Cold-drawn Special-shaped Steel Pipe Testing

Cold-drawn special-shaped steel pipe inspection mainly conducts scientific testing and analysis on the geometric dimension and shape inspection, surface quality inspection, mechanical property testing, chemical composition analysis and metallographic structure analysis, 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 inspection: This is the basis of the quality control of the cold-drawn special-shaped steel pipe, mainly detecting the outer diameters, wall thicknesses, side lengths, angles, corner radii and cross-section shape deviations of the pipe materials. For the complex cross-sections (such as the square, rectangular, elliptical and triangular), the dedicated measuring tools or coordinate measuring machines need to be used for the multi-point precise measurements, ensuring that they meet the tolerance ranges specified in the drawings or standards.

Surface quality inspection: Including the comprehensive inspection of the inner and outer surface defects of the steel pipe. The testing items cover the cracks, folds, scars, pockmarks, scratches, pits and residual oxide scales. Usually, the visual inspection (VT) is carried out with the endoscopes, and the depths, lengths and distributions of the defects are rated according to the relevant standards (such as GB/T 5777), to judge their effects on the use safety.

Mechanical property testing: Used to evaluate the ability of the steel pipe to bear the loads and deformations, and is the key to judging whether it meets the use requirements. The core testing items include the tensile strength, yield strength, elongation after fracture and reduction of area. The stress-strain curves of the material are obtained through the tensile testing, providing the data support for the structural design and safety evaluation.

Chemical composition analysis: Aims to verify whether the material compositions of the steel pipe meet the designation provisions. Through the means such as the spectrometers and carbon-sulphur analysers, the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulphur (S), as well as the alloying elements such as the chromium (Cr), nickel (Ni) and molybdenum (Mo), are precisely determined. The composition deviations directly affect the machinability, welding performance and final mechanical properties of the steel pipe.

Metallographic structure analysis: Through the sampling, mounting, grinding, polishing, etching and microscope observation, the microstructures of the steel pipe are analysed, such as the grain sizes, non-metallic inclusion grades, banded structures and decarburized layer depths. The metallographic structures directly reflect the advantages and disadvantages of the cold-drawing process and the heat treatment states, and are the important basis for evaluating the internal quality and property uniformity of the material.

Testing Scope

Classification by cross-section shapes: The testing scope covers all the non-circular cross-section cold-drawn steel pipes, mainly including the square tubes, rectangular tubes, elliptical tubes, flat elliptical tubes, D-shaped tubes, hexagonal tubes, triangular tubes, sector tubes and various complex special-shaped cross-section tubes. The testing emphases of the different cross-sections are different. For example, the rectangular tubes need to emphatically test the diagonal differences and straightness.

Classification by material designations: Covering the special-shaped tubes made of the various materials such as the carbon structural steels (such as 20# and 45#), alloy structural steels, stainless steels (such as 304 and 316) and special alloy steels. The testing standards, method emphases and judgement indicators of the different materials are different, and need to be implemented according to the corresponding material standards.

Classification by product states: Including the steel pipes of the different heat treatment states such as the cold-drawn states (hard state and semi-hard state), annealed state and normalized state. The states are different, and their mechanical properties, residual stresses and microstructures are quite different. During the testing, the product delivery states need to be clearly identified, and the targeted testing of the items such as the hardness and bending needs to be carried out.

Classification by application fields: The testing needs to determine the strictness grades according to the final uses of the steel pipes. For example, the special-shaped tubes used for the mechanical structural parts (such as the guide rails and frames) focus on the dimension precision and strength; those used for the fluid conveying focus on the air tightness and pressure resistance; those used for the decoration pay more attention to the surface finish and appearance.

By production batches and specifications: The testing needs to cover all the specification products of the same melting furnace number and the same heat treatment batch. For the newly trial-produced specifications, small-batch multi-specification products or the products with the special technical agreements of the customers, the tightened or full-item testing needs to be implemented, ensuring the quality consistency.

