Cold-rolled Steel Pipe Testing

Cold-rolled steel pipe inspection mainly conducts scientific testing and analysis on the dimension and shape inspection, surface quality inspection, chemical composition analysis, mechanical property testing, metallographic structure examination and non-destructive 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: Detects the outer diameters, inner diameters, wall thicknesses, lengths, and shape dimensions such as the ovality and straightness of the steel pipe. These are the basic geometric parameters, directly affecting the assembly performance and use functions of the steel pipe. The high-precision measuring tools need to be used, ensuring that they meet the tolerance requirements of the drawings or relevant standards (such as GB/T 3639 and ASTM A269).

Surface quality inspection: Through the visual inspection or with the aid of the tools such as the low-power magnifying glasses and endoscopes, it is checked whether there are the defects such as the cracks, folds, scars, scratches, pits, pockmarks and rust on the inner and outer surfaces of the steel pipe. The surface quality is related to the corrosion resistance, fatigue strength and appearance grades of the steel pipe, and is the important intuitive indicator for judging whether it is qualified.

Chemical composition analysis: The equipment such as the spectrometers and carbon-sulphur analysers are used for the quantitative analysis of the chemical compositions of the steel pipe, determining the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulphur (S), chromium (Cr) and nickel (Ni). The chemical composition is the foundation determining the mechanical properties and corrosion resistance of the steel pipe, and must meet the standard provisions of the corresponding steel grades.

Mechanical property testing: This is the key to evaluating the load-bearing capacity of the steel pipe, mainly including the tensile testing (determining the tensile strength, yield strength and elongation after fracture), hardness testing (Brinell, Rockwell or Vickers hardness), as well as the process property tests such as the flattening, flaring and bending, to verify whether it meets the strength, plasticity and toughness requirements specified in the standards.

Metallographic structure examination: Through the preparation of the specimens, the microstructures of the steel pipe (such as the grain sizes, banded structures, decarburized layer depths and non-metallic inclusion grades) are observed under the optical metallographic microscopes or scanning electron microscopes. The metallographic structures are the bridge connecting the chemical compositions and mechanical properties, and can effectively reveal the internal quality of the material and whether the heat treatment processes are appropriate.

Non-destructive testing: Detects the internal and surface defects without damaging the steel pipe. The common methods include the eddy current testing (applicable to the fast screening of the surface and near-surface defects), ultrasonic testing (sensitive to the internal cracks and delaminations) and magnetic flux leakage testing. For the pipes with the high requirements for the fluid conveying or pressure-bearing, the non-destructive testing is the mandatory quality assurance link.

Testing Scope

Steel pipes classified by use: The testing scope covers the cold-rolled precision steel pipes for the mechanical structures (such as GB/T 3639), precision inner diameter steel pipes for the hydraulic and pneumatic cylinder tubes, tubes for the heat exchangers, tubes for the bearings, automobile shock absorber tubes and tubes for the fluid conveying. For the steel pipes of the different uses, the testing items and acceptance criteria have the significant differences, and need to be implemented in a targeted manner.

Steel pipes classified by material: Including the cold-rolled steel pipes made of the high-quality carbon structural steels (such as 10#, 20# and 45#), alloy structural steels (such as 20Cr and 40Cr) and stainless steels (such as 304 and 316L). The testing emphases of the different materials are different. For example, the stainless steels additionally need the intergranular corrosion testing, and the alloy steels need to focus on the hardenability.

Steel pipes classified by dimension specifications: Covering the various types of cold-rolled steel pipes from the small diameters (such as the capillary tubes with the outer diameters <10 mm) to the large diameters (outer diameters >200 mm), and from the thin walls (wall thickness to outer diameter ratio ≤1/20) to the thick walls. The testing methods of the different specifications need to be adapted. For example, the dimension measurement of the thin-wall pipes needs to be more careful to prevent the deformation.

Steel pipes classified by the production process stages: The testing runs through the incoming inspection of the raw materials (tube blanks), on-line inspection during the cold rolling (such as the diameter measurement and eddy current flaw detection), final inspection before the ex-factory of the finished products and the incoming re-inspection of the users. The whole-process quality control is the necessary means of ensuring the qualification of the final products.

Steel pipes classified by the special requirements: For the special steel pipes used in the high-temperature, low-temperature, high-pressure, corrosive or clean environments, the testing scope needs to be extended to the specific items, such as the high-temperature creep testing, low-temperature impact testing, hydrostatic testing to the burst, and cleanliness analysis (particle detection).

Testing Methods

Dimension measurement method: The outer diameters and wall thicknesses are commonly measured on-line with the digital display outer diameter micrometers and laser diameter measuring instruments; the inner diameters are measured with the inner diameter dial indicators or pneumatic gauges; the lengths are measured with the laser length measuring instruments or large callipers; the ovality and straightness are measured through the V-blocks combined with the dial indicators or dedicated straightness measuring instruments. The measurement needs to be carried out at the standard temperature, and the sufficient numbers of cross-sections and positions should be selected.

Surface inspection method: Mainly based on the visual inspection under the natural light or standard illumination, and the 5-10 times magnifying glasses are used to assist the inspection of the suspicious areas. The inner surface inspection can be carried out with the aid of the endoscopes with the light sources. For the steel pipes with the high standard requirements, the pickling (such as GB/T 226) or blue dot method can be used to inspect the oxide film integrity of the stainless steel, or the surface roughness testers are used for the quantitative evaluation.

