Longitudinal Submerged Arc Welded Steel Pipe Testing

Longitudinally welded steel pipe inspection mainly conducts scientific testing and analysis on the dimension and geometric shape inspection, surface quality inspection, weld quality non-destructive testing, chemical composition analysis, mechanical property testing, hydrostatic testing or non-destructive flaw detection substitute 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 geometric shape inspection: Checks the outer diameters, wall thicknesses, lengths, ovality and straightness of the steel pipe. Ensuring that the pipe body dimensions meet the technical requirements of the product standards or order contracts is the basic testing item of evaluating the geometric precision of the steel pipe and guaranteeing the subsequent assembly and construction.

Surface quality inspection: Through the visual inspection or auxiliary tools, whether there are the defects such as the cracks, scars, folds, scratches, press marks and rust on the inner and outer surfaces of the steel pipe is checked. The surface quality directly affects the corrosion resistance, fatigue strength and service life of the steel pipe, and is the mandatory inspection item of the ex-factory inspection.

Weld quality non-destructive testing: The non-destructive testing of the longitudinal welds is carried out, and is the core of guaranteeing the integrity and compactness of the welds. The main methods include the ultrasonic testing, radiographic testing, magnetic particle testing and penetrant testing, to find the various discontinuity defects inside and on the surfaces of the welds.

Chemical composition analysis: Through the methods such as the spectrometry analysis and chemical titration, the contents of the carbon, silicon, manganese, phosphorus, sulphur and alloying elements of the steel pipe base material are determined. The chemical composition is the foundation determining the mechanical properties, welding performance and corrosion resistance of the steel pipe, and must meet the corresponding steel grade standards.

Mechanical property testing: The tests such as the tensile, bending and impact are carried out on the pipe bodies or specimens. The tensile testing determines the yield strength, tensile strength and elongation; the bending testing inspects the plasticity of the welds and base material; the impact testing evaluates the toughness of the material at the low temperatures, ensuring that the steel pipe meets the strength and toughness requirements of the use working conditions.

Hydrostatic testing or non-destructive flaw detection substitute testing: Verifies the pressure-bearing capacity and sealing of the steel pipe at the specific pressures. The hydrostatic testing is the traditional verification method; according to the standard permissions, the non-destructive testing methods such as the combined ultrasonic and eddy current flaw detection can also be adopted as the substitute, to inspect the overall integrity of the whole steel pipe.

Metallographic structure examination: The microstructures of the steel pipe, such as the grain sizes, non-metallic inclusion grades, banded structures and the structure morphologies of the welds and heat-affected zones, are observed through the microscopes. The metallographic analysis is used to evaluate the heat treatment quality, process stability and potential failure risks of the material.

Testing Scope

Oil and natural gas conveying pipes (line pipes): Used for the onshore and submarine long-distance oil and gas transmission pipelines, with the extremely high testing requirements. In addition to the routine items, the special tests such as the crack arrest toughness under the high strength, resistance to the hydrogen induced cracking (HIC) and sulphide stress corrosion cracking (SSCC) are particularly emphasized, and the standards such as API Spec 5L and GB/T 9711 need to be met.

Steel pipes for the structures: Applied to the steel structures such as the buildings, bridges and towers and masts. The testing emphasis is on the dimension precision, mechanical properties (especially the yield strength) and weldability. The standards such as GB/T 13793 and ASTM A500 need to be met, ensuring the load-bearing safety and structural stability.

Steel pipes for the fluid conveying: The pipelines used to convey the non-corrosive fluids such as the water, steam and air. The testing focuses on the dimension tolerances, hydrostatic testing or non-destructive flaw detection substitute testing, and the inner and outer surface quality, and the standards such as GB/T 3091 and ASTM A53 need to be met.

Precision steel pipes for the mechanical structures: Used to make the precision mechanical components such as the hydraulic cylinders and cylinder tubes. With the extremely strict requirements for the dimension precision (especially the inner diameter tolerances and wall thickness uniformity), surface roughness, straightness and mechanical properties, the higher-precision standards such as GB/T 3639 and DIN 2391 are often implemented.

Steel pipes serving at the low temperatures: Used for the occasions such as the liquefied natural gas (LNG) and low-temperature storage tanks. The testing core is the Charpy V-notch impact testing at the design minimum temperatures, ensuring that the material has the sufficient low-temperature impact toughness and preventing the brittle fracture, and the low-temperature impact energy indicators of the relevant standards need to be met.

Corrosion-resistant alloy steel pipes: Applied to the corrosive environments, such as the chemical and offshore platforms. In addition to the routine testing, the specific corrosion property tests such as the intergranular corrosion testing and pitting corrosion testing need to be carried out, and it is verified whether the alloy compositions meet the requirements of the stainless steel or duplex steel in the standards such as ASTM A213 and ASTM A269.

Testing Methods

Ultrasonic testing (UT): Utilizes the principle that the high-frequency sound waves reflect when encountering the defects during the propagation in the material. Mainly used to detect the internal volumetric defects (such as the blowholes and slag inclusions) and planar defects (such as the cracks and lack of fusion) of the welds and base material. The automated ultrasonic testing (AUT) has become the mainstream method of the long-distance pipeline welded pipe testing due to its high efficiency and precision.

