Ceramic-lined Composite Steel Pipe Testing

Ceramic composite steel pipe inspection mainly conducts scientific testing and analysis on the micro-structure and interface analysis, ceramic layer hardness testing, wear resistance testing, compressive and bending strength testing, bonding strength 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

Micro-structure and interface analysis: Through the metallographic microscopes or scanning electron microscopes (SEM), the thicknesses, compactness, porosity of the ceramic layers (such as the corundum and silicon carbide) and their bonding interfaces with the steel pipe substrate are observed. A good interface should be the metallurgical or mechanical bonding, without the through cracks, delaminations or large-area pores. This is the basis for ensuring the mechanical and corrosion-resistant properties of the composite pipe.

Ceramic layer hardness testing: The Rockwell hardness testers or Vickers hardness testers are used to measure the surface hardness of the ceramic layer. The core advantage of the ceramic composite steel pipe lies in its extremely high surface hardness, usually reaching HV 1100-1800, far higher than the ordinary steels. This is the direct quantitative indicator of its wear resistance. During the testing, the multi-point measurements need to be carried out in the different areas to evaluate the uniformity.

Wear resistance testing: In the laboratories, the actual working conditions are simulated, and the abrasive wear testing machines (such as the MLS-23 wet sand rubber wheel type wear testing machine) are commonly adopted for the testing. By measuring the mass losses or volume losses of the specimens under the specified test conditions, the relative wear resistance is calculated, to scientifically evaluate its ability to resist the material scouring and wear.

Compressive and bending strength testing: The compression and bending tests are carried out on the composite pipe as a whole or its prepared specimens, evaluating its ability to bear the external loads. The emphasis is on whether the ceramic layer has the brittle spalling or cracking under the loads, and whether the steel pipe substrate has the plastic deformation. This is related to the structural safety of the pipeline during the installation and use.

Bonding strength (compression shear strength) testing: This is the key item evaluating the bonding quality between the ceramic layer and the metal substrate. The compression shear testing method is usually adopted. The specimen is placed in a dedicated mould and the vertical pressure is applied until the interface is damaged, recording the maximum loads and calculating the bonding strength. The excellent bonding strength is the premise of preventing the ceramic layer from falling off under the impacts.

Corrosion resistance testing: According to the expected service environments (such as the acidic, alkaline or chloride ion containing media), the chemical immersion method or electrochemical methods (such as the potentiodynamic polarization curve testing) are adopted for the testing. The isolation effects of the ceramic layer on the corrosive media and the corrosion rates of the steel pipe substrate are evaluated, ensuring its long-term stability under the combined corrosion and wear working conditions.

Testing Scope

Centrifugal self-propagating high-temperature synthesis (SHS) ceramic-lined composite steel pipes: This type of composite pipes is the mainstream product, and the testing scope covers the whole process from the raw materials (thermite and steel pipes) to the finished products. The emphasis is on the phase compositions, thickness uniformity, residual stresses and possible macro defects of the ceramic layers (mainly α-Al₂O₃ and a small amount of FeAl₂O₄) formed by the SHS reactions.

Thermally sprayed (plasma and flame spraying) ceramic coated steel pipes: The testing focuses on the coating quality formed by the spraying processes. Including the coating thicknesses, porosity, oxide contents, bonding strengths with the substrates (commonly tested with the tensile method or scratch method) and the micro-hardness of the coatings themselves, evaluating their thermal shock resistance and fatigue spalling resistance.

Diffusion welded or brazed composite ceramic steel pipes: Applicable to the occasions requiring the high bonding strengths and high precision. The testing scope includes the compositions and uniformity of the intermediate transition layers, the integrity of the welding/brazing interfaces, whether there are the harmful brittle intermetallic compounds, and the dimensional stability of the composite pipes under the high and low temperature cycles.

Extrusion formed or centrifugal cast ceramic composite pipes: The effects of the ceramic slurry formulations and forming processes on the final product properties are tested. The emphasis is on the densities of the ceramic layers, shrinkage rates after the sintering, clearance fits with the metal sleeves, and the effectiveness of the end face sealing treatments.

Testing of the different specifications and connection parts: The testing needs to cover the different pipe diameters (DN50-DN1000), wall thicknesses and pipe fittings such as the straight pipes, elbows, tees and reducers. Especially for the outer arc and inner arc sides of the special-shaped parts such as the elbows, due to the different forming processes, the ceramic layer thicknesses and qualities need to be emphatically sampled and inspected.