Testing Methods

Dimension measurement method: For the routine dimensions, the traditional measuring tools such as the vernier callipers, micrometers, R gauges and universal angle rulers are used for the contact measurement. For the high-precision or complex profile special-shaped tubes, the optical projectors, image measuring instruments or coordinate measuring machines (CMM) are adopted for the non-contact scanning, obtaining the three-dimensional data and comparing with the CAD models, with the precision reaching the micron level.

Non-destructive testing method: Under the premise of not damaging the workpieces, the internal and surface defects are detected. The common methods include the ultrasonic flaw detection (UT, for detecting the internal cracks and inclusions), eddy current flaw detection (ET, for the fast screening of the surface and near-surface defects) and liquid penetrant flaw detection (PT, for the display of the open surface defects). The magnetic particle flaw detection (MT) is mainly used for the surface testing of the ferromagnetic materials.

Physical and chemical testing method: The laboratory analysis is carried out through the destructive sampling. The mechanical properties are tested with the universal material testing machines for the tensile, bending or flattening; the chemical compositions are quickly qualitatively and quantitatively analysed with the direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF); the metallographic structures need to be observed and rated with the optical or electron microscopes after the sample preparation in the laboratories.

Surface inspection method: Mainly based on the visual inspection, supplemented with the 10 times magnifying glasses when necessary. The inner surface inspection needs to use the pipeline endoscopes. For the precision special-shaped tubes with the surface roughness requirements, the surface roughness testers need to be used for the multi-point measurements at the specified parts, obtaining the parameter values such as the Ra and Rz.

Pressure and air tightness testing method: For the special-shaped tubes with the pressure-bearing requirements, the hydrostatic testing or pneumatic testing needs to be carried out. One end of the steel pipe is sealed, and the liquid (usually water) or gas is injected from the other end, pressurized to the specified pressure and maintained for a certain time, checking whether there are the leakages or permanent deformations, to verify its pressure-bearing capacity and sealing integrity.

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

6. GB/T 5777, Seamless steel pipe and tubing methods for ultrasonic testing

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: The core equipment includes the high-precision digital display vernier callipers (0-1000 mm), outer diameter micrometers, wall thickness gauges, coordinate measuring machines (CMM) and laser scanners. The CMM can carry out the automated and high-precision measurement of the spatial dimensions and geometric tolerances of the special-shaped cross-sections, and is the necessary equipment for the testing of the complex special-shaped tubes.

Mechanical property testing equipment: The core is the microcomputer-controlled electronic universal testing machine, equipped with the high-precision transducers and extensometers, used to complete the tests such as the tensile, compression and bending. The supporting equipment includes the impact testing machines (for determining the low-temperature impact toughness) and Rockwell/Brinell/Vickers hardness testers (for the measurement with the different hardness scales).

Chemical composition and metallographic analysis equipment: The direct-reading spectrometer is the mainstream equipment for the fast composition analysis. The metallographic laboratories need to be equipped with the metallographic specimen cutting machines, mounting machines, grinding and polishing machines, etching devices, as well as the metallographic microscopes or scanning electron microscopes (SEM) with the image analysis software, used for the micro-structure observation and quantitative analysis.

Non-destructive testing equipment: Including the multi-channel digital ultrasonic flaw detectors, through-type or point-type eddy current flaw detectors, magnetic particle flaw detectors with the ultraviolet lamps, and penetrant flaw detection reagent kits. The modern ultrasonic flaw detectors are often equipped with the automatic scanning devices and imaging systems, which can intuitively display the defect positions and morphologies.

Dedicated testing and auxiliary equipment: Including the pipeline endoscopes (for checking the inner walls), surface roughness measuring instruments, Leeb hardness testers (for the fast on-site hardness testing), hydrostatic testing pump stations and air tightness testing devices. In addition, the standard mechanical property specimen processing equipment such as the wire cutting machines and lathes also needs to be equipped, ensuring that the specimens meet the standard requirements.