Radiographic testing (RT): The X-rays or γ-rays are adopted to penetrate the steel pipe, and the internal structure images are recorded through the films or digital imaging plates. It can intuitively display the shapes, sizes and positions of the defects, with the high detection sensitivity for the volumetric defects, commonly used for the weld quality rating and process appraisal, but the radiation safety protection needs to be noted.

Magnetic particle testing (MT): Detects the surface and near-surface defects of the ferromagnetic material steel pipes. After the magnetization, the magnetic flux leakage at the defects attracts the magnetic particles to form the magnetic indications. This method has the simple equipment, convenient operation and intuitive display, particularly suitable for detecting the surface cracks of the welds and heat-affected zones.

Penetrant testing (PT): The capillary action is used to make the penetrant penetrate into the surface open defects, and the visible indications are formed after the adsorption of the developers. Applicable to all the non-porous metal materials, used to detect the open defects such as the cracks and pinholes on the outer surfaces of the steel pipes (especially the non-ferromagnetic austenitic stainless steel pipes).

Eddy current testing (ET): Based on the electromagnetic induction principle. The probe coils generate the alternating magnetic fields to induce the eddy currents in the steel pipe, and the defects disturb the eddy current distributions, thereby changing the coil impedances. Commonly used for the automated on-line testing, quickly finding the defects such as the surface and near-surface cracks and pits, and can measure the wall thicknesses.

Hydrostatic testing: The steel pipe is filled with water, pressurized to the specified test pressure (usually 1.5 times the nominal pressure or the values specified in the standards), and maintained for a certain time, checking whether there are the leakages or permanent deformations. This is the traditional and effective method of verifying the overall strength and sealing of the steel pipe.

Mechanical property testing method: The tensile testing is carried out on the universal testing machines according to GB/T 228.1; the bending testing bends the specimen around the bending mandrel of a certain diameter to the specified angle according to GB/T 244; the Charpy impact testing is carried out on the impact testing machines according to GB/T 229, and the specimens need to be kept warm at the specified temperatures.

Testing Standards

Domestic Standards:

1. GB/T 9711, Petroleum and natural gas industries — Steel pipe for pipeline transportation systems

2. GB/T 3091, Welded steel pipe for low pressure liquid delivery

3. GB/T 13793, Longitudinal welded steel pipes

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

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

6. GB/T 244, Metallic materials — Tube — Bend test

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

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

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

International Standards:

1. API 5L, Specification for line pipe

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

4. ASTM A53, Standard specification for pipe, steel, black and hot-dipped, zinc-coated, welded and seamless

5. ASTM A500, Standard specification for cold-formed welded and seamless carbon steel structural tubing in rounds and shapes

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

Automated ultrasonic flaw detection system: Consisting of the multi-channel ultrasonic flaw detectors, probe arrays (mostly adopting the focused probes), mechanical transmission devices and computer analysis systems. It can realize the high-speed and full-coverage scanning of the full-length welds and pipe bodies of the steel pipe, automatically mark and record the defects. It is the key equipment of the modern welded pipe production lines.

X-ray real-time imaging system: Including the X-ray machines, digital flat panel detectors, mechanical support mechanisms and image processing software. Compared with the traditional film radiographic testing (RT), this system can realize the real-time display, storage and digital rating of the weld images, with the higher efficiency and no environmental pollution.

Universal material testing machine: Used for the mechanical property tests such as the tensile, bending and compression. The equipment is equipped with the high-precision load transducers and extensometers, controlled by the computers and automatically acquiring the stress-strain curves, precisely calculating the key mechanical parameters such as the yield strength, tensile strength and elongation.

Pendulum impact testing machine: Used to determine the ability of the metal materials to resist the impact damage under the dynamic loads, i.e. the impact absorbed energy. The equipment needs to be equipped with the low-temperature tanks to meet the low-temperature impact testing requirements, ensuring the accurate test temperatures. It is the necessary equipment for evaluating the toughness of the materials such as the pipeline steels.

Direct-reading spectrometer: Used for the fast and accurate chemical composition analysis of the steel pipe. The samples are excited to generate the characteristic spectra, and the spectral line intensities of the various elements are detected through the spectroscopic systems, thereby calculating the element contents. It has the fast analysis speed and high precision, and is the important tool of the furnace-front analysis and finished product re-inspection.

Metallographic microscope and sample preparation equipment: Including the cutting machines, mounting machines, grinding and polishing machines, etching devices and metallographic microscopes with the image analysis software. Used to prepare the cross-section metallographic specimens of the steel pipe, observe and rate its microstructures, grain sizes, non-metallic inclusions and weld micro-structures.

Precision dimension measuring tools: Including the outer diameter micrometers, wall thickness micrometers, ultrasonic thickness gauges, π tapes, large callipers and straightness measuring instruments. Used for the manual or semi-automatic measurement of the geometric dimensions such as the outer diameters, wall thicknesses, lengths, ovality and straightness of the steel pipe, ensuring that they meet the precision requirements.