Chemical composition analysis method: The photoelectric direct-reading spectrometry (OES) is mainly adopted for the fast multi-element simultaneous analysis; the carbon and sulphur contents are precisely determined with the high-frequency infrared carbon-sulphur analysers; for the arbitration or higher precision requirements, the inductively coupled plasma atomic emission spectrometry (ICP-OES) or X-ray fluorescence spectrometry (XRF) can be applied. The sampling needs to be representative, usually drilling the chip samples from the pipe ends or specimens.

Mechanical property testing method: The tensile testing is carried out on a universal material testing machine according to GB/T 228.1 (ISO 6892-1), and the specimens can be taken from the longitudinal or transverse directions of the steel pipe and machined into the standard proportional specimens. The hardness testing is carried out according to GB/T 230.1 (Rockwell) or GB/T 231.1 (Brinell). The process tests such as the flattening, flaring and bending are carried out under the specific tooling according to the standards such as GB/T 246, and the absence of the cracks is qualified.

Metallographic examination method: According to the provisions of GB/T 13298, the specimens are cut, and the metallographic specimens are prepared after the mounting, grinding, polishing and etching (such as using the 4% nitric acid alcohol solution). Under the microscope, the grain sizes are rated according to GB/T 6394, the non-metallic inclusions are rated according to GB/T 10561, the decarburized layer depths are determined according to GB/T 224, and whether the structure morphologies are normal is observed.

Non-destructive testing method: The eddy current testing (ET) is carried out according to GB/T 7735, using the electromagnetic induction principle to detect the surface and near-surface defects; the ultrasonic testing (UT) is carried out according to GB/T 5777, using the high-frequency sound wave reflections to detect the internal defects; the magnetic flux leakage testing (MFL) is sensitive to the longitudinal defects. The method selection depends on the defect types, detection speeds and standard requirements.

Testing Standards

Domestic Standards:

1. GB/T 3639, Cold drawn or cold rolled precision steel tubes

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

3. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

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

5. GB/T 246, Metallic materials — Tube — Flattening test

6. GB/T 224, Determination of decarburized layer depth of steel

7. GB/T 226, Etched structures and defects of steel — Method of acid etch examination

8. GB/T 13298, Inspection methods of microstructure for metals

9. GB/T 6394-2017, Determination of estimating the average grain size of metal

10. GB/T 10561-2023, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

11. GB/T 7735, Steel tubes — Automated eddy current testing for the detection of imperfections

12. 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

3. ASTM A269, Standard specification for seamless and welded austenitic stainless steel tubing for general service

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

Dimension measuring equipment: The core equipment includes the high-precision digital display outer diameter micrometers (resolution 0.001 mm), wall thickness micrometers, inner diameter dial gauges, large vernier callipers and laser scanning diameter measuring instruments (for the on-line high-speed measurement). For the batch testing, the dedicated gauges such as the go/no-go gauges and ring gauges can be used for the fast judgement. The coordinate measuring machines (CMM) are used for the precision measurement of the complex dimensions and geometric tolerances.

Mechanical property testing equipment: The core is the microcomputer-controlled electronic universal testing machine, used for the tensile, compression and bending tests, equipped with the extensometers for the precise deformation measurement. In addition, the Brinell hardness testers, Rockwell hardness testers or Vickers hardness testers are needed. The flattening testing needs the presses with the dedicated compression plates, and the flaring testing needs the taper drift pins and pressure devices.

Chemical composition analysis equipment: The direct-reading spectrometer is the mainstream equipment of the production sites and laboratories, with the fast analysis speed and high precision. The infrared carbon-sulphur analysers are specially used for the precise determination of the carbon and sulphur contents. For the trace elements or the analysis of the wider element ranges, the inductively coupled plasma atomic emission spectrometers (ICP-OES) are used. The supporting sampling equipment includes the lathes, drilling machines and sample grinding machines.

Metallographic analysis equipment: Including the sample preparation equipment such as the metallographic cutting machines, mounting machines, pre-grinding machines and polishing machines. The observation core is the optical metallographic microscopes, equipped with the digital camera systems and image analysis software for the photographing, storage and analysis of the structure images. For the higher-resolution micro analysis, the scanning electron microscopes (SEM) and energy dispersive spectrometers (EDS) need to be used.

Non-destructive testing equipment: The eddy current testing uses the through-type or point-type probe eddy current flaw detectors, often equipped with the automatic feeding and sorting devices. The ultrasonic flaw detectors (digital type) are used with the probes of the different frequencies and angles (such as the straight-beam probes and angle-beam probes). The magnetic flux leakage testing lines integrate the magnetization, detection, demagnetization and signal processing units, suitable for the automated testing of the large batches of steel pipes.

Other auxiliary equipment: Including the high-pressure testing pump systems for the hydrostatic testing, particle collection and counting devices for the cleanliness analysis, salt spray test chambers or electrochemical workstations for the corrosion testing, as well as the low-temperature tanks and impact testing machines for the low-temperature impact testing. These equipment provide the necessary means for meeting the specific testing requirements.