Thin-wall Stainless Steel Pipe Testing

Thin-wall stainless steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, dimension and shape inspection, corrosion resistance testing, surface quality inspection 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

Chemical composition analysis: The chemical composition testing of the base materials of the thin-wall stainless steel pipes is the basis of ensuring their corrosion resistance, weldability and mechanical properties. Mainly according to the standards such as GB/T 20878 or ASTM A240, the spectrometers are used to precisely determine the contents of the key elements such as the carbon (C), chromium (Cr), nickel (Ni) and molybdenum (Mo), to verify whether they meet the designation requirements, such as 304, 304L, 316 and 316L.

Mechanical property testing: Evaluating the performance of the thin-wall stainless steel pipes under the stressed states is the key of judging whether they can meet the service strength and plasticity requirements. The core testing items include 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 testing and flaring testing, ensuring that the pipes do not undergo the brittle fracture or excessive deformation during the installation and use.

Dimension and shape inspection: The precise measurement of the outer diameters, wall thicknesses, ovality, camber, lengths and end face states of the thin-wall stainless steel pipes. The tools such as the micrometers, callipers, wall thickness gauges and laser measuring instruments are usually used, strictly referring to the product standards such as GB/T 18704 or CJ/T 151, ensuring that their dimension tolerances and geometric precision meet the sealing and stability requirements of the pipeline system connection, installation and fluid conveyance.

Corrosion resistance testing: This is the core indicator of measuring the service lives and safety of the thin-wall stainless steel pipes. The commonly used testing methods include the intergranular corrosion testing (such as the sulfuric acid-copper sulfate method), salt spray testing, pitting potential measurement and actual working condition simulated corrosion testing. Through these tests, the corrosion resistance of the material in the specific corrosive media (such as the chloride ion environments) is evaluated, predicting its long-term service performance.

Surface quality inspection: The smoothness of the inner and outer surfaces of the thin-wall stainless steel pipes, and whether the defects such as the scratches, pits, cracks, scars and oxide scales exist are checked. The visual inspection is usually adopted, and if necessary, the non-destructive testing means such as the endoscopes, magnifying glasses or eddy current flaw detection are used. The surface qualities not only affect the appearance, but also are related to the corrosion initiation points and the resistance and cleanliness of the fluid conveyance.

Non-destructive testing: Under the premise of not destroying the pipes, the potential defects inside and near the surfaces are detected. For the thin-wall stainless steel pipes with the higher requirements, the eddy current testing (ET) is often used to detect the surface and near-surface cracks and inclusions; for the welded pipes, the ultrasonic testing (UT) or liquid penetrant testing (PT) may also be used to check the weld qualities, ensuring the structural integrity of the pipes.

Hydrostatic testing or air tightness testing: Simulating the sealing and pressure resistance of the pipes under the actual pressure-bearing working conditions. The hydrostatic testing is to fill the pipes with the liquid such as the water and pressurize to the specified test pressure, holding the pressure and checking whether the leakage or permanent deformation occurs. The air tightness testing uses the gas media, with the higher sensitivity, often used to check the tiny leakages. This is the final comprehensive inspection of verifying the pressure-bearing capability of the pipes before the factory delivery.

Testing Scope

Building water supply and gas pipelines: Mainly testing their hygienic properties (the precipitation of the heavy metals such as the lead and chromium), pressure resistance strengths, connection sealing and long-term water corrosion resistance. Need to comply with the standards such as the Code for design of building water supply and drainage and CJ/T 151, ensuring the safety of the drinking water and the reliability of the gas conveyance. It is the preferred material of the direct drinking water systems and gas indoor pipelines in the modern buildings.

Food and pharmaceutical industry pipelines: In this field, the testing emphasis is the extremely high hygienic standards. In addition to the routine chemical compositions and mechanical properties, the surface roughness (Ra value) testing and cleanliness testing need to be additionally carried out, and whether they meet the relevant FDA or GMP requirements is verified, ensuring that the pipe inner walls are smooth, without retention, and easy to clean and disinfect, preventing the microbial growth and product contamination.

