Square Steel Pipe Testing

Square steel pipe inspection mainly conducts scientific testing and analysis on the geometric dimension and shape deviation testing, chemical composition analysis, mechanical property testing, surface quality and defect testing, internal defect non-destructive testing and process property 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

Geometric dimension and shape deviation testing: Mainly including the measurement of the side lengths, wall thicknesses, diagonal differences, straightness, twist degrees and end face bevel cutting degrees. These are the most basic structural parameters of the square steel pipe, directly related to its assembly precision, load-bearing capacity and structural stability. Through the precise measurement, ensuring that they meet the tolerance ranges specified in the drawings or standards is the prerequisite of the subsequent processing and applications.

Chemical composition analysis: Aims to determine the contents of the chemical elements (such as C, Si, Mn, P, S, Cr and Ni) of the steel used for the square steel pipe. This testing is the fundamental basis for judging whether the material designations meet the standards, evaluating their weldability, corrosion resistance, mechanical properties and heat treatment process adaptability, usually completed with the spectrometry analysis or wet chemical analysis.

Mechanical property testing: This is the core item of evaluating the use properties of the square steel pipe, mainly including the tensile testing (determining the tensile strength, yield strength and elongation after fracture), hardness testing (Brinell, Rockwell or Vickers hardness) and the possible impact testing (determining the low-temperature impact toughness). These data directly reflect the ability of the material to resist the deformation and fracture.

Surface quality and defect testing: Checks whether there are the defects such as the cracks, scars, folds, scratches, rust and pockmarks on the inner and outer surfaces of the square steel pipe. The serious surface defects will form the stress concentration points, significantly reducing the fatigue life and load-bearing capacity of the steel pipe, and even causing the early failures. The inspection methods include the visual inspection, endoscopes and non-destructive testing technologies.

Internal defect non-destructive testing: Used to detect the macro defects inside or near the surfaces of the square steel pipe, such as the delaminations, inclusions, blowholes and internal cracks. The common methods include the ultrasonic testing (UT), utilizing the propagation characteristics of the high-frequency sound waves in the material to find the defects; as well as the eddy current testing (ET), suitable for the fast screening of the surface and near-surface defects of the conductive materials.

Process property testing: According to the use requirements, the specific process property tests may need to be carried out on some square steel pipes, such as the flattening testing (inspecting their ability to bear the plastic deformations of the specified degrees), flaring testing (inspecting the plastic deformation ability of the pipe end expansion) or bending testing, to evaluate their processing and forming adaptability.

Testing Scope

Classification by production methods: The testing scope covers the square steel pipes produced by the different processes, mainly including the hot-rolled seamless square pipes, cold-drawn (rolled) seamless square pipes and welded square pipes (such as the high-frequency welded and submerged arc welded square pipes). The products of the different processes have the differences in the residual stresses, dimension precision and mechanical property distributions, and the testing emphases and methods need to be adjusted accordingly.

Classification by materials and designations: The testing is applicable to the square steel pipes made of the different materials such as the carbon structural steels (such as Q235), low-alloy high-strength structural steels (such as Q355), alloy structural steels and stainless steels (such as 304 and 316). The testing standards, chemical composition limits and mechanical property indicators of the different materials are different.

Classification by dimension specifications: The testing covers from the thin-wall small-diameter square pipes for the small precision instruments to the large-diameter thick-wall square pipes for the large building structures and heavy machinery. For the square steel pipes of the different specifications, the parameter thresholds of the testing items, sampling positions and testing equipment (such as the probe sizes and frequencies) all need to be selected according to the standard provisions.

Classification by application fields: The testing needs to formulate the corresponding plans for the square steel pipes of the different final uses, such as the building steel structures, machinery manufacturing, vehicle frames, bridge construction, power transmission towers and curtain wall supports. The different fields have the specific requirements for the strength, toughness, weather resistance and fatigue properties of the steel pipes.

Classification by quality grades and standards: The testing needs to be carried out according to the specific product standards or purchase technical agreements, such as the national standards (GB/T), international standards (ISO), American Society for Testing and Materials standards (ASTM) and European standards (EN). The different standards may have the differences in the provisions of the testing items, methods, frequencies and acceptance criteria.

Classification by delivery states: The square steel pipes may be delivered in the different heat treatment states such as the hot-rolled, cold-drawn, annealed, normalized or quenched and tempered states. During the testing, their delivery states must be considered, because the heat treatment processes will significantly change the microstructures and mechanical properties of the material.

Testing Methods

Dimension measurement method: The high-precision callipers, micrometers, wall thickness gauges and steel tape measures are used to directly measure the side lengths and wall thicknesses; the diagonal rulers or combined measuring tools are used to measure the diagonal differences; the square steel pipe is placed on the platform and the feeler gauges and right-angle rulers are used to detect the straightness and end face bevel cutting degrees; for the large-batch fast testing, the on-line automatic dimension measuring systems can be adopted.

