Interlocking Steel Grating Testing

Interlocking steel grating inspection mainly conducts scientific testing and analysis on the appearance and dimension testing, welding/interlocking quality testing, mechanical property testing, surface treatment layer testing, raw material material verification and marking and packaging inspection, 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

Appearance and dimension testing: Through the tools such as the visual inspection, callipers and steel tape measures, whether the surfaces of the interlocking steel grating are flat and free of the defects such as the warping, cracks and burrs is checked. The key dimensions such as the lengths, widths, diagonal deviations, load-bearing flat steel thicknesses and widths and cross bar spacings are precisely measured, ensuring that they meet the requirements of the design drawings and relevant standards (such as YB/T 4001.1).

Welding/interlocking quality testing: Emphatically checks the connection parts between the load-bearing flat steels and cross bars. For the welded types, the continuity and fullness of the welds need to be checked for the false welding, missed welding and weld slag spatter. For the interlocking types, the slot depths and precision of the flat steel openings, the tightness of the interlocking fits, and the firmness of the locking or press welding need to be checked, ensuring that the connection points can effectively transfer the loads.

Mechanical property testing: This is the core of evaluating the load-bearing capacity. The main items include: 1) bending testing, determining the maximum deflections and residual deformations under the uniformly distributed loads; 2) load testing, verifying its safety under the design loads; 3) impact resistance testing, simulating the structural integrity under the unexpected impact conditions. These tests are usually carried out on the universal material testing machines.

Surface treatment layer testing: If the steel grating is treated with the anti-corrosion treatments such as the hot-dip galvanizing and spraying, the coating quality needs to be tested. Including the coating appearances (uniformity, gloss, and whether there are the missed plating and bubbles), coating thicknesses (measured with the magnetic or eddy current thickness gauges, and the average thicknesses of the galvanized layers are usually not less than 70 μm), coating adhesion (such as the cross-cut testing) and corrosion resistance (salt spray testing), etc.

Raw material material verification: Through the spectrometers or chemical analysis methods, the sampling inspection of the chemical compositions of the raw materials such as the flat steels and cross bars constituting the steel grating is carried out, confirming whether their designations (such as Q235B and Q355B) meet the standard provisions. This is the basis for guaranteeing the final mechanical properties and durability of the product.

Marking and packaging inspection: Checks whether the information on the product nameplates or labels is complete, including the manufacturers, product models, specification dimensions, implemented standards and load-bearing capacities. At the same time, whether the packaging is firm and can effectively prevent the deformation and surface damages during the transportation is checked, and the accompanying quality certificate documents are checked.

Testing Scope

Manufacturing process inspection: Covering the whole process from the incoming raw materials to the ex-factory finished products. Including the raw material re-inspection, first inspection of the blanking dimensions, patrol and sampling inspections of the welding/interlocking processes, and process control point inspections before and after the hot-dip galvanizing, ensuring that each process is in the controlled state and realizing the process quality control.

Ex-factory/delivery acceptance testing: The final inspection carried out before the product packaging and shipment. Usually, according to the purchase contracts, technical agreements and relevant product standards, the batch sampling is carried out on the products, comprehensively checking their appearances, dimensions, markings and document materials. This is the key link of the manufacturers proving the product qualification to the users.

Arrival/warehousing re-inspection: The verification testing carried out by the user units or construction parties when the steel gratings arrive at the construction sites or warehouses. Emphatically checking whether the product quantities and specification models are consistent with the orders, and carrying out the necessary appearance inspections and dimension sampling inspections, preventing the unqualified products or wrong products from entering the installation links.

Installation process and completion acceptance testing: During the steel grating installation, the laying flatness, lap lengths with the support structures, and the firmness and standardization of the fixing methods (welding or installation clips) are checked. At the completion of the projects, the comprehensive evaluation of the installation quality and safety properties of the whole platforms or walkways is carried out.

In-service safety property evaluation: For the steel gratings that have been in use for many years, the periodic safety testing needs to be carried out. Mainly checking whether there are the section weakenings caused by the corrosion, deformations and cracks caused by the overloads or fatigue, and the loosening or failures of the connection parts, providing the basis for the maintenance, replacement or reinforcement.

Special application environment special testing: For the steel gratings used in the special environments such as the humid, corrosive (chemical), high-temperature, low-temperature (cold storage), flammable and explosive or densely populated, in addition to the routine items, the special testing such as the chemical medium corrosion resistance testing, fire resistance testing and anti-slip property (serrated plates) precise measurement also needs to be added.

