Clad Steel Plate Testing

Clad steel plate inspection mainly conducts scientific testing and analysis on the cladding layer bonding strength testing, cladding layer thickness measurement, interface defect testing, chemical composition analysis, mechanical property testing and corrosion resistance assessment, 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

Cladding layer bonding strength testing: This is the core item of the clad steel plate testing, directly related to the service safety and reliability of the material. It mainly evaluates the interfacial bonding quality between the cladding layer (such as the stainless steel, titanium and nickel-based alloy) and the base layer (such as the carbon steel and low-alloy steel), preventing the failures such as the delamination and peeling during the use. The testing is usually carried out through the shear testing or tensile testing, and the bonding strength is quantitatively evaluated according to the relevant standards.

Cladding layer thickness measurement: Precisely measuring the cladding layer thickness is the key to ensuring that the clad steel plate meets the design and use requirements. The insufficient cladding layer thickness may lead to the degradation of the corrosion resistance or wear resistance, while the excessive thickness will cause the unnecessary cost waste. The common non-destructive measurement methods include the ultrasonic thickness measurement method, magnetic thickness measurement method (applicable to the ferromagnetic cladding/non-magnetic base layer combinations) and metallographic microscope measurement method.

Interface defect testing: Aims to find the internal discontinuity defects existing at the bonding interface of the clad steel plate, such as the non-bonding, weak bonding, inclusions and blowholes. These defects are the stress concentration points, which will significantly reduce the overall strength and fatigue resistance of the material. The testing mainly relies on the ultrasonic flaw detection technology, utilizing the changes of the reflection or transmission characteristics of the ultrasonic waves at the defect interfaces to judge the existence, positions and sizes of the defects.

Chemical composition analysis: The chemical composition analyses are respectively carried out on the cladding layer and base layer materials of the clad steel plate, to verify whether they meet the material standard requirements. This is the basis for ensuring that the material has the expected corrosion resistance, mechanical properties and welding performance. The spectrometry analysis method (such as the direct-reading spectrometer) is usually adopted to analyse the samples of the cladding layer and base layer, ensuring that the key element contents are within the ranges allowed by the standards.

Mechanical property testing: Including the tensile, bending, impact and hardness tests carried out on the whole clad steel plate and respectively on the base layer and cladding layer materials. The overall mechanical property testing (such as the tensile) reflects the comprehensive properties of the composite material; the separate testing is used to verify whether the base layer and cladding layer each meet the standards. The bending testing especially focuses on whether the cracking or delamination appears at the bonding interface after the bending.

Corrosion resistance assessment: Evaluates the corrosion resistance of the cladding layer material for the specific service environments. For the cladding layers such as the stainless steel and nickel-based alloys, the common tests include the intergranular corrosion testing, pitting potential measurement and salt spray testing. This testing item is the key basis for verifying whether the clad steel plate can work stably for a long time in the corrosive environments such as the chemical industry and marine.

Metallographic structure examination: Through the sampling, mounting, grinding, polishing and etching, the microstructures of the cladding layer, base layer and bonding interface are observed under the optical microscope or scanning electron microscope. This examination can intuitively evaluate the morphology of the bonding interface (such as whether the metallurgical bonding is formed), the grain sizes of the cladding layer and base layer, whether there are the harmful phase precipitations and the structure changes of the heat-affected zones.

Testing Scope

Explosive clad steel plates: The high-pressure and high-speed shock waves generated by the explosive detonation are utilized to make the cladding layer and base layer metals achieve the metallurgical bonding in an instant. The testing emphasis is on the large-area interface bonding rate, the hardened layers possibly produced by the explosion process, and the effects of the wavy bonding morphologies near the interfaces on the properties, and the comprehensive ultrasonic flaw detection is usually required.

Roll-bonded clad steel plates: Through the hot rolling or cold rolling processes, the cladding layer and base layer metals realize the bonding through the diffusion welding under the high temperatures and high pressures. The testing needs to focus on the interface oxidation possibly caused by the rolling process, the thicknesses and compositions of the element diffusion layers, and the bonding strength differences caused by the uneven rolling deformation. The metallographic examination is particularly important in the evaluation of this type of materials.

