High-pressure Steel Pipe Testing

High-pressure steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile mechanical property testing, impact toughness testing, hardness testing, macro and micro metallographic examination and non-metallic inclusion rating, 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

Chemical composition analysis: The element quantitative analysis is carried out on the high-pressure steel pipe base material with the spectrometers, ensuring that the contents of the main elements and trace elements such as C, Si, Mn, P, S, Cr, Mo, Ni and V meet the material standards (such as GB/T 9948 and ASTM A335). This item is the basic basis for judging the metallurgical quality, heat resistance, corrosion resistance and welding performance of the material, and is crucial for the subsequent heat treatment processes and service safety.

Tensile mechanical property testing: According to the GB/T 228.1 or ASTM A370 standards, the yield strength, tensile strength, elongation after fracture and reduction of area of the steel pipe are determined on a universal material testing machine. These parameters directly reflect the anti-deformation and anti-fracture abilities of the material under the high-pressure loads, and are one of the core indicators of evaluating the structural integrity and pressure-bearing margins.

Impact toughness testing: The Charpy V-notch impact testing is usually adopted. The impact absorbed energies of the steel pipe (especially the welds and heat-affected zones) at the specified low temperatures (such as -20 ℃ and -46 ℃) are determined on the standard impact testing machines. This indicator is used to evaluate the ability of the material to resist the brittle fracture at the low temperatures or under the dynamic loads, especially important for the pipelines in the cold regions or bearing the alternating loads.

Hardness testing: The Brinell (HBW), Rockwell (HRC/B) or Vickers (HV) hardness testers are used for the multi-point testing of the steel pipe base material, welds and heat-affected zones. The hardness values can indirectly reflect the strength, wear resistance and heat treatment effects of the material, and are also used to evaluate the homogeneity of the welded joints, avoiding the stress concentrations or early failures caused by the locally too hard or too soft parts.

Macro and micro metallographic examination: Through the stereomicroscopes and optical metallographic microscopes, the low-power structures (such as the shrinkage cavities, segregation and cracks) and high-power micro-structures (such as the grain sizes, phase compositions and non-metallic inclusion grades) of the steel pipe are checked. The metallographic analysis is the key means of revealing the internal metallurgical defects of the material, evaluating the heat treatment process quality and diagnosing the failure causes.

Non-metallic inclusion rating: According to the GB/T 10561 or ASTM E45 standards, the types, morphologies, sizes and distributions of the non-metallic inclusions such as the sulphides, oxides and silicates in the steel pipe specimens are rated. The excessively high inclusion contents will seriously reduce the fatigue strength and toughness of the material, and are the potential crack sources in the high-pressure environments.

Testing Scope

Seamless high-pressure steel pipes: Mainly used for the extreme working conditions such as the petrochemical, power station boilers, high-pressure steam pipelines and ultra-supercritical units. The testing needs to cover the full lengths from the pipe bodies to the ends, emphatically focusing on the wall thickness unevenness, internal and external folds, scratches possibly produced after the hot rolling or cold drawing, as well as the internal defects caused by the improper piercing processes.

Welded high-pressure steel pipes: Including the longitudinally submerged arc welded (LSAW) and spiral submerged arc welded (SSAW) steel pipes, commonly used for the long-distance oil and gas transmission pipelines and urban gas pipe networks. The core testing areas are the welds and the heat-affected zones on both sides, and the continuity, compactness of the welded joints and the property matching with the base materials need to be systematically evaluated.

Pipe fittings and connection parts: Covering the high-pressure pipe fittings such as the elbows, tees, reducers and pipe caps, as well as the girth welds installed on site. These areas have the complex geometric shapes and significant stress concentrations, and are the emphases and difficulties of the testing. The comprehensive non-destructive testing and mechanical property sampling inspections need to be carried out.

In-service high-pressure pipelines: The periodic inspections and condition monitoring are carried out on the high-pressure pipelines already in operation. The testing emphasis is on the wall thickness reductions, crack initiations and propagations and material deteriorations caused by the medium corrosion, erosion, high-temperature creep and fatigue loads, evaluating their remaining strengths and lives.

Raw materials and semi-finished products: The testing scope extends to the steel ingots, continuous casting billets or hot-rolled coils used to make the high-pressure steel pipes. Through the chemical compositions, low-power structure and ultrasonic testing of the raw materials, the quality can be controlled from the source, avoiding the defects flowing into the subsequent pipe-making processes and improving the overall qualification rate of the products.

Steel pipes serving in the special environments: Applicable to the steel pipes for the specific working conditions such as the acidic environments (H2S stress corrosion), high-temperature high-pressure hydrogen environments and deep-sea high-pressure environments. In addition to the routine items, the special testing such as the HIC (hydrogen induced cracking), SSC (sulphide stress corrosion) testing and high-temperature creep rupture strength testing also needs to be added.

