Steel Grating Testing

Steel grating inspection mainly conducts scientific testing and analysis on the shape dimension and deviation testing, load-bearing flat steel thickness and width testing, cross bar, surface flatness, welding quality and firmness testing and surface treatment quality 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

Shape dimension and deviation testing: Mainly testing the shape dimensions of the steel gratings such as the lengths, widths and diagonal differences, as well as the differences of the diagonal lengths. The high-precision steel tapes or laser distance meters need to be used, and the measurement results need to meet the tolerance ranges specified in the relevant standards such as GB/T 700-2006, ensuring that the products can precisely match the installation sites.

Load-bearing flat steel thickness and width testing: The ultrasonic thickness gauges or digital vernier callipers are used for the multi-point measurement of the thicknesses and widths of the load-bearing flat steels. This item is the basis of evaluating the load-bearing capacities of the steel gratings. The negative deviations of the flat steel dimensions directly affect their section moduli and final load performances, and must be strictly controlled.

Cross bar (twisted square steel or round steel) dimension and spacing testing: The diameters, heights of the cross bars and their welding firmness with the load-bearing flat steels are tested, and the center spacings of the cross bars are precisely measured. The uniformity of the spacings affects the overall rigidity and anti-slip properties of the steel gratings. During the testing, multiple measurement points need to be randomly selected along the panel length and width directions.

Surface flatness (deflection) testing: Under the specific spans, after the standard preloads are applied to the steel gratings, the levels or dedicated deflection measuring instruments are used to detect the bending deformation amounts of their surfaces. The excessive flatness deviations will affect the walking safety feeling and appearance, and are the important indicators of evaluating the manufacturing processes and initial rigidity.

Welding quality and firmness testing: The continuity and fullness of the welds are checked through the visual inspection, and whether the defects such as the missing welds, false welds and undercuts exist. When necessary, the tapping sound method is used or the destructive sampling testing (such as the tensile and shear testing) is carried out, to verify whether the shear strengths of each weld point meet the requirements.

Surface treatment quality testing: For the steel gratings with the surface treatments such as the hot-dip galvanizing and painting, the galvanized layer thicknesses (using the magnetic thickness gauges), adhesion (cross-cut testing), uniformity, and whether the appearance defects such as the missing galvanizing, peeling and runs exist need to be tested. The coating qualities are directly related to the corrosion resistance service lives of the products.

Testing Scope

Classification by the production processes: Covering the pressure resistance welded steel gratings, pressure welded locked-edge steel gratings, plug-in steel gratings and composite steel gratings, etc. The testing emphases of the products of the different processes are different. For example, the welded gratings emphatically test the weld point strengths, and the plug-in gratings need to pay attention to the flat steel slotting precision and interlocking tightness.

Classification by the load-bearing flat steel shapes: Including the plain type (I-type flat steel) steel gratings and serrated (anti-slip) steel gratings. For the serrated steel gratings, the uniformity and consistency of the tooth heights and tooth spacings need to be additionally tested, ensuring the effectiveness of their anti-slip functions.

Classification by the surface treatment states: The testing scope covers the steel gratings of the different surface states such as the black pieces (untreated), hot-dip galvanizing, electrogalvanizing, painting and powder spraying. The testing items need to be adjusted according to the treatment processes, such as the galvanized layer testing or paint film property testing.

Classification by the use occasions and load requirements: Including the steel gratings used for the different scenarios such as the industrial platforms, stair treads, trench covers, ship decks and elevated walkways. The testing needs to carry out the differentiated evaluation according to their design load grades (such as the light and heavy duty) and specific industry standards (such as the marine and mining use).

Raw material incoming inspection scope: The testing scope extends to the raw materials before the production, such as the material certificate documents (quality certificates) of the flat steels and square steels, and the re-inspection of the chemical compositions and mechanical properties (tensile strength and yield strength), controlling the product qualities from the sources.

Post-installation on-site testing: The testing scope is not only limited to the ex-factory products, but also includes the steel gratings after the installation and positioning. The installation firmness, matching with the support structures, and whether the abnormal deformations or damages exist need to be checked, ensuring the use safety.

