Steel Coil Nail Testing

Collated nail inspection mainly conducts scientific testing and analysis on the dimensional precision testing, surface hardness testing, bending resistance testing, coating quality and corrosion resistance testing, nail tip sharpness and penetration force testing, and weld point strength testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dimensional precision testing: Measures the total length, shank diameter, tip angle and the head diameter and thickness of the collated nail. These dimensional parameters directly affect its compatibility with the nail magazines of the pneumatic or electric tools, as well as the final connection strength and driving performance. The excessive dimensional deviations will cause the nail jamming, poor feeding or insufficient withdrawal resistance.

Surface hardness testing: A Rockwell hardness tester (HRC scale) is usually adopted for the multi-point testing at the middle of the shank. The hardness is the key indicator for measuring the bending and penetration resistance of the collated nail. The excessively low hardness is easy to cause the shank bending, and the excessively high hardness may make the nail body brittle and fracture under the impact. A balance between the toughness and rigidity needs to be achieved.

Bending resistance testing: A single nail is supported on the two supports at the specified distance, and the load is applied vertically at the midpoint until it fractures or reaches the specified deflection. By measuring the maximum bending load or bending angle, the plastic deformation ability of the nail when bearing the lateral forces and the toughness resisting the fracture are evaluated.

Coating quality and corrosion resistance testing: Checks the uniformity, adhesion and thickness of the galvanized layer or other anti-corrosion coatings. The common methods include the coating thickness gauge measurement, copper sulphate dropping test or salt spray testing. The good coating can effectively prevent the nails from rusting during the storage and use, ensuring their long-term connection reliability.

Nail tip sharpness and penetration force testing: Through the dedicated instruments or the simulation of the actual driving into the wood of the specified hardness (such as the pine and oak), the sharpness of the nail tip and the penetration resistance are evaluated. The sharp nail tip can reduce the impact force required for the driving, prevent the wood splitting, and improve the construction efficiency.

Weld point strength testing: For the collated nails arranged in rows through the welding method, the firmness of the weld points needs to be detected. By applying the shear force or peel force, it is tested whether the weld points can maintain the connection under the impact of the high-speed automatic nail feeding, ensuring that the whole row of nails will not scatter during the use.

Material chemical composition analysis: A spectrometer is used for the chemical composition analysis of the nail body material, mainly confirming the contents of the elements such as the carbon (C), manganese (Mn), silicon (Si), phosphorus (P) and sulphur (S). The composition determines the hardenability, strength and machinability of the steel, and is the basis for ensuring its comprehensive mechanical properties.

Testing Scope

Straight collated nails: The most common type, with the straight shanks, widely used for the ordinary wood structure connections, pallet making, furniture assembly and building formwork fixing. The testing emphasis is on the dimensional consistency, hardness and straightness, ensuring that they can be smoothly driven out of the nail magazines.

Threaded collated nails: The shanks have the ring-shaped or spiral threads, which can provide the withdrawal resistance (gripping force) far higher than the straight nails after the driving. During the testing, in addition to the basic items, the depth, completeness and continuity of the threads, as well as the actual improvement effect on the connection strength, need to be additionally focused on.

Staples and U-shaped nails: Mainly used to fix the wires, meshes, canvases, insulation materials and wooden box edge banding. The testing items include the crown width, leg length and parallelism, tip shape and the overall forming angle, ensuring that they can firmly clamp the fixed objects.

Coil nails: The nails supplied in the coil form, usually used for the occasions requiring the continuous high-speed driving, such as the roof construction and floor laying. The testing needs to focus on the connection strength between the single nails (glue connection or wire connection), the smoothness of the whole coil and the separation characteristics of the single nails.

Special collated nails: Such as the hardened concrete nails and heavy-duty anti-corrosion nails. According to their special uses, the testing scope needs to be expanded. For example, the hardened concrete nails need to carry out the quenching hardness testing and the special performance testing of the driving into the concrete; the heavy-duty anti-corrosion nails need to carry out the more stringent salt spray testing.

Different materials and coating types: Covering the products of the different base materials such as the low-carbon steel, medium-carbon steel and stainless steel, as well as the different surface treatment processes such as the electro-galvanizing, hot-dip galvanizing and phosphating. The testing methods need to be adjusted according to the material characteristics. For example, the stainless steel nails need to increase the pitting resistance testing.

Different specifications and packaging forms: The testing scope covers all the specifications from the lengths of over ten millimetres to the hundreds of millimetres with the different wire diameters. At the same time, for the products in the different packaging forms such as the boxes, bags and coils, the packaging integrity and the effectiveness of the anti-rust measures also need to be checked.