Chemical composition analysis method: The spark discharge atomic emission spectrometry (OES) is mainly adopted, which can carry out the fast and multi-element simultaneous analysis, suitable for the fast judgement of the furnace-front and finished products; the inductively coupled plasma atomic emission spectrometry (ICP-OES) has the higher precision; for the arbitration or high-precision requirements, the classic carbon-sulphur analysers and spectrophotometers and other wet chemical analysis are adopted.

Mechanical property testing method: The tensile testing is carried out according to the standards such as GB/T 228.1, loading the standard specimens on the universal material testing machines, recording the load-displacement curves and calculating the various strength and plasticity indicators; the hardness testing selects the suitable hardness testers and scales (HBW, HRB and HV) according to the specimen sizes and materials, carrying out the multi-point measurements on the flat surfaces and taking the average values.

Surface and near-surface defect testing method: The visual inspection is the basis, supplemented with the lighting and magnifying glasses; the eddy current testing (ET) utilizes the electromagnetic induction principle, very sensitive to the surface and near-surface defects such as the cracks and folds of the conductive materials, can realize the high-speed automated testing; the magnetic particle testing (MT) is applicable to the ferromagnetic materials, and can detect and display the magnetic indications of the surface and near-surface defects.

Internal defect non-destructive testing method: The ultrasonic testing (UT) is the core method, adopting the pulse reflection method. The probes transmit and receive the ultrasonic waves, and the positions and equivalent sizes of the internal defects (such as the delaminations and inclusions) are judged according to the positions, amplitudes and waveforms of the echo signals. This method has the strong penetration and high sensitivity, particularly suitable for the thick-wall pipe testing.

Process property testing method: The flattening testing places the pipe section specimens between two parallel compression plates, pressing to the specified heights or until the specimen rupture, checking whether there are the cracks; the flaring testing uses the taper drift pins of the specified tapers to expand the pipe ends to the specified outer diameter flaring rates, observing whether there is the cracking; the bending testing bends the specimens around the specified bending mandrels to the required angles, checking the outer surfaces.

Testing Standards

Domestic Standards:

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

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

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

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

5. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: 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

Geometric dimension measuring equipment: Including the vernier callipers, outer diameter micrometers, wall thickness ultrasonic thickness gauges, steel tape measures, right-angle rulers, platforms with the feeler gauge sets and diagonal measuring instruments. For the high-precision or automated production lines, the laser scanners, vision measuring systems or on-line real-time diameter and thickness measuring instruments are often equipped, realizing the non-contact and high-efficiency measurement and data recording.

Chemical composition analysis instruments: The desktop or mobile direct-reading spectrometers are the main equipment of the fast analysis on site and in the laboratories; the inductively coupled plasma atomic emission spectrometers (ICP-OES) are used for the higher-precision and wider element range analysis; the carbon-sulphur analysers are specially used for the precise determination of the carbon and sulphur contents; in addition, the supporting equipment such as the spectrophotometers, balances and high-temperature furnaces are included.

Mechanical property testing equipment: The electronic universal material testing machine is the core of the tensile, compression and bending tests, equipped with the high-precision load transducers and extensometers; the pendulum impact testing machines are used to determine the impact absorbed energies of the Charpy V-type or U-type notch specimens; the various hardness testers (Brinell, Rockwell, Vickers and Leeb) are used for the hardness testing in the different occasions.

Non-destructive testing equipment: The ultrasonic flaw detectors (digital type) used with the probes of the various angles (straight-beam probes and angle-beam probes) are the key to the internal defect testing; the eddy current flaw detectors with the through-type and rotary point-type probes are used for the high-speed testing of the surface and near-surface defects; the magnetic particle flaw detectors (including the fluorescent magnetic particle equipment) are used for the surface defect testing of the ferromagnetic materials. The X-ray real-time imaging systems can also be used for the internal quality inspection of the important structures.

Macro and micro examination equipment: The stereomicroscopes and metallographic microscopes are used to observe the surface defect morphologies, fracture characteristics and microstructures; the sampling equipment (such as the cutting machines, mounting machines and grinding and polishing machines) is used to prepare the metallographic specimens; the etching solutions are used to display the grain boundaries and phase compositions of the metal, evaluating the non-metallic inclusions, grain sizes and decarburized layer depths.

Environmental and auxiliary equipment: The high and low temperature environmental test chambers are used to simulate the property tests of the material at the different temperatures, especially the impact testing; the coating thickness gauges are used to detect the galvanized layer and coating thicknesses on the square steel pipe surfaces; the salt spray test chambers are used to evaluate their corrosion resistance. In addition, the specimen processing lathes, standard samples and data acquisition and processing systems are included.