Testing Methods

Dimension measurement method: The measuring tools conforming to the metrology requirements such as the steel tapes, vernier callipers, micrometers, feeler gauges and squares are used for the direct measurement. During the measurement, the effects of the ambient temperatures on the measuring tools need to be noted, and the principle of "multiple measurements taking the average values" is followed to reduce the human errors.

Visual and touch inspection method: Under the good lighting conditions, whether the steel grating surfaces have the obvious defects such as the cracks, burrs, sharp edges, welding slag, spatters and coating missing areas is checked. At the same time, the surface roughness and weld point protrusions are perceived through the touch. This is a basic and effective qualitative testing method.

Load property testing method: According to the standards such as GB/T 14452 or ASTM A796, carried out on the universal material testing machines or dedicated loading platforms. Through the application of the uniformly distributed loads or concentrated loads, the deflections and residual deformation amounts are measured, and the ultimate load-bearing capacity testing is carried out, to verify whether their mechanical properties reach the standards.

Non-destructive testing method: For the important welded structures or the steel gratings suspected of having the internal defects, the penetrant testing (PT) can be used to check the surface open defects, or the ultrasonic flaw detection (UT) can be used to detect the defects such as the possible lack of fusion and blowholes inside the welds.

Chemical and metallographic analysis method: The spectrometers are used for the chemical composition analysis of the raw materials, verifying whether they meet the designation requirements (such as Q235B and SS400). When necessary, the samples can be cut to make the metallographic specimens, and their micro-structures are observed under the microscopes, judging whether the heat treatment processes are appropriate and whether the phenomena such as the overburning and decarburization exist.

Coating property testing method: The magnetic method or eddy current method is used to measure the galvanized layer thicknesses; the copper sulfate immersion testing is carried out to evaluate the coating uniformity; the cross-cut testing, bending testing or hammer testing is carried out to verify the adhesion of the galvanized layers or coatings; the neutral salt spray testing (NSS) is carried out to simulate the accelerated corrosion, evaluating their long-term corrosion resistance.

Testing Standards

Domestic Standards:

1. YB/T 4001.1, Steel grating — Part 1: Steel grating

2. GB/T 700, Carbon structural steels

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

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

5. GB/T 1839, Test methods for mass of galvanized coating on steel wire and steel wire products

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

3. ASTM A796, Standard practice for structural design of corrugated steel pipe, pipe-arches, and arches for storm and sanitary sewers and other buried applications

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

Geometric quantity measuring instruments: Including the high-precision electronic digital vernier callipers (0-300mm), outer diameter micrometers, calibrated steel tapes, laser distance meters, platforms and dial indicators (used to measure the flatness) and feeler gauge sets, etc. These are the basic equipment for completing the dimension and geometric tolerance testing.

Mechanical property testing equipment: The core equipment is the microcomputer-controlled electronic universal material testing machines, used for the tensile testing of the flat steels and load bending testing of the steel gratings. The supporting equipment includes the deflection testers, residual deformation measuring instruments, as well as the pendulum impact testing machines (when necessary) for the impact testing.

Coating testing instruments: The magnetic thickness gauges or eddy current thickness gauges are used for the non-destructive measurement of the metal coating or non-conductive coating thicknesses; the cross-cut testers and tapes are used for the adhesion testing; the salt spray test chambers are used to simulate the accelerated corrosion environments and evaluate the protection lives; the gloss meters can be used to quantify the coating appearances.

Material analysis instruments: The direct-reading spectrometers (OES) can quickly and accurately analyse the chemical compositions of the metal materials; the metallographic microscopes equipped with the image analysis systems are used to observe and rate the micro-structures, grain sizes and coating cross-section morphologies of the materials.

Welding quality testing tools: Including the welding inspection rulers (can measure the weld leg sizes and misalignment amounts, etc.), magnifying glasses (used to observe the weld surface qualities), as well as the penetrant testing kits (including the cleaners, penetrants and developers), used to find the surface open defects.

On-site and auxiliary equipment: The high-brightness lighting lamps, endoscopes (used to check the closed or difficult-to-reach areas), hardness testers (such as the Leeb or Brinell hardness testers, used for the material hardness sampling inspection), as well as the environmental temperature and humidity recorders, ensuring that the testing is carried out under the standard environments and the data are traceable.