Surfacing clad steel plates: The surfacing processes (such as the strip electrode surfacing and plasma surfacing) are adopted to deposit an alloy layer with the special properties on the base layer surface. In addition to the bonding strength, the testing scope more emphasizes the chemical composition uniformity of the surfacing layer, dilution rate control, whether there are the welding defects such as the blowholes and slag inclusions inside the surfacing layer, and the thickness uniformity of the surfacing layer.

Clad plates of the different material combinations: Such as the stainless steel/steel, titanium/steel, copper/steel and nickel-based alloy/steel. The testing needs to select the corresponding methods according to the specific material combinations. For example, the titanium/steel clad plates need to particularly focus on the formation of the brittle intermetallic compounds; for the combinations of the non-ferromagnetic cladding layers (such as the stainless steel and titanium) with the ferromagnetic base layers, the eddy current method can be used for the auxiliary detection of the cladding layer defects.

In-service clad equipment and components: The periodic inspections or failure analyses are carried out on the pressure vessels, pipelines and reactors manufactured from the clad steel plates already in use. The testing scope focuses on the defects possibly produced under the service conditions such as the corrosion, stress and high temperatures, such as the cladding layer thinning, stress corrosion cracking, interfacial fatigue crack propagation and the evolution of the original defects.

Welded joints of the clad steel plates: The weld areas of the clad steel plates are the weak links of the structures, and the testing scope includes the weld quality itself and the effects of the welding heat-affected zones on the bonding interfaces. The non-destructive testing (UT, RT and PT) needs to be carried out on the butt welds and fillet welds, and the chemical composition changes of the transition layers after the welding and whether the interfacial bonding is weakened by the welding thermal cycles are evaluated.

Testing Methods

Ultrasonic testing method: This is the preferred and most important method for the internal defect testing of the clad steel plate, especially the interfacial non-bonding defect testing. The longitudinal wave straight-beam probes or twin-crystal straight-beam probes are adopted for the scanning from the base layer side, and the bonding quality is judged by observing the changes of the back-wall echo signals or the interface echo signals. This method can effectively detect the planar non-bonding defects, and can locate and quantitatively evaluate the defects.

Shear testing method: Used to quantitatively determine the bonding strength between the cladding layer and base layer. Usually in accordance with the standards such as ASTM A264 and GB/T 6396, the specimens of the specific shapes are prepared, and the shear forces are applied on a universal testing machine until the interface separation. The maximum shear load is recorded and the shear strength is calculated. This value is the most direct mechanical indicator for evaluating the bonding quality of the clad plate.

Tensile testing method: Another method of measuring the bonding strength, applicable to the evaluation of the thinner cladding layers or specific requirements. The specimen is machined into a specific shape, and the tensile force perpendicular to the interface is applied on a tensile testing machine, measuring the stress required to separate the interface. In addition, the transverse or longitudinal tensile testing of the whole clad plate can obtain the data such as the tensile strength, yield strength and elongation of the composite material.

Metallographic examination method: A destructive but convincing testing method. Through the cutting, mounting, grinding, polishing and etching, the metallographic specimens containing the complete interfaces are prepared, and under the microscope, the micro-morphologies of the bonding interfaces, bonding types (metallurgical bonding and mechanical bonding), diffusion layer thicknesses, and whether there are the oxide inclusions or cracks are observed, providing the micro evidence for the other testing results.

Eddy current testing method: Mainly used for the testing of the surface and near-surface defects of the conductive cladding layers (such as the stainless steel, titanium and copper), and is sensitive to the pitting, cracks and pits of the cladding layer materials. When the cladding layer is the non-ferromagnetic material and the base layer is the ferromagnetic material, this method can also be used to measure the cladding layer thickness, based on the principle that the probe coil impedance changes with the cladding layer thickness.

