Carbon Steel Pipe Testing

Carbon steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, dimension and shape inspection, surface quality examination, 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

Chemical composition analysis: Through the spectrometry or wet analysis, the contents of the key elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S) are determined. This is the basis for verifying the conformity of the steel pipe material designations (such as Q235, 20# and 45#), ensuring that the material has the expected mechanical properties and process properties, such as the weldability and cold workability.

Mechanical property testing: Mainly tests the strength, plasticity and toughness indicators of the steel pipe, including the tensile strength, yield strength, elongation after fracture and reduction of area. These data directly reflect the load-bearing capacity and deformation ability of the steel pipe under the loads, and are the key basis for evaluating whether it can meet the structural design or pressure-bearing requirements.

Dimension and shape inspection: The tools such as the callipers, micrometers, thickness gauges and ring gauges are used to precisely measure the outer diameters, wall thicknesses, lengths, ovality and straightness of the steel pipe. The dimension precision directly affects the assembly, connection quality and subsequent processing of the pipeline, and is the basic link of the production quality control.

Surface quality examination: Through the visual inspection or with the aid of the equipment such as the magnifying glasses and endoscopes, whether there are the defects such as the cracks, folds, scars, scratches and rust on the inner and outer surfaces of the steel pipe is checked. The surface defects are often the stress concentration points, which will significantly reduce the fatigue life and corrosion resistance of the steel pipe, and even cause the early failures.

Non-destructive testing: Under the premise of not damaging the steel pipe, the methods such as the ultrasonic, radiographic, eddy current, magnetic particle or penetrant are used to detect the discontinuity defects inside and near the surfaces. This testing is crucial for the high-pressure, high-temperature or key pressure-bearing pipes, and can effectively find the hidden defects such as the inclusions, blowholes, delaminations and incomplete penetration.

Process property testing: The specific tests are carried out according to the uses of the steel pipe, such as the flattening testing, flaring testing, bending testing and flanging testing. These tests simulate the plastic deformation processes that the steel pipe may experience in the processing or use, evaluating its ability to resist the cracking and maintain the integrity.

Metallographic structure examination: Through the specimen preparation, polishing and etching, the microstructures of the steel pipe, such as the morphologies, sizes and distributions of the ferrite, pearlite and bainite, are observed under the microscope. The metallographic structures are the bridge connecting the chemical compositions and mechanical properties, and can analyse the heat treatment effects and evaluate the internal quality of the material.

Testing Scope

Classification by manufacturing processes: Covering the two major categories of the seamless steel pipes (hot-rolled and cold-drawn) and welded steel pipes (longitudinally welded and spiral welded). The steel pipes produced by the different processes have the different testing emphases. The seamless pipes focus on the internal metallurgical quality and wall thickness uniformity, while the welded pipes emphasize the weld quality and the properties of the heat-affected zones.

Classification by uses: Including the structural steel pipes (buildings and machinery), fluid conveying pipes (water, gas and oil), boiler and heat exchanger pipes, and oil and natural gas industry pipes (line pipes, casings and drill pipes). The different uses correspond to the different product standards and testing specifications, with the different strictness requirements.

Classification by dimension specifications: From the small-diameter capillary tubes to the ultra-large-diameter thick-wall pipes, all are within the testing scope. The testing methods need to adapt to the different specifications. For example, the ultrasonic testing of the large-diameter pipes needs to adopt the multi-probe or automatic scanning systems, while the small-diameter pipes may adopt the eddy current testing.

Classification by delivery states: The testing objects include the steel pipes of the different heat treatment states such as the hot-rolled, cold-drawn, normalized, annealed and quenched and tempered states. The delivery states determine the final properties of the steel pipe, and the testing items need to be adjusted accordingly. For example, the quenched and tempered pipes need to add the hardness or impact toughness testing.

Raw materials and semi-finished products: The testing scope extends upward to the steelmaking billets (steel ingots and continuous casting billets) and the steel strips/plates for making the pipes. The inspection of the chemical compositions, low-power structures and surface qualities of the raw materials is the effective means of controlling the steel pipe quality from the source.

In-service pipeline testing: For the carbon steel pipelines already in use, the periodic or non-periodic condition monitoring and evaluation are carried out, such as the wall thickness ultrasonic thickness measurement, corrosion condition inspection and non-destructive flaw detection re-inspection, to ensure the operation safety and predict the remaining lives.

Testing Methods and Standards

Chemical composition analysis method: Mainly based on the standards such as the GB/T 223 series and ASTM A751. The common methods include the spark discharge atomic emission spectrometry (OES) for the fast multi-element analysis, the carbon-sulphur analysers for determining the C and S contents, and the inductively coupled plasma atomic emission spectrometry (ICP-OES) for the more precise trace element determination.

