Stainless Steel Flange Testing

Stainless steel flange inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, dimension and geometric precision testing, surface quality and defect inspection, non-destructive testing and corrosion resistance 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 methods such as the spectroscopic analysis, the contents of the key elements such as the chromium, nickel, molybdenum and carbon in the stainless steel flange materials are precisely determined. This is the basis of verifying the conformity of the material designations (such as 304 and 316L), directly affects the corrosion resistance, mechanical properties and weldability of the flanges, and is the primary basis of judging whether they are suitable for the specific corrosive environments.

Mechanical property testing: Mainly evaluating the performance of the flange materials under the stressed states, including the room temperature tensile testing (determining the tensile strengths, yield strengths and elongations after fracture), impact toughness testing (evaluating the anti-brittle fracture capability of the material under the low temperatures or dynamic loads), as well as the hardness testing (Brinell, Rockwell or Vickers hardness). These data are the key of ensuring that the flanges have the sufficient load-bearing capabilities and safety margins in the pipeline systems.

Dimension and geometric precision testing: The comprehensive measurement of the outer diameters, inner diameters, bolt hole pitch circle diameters, hole diameters, flange thicknesses and sealing face dimensions (such as the raised face and recessed face heights) of the flanges. The strict dimension tolerance control is the prerequisite of ensuring that the flanges and the matching parts (such as the valves, pipelines and other flanges) can be correctly assembled and form the effective seals, preventing the leakage or installation difficulties caused by the dimension deviations.

Surface quality and defect inspection: The methods such as the visual inspection, penetrant testing (PT) or magnetic particle testing (MT) are used to check the defects on the flange sealing faces and outer surfaces. The focus is on whether the cracks, scratches, pits, rust, laps and inclusions exist. In particular, any tiny defect on the sealing faces may become the leakage channel of the pressure media and must be strictly controlled.

Non-destructive testing (NDT): Detecting the internal and surface defects under the premise of not destroying the flanges. The commonly used methods include: the ultrasonic testing (UT) for finding the internal cracks, lamination and slag inclusions; the radiographic testing (RT) for detecting the internal volume defects (such as the blowholes and shrinkage cavities); the penetrant testing (PT) and magnetic particle testing (MT) for the surface open defects. This is the core link of ensuring the structural integrity of the flanges.

Corrosion resistance testing: For the stainless steel flanges with the special corrosion resistance requirements, the specific corrosion testing needs to be carried out. The commonly used methods include the intergranular corrosion testing (such as the sulfuric acid-copper sulfate method), pitting potential measurement and salt spray testing, used to evaluate the stability of the material in the specific corrosive media (such as the chloride ion environments), verifying whether they meet the design life requirements.

Pressure testing: The final verification testing carried out on the finished flanges or representative specimens before the factory delivery, usually including the hydrostatic testing or pneumatic testing. Through the application of the test pressures higher than the design pressures and the pressure holding, whether the flange bodies and connection parts have the leakage, permanent deformation or fracture is checked. It is the direct means of simulating the actual working conditions and verifying their pressure-bearing safety.

Testing Scope

Material verification: The testing covers the raw materials (forgings, castings and plates) and finished products of the flanges. Ensuring that from the raw material incoming to the final product delivery, their chemical compositions and metallographic structures meet the provisions of the corresponding standards (such as ASTM A182 and the GB/T 9115 series), preventing the substitution of the inferior goods or material misuse.

Production process monitoring: The testing runs through the whole manufacturing process. Including the blank inspection after the forging/casting, the mechanical property and hardness sampling inspection after the heat treatment (solution treatment, quenching and tempering), the dimension patrol inspection during the machining, and the non-destructive testing of the welded parts (if any), realizing the whole process quality control.

Finished product comprehensive inspection: The 100% dimension inspection, surface quality inspection and hardness inspection are carried out on the completed flanges. At the same time, the sampling is carried out according to the batches or standard requirements, and the destructive testing (mechanical properties and chemical analysis) and non-destructive testing are carried out on the samples, to represent the quality levels of the whole batches of products.

Targeted testing of the different types of flanges: The testing scope needs to be adjusted according to the flange types. For example, for the weld-neck flanges, the non-destructive testing of the welded joints needs to be emphatically focused on; for the threaded flanges, the thread precision and engagement degrees need to be tested; for the lap-joint flanges, the fit of the stub ends and flange rings and the welding quality need to be checked; for the hub flanges, the stress concentration parts of the transition areas between the hubs and flange faces need to be checked.

Additional testing for the special working condition applications: For the flanges used in the extreme or special working conditions such as the low temperatures, high temperatures, high pressures, highly corrosive media (such as the H2S acid environments), alternating loads or nuclear power plants, the testing scope needs to be expanded. Including the low-temperature impact testing, high-temperature creep testing, stress corrosion cracking (SCC) testing and fatigue testing, to meet the stricter safety specifications.

Traceability and compliance inspection: The testing scope is not limited to the products themselves, but also includes the audit of the accompanying quality documents (such as the material certificates, heat treatment reports, non-destructive testing reports and pressure testing reports), ensuring that all the testing activities are traceable and comply with the procurement technical agreements, product standards and relevant safety regulations (such as the TSG and PED directives).

Testing Methods

Spectroscopic analysis method: Used for the chemical composition analysis, mainly including the spark direct-reading spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES has the high precision and is the main laboratory method; the XRF is portable and fast, suitable for the on-site fast material designation screening. Before the analysis, the specimen surfaces need to be standardized ground to eliminate the contamination and ensure the accurate data.

