🔍 How to distinguish inferior fasteners (bolts/screws/nuts)?

Common tricks for low-quality fasteners include falsely labeling grades, using recycled materials, avoiding heat treatment, and cutting corners in coating. On site inspection can be quickly carried out through four steps: observation, measurement, testing, and verification.

1、 Look – Appearance and Identification (most intuitive)

① Head performance level marking
The head of regular carbon steel high-strength bolts (≥ M5) should have clear stamping grade: 4.8 8.8 10.9 12.9; Stainless steel standard A2-70 A4-80.

Poor quality products have no markings, unclear or skewed markings, or false markings (printed with 8.8 but actually 4.8 grade ordinary iron nails).

② Surface and Coating
High quality: The galvanized/Dacromet layer is uniformly shiny or matte, without peeling, black spots, or whitening.

• Poor quality: The coating is mottled and easy to scrape off (nails can be peeled off), with rust spots, oxide skin, and obvious burrs.

③ Thread profile
The thread should have a full and symmetrical tooth shape, without broken teeth, flat teeth, or obvious burrs. Poor quality threads are often crooked and uneven, and the filling is exposed when tested with a stop gauge.

2、 Quantity – Size and Accuracy

Measure the width of the opposite edge, thickness of the head, diameter of the rod, and length with a caliper, and compare them with the tolerances of national standards (GB/T 5782, etc.). If they are too thin or too short, it is often considered as material theft.

• Use threaded gauges: The gauge should be smoothly screwed in, and the gauge should be screwed in ≤ 2-3 threads. If the thread is too small, if the gauge is screwed in too much, the thread will be too large – both are of poor quality.

3、 Test – Texture and Simple Physical Discrimination

Method operation judgment

Tighten the standard nut with hand feel and ensure even and smooth resistance when manually screwed in; Stuck/too loose and swaying=poor quality

Hardness scratch test file/steel saw blade lightly scratches the non stress part of the bolt with a hardness of 8.8 or above, which has been quenched and tempered and has high hardness and is difficult to leave marks; Poor quality ordinary steel has obvious grooves when scraped

Listening to the sound of falling freely from a distance of about 50cm from the ground, high-quality high-strength bolts sound crisp and bounce well; Poor quality sound, dull and hoarse

The stainless steel magnet method for 304/316 stainless steel should be non-magnetic or only weakly magnetized (with normal micro magnetism after cold processing); Strong magnetic attraction is mostly 201 or carbon steel impersonating 304

4、 Inspection – Document and Destructive Sampling (Key Procurement Control)

• Proof request: The supplier is required to provide a material report (spectral analysis) and a mechanical performance testing report (tensile/yield/hardness), stamped with CNAS/testing seal.

Salt spray test: Galvanized parts require no white rust in neutral salt spray for 48-96 hours (depending on the usage environment), and inferior coatings will rust within a few hours.

-Tensile testing (important parts): The 8.8 grade fracture surface should have plastic deformation (necking); If untempered low-carbon steel is impersonated, it will suddenly fracture without deformation.

⚠️ Key points for avoiding pitfalls in procurement

1. Refuse the three no loose items: formal packaging indicating the factory name, specifications, grade, and batch number.
2. Beware of abnormally low prices: The price of grade 8.8 of the same specification is significantly lower than the market average → highly suspected to be of poor quality.
3. Key stress points (steel structure nodes/cylinder heads/suspension points) must obtain a quality assurance certificate and leave samples. For ordinary home decoration, a preliminary screening can be conducted based on appearance and feel.

Quick mnemonic ✅
Check if the label is clear or not → Check the thread coating → Measure the size → Tighten smoothly → Check the quality assurance certificate and hardness report for important parts