Mechanical property testing method: The tensile testing follows GB/T 228.1 (ISO 6892-1 and ASTM A370), carried out on the universal testing machines. The specimens are usually cut from the steel pipe in the transverse or longitudinal directions and machined into the standard proportional or non-proportional specimens, to obtain the accurate strength and plasticity data.

Non-destructive testing method: The ultrasonic testing (UT) is carried out according to GB/T 5777 (ASTM A435/A435M), used to detect the internal defects; the eddy current testing (ET) is carried out according to GB/T 7735, commonly used for the fast screening of the surface and near-surface defects; the radiographic testing (RT) is carried out according to GB/T 3323, providing the intuitive image records of the defects.

Dimension and surface inspection method: The dimension measurement is based on the tolerance requirements specified in the product standards such as GB/T 17395. The surface inspection is usually carried out with the visual inspection under the good lighting conditions. For the steel pipes with the high standard requirements, the magnetic particle testing (MT, according to GB/T 15822) or penetrant testing (PT, according to GB/T 18851) may be adopted to find the surface open defects.

Process property testing method: The flattening testing is carried out according to GB/T 246, flattening the pipe sections to the specified heights and checking whether there are the cracks; the flaring testing is carried out according to GB/T 242, pressing the taper drift pins into the pipe ends for the flaring, evaluating their plastic deformation ability; the bending testing bends the specimens around the bending mandrels of a certain diameter according to the relevant standards, checking whether the outer sides are cracked.

Metallographic examination method: The metallographic specimens are prepared according to the standards such as GB/T 13298. The non-metallic inclusions (rated according to GB/T 10561), grain sizes (rated according to GB/T 6394) and microstructures are observed through the optical microscopes or scanning electron microscopes, providing the basis for the quality analysis and failure diagnosis.

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 5777, Seamless steel pipe and tubing methods for ultrasonic testing

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

4. GB/T 3323, Non-destructive testing of welds — Radiographic testing

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

6. GB/T 242, Metallic materials — Tube — Flaring test

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

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

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

10. GB/T 229-2020, Metallic materials — Charpy pendulum impact 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

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

4. ASTM A751, Standard test methods, practices, and terminology for chemical analysis of steel products

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

Chemical composition analysis equipment: The core equipment includes the direct-reading spectrometers (OES), carbon-sulphur analysers, X-ray fluorescence spectrometers (XRF) and inductively coupled plasma spectrometers (ICP-OES/OES). These equipment realize the fast and precise quantitative analysis from the major elements to the trace elements, and are the necessary instruments of the laboratories.

Mechanical property testing equipment: The electronic universal material testing machine is the main equipment for the tensile, compression and bending tests, equipped with the extensometers for the precise deformation measurement. In addition, the impact testing machines (Charpy pendulum type) are used to determine the toughness (impact energy) of the material under the impact loads, and the Brinell, Rockwell and Vickers hardness testers are used to measure the hardness values in the different ranges.

Non-destructive testing instruments: Including the multi-channel digital ultrasonic flaw detectors, eddy current flaw detectors and X-ray or γ-ray real-time imaging systems. The automated non-destructive testing systems, such as the on-line ultrasonic/eddy current combined flaw detection equipment of the steel pipes, can realize the high-speed and full-coverage testing of the steel pipes in the large-batch production.

Dimension and geometric quantity measuring tools: Covering the outer diameter micrometers, wall thickness ultrasonic thickness gauges, laser diameter measuring instruments, large callipers, π tapes, radius templates and straightness testing platforms. The coordinate measuring machines (CMM) can be used for the high-precision detection of the complex dimensions and geometric tolerances.

Metallographic sample preparation and observation equipment: Mainly including the metallographic specimen cutting machines, mounting machines, grinding and polishing machines and etching devices. The observation core is the optical metallographic microscopes, and for the advanced analysis, the scanning electron microscopes (SEM) equipped with the energy dispersive spectrometers (EDS) need to be used for the micro-area morphology observation and composition analysis.

Process property testing devices: Such as the flattening testing machines, flaring testing machines, bending testing machines (guided bending or free bending) and flanging testing tools. These dedicated devices usually have the simple structures but require the good rigidity, and can strictly apply the loads to the specimens according to the rates and modes specified in the standards.

Macro examination and surface inspection equipment: Including the pickling tanks or electrolytic etching devices for the low-power structure examination (such as the dendrites, porosity and segregation), as well as the endoscopes and industrial video endoscopes for the remote visual inspection of the steel pipe inner surfaces. The good lighting systems (such as the LED cold light sources) are the basis of the surface visual inspection.