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. It is the preferred method of detecting the internal volumetric defects (such as the blowholes, slag inclusions and incomplete penetration) and planar defects (such as the cracks and delaminations) of the high-pressure steel pipes. The automated ultrasonic testing (AUT) can be adopted for the full-coverage scanning of the welds, or the manual ultrasonic testing (MUT) can be used for the precise quantification and positioning of the suspicious areas.

Radiographic testing (RT): The X-rays or γ-rays are usually adopted to penetrate the workpieces, and the internal structure images are recorded through the films or digital imaging plates (DR/CR). This method displays the volumetric defects (especially the blowholes and slag inclusions) intuitively with the high sensitivity, commonly used for the quality rating of the welded joints, but the radiation safety protection needs to be noted.

Magnetic particle testing (MT): Applicable to the detection of the surface and near-surface defects of the ferromagnetic material high-pressure steel pipes. After the magnetization of the steel pipe, the magnetic flux leakage at the defects will attract the magnetic particles to form the magnetic indications. This method is extremely sensitive to the linear defects such as the surface cracks, folds and hair lines, with the simple and fast operation, widely applied to the surface testing of the pipe bodies and welds.

Penetrant testing (PT): Used to detect the surface open defects of the non-ferromagnetic high-pressure steel pipes (such as the austenitic stainless steel pipes). The penetrant is applied to the workpiece surfaces, and the capillary action makes it penetrate into the defects. After the cleaning, the developer adsorbs the penetrant in the defects to form the indications. This method has the simple equipment, but can only detect the surface open defects.

Eddy current testing (ET): Utilizes the electromagnetic induction principle to detect the surface and near-surface defects of the conductive material high-pressure steel pipes. The probe coils generate the alternating magnetic fields to induce the eddy currents in the pipe walls, and the defects disturb the eddy currents, leading to the impedance changes of the coils. This method has the fast speed and is easy to automate, commonly used for the on-line large-batch testing, but is insensitive to the deep internal defects.

Macro corrosion (acid etching) examination: After the cross-sections or longitudinal sections of the steel pipe specimens are corroded with the specific acid solutions, the low-power structure defects such as the centre shrinkage cavities, ingot segregation, white spots and cracks are checked under the low magnification. This is a destructive but very intuitive and effective method of inspecting the metallurgical quality of the material.

Testing Standards

Domestic Standards:

1. GB/T 9948, Seamless steel tubes for petroleum cracking

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

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

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

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

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

International Standards:

1. ASTM A335, Standard specification for seamless ferritic alloy-steel pipe for high-temperature service

2. ASTM A370, Standard test methods and definitions for mechanical testing of steel products

3. ASTM E45, Standard test methods for determining the inclusion content of steel

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

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 the mechanical property tests such as the tensile, compression and bending. The modern equipment is equipped with the high-precision transducers and computer control systems, can automatically acquire the stress-strain curves, calculate the various property parameters, and meet the requirements of the various international standards such as GB, ISO and ASTM.

Impact testing machine: The dedicated equipment for determining the impact toughness of the material. Usually of the pendulum type. During the testing, the pendulum converts the stored potential energy into the kinetic energy to break the notched specimens, and the impact absorbed energy is calculated by measuring the energy consumed to break the specimens. The equipment needs to be equipped with the low-temperature tanks to meet the testing requirements at the different temperatures.

Direct-reading spectrometer (OES): The key instrument for the fast quantitative chemical composition analysis. The samples are excited to generate the characteristic spectra, and the element contents are determined according to the spectral line intensities. It has the fast analysis speed and high precision, and can analyse dozens of elements simultaneously. It is the necessary equipment for the furnace-front fast analysis and finished product re-inspection of the iron and steel enterprises.

Automatic ultrasonic flaw detection system: Consisting of the multi-channel ultrasonic flaw detectors, probe arrays, precision mechanical transmission devices and computer analysis software. It can realize the fully automatic, high-speed and full-coverage flaw detection of the steel pipes. The system can display the defect positions and equivalent sizes in real time, and automatically mark, sort and generate the test reports. The detection efficiency and reliability are far higher than the manual testing.

Digital radiographic imaging system (DR/CR): The advanced non-destructive testing equipment replacing the traditional film photography. The DR system directly converts the X-rays into the digital images through the flat panel detectors; the CR system uses the imaging plates (IP plates) to record the latent images and then reads them through the scanners. Both have the advantages of the fast imaging, wide dynamic ranges, digital processing and storage, and environmental friendliness without the waste liquids.

Metallographic microscope system: Including the optical microscopes, image acquisition cards, computers and professional metallographic analysis software. Used to observe and analyse the microstructures of the material. The software can assist the quantitative metallographic analysis such as the automatic grain size rating, phase area percentage calculations and inclusion statistics, making the testing results more objective and accurate.