Bimetal Composite Pipe Testing

Bimetal composite pipe inspection mainly conducts scientific testing and analysis on the dimension and geometric precision testing, base pipe and cladding material chemical analysis, mechanical property testing, bonding strength, interface micro-structure and defect testing 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

Dimension and geometric precision testing: The inner diameters, outer diameters, wall thicknesses, out-of-roundness, straightness and lengths of the bimetal composite pipes are checked. Ensuring that their dimensions meet the design drawings and product standard requirements is the basis of guaranteeing the subsequent processing, installation and service properties, especially paying attention to the uniformity of the composite layer thicknesses.

Base pipe and cladding material chemical analysis: Through the methods such as the spectroscopic analysis and wet chemical analysis, the chemical compositions of the base pipes (usually the carbon steels or low-alloy steels) and cladding layers (such as the stainless steels, nickel-based alloys and copper alloys) are determined. Verifying whether the material designations meet the standards is the root of guaranteeing the comprehensive properties such as the corrosion resistance and strengths of the material.

Mechanical property testing: The sampling testing is carried out on the composite pipes as a whole or separately on the base pipes and cladding layers, including the tensile testing (determining the tensile strength, yield strength and elongation), hardness testing (Brinell, Rockwell or Vickers hardness, including the interface area hardness gradients), as well as the process property testing such as the flattening, bending and flaring. The load-bearing capacities and processing adaptability are evaluated.

Bonding strength (shear strength) testing: This is the core testing item of the bimetal composite pipes. Through the shear testing or bonding strength tensile testing, the bonding forces between the base pipe and cladding metal interfaces are quantitatively determined, ensuring that they reach the minimum values specified in the standards (such as the requirements of ASTM A265 and ISO 15614), preventing the delamination failures in service. Interface micro-structure and defect testing: The metallographic microscopes and scanning electron microscopes (SEM) are used to observe the micro morphologies of the composite interfaces, diffusion layer thicknesses, and whether there are the defects such as the non-bonding, voids, cracks and inclusions. The bonding mechanisms (metallurgical bonding or mechanical bonding) are analysed, providing the basis for the process optimization and quality evaluation.

Non-destructive testing (NDT): The methods such as the ultrasonic testing (UT, especially the ultrasonic testing of the interface bonding states), eddy current testing (ET, mainly targeting the cladding surface and near-surface defects), penetrant testing (PT) or radiographic testing (RT) are applied. Under the premise of not destroying the pipe bodies, the internal and interface defects are comprehensively screened.

Testing Scope

Metallurgically bonded composite pipes: Mainly testing the pipes realizing the metallurgical bonding through the processes such as the explosive cladding, roll bonding and centrifugal casting + hot extrusion. The testing emphases are the interface diffusion layers, micro defects, bonding strengths, as well as the base pipe property changes possibly caused by the thermal processes.

Mechanically bonded composite pipes: Mainly referring to the pipes bonded through the mechanical forces such as the hydraulic expansion, cold drawing and diameter reduction. The testing core is the interface fit tightness, residual contact stresses and hydraulic peeling resistance, and their stability under the temperature and pressure cycles needs to be evaluated.

Pipes for the petroleum and natural gas industry: Covering the oil and gas transmission pipes, downhole pipe strings (tubing and casing) and gathering and transmission pipelines. The testing strictly follows the standards such as API 5LD and ISO 13680, focusing on the corrosion resistance (such as the resistance to H2S and CO2), high-pressure sealing, full-size burst testing and bonding reliability under the harsh environments.

Pipes for the chemical and process industry: Applied to the reactors, heat exchangers and process pipes for the corrosive medium transmission. The testing emphases are the corrosion resistance of the cladding layers to the specific chemicals (such as the pitting and intergranular corrosion testing), the bonding strengths at the high temperatures, as well as the thermal fatigue properties.

Pipes for the power and seawater desalination: Including the power plant boiler feedwater heater pipes, condenser pipes and seawater desalination heat transfer pipes. The testing needs to focus on the erosion-corrosion resistance, anti-biofouling ability of the cladding layers, as well as the vibration wear resistance and erosion resistance under the high flow velocities and multiphase flow conditions.

Newly manufactured pipe ex-factory inspection: The comprehensive dimension, physical and chemical property, bonding strength and non-destructive testing is carried out on each batch or by a certain proportion of the sampling inspection of the newly produced composite pipes, issuing the inspection reports meeting the product standards or purchase technical agreements.

In-service pipeline periodic inspection and life evaluation: The periodic testing is carried out on the composite pipes already in use, evaluating their corrosion thinning conditions, cladding integrity, and whether the interface degradation or peeling occurs. Combined with the residual strength analysis and fitness-for-service evaluation, the remaining lives are predicted.

Testing Methods and Standards

Bonding strength testing method (shear method): According to the standards such as ASTM A265 and GB/T 12763, the shear forces are applied to the cladding layers through the dedicated fixtures until the interface separation, recording the maximum loads and calculating the shear strengths. This is the most direct and quantitative method of evaluating the interface bonding quality.