Heating and solar energy system pipelines: The testing needs to pay attention to their performance under the cold and hot water alternating circulation and high-temperature water environments. The thermal cycling testing, high-temperature oxidation resistance testing and thermal expansion coefficient determination are emphatically carried out. The pipes are required to have the good thermal fatigue resistance, stable mechanical properties and low heat losses, ensuring the long-term efficient and stable operation of the heating systems or solar hot water systems.

Ship and ocean engineering pipelines: The application environments are extremely harsh, and the testing requirements are the highest. In addition to the routine items, the strict seawater corrosion resistance testing (such as the artificial seawater immersion), pitting and crevice corrosion resistance evaluation must be carried out, as well as the stricter mechanical and corrosion performance testing of the welded joints, ensuring the ultra-long service lives in the high-temperature, high-humidity and high-salt-spray marine environments.

Decorative and structural thin-wall pipes: The testing focuses on the appearance aesthetics and certain structural strengths. The surface color uniformity, glossiness and whether the surface defects exist need to be strictly checked. At the same time, for the load-bearing or stressed components, the mechanical properties (such as the yield strengths and stiffness) and the strength testing of the welded parts are equally indispensable, to meet the dual requirements of the safety and aesthetics of the applications such as the building curtain walls, guardrails and furniture.

Medical gas and high-purity gas conveyance pipelines: The testing core is the extremely high cleanliness and sealing. The strict particulate contamination testing, oil residue testing and helium mass spectrometry leak testing need to be carried out. The pipe inner surfaces must undergo the special polishing and passivation treatments and pass the relevant certifications, ensuring the purity and leak-free safety of the conveyed medical oxygen, special gases or high-purity process gases.

Testing Methods

Spectroscopic analysis method (such as the ICP-OES and XRF): Used for the fast and accurate determination of the chemical compositions of the thin-wall stainless steel pipes. The inductively coupled plasma optical emission spectrometry (ICP-OES) has the high precision and can analyse the multiple trace elements; the X-ray fluorescence spectrometry (XRF) is the non-destructive fast screening method. Through the excitation of the samples to generate the characteristic spectra, the qualitative and quantitative analysis is carried out by comparing with the standard spectral lines. It is the main means of the material designation identification.

Tensile testing method: According to the GB/T 228.1 standard, the axial tensile loads are applied to the specimens cut from the pipes on the universal material testing machines until the fracture. Through the recording of the load-displacement curves, the tensile strengths, proof strengths at the plastic extension (Rp0.2, commonly known as the yield strength) and elongations after fracture of the material are precisely calculated, thereby comprehensively evaluating the matching relationship between its strengths and plasticity.

Metallographic inspection method: Through a series of sample preparation steps such as the sampling, mounting, grinding, polishing and etching, the micro-structures of the material are observed under the optical metallographic microscopes or electron microscopes. Used to rate the grain size grades, detect the non-metallic inclusions, observe the phase compositions, and identify whether the micro defects such as the overheating, overburning and intergranular corrosion tendency exist. It is the important bridge connecting the material properties and processes.

Salt spray testing method: A commonly used artificial accelerated corrosion testing method, carried out according to the GB/T 10125 standard. The specimens are placed in the closed salt spray test chambers, and a certain concentration of sodium chloride solution is continuously or intermittently sprayed to form the salt spray environments. Through the observation of the time and areas of the corrosion product appearance on the specimen surfaces after a certain time, the corrosion resistance differences of the different materials or different surface treatment processes are qualitatively compared.

Eddy current testing method: An electromagnetic non-destructive testing method, especially suitable for the high-speed online testing of the conductive thin-wall stainless steel pipes. The probe coils are energized with the alternating currents to generate the alternating magnetic fields, inducing the eddy currents in the pipes; when the defects exist, the eddy current fields will be distorted, causing the impedance changes of the detection coils. This method can efficiently detect the defects such as the surface and near-surface cracks, pits and inclusions, but is not sensitive to the deep internal defects.

Liquid penetrant testing method: Mainly used to detect the open defects on the surfaces of the non-porous materials. The penetrant containing the fluorescent or colored dyes is applied to the surfaces of the pipes to be inspected, and the capillary action makes the penetrant seep into the defects; after cleaning the excess penetrant, the developer is applied to adsorb the penetrant in the defects to the surfaces, thereby forming the observable defect indications. This method has the simple equipment and convenient operation, and is particularly sensitive to the surface cracks.