Testing Methods

Visual and manual measurement method: The most basic and widely applied method. The inspectors use their experience and simple tools (such as the weld inspection gauges, feeler gauges and callipers) for the fast judgement and preliminary measurement of the product appearances, obvious defects and approximate dimensions. This method has the high efficiency, but has a certain dependence on the personnel experience and lighting conditions.

Instrument precise measurement method: The high-precision instruments are used to obtain the objective data. Such as using the digital callipers and micrometers to measure the flat steel thicknesses; using the laser range finders or large platforms combined with the height gauges to measure the flatness and diagonals; using the ultrasonic thickness gauges to measure the base material thicknesses before the galvanizing, ensuring that they are not lower than the nominal thicknesses minus the negative deviations.

Non-destructive testing (NDT) method: Probes the internal or surface fine defects under the premise of not damaging the products. The common methods include: magnetic particle testing (MT) for finding the surface and near-surface cracks; penetrant testing (PT) for the surface open defect inspection of the non-magnetic materials; when necessary, the ultrasonic testing (UT) can be adopted to evaluate the internal welding quality.

Destructive testing method: Through the sampling, the specimens are destroyed in the laboratories to obtain their property limits. Such as the bending testing placing the specimens on two supports and loading at the centres to the specified loads or failures, measuring the deflections; the tensile testing cutting the flat steel specimens, determining the mechanical indicators such as the tensile strengths, yield strengths and elongation after fracture.

Chemical and metallographic analysis method: Used for the verification of the material essence. The chemical analysis methods (such as the spectrometers) can quickly determine the contents of the various elements in the material. The metallographic analysis methods observe the microstructures of the material under the microscopes through the sample preparation, polishing and etching, judging their heat treatment states or whether there are the abnormal structures, and analysing the failure causes.

Coating property testing method: Evaluates the quality of the anti-corrosion layers. The coating thicknesses are measured with the magnetic method (iron and steel substrates) or eddy current method (non-ferrous metal substrates); the adhesion commonly uses the cross-cut method, pasting the tapes on the cross-cut areas and then tearing, observing the coating falling off conditions; the salt spray resistance is tested in the salt spray test chambers simulating the accelerated corrosion environments.

Testing Standards

Domestic Standards:

1. YB/T 4001.1, Steel grating and its associated products — Part 1: Steel grating

2. GB/T 13912, Metallic coatings — Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods

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

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

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

International Standards:

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

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

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

Dimension and geometric measurement equipment: Including the vernier callipers, outer diameter micrometers and steel tape measures (need to be calibrated) for the routine dimension measurement; platforms, square boxes and dial indicators/micrometers for detecting the geometric tolerances such as the flatness and parallelism; laser range finders and total stations for the fast measurement of the overall dimensions and flatness after the installation of the large steel gratings.

Mechanical property testing machine: The core equipment is the microcomputer-controlled electronic universal testing machine, which can carry out the various tests such as the bending, tensile and compression, precisely record the load-displacement curves and automatically calculate the results such as the strengths and deflections. The impact testing machines are used to determine the toughness of the material under the dynamic loads, evaluating its ability to resist the sudden impacts.

Non-destructive testing (NDT) instruments: The magnetic particle flaw detectors (including the yokes, magnetic particles and UV lamps) are used for the surface crack testing of the welds and heat-affected zones; the colour penetrant testing kits (cleaners, penetrants and developers) are used to check the surface open defects of the non-porous materials; the ultrasonic flaw detectors can be used for the detection and positioning of the internal defects of the thicker materials.

Coating testing instruments: The coating thickness gauges are the necessary tools, divided into the magnetic thickness gauges and eddy current thickness gauges, and the selection is made according to the substrates during the use. The adhesion testers (cross-cut testers) provide the cutting blades with the standard spacings. The salt spray test chambers are used to simulate the marine or industrial atmospheric environments, accelerating the evaluation of the corrosion-resistant service lives of the coatings.

Material composition and structure analysis equipment: The portable spectrometers can quickly carry out the designation identification and semi-quantitative composition analysis of the raw materials on site. The laboratories may be equipped with the more precise photoelectric direct-reading spectrometers. The metallographic microscopes combined with the sample preparation equipment such as the cutting machines, mounting machines, polishing machines and etchants are used to observe and analyse the micro-structures of the material.

Auxiliary and general equipment: Including the high-precision electronic balances (weighing method for measuring the galvanized layer masses), surface roughness comparison blocks or instruments, illuminance meters (ensuring the sufficient light for the visual inspection), environmental temperature and humidity meters (some tests have the environmental requirements), as well as the calibration blocks and standard specimens of all the instruments, ensuring the traceability and accuracy of the testing results.