Ultrasonic testing (UT) method: The longitudinal wave or shear wave probes are adopted, using the differences of the reflection/transmission characteristics of the ultrasonic waves at the well-bonded interfaces and defective interfaces for the testing. The standards such as ISO 13665 and ASTM E797 can effectively detect the planar defects such as the non-bonding and delamination, and evaluate their sizes and positions.

Eddy current testing (ET) method: According to the standards such as ASTM E309 and ISO 15549, the electromagnetic induction principle is used to detect the defects such as the cracks and holes on the cladding surfaces and near the surfaces, as well as the cladding thickness changes. Suitable for the fast scanning of the non-ferromagnetic cladding materials such as the stainless steels and copper alloys.

Corrosion property evaluation method: The corresponding standards are selected according to the service environments for the testing. Such as the pitting and crevice corrosion testing according to ASTM G48, the intergranular corrosion testing according to ASTM A262 (such as the oxalic acid etching and sulfuric acid-copper sulfate bending methods), and the uniform corrosion weight loss testing according to ASTM G31.

Metallographic inspection method: According to the standards such as ASTM E3 and GB/T 13298, the cross-section metallographic specimens containing the composite interfaces are prepared. After the grinding, polishing and etching, the observation is carried out under the optical microscopes or SEM. The bonding layer thicknesses and diffusion layer depths can be precisely measured, and the bonding types and defects are qualitatively analysed.

Full-size pressure (burst) testing: For the composite pipes of the important uses, according to the requirements of API 5LD or project specifications, the full-size hydrostatic testing is carried out to the specified pressures or until the burst. The overall strengths and sealing of the pipe bodies are verified, and it is one of the most comprehensive verification tests.

High-temperature bonding stability testing: Simulating the high-temperature service conditions, the specimens are maintained at the specific temperatures for a certain time and then cooled to the room temperatures to test their bonding strength changes. Used to evaluate the bonding property degradation caused by the brittle phase precipitation that may occur at the metallurgical bonding interfaces under the long-term thermal exposure.

Testing Standards

Domestic Standards:

1. GB/T 13298, Inspection method of microstructure for metals

2. GB/T 12763, Bimetal composite pipes

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

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. API 5LD, Specification for CRA clad or lined steel pipe

2. ISO 13680, Petroleum and natural gas industries — Corrosion-resistant alloy seamless tubes for use as casing, tubing and coupling stock

3. ISO 13665, Seamless and welded steel tubes for pressure purposes — Ultrasonic testing of the tube ends for the detection of laminar imperfections

4. ASTM A265, Standard specification for steel and iron for fusion-welded and clad steel piping

5. ASTM E797, Standard practice for measuring thickness by manual ultrasonic pulse-echo contact method

6. ASTM E309, Standard practice for eddy-current examination of steel tubular products using magnetic saturation

7. 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: Used to carry out the mechanical property testing such as the bonding strength (shear/tensile), tensile testing and flattening testing. The equipment needs to be equipped with the dedicated composite pipe bonding strength testing fixtures, extensometers and high-precision load transducers, to meet the requirements of the standards such as ASTM A265 for the loading rates and measurement precision.

Ultrasonic testing system: Including the multi-channel or single-channel ultrasonic flaw detectors, high-frequency focused probes (such as 10-20MHz), comparison test blocks (with the artificial defects) and mechanical scanning devices. The advanced phased array ultrasonic testing (PAUT) systems can more intuitively image and display the interface bonding states and defect morphologies.

Eddy current testing instrument: The portable or bench-type eddy current instruments equipped with the differential or absolute probes, suitable for the online or offline testing of the pipes. For the non-ferromagnetic cladding layers on the ferromagnetic base pipes, the remote field eddy current (RFT) or magnetic saturation technologies need to be adopted to suppress the interference of the base pipe signals.

Spectrometer and chemical composition analysis equipment: The mobile or bench-type direct-reading spectrometers (OES) are used for the fast element analysis on site or in the laboratories. For the more precise analysis, the wet chemical analysis equipment such as the inductively coupled plasma optical emission spectrometers (ICP-OES) or carbon-sulfur analysers need to be used.

Metallographic microscope and scanning electron microscope (SEM): The optical metallographic microscopes are used for the routine interface morphology observation and low-power structure analysis. The scanning electron microscopes equipped with the energy dispersive spectrometers (EDS) can be used for the higher-resolution micro-structure observation, micro-area composition analysis and fracture morphology analysis, to study the bonding mechanisms and failure causes.

Corrosion testing equipment: Including the constant-temperature water baths, electrochemical workstations (for the electrochemical testing such as the potentiodynamic polarization and electrochemical impedance spectroscopy), high-temperature and high-pressure autoclaves (simulating the oil and gas field corrosion environments) and salt spray test chambers, etc., used for the comprehensive evaluation of the corrosion resistance of the cladding materials and composite interfaces.

Dimension and geometric quantity measurement tools: The high-precision outer diameter micrometers, inner diameter dial indicators, wall thickness ultrasonic thickness gauges (can measure the layers), laser diameter gauges, large platforms and height gauges (used to measure the straightness), etc. Ensuring that the geometric dimensions of the pipes are strictly controlled within the tolerance ranges.