Hydrostatic testing method: The traditional and effective method of verifying the pressure-bearing capability of the pipes. According to the standard requirements, the two ends of the thin-wall stainless steel pipes are sealed, filled with the water and the air is exhausted. Then the pressure pumps are used to slowly pressurize to the specified test pressure (usually 1.5 times the nominal pressure or higher), holding the pressure for a certain time. During the period, whether the pipe bodies have the leakage, sweating or visible permanent deformation is checked. It is the key factory delivery test of ensuring the use safety.

Testing Standards

Domestic Standards:

1. GB/T 20878, Stainless and heat-resisting steels — Designation and chemical composition

2. GB/T 18704, Light gauge stainless steel pipes for building water supply

3. CJ/T 151, Thin-wall stainless steel pipes for building water supply

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

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ASTM A240, Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications

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

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

Direct-reading spectrometer (OES): The core equipment of the chemical composition analysis. Through the arc or spark excitation of the samples, the atoms emit the light of the characteristic wavelengths. After the grating dispersion, the CCD detectors receive and convert them into the electrical signals. Compared with the built-in standard curves, the percentage contents of more than ten elements in the steel such as the carbon, silicon, manganese, phosphorus, sulfur, chromium and nickel can be simultaneously determined within tens of seconds, with the fast analysis speeds and high precision.

Universal material testing machine: The main equipment of carrying out the mechanical property testing. Equipped with the high-precision sensors and control systems, it can complete the various testing such as the tensile, compression and bending. When used for the thin-wall stainless steel pipe testing, the corresponding pipe-specific fixtures and extensometers need to be equipped, to precisely measure the loads and deformations during the testing processes, thereby calculating the accurate strength and plasticity indicators.

Metallographic microscope and sample preparation equipment: The necessary tools of the metallographic analysis. The system includes the cutting machines, mounting machines, pre-grinding machines, polishing machines, etching devices and metallographic microscopes. The modern metallographic microscopes are often equipped with the camera systems and image analysis software, which can carry out the automatic or semi-automatic quantitative analysis of the grain sizes, phase ratios and inclusion grades, providing the micro basis for the material quality control and process improvements.

Salt spray test chamber: The dedicated equipment of simulating the marine or industrial atmospheric corrosion environments. The chambers are made of the corrosion-resistant materials, equipped with the spraying systems, heating systems, saturation barrels and control systems. The chamber temperatures, salt spray sedimentation amounts and spraying/stopping cycles can be precisely controlled, realizing the various testing conditions such as the neutral salt spray testing (NSS), acetic acid salt spray testing (ASS) and copper-accelerated acetic acid salt spray testing (CASS).

Eddy current flaw detector and feed-through/rotating probes: The key equipment of the automated non-destructive testing. The eddy current flaw detectors generate the excitation signals and process the detection signals, indicating the defects through the impedance plane diagrams or signal amplitudes. For the thin-wall stainless steel pipes, the feed-through coils are often used for the high-speed online testing, or the rotating probes (probe rotation or pipe rotation) are used for the more precise off-line testing, to cover the full circumferential and axial areas of the pipes.

Ultrasonic thickness gauge and dimension measuring instrument: Used for the fast and non-destructive measurement of the pipe wall thicknesses and outer diameters. The ultrasonic thickness gauges are based on the pulse reflection principle, with the precision up to 0.01mm. The laser scanning diameter gauges or high-precision callipers and micrometers are commonly used for the outer diameter and ovality measurement. These portable instruments are the common tools of the process inspection and finished product sampling inspection at the production sites, ensuring that the dimensions are continuously controlled.

Hydrostatic testing machine: The device specially used for the pipe hydrostatic or pneumatic testing. The system includes the high-pressure pumps, pressure vessels, pressure transducers, safety valves, pipelines and clamping devices. The highly automated hydrostatic testing machines can automatically complete the whole process of the loading, sealing, filling, pressurizing, pressure holding, pressure releasing, leak checking and unloading, and record the pressure-time curves, realizing the efficient and reliable pressure-bearing performance verification.