C-section Steel Testing

C-section steel inspection mainly conducts scientific testing and analysis on the dimension and shape inspection, mechanical property testing, chemical composition analysis, surface quality inspection and coating performance 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

Dimension and shape inspection: This testing aims to verify whether the cross-section dimensions of the C-section steel meet the standard or drawing requirements, and is the basis for ensuring the interchangeability of the components and the installation precision. It mainly includes the precise measurement of the key dimensions such as the height, width, flange width and thickness, as well as the evaluation of the shape deviations such as the bending and flatness. Any out-of-tolerance may affect the structural stability or the subsequent installation.

Mechanical property testing: This is the key link for evaluating the load-bearing capacity of the C-section steel. The core indicators such as the yield strength, tensile strength and elongation after fracture are mainly determined through the tensile testing, to judge the deformation and fracture behaviour of the material under the load. These data are the fundamental basis for the strength and stiffness calculations in the structural design.

Chemical composition analysis: Through the spectrometry analysis or chemical titration methods, the specific contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S) contained in the C-section steel are precisely determined. The chemical composition directly affects the strength, toughness, weldability and corrosion resistance of the steel, and is the primary basis for judging whether the material designation is qualified.

Surface quality inspection: The surface of the C-section steel is comprehensively inspected through the visual or instrumental methods, identifying whether there are the defects such as the cracks, scars, folds, pitting, corrosion and excessively thick oxide scales. The serious surface defects will weaken the effective cross-section, become the stress concentration points and crack sources, and significantly reduce the fatigue life and durability of the component.

Coating performance testing: For the galvanized, galvalume-coated or painted C-section steel, the coating quality needs to be quantitatively evaluated. The key items include the coating thickness (measured with a magnetic or eddy current thickness gauge), coating adhesion (cross-cut method or bending testing), and the salt spray corrosion resistance. The excellent coating is the key guarantee for extending the service life of the steel structure.

Testing Scope

Classification by the material designations: The testing covers the C-section steels made of the carbon structural steels and low-alloy high-strength steels commonly used in the building structures, such as the Q235 and Q355. The different designations correspond to the different chemical composition and mechanical property requirements, and the testing needs to be strictly carried out according to the specified indicators in the corresponding national standards (such as GB/T 6725).

Classification by the cross-section specifications: The testing scope includes the C-section steels of the various common cross-section dimensions, such as the C80, C100, C120, C140 and C160 series. For the different specifications, the allowable dimension deviations and the testing reference values of the cross-section characteristic parameters (such as the moment of inertia and section modulus) also change accordingly.

Classification by the production processes: Including the C-section steels produced by the two main processes of the cold bending forming (the cold-rolled strips or steel plates formed by the roll bending) and the hot rolling direct forming. The cold-bent C-section steels need to emphatically focus on the influence of the cold working hardening on the mechanical properties and residual stresses, while the hot-rolled products focus more on their intrinsic metallurgical quality.

Classification by the application fields: The testing needs to cover the C-section steels of the different application scenarios, such as the building purlins, wall beams, frames of the light steel structure houses, supporting components of the mechanical equipment and shelf beams. The testing emphasis and standard requirements for the C-section steels in the different application fields are different.

Classification by the coating types: For the anti-corrosion C-section steels, the testing scope includes the products with the various anti-corrosion forms such as the hot-dip galvanizing (zinc coating weight or thickness), galvalume (AZ), zinc-aluminium-magnesium (ZM) and the subsequent spray coatings, and the special testing needs to be carried out according to the corresponding coating standards.

Testing Methods and Standards

Dimension measurement method: The vernier callipers, micrometers, tape measures with the precision not lower than 0.02 mm, or the more advanced optical projectors and laser scanners are usually used. The measurement needs to be carried out at the cross-section not less than 500 mm from the ends, and follows the measurement positions and tolerance judgement methods specified in GB/T 6725 "General technical requirements for cold-formed steel sections".

Tensile testing method: Carried out in accordance with GB/T 228.1 "Metallic materials — Tensile testing — Part 1: Method of test at room temperature". The standard tensile specimens are prepared by sampling from the web or flange of the C-section steel, and the testing is carried out on a universal material testing machine, loaded at a constant rate until the fracture, with the automatic recording of the load-displacement curves, and the various mechanical property parameters are calculated.

Chemical composition analysis method: The direct-reading optical emission spectrometry (OES) is mainly adopted for the fast and accurate determination of the multiple elements. The traditional chemical analysis methods (such as the carbon-sulphur analysers and spectrophotometers) can also be adopted for the arbitration or verification. The testing is based on GB/T 4336 "Carbon and low-alloy steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method", etc.

Surface and coating testing method: The surface defects mainly rely on the visual inspection, supplemented by the penetrant testing (PT) when necessary. The coating thickness is measured by the magnetic thickness measurement method (GB/T 4956) or eddy current thickness measurement method (GB/T 4957). The adhesion is commonly rated with the cross-cut method (GB/T 9286), and the corrosion resistance is accelerated and evaluated through the neutral salt spray testing (GB/T 10125).

Bending and process performance testing: To evaluate the cold working performance and toughness of the C-section steel, the cold bending testing (GB/T 232) or repeated bending testing may be required. For the components with the welding requirements, the welding procedure qualification also needs to be carried out, detecting the mechanical properties of the welded joints, to ensure that they meet the structural design requirements.

Testing Standards

Domestic Standards:

1. GB/T 6725, General technical requirements for cold-formed steel sections

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

3. GB/T 4336, Carbon and low-alloy steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method

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

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

6. GB/T 4957, Non-conductive coatings on non-magnetic basis metals — Measurement of coating thickness — Eddy current method

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

8. 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 for carrying out the mechanical property tests such as the tensile, bending and compression of the C-section steel. The modern models are equipped with the high-precision sensors and computer control systems, which can automatically collect and process the data and output the detailed test reports, ensuring the accuracy and reliability of the mechanical property testing.

Direct-reading optical emission spectrometer: The essential instrument for the rapid chemical composition analysis. The characteristic spectra are generated by exciting the sample, received by the photomultiplier tubes or CCD detectors and converted into the electrical signals, and the percentage contents of the various elements are directly displayed after the computer processing. It has the fast analysis speed and high precision, and is suitable for the rapid analysis before the furnace and the finished product inspection.

Precision dimension measuring tools and measurement systems: Including the high-precision digital vernier callipers, outer diameter micrometers, wall thickness gauges, radius gauges and feeler gauges, as well as the automated equipment such as the coordinate measuring machines (CMM) and laser profile scanners. The latter can efficiently obtain the three-dimensional point cloud data of the whole cross-section of the C-section steel, carrying out the comprehensive dimension and geometric tolerance analysis.

Coating thickness gauge and corrosion test chamber: The magnetic/eddy current dual-purpose coating thickness gauge is used for the fast non-destructive measurement of the non-magnetic coating thickness on the C-section steel with the steel substrate on site or in the laboratory. The salt spray corrosion test chamber is used to simulate the marine or industrial atmospheric environments, accelerating the evaluation of the corrosion resistance of the coating or substrate through the continuous spraying. It is the key equipment for evaluating the protection life.

Macroscopic and microscopic inspection equipment: Including the stereomicroscopes and metallographic microscopes, used for the observation and analysis of the surface defects, fracture morphologies and microstructures of the C-section steel. For example, through the metallographic analysis, the grain size, non-metallic inclusion grade and decarburization layer depth of the material can be judged, which are the important basis for evaluating the intrinsic quality of the material.