Testing of the in-service and repaired pipelines: The periodic testing is carried out on the in-service pipelines, evaluating the wear thicknesses, local damage conditions and substrate corrosion states of the ceramic layers. For the areas repaired with the on-site repair technologies (such as the ceramic adhesive repair and micro-arc spark deposition), the bonding qualities and property matching degrees between the repair layers and the original ceramic layers need to be tested.

Testing Methods

Non-destructive testing methods: Mainly including the ultrasonic testing (UT) and industrial CT scanning. The ultrasonic testing is used to detect the internal interlayers, lack of fusion and debonding defects between the ceramic layer and the substrate; the industrial CT can three-dimensionally visualize the thickness distributions, pore morphologies of the ceramic layers and the internal defects of the complex structures. It is the high-end precision testing means.

Metallographic micro-analysis method: The cross-section metallographic specimens of the composite pipe are prepared, and after the grinding, polishing and etching, the observation is carried out under a metallographic microscope. This method can intuitively measure the ceramic layer thicknesses and gradient transition layer structures, observe the grain morphologies and sizes, and qualitatively analyse the interface bonding forms. It is the basic and important testing method.

X-ray diffraction (XRD) phase analysis: Used to determine the crystalline phase compositions of the ceramic layers. For example, analysing the relative contents of the corundum (α-Al₂O₃) and hercynite (FeAl₂O₄) in the SHS ceramic layers, or detecting the amorphous phase contents in the thermally sprayed coatings. The phase compositions directly affect the hardness, toughness and chemical stability of the material.

Scratch method bonding strength testing: A scratch testing machine equipped with acoustic emission detectors is used. The diamond indenter scratches across the coating surface at the constant or increasing loads. By monitoring the acoustic emission signal mutations and friction force changes, combined with the microscopic observation of the scratch morphologies, the bonding strengths and failure critical loads between the coating and the substrate are quantitatively evaluated.

Erosion wear testing method: Simulates the erosion working conditions of the gas or liquid carrying the solid particles scouring the pipe walls. Under the specific temperature, flow velocity, impact angle and abrasive conditions, the specimen is eroded for a certain time, and the erosion rates are calculated through the weight loss method, evaluating the erosion resistance of the material under the specific working conditions. It is closer to the reality than the ordinary abrasive wear.

Thermal shock performance testing: The specimen is repeatedly subjected to the rapid heating and cooling cycles in the high temperature (such as 800 ℃) and room temperature water (or air), observing whether there are the cracks, warping or spalling on the ceramic layer surfaces. This testing is used to evaluate the reliability of the composite pipe under the conditions with the drastic temperature changes (such as the high-temperature material conveying and rapid cooling).

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 232-2010, Metallic materials — Bend test method

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

4. 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. ASTM G105, Standard test method for conducting wet sand/rubber wheel abrasion 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

Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS): The SEM provides the high-resolution morphology information of the ceramic layer surfaces and cross-sections, observing the micro cracks, pores and particle bonding states. The EDS can carry out the qualitative and semi-quantitative element analysis of the micro-areas, used to identify the foreign inclusions and analyse the element diffusion at the interfaces. It is the core equipment of the micro analysis.

Micro-hardness tester and macro-hardness tester: The micro-hardness tester (Vickers or Knoop) is used to measure the hardness of the micro-areas of the ceramic layers, evaluating their uniformity. The macro-hardness tester (Rockwell) is used for the fast testing of the overall or substrate hardness of the composite pipe. Usually, the automatic turrets and image measurement systems need to be equipped to improve the efficiency and precision.

Universal material testing machine: Equipped with the dedicated fixtures and indenters, used to carry out the mechanical property tests of the composite pipe such as the compression shear strength testing, three-point bending testing and compression testing. The equipment needs to have the high rigidity and precise displacement and load control abilities, and can record the complete stress-strain curves.

Wear testing machine series: The different types of wear testing machines are selected according to the testing standards. Such as the rubber wheel wet sand wear testing machine (ASTM G105), pin-on-disc wear testing machine and erosion wear testing machine. These equipment can simulate the different wear mechanisms, and are the indispensable tools for quantifying the wear resistance.

Ultrasonic flaw detector and thickness gauge: The ultrasonic flaw detector is used for the non-destructive flaw detection of the internal defects, and the suitable probe frequencies and angles need to be selected according to the acoustic characteristics of the ceramic layers. The ultrasonic thickness gauge is used for the fast and non-destructive measurement of the remaining thicknesses of the ceramic layers or the whole pipe walls, especially widely applied in the in-service testing.

Electrochemical workstation: Used for the precision testing of the corrosion resistance of the composite pipe. Through the tests such as the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), the protective properties of the ceramic layers in the electrolytes, porosity and the corrosion current densities of the substrate metals can be quickly evaluated, providing the scientific basis for the material selection.