not under the head. |
| Proof load | The bolt is loaded to a set proof load (a little under yield) and released. It must not stretch permanently beyond a tiny limit. |
| Wedge tensile | The bolt is pulled with a wedge under the head (usually 6° or 10°). Proves the head-to-shank joint is sound and not brittle. |
| Hardness | Vickers, Brinell or Rockwell on a cross section or end. Must fall within a set range for the class: too low is weak, too high is brittle. |
| Decarburisation | A polished section through the thread is examined to make sure the surface has not lost too much carbon. |
| Impact (Charpy) | Toughness at a set temperature, for larger bolts and low-temperature service. |
Dimensional and thread checks
Threads are checked with GO and NO-GO ring gauges (bolts) and plug gauges (nuts) to tolerance class 6g / 6H, or the coated-thread allowances for galvanised parts. Head height, across flats, across corners, length and straightness are measured against the product standard, such as ISO 4014, ISO 4017 or DIN 931. Surface discontinuities such as seams and forging cracks are judged against ISO 6157.
Coating tests
- Coating thickness by magnetic or X-ray gauge.
- Neutral salt spray (ISO 9227): hours to white and red rust.
- Adhesion and appearance checks.
- Hydrogen embrittlement checks for high-strength plated parts, by holding tightened parts under load for a set time.
- Friction / torque-tension (ISO 16047), where a set friction coefficient is specified.
Head markings: what to look for
Hex bolts and socket screws of class 8.8 and above, from M5 upward, must carry the property class (8.8, 10.9, 12.9) and the maker's identification mark on the head. Stainless fasteners show the steel group and class, such as A2-70 or A4-80. A bolt without a maker's mark is a warning sign. Our head markings guide shows the common marks.
Sampling and certificates
Acceptance inspection usually follows ISO 3269, which sets how many parts per lot are checked for each feature. Results are reported on an inspection certificate to EN 10204:
- 2.1: declaration of compliance, no test results.
- 2.2: test report with typical (non-specific) results.
- 3.1: specific test results for your lot, validated by the maker's own independent quality department. This is the most common requirement.
- 3.2: as 3.1, but also validated by the purchaser's or a third-party inspector.
Testing and certification in India
- IS 1367 (Part 17) sets the inspection, sampling and acceptance procedure, and IS 1367 (Part 9 and Part 10) cover surface discontinuities on bolts and nuts.
- BIS (ISI) licence. Makers of products under the Fasteners QCO need a BIS licence, with in-house testing facilities and BIS surveillance.
- NABL-accredited labs are used for independent tensile, hardness, chemical and salt spray testing.
- Project inspection. Railway, power transmission, bridge and PSU projects often require third-party or customer inspection before dispatch, which is equivalent to an EN 10204 3.2 certificate.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.