Magnetic thickness measurement method: A non-destructive method specially used to measure the thicknesses of the non-magnetic cladding layers (such as the stainless steel, aluminium and plastic coatings) on the ferromagnetic base layers. The magnetic circuit reluctance at the thickness gauge probe increases with the thickness of the non-magnetic cladding layer, and the cladding layer thickness can be converted by measuring the changes of the reluctance or magnetic flux. This method is fast and convenient, suitable for the on-site mass testing.

Chemical analysis and spectrometry method: Used to verify the material compositions. The portable or bench-type direct-reading spectrometers are usually adopted for the fixed-point excitation of the cladding layer and base layer, and the element types and contents are determined by analysing the wavelengths and intensities of the characteristic spectra. For the surfacing layers, the electron probe or energy spectrum analysis is also needed to determine the element distributions and dilution rates of the specific areas.

Testing Standards

Domestic Standards:

1. GB/T 6396, Clad steel plates — Mechanical and technological property test method

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 232-2010, Metallic materials — Bend test method

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

6. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

1. ASTM A264, Standard specification for stainless chromium-nickel steel-clad plate, sheet, and strip

2. ASTM E8/E8M, Standard test methods for tension testing of metallic materials

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

4. 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

Ultrasonic flaw detector: The key equipment of the clad steel plate testing, required to have the high resolution, high signal-to-noise ratio and good near-surface resolution ability. The digital ultrasonic flaw detectors are usually equipped with the twin-crystal straight-beam probes (for detecting the near-surface defects) and single-crystal straight-beam probes, with the A-scan display, gate alarm and data recording functions. Some high-end equipment supports the C-scan imaging, which can intuitively display the planar distributions of the defects.

Universal material testing machine: Used for the mechanical property tests such as the shear testing, tensile testing and bending testing of the clad steel plates. The equipment should have the sufficient load capacity and precise load and displacement measurement systems, and be equipped with the dedicated fixtures applicable to the different test standards (such as the shear testing fixtures). The testing process needs to meet the requirements of the standards such as GB/T 228.1 and ASTM E8.

Metallographic microscope and sample preparation equipment: Including the cutting machines, mounting machines, pre-grinding machines, polishing machines, etching devices and optical metallographic microscopes or digital microscope systems. The sample preparation equipment is used to prepare the interface specimens without the scratches and disturbances; the microscope is used to observe and photograph the microstructures, analyse the interfacial bonding states, and measure the cladding layer thicknesses and diffusion layer widths.

Cladding layer thickness gauge: Mainly divided into the magnetic thickness gauges and eddy current thickness gauges according to the measurement principles. The magnetic thickness gauges are used to measure the non-magnetic cladding layers on the iron substrates; the eddy current thickness gauges are used to measure the non-conductive cladding layers on the non-ferromagnetic metal substrates, or the non-conductive cladding layers on the conductive substrates. The instruments need to be regularly calibrated with the standard thickness foils to ensure the measurement precision.

Direct-reading spectrometer: Used for the fast and accurate analysis of the chemical compositions of the cladding layer and base layer on site or in the laboratory. The equipment consists of the excitation light sources, optical systems, detection systems and data processing systems. For the on-site testing, the portable spectrometers are mostly used, which can directly carry out the spark excitation on the material surfaces and obtain the quantitative analysis results of the multiple elements in a short time.

Hardness tester: Used to measure the hardness of the cladding layer, base layer and heat-affected zones of the clad steel plate, evaluating the strength uniformity and work hardening conditions of the material. The common types include the Vickers hardness testers, Rockwell hardness testers and Leeb hardness testers. For the thin cladding layers, the small-load Vickers hardness testers need to be used for the measurement, to avoid the effects of the base layer on the test results.

Corrosion testing equipment: Configured according to the testing requirements, such as the salt spray test chambers for simulating the marine atmospheric environments and evaluating the uniform corrosion resistance of the cladding layers; electrochemical workstations for measuring the electrochemical parameters such as the pitting potentials and re-passivation potentials of the materials; and high-temperature and high-pressure autoclaves for simulating the corrosion behaviour studies under the chemical medium environments.