Testing Methods

Dimension measurement method: The vernier callipers, micrometers or optical projectors with the resolution not lower than 0.01 mm are used for the sampling measurement of the various part dimensions of the collated nail. The length, shank diameter and head dimensions need to be randomly sampled with the sufficient samples in the different batches, and their average values and standard deviations are calculated, compared with the product standards to judge whether they meet the tolerance ranges.

Hardness testing method: Mainly carried out in accordance with GB/T 230.1 or ASTM E18, using a Rockwell hardness tester (C scale). Before the testing, the tested points on the shank need to be ground flat, and each sample is tested at least three times at the different positions, taking the average value. During the testing, it is necessary to ensure that the load direction is perpendicular to the shank axis, avoiding the result errors caused by the improper support.

Static load bending testing method: With reference to the industry standards such as QB/T 3658, the nail is placed on the two support rollers of a bending testing device with the adjustable span. The load is applied vertically at the midpoint at a constant rate, and the load-displacement curve is recorded until the specimen fractures or the bending angle reaches the specified value (such as 90°), and the maximum bending force is calculated or the fracture morphology is observed.

Salt spray testing method: In accordance with the neutral salt spray testing standard GB/T 10125 (equivalent to ISO 9227), the corrosion resistance of the coating is evaluated. The specimens are placed in the salt spray chamber, and the 5% sodium chloride solution is continuously sprayed at 35±2 ℃. After the specified periods (such as 24 h, 48 h and 72 h), the specimens are taken out to observe the grades and areas of the rust and white rust on the surfaces, judging the protective performance of the coating.

Coating thickness measurement method: For the galvanized layers, the magnetic thickness measurement method (according to GB/T 4956) or metallographic microscope method can be adopted. The magnetic method is fast and non-destructive, and is suitable for the on-site and batch detection; the metallographic method directly observes the coating cross-section thickness under a microscope through the sample preparation, mounting, polishing and etching, with the more precise results, but belongs to the destructive testing.

Weld point shear strength testing: A dedicated fixture is used to fix the two ends of the weld point of the collated nail, and the shear force is applied on a material testing machine until the weld point fractures. The maximum shear load is recorded, and the fracture position (at the weld point or the base material) is observed, thereby evaluating the reliability and consistency of the welding process.

Chemical composition spectrometry analysis method: A direct-reading optical emission spectrometer or X-ray fluorescence spectrometer is used for the fast and non-destructive analysis of the nail material. The clean position of the shank is used as the excitation point, and the instrument quantitatively or semi-quantitatively gives the contents of the various elements by analysing the generated characteristic spectra, compared with the corresponding steel designation standards.

Testing Standards

Domestic Standards:

1. QB/T 3658, Collated nails

2. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

3. GB/T 4956, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method

4. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

5. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

6. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. ASTM E18, Standard test methods for Rockwell hardness of metallic materials

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Precision dimension measuring instruments: Including the digital vernier callipers, outer diameter micrometers, universal tool microscopes or two-dimensional image measuring instruments. These equipment can obtain the geometric parameters of the collated nail with the high precision, and the image measuring instrument is especially suitable for the fast and batch detection of the complex profiles such as the nail tip angles.

Hardness tester: The Rockwell hardness tester is the core equipment and needs to be regularly calibrated with the standard hardness blocks. For the smaller specification nails or specific parts, the Vickers or superficial Rockwell hardness testers may be needed. The portable Leeb hardness tester is suitable for the fast on-site screening.

Mechanical property testing machine: The electronic universal material testing machine, equipped with the three-point bending fixtures and the corresponding force and displacement sensors. It is used for the bending resistance testing and weld point shear testing, and can precisely control the loading rate, record the data in real time and generate the test reports.

Corrosion testing equipment: The salt spray test chamber is the key equipment for evaluating the corrosion resistance, requiring the uniform and controllable temperature, spray amount and sedimentation rate in the chamber. The supporting equipment also includes the cutting machines and mounting machines for the specimen preparation, as well as the magnifying glasses and standard rating charts for the result rating.

Coating thickness gauge: According to the coating and substrate properties, the magnetic thickness gauge (for the non-magnetic coatings on the steel substrates) or eddy current thickness gauge (for the insulating coatings on the non-ferrous metal substrates) is selected. The equipment needs to be calibrated with the standard thickness foils before use, ensuring the measurement accuracy.

Spectrometer: The bench-type direct-reading optical emission spectrometer can carry out the fast and accurate quantitative composition analysis of the nail material, and is a sharp tool for the incoming inspection and material identification. The handheld XRF spectrometer is more suitable for the on-site material sorting and large-batch screening.

Optical observation and analysis equipment: The stereomicroscope is used to observe the nail tip sharpness, surface defects, coating uniformity and weld point morphology. The metallographic microscope, combined with the sample preparation equipment, is used to analyse the microstructures of the coating and substrate, measure the coating thickness and observe the metallographic structures after the heat treatment.