Mechanical property testing method: According to the standards (such as GB/T 228.1 and ASTM A370), the standard tensile specimens and impact specimens are prepared, and the testing is carried out on the universal material testing machines and impact testing machines. The tensile testing obtains the stress-strain curves; the impact testing (Charpy V-notch) is carried out at the specific temperatures, determining the absorbed energies. The hardness testing needs to be measured at the specified positions of the flanges (usually the flange backs or the specimens) according to the grid point distribution.

Dimension measurement method: Combining the traditional measuring tools and modern instruments. The vernier callipers, micrometers, height gauges and thread gauges are used for the basic measurement; the coordinate measuring machines (CMM) are used for the high-precision digitalized detection of the complex geometric shapes and position degrees (such as the bolt hole uniform distribution), with the excellent efficiency and precision.

Liquid penetrant testing method: Suitable for the detection of the surface open defects of all the non-porous metal materials. The penetrant is applied to the cleaned flange surfaces, and the capillary action makes it seep into the defects. After removing the excess penetrant, the developer is applied, and the penetrant in the defects is sucked out to form the visible indications. This method is simple to operate and sensitive to the fine cracks.

Magnetic particle testing method: Only suitable for the ferromagnetic materials (martensitic stainless steels, etc.). The flanges are energized or magnetized, and the surface or near-surface defects will generate the leakage magnetic fields, adsorbing the applied magnetic particles to form the magnetic indication displays. This method has the high sensitivity to the linear defects (such as the cracks), but attention needs to be paid to the magnetization directions and magnetic field intensities.

Ultrasonic testing method: Using the principle that the high-frequency sound waves generate the reflection or attenuation when meeting the defects during the propagation in the material. The longitudinal wave straight-beam probes are used to detect the internal defects parallel to the detection surfaces, and the transverse wave angle-beam probes are used to detect the welds and near-surface defects. This method has the strong penetrating power, high sensitivity and thickness measurement capability, but requires the high operator skills and specimen surface smoothness.

Radiographic testing method: The X-rays or γ-rays are used to penetrate the flanges. Since the absorption of the rays by the defect parts and the matrix is different, the contrast images are formed on the films or digital imaging plates. It can intuitively display the shapes, sizes and positions of the defects, and is especially good at detecting the volume defects, but has the radiation safety protection requirements and higher costs.

Testing Standards

Domestic Standards:

1. GB/T 9115, Steel pipe flanges with weld-neck for petroleum, chemical and allied industries

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

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

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

5. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: 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 A182, Standard specification for forged or rolled alloy and stainless steel pipe flanges, forged fittings, and valves and parts for high-temperature service

4. ASTM A370, Standard test methods and definitions for mechanical testing 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

Direct-reading spectrometer: The core equipment of the chemical composition analysis. Through the arc excitation of the samples to generate the characteristic spectra, the photomultiplier tubes or CCD detectors receive them after the grating dispersion, and the computer systems quickly and quantitatively analyse the contents of the various elements. The modern spectrometers have the fast analysis speeds and high precision, and contain the multiple material standard databases.

Universal material testing machine: The necessary equipment of carrying out the mechanical testing such as the tensile, compression and bending. Through the servo motors or hydraulic systems, the precisely controllable loads are applied, combined with the high-precision transducers to measure the force values and displacements, automatically drawing the curves and calculating the various performance parameters. It needs to be regularly calibrated by the metrology institutions to ensure the accurate and reliable data.

Impact testing machine: The pendulum-type equipment used to determine the impact toughness of the material. The standard notched specimens are placed on the supports, and the pendulums released from a certain height break them. Through the angle difference of the pendulum rising after breaking the specimens, the impact energies (KV2) absorbed by the specimens are calculated. The testing machines need to keep the pendulum shaft friction and air resistance to the minimum, and the testing environmental temperatures need to be precisely controlled.

Coordinate measuring machine: The modernized equipment of the high-precision dimension and geometric tolerance testing. Through the probe movement on the three mutually perpendicular guide rails, the flange surface points are detected by the contact or non-contact (laser and optical) methods. The computer software reconstructs the three-dimensional models according to the measured point cloud data, and carries out the comparative analysis with the CAD digital models or theoretical dimensions.

Ultrasonic flaw detector: One of the main equipment of the non-destructive testing. Consisting of the pulse generators, probes, amplifiers, displays and data storage units. The digital ultrasonic flaw detectors have the A/B/C-scan displays, automatic defect evaluation, data storage and playback, test report generation and other functions, greatly improving the objectivity and reliability of the testing.

Digital radiographic imaging system: The digitalized upgrade of the traditional film radiography. The flat panel detectors (DR) or imaging plates (CR) are used instead of the films to receive the rays, directly converting the radiation signals into the digital images. With the advantages of the fast imaging, wide dynamic ranges, image enhancement processing and environmental friendliness (no chemical processing), it is convenient for the archiving and remote film reading.

Portable hardness tester: Used for the fast hardness testing at the sites or production lines. The common ones include the Leeb hardness testers (measuring the hardness through the rebound speeds of the impact bodies) and ultrasonic contact impedance hardness testers (measuring the hardness through the vibration rod frequency changes). Before the use, the calibration needs to be carried out on the standard hardness blocks, and attention needs to be paid to the conversions between the hardness testers of the different principles and the material adaptability.

Pressure testing system: Consisting of the high-pressure pumps, pressure vessels or fixtures, pressure transducers, safety valves and data acquisition systems. Used for the hydrostatic or pneumatic testing of the flange assemblies or test sealing plugs. The system must be equipped with the overpressure protection devices, and the test pressures and pressure holding times need to be strictly set and recorded according to the product standards or specification requirements.