Light Steel Keel Testing

Light steel keel inspection mainly conducts scientific testing and analysis on the appearance and dimension deviation testing, mechanical property testing, galvanized layer quality testing, coating property testing, straightness and flatness 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

Appearance and dimension deviation testing: The basic dimensions of the light steel keels such as the lengths, widths and heights, as well as the key geometric dimensions such as the cross-section shapes, side heights and thicknesses are measured, ensuring that they meet the design drawings or the provisions of the relevant standards. At the same time, the surface qualities are checked, such as whether the galvanized layers are uniform and whether the defects affecting the use such as the cracks, burrs, rust and indentations exist.

Mechanical property testing: This is the core item of evaluating the load-bearing capability and structural safety of the light steel keels. Mainly including the determination of the indicators such as the tensile strengths, yield strengths and elongations, to verify whether the mechanical properties of the keel materials meet the building load requirements, ensuring that they can stably support in the suspended ceiling or partition wall systems.

Galvanized layer quality testing: The light steel keels generally adopt the hot-dip galvanizing or electro-galvanizing processes for the anti-corrosion treatment. The testing items include the thicknesses of the galvanized layers (zinc layer masses per unit area), adhesion and uniformity. The insufficient thicknesses will lead to the decreased anti-corrosion capability, and the poor adhesion is easy to cause the peeling during the processing or transportation.

Coating property testing (as applicable): For the keels with the color coatings or special anti-corrosion coatings, the coating thicknesses, hardnesses, adhesion, impact resistance and corrosion resistance (such as the salt spray testing) need to be tested. These items are directly related to the durability of the keels and the service lives in the specific environments.

Straightness and flatness testing: Measuring the bending degrees (straightness) of the keels in the length direction, as well as the levelness degrees (flatness) of the planes such as the flanges and webs. The out-of-tolerance straightness or flatness will lead to the installation difficulties and uneven skeletons, seriously affecting the installation quality and final levelness of the subsequent facing plates.

Testing Scope

By cross-section shapes: The testing scope covers the light steel keels of the various common cross-section shapes, mainly including the U-type (load-bearing keels and furring keels), C-type (studs), T-type (aluminum alloy keels or painted keels), L-type (angle beads) and various special-shaped keels. The targeted testing needs to be carried out for their unique functional parts.

By use positions: The testing needs to distinguish the keels for the suspended ceiling systems and the keels for the partition wall systems. For the ceiling keels, their suspension load-bearing capacities and bending resistance properties are mainly tested; for the partition wall keels, their axial compression resistance, impact resistance and the strengths of the connection parts with the walls need to be additionally paid attention to.

By surface treatments: The testing scope covers the keels of the different anti-corrosion processes, such as the hot-dip galvanized keels (Z-type), electro-galvanized keels (D-type), color-coated keels and fireproof-coated keels. For the different treatment methods, the key testing items (such as the coating thicknesses and fire resistance ratings) and standard requirements are different.

By product specifications: The testing needs to cover all the common specification models in the production and use, including the products of the different lengths, widths, thicknesses and unit masses. For the non-standard customized products, the special testing needs to be carried out according to their design technical parameters and contract agreements.

By materials and processes: The testing is not only for the finished keels, but also the traceability testing of the chemical compositions and mechanical properties of their raw materials (cold-rolled steel strips) should be carried out. At the same time, the key process parameters in the production process (such as the rolling precision, galvanizing temperatures and times) are monitored, belonging to the scope of the process quality control.

Testing Methods

Dimension and geometric tolerance measurement method: The measuring tools such as the vernier callipers, micrometers, tapes, feeler gauges and radius gauges are used for the direct measurement. For the straightness, the keels can be placed flat on the testing platforms, and the maximum gaps between them and the platforms are measured with the feeler gauges; the flatness can be inspected using the straight edges or surface plates combined with the feeler gauges.

Mechanical property testing method: According to the GB/T 228.1 standard (Metallic materials — Tensile testing), the tensile testing is carried out on the specimens cut from the keels on the universal material testing machines, obtaining the data such as the tensile strengths, yield strengths and elongations after fracture. The bending resistance testing uses the three-point bending method to determine the bending strengths and deflections of the keels.

Galvanized layer testing method: The galvanized layer thicknesses are usually measured non-destructively with the magnetic thickness gauges, or the zinc layer masses per unit area are calculated on the specimens through the dissolution and weighing method (GB/T 1839). The adhesion testing commonly uses the hammering testing method to observe whether the coatings are peeling or flaking; the uniformity can be qualitatively judged through the copper sulfate solution etching testing.

Chemical composition analysis method: The spectroscopic analysers (such as the direct-reading spectrometers) or chemical titration method are used for the quantitative analysis of the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S) of the keel base materials, ensuring that the raw materials meet the corresponding steel grade standards. This is the basis of guaranteeing the mechanical properties.

Coating property testing method: The coating thicknesses are measured with the eddy current thickness gauges; the adhesion is commonly evaluated with the cross-cut method (GB/T 9286); the impact resistance is tested through the falling weight impact testing; the corrosion resistance is tested through the neutral salt spray testing (GB/T 10125) to simulate the harsh environments, regularly observing the rust and blistering conditions of the coating surfaces.

Testing Standards

Domestic Standards:

1. GB/T 11981, Light gauge steel keels for buildings

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

3. GB/T 1839, Test methods for determination of mass of zinc coating on galvanized iron and steel products

4. GB/T 9286, Paints and varnishes — Cross-cut test

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

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

Universal material testing machine: The core equipment of carrying out the mechanical property testing such as the tensile, bending and compression. It can precisely control the loading rates, record the load-displacement curves in real time, and automatically calculate the various strength indicators and plasticity indicators of the material. It is the necessary instrument of evaluating the load-bearing capability of the keels.

Coating thickness gauge: Including the magnetic thickness gauges and eddy current thickness gauges. The magnetic thickness gauges are used to measure the thicknesses of the non-magnetic coatings (such as the zinc) on the steel matrices; the eddy current thickness gauges are suitable for measuring the insulating coating thicknesses on the non-conductive matrices, or distinguishing the coatings on the non-ferromagnetic metals.

Precision dimension measuring tools: Including the high-precision electronic digital callipers (resolution 0.01mm), outer diameter micrometers, depth gauges, radius gauges, feeler gauges, as well as the large platforms and knife-edge straight edges. These tools together constitute the basic equipment for the comprehensive and accurate measurement of the geometric dimensions and geometric tolerances of the keels.

Salt spray test chamber: The key equipment used to evaluate the corrosion resistance of the keels and their coatings. It can simulate the marine or industrial atmospheric environments. Through the continuous or alternating spraying of a certain concentration of sodium chloride solution, under the set temperature and humidity conditions, the corrosion processes of the samples are accelerated and tested.

Spectroscopic analyser: Usually referring to the direct-reading spectrometers, which can quickly and accurately carry out the qualitative and quantitative analysis of the chemical compositions of the keel metal materials. It plays a crucial role in the incoming inspection of the production raw materials and quality traceability, ensuring that the material compositions meet the standard requirements.