Welded Steel Plate Testing

Welded steel plate inspection mainly conducts scientific testing and analysis on the appearance quality testing, weld dimension testing, internal defect testing, mechanical property testing, metallographic structure analysis and chemical composition analysis, 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

Appearance quality testing: Mainly checks the surface and weld appearance defects of the welded steel plate. Including whether the weld formation is uniform, and whether there are the undercuts, weld reinforcements, arc strikes, surface blowholes, cracks, lack of fusion and welding spatter. The visual testing (VT) is the most basic testing method of this item, often supplemented with the magnifying glasses to observe the fine defects.

Weld dimension testing: Aims to verify whether the geometric dimensions of the weld conform to the design or standard requirements. The key measurement indicators include the weld reinforcement, weld width, fillet weld leg sizes and throat heights, weld misalignment and bevel clearances. The dedicated measuring tools such as the welding inspection gauges and weld callipers are usually used for the precise measurement.

Internal defect testing: Used to explore the invisible defects inside the welds and heat-affected zones, and is crucial for evaluating the structural integrity. The common internal defects include the incomplete penetration, lack of fusion, internal blowholes, slag inclusions and internal cracks. This item mainly relies on the non-destructive testing technologies, such as the radiographic testing and ultrasonic testing.

Mechanical property testing: Used to evaluate the performance of the welded joints under the actual loads. The core testing items include the tensile testing (determining the tensile strength and yield strength), bending testing (evaluating the plasticity and bonding quality), impact testing (determining the low-temperature impact toughness) and hardness testing (evaluating the performance changes of the heat-affected zones). The samples are usually taken from the welding test plates.

Metallographic structure analysis: The metal structure morphologies of the various areas of the welded joints (weld, fusion line, heat-affected zone and base material) are observed through the micro means. The analysis contents include the grain sizes, phase compositions, micro defects (such as the micro cracks and inclusions) and the structure changes caused by the welding thermal cycles. It is the important basis for evaluating the welding process quality and material properties.

Chemical composition analysis: Detects the chemical compositions of the weld metal, to verify their matching with the base material and whether they meet the standard requirements. The contents of the harmful elements such as the carbon, sulphur and phosphorus, as well as the burning losses of the alloying elements, are particularly focused on. The common methods include the spectrometry analysis and chemical titration. For the corrosion-resistant steels, the specific element analyses also need to be carried out.

Corrosion resistance testing: Carried out for the welded steel plates used in the corrosive environments (such as the stainless steels and weathering steels). The main methods include the salt spray testing, intergranular corrosion testing (such as the sulphuric acid-copper sulfate method) and pitting potential determination, to evaluate whether the corrosion resistance of the welded joints (especially the heat-affected zones) is degraded by the welding thermal processes.

Testing Scope

Butt welded joints: Applicable to the joints formed by welding two steel plates on the same plane through the butt bevels. This is the most common joint form, and the testing scope covers the whole weld and the heat-affected zones on both sides, emphatically testing the penetration depths, internal defects and mechanical property uniformity of the welds.

Corner and T-type welded joints: Applicable to the joints where two steel plates are connected at the right angles or in the T-shape. The testing scope concentrates on the fillet weld leg sizes, throat thicknesses, weld shapes and the fusion conditions at the roots. The stress concentrations of this type of joints are obvious, and the crack testing at the corners needs to be particularly noted.

Lap welded joints: Applicable to the joints where one steel plate is lapped on another steel plate for the fillet welding. The testing emphasis is on whether the lap lengths are sufficient, and whether the welds along the lap edges are continuous and free of the defects such as the lack of fusion, ensuring the effective load transfer.

Welding heat-affected zones: Referring to the areas where the metallographic structures and mechanical properties of the base material change due to the thermal effects (without melting) during the welding process. This area is the key range of the testing, especially focusing on its hardness changes, toughness degradation and the possible reheat cracks or softening phenomena.

Welded joints of the different thickness steel plates: When the thicknesses of the butt-connected steel plates are different, the testing scope needs to particularly focus on the bevel machining precision of the thick plate side, the burn-through risks of the thin plate side, and the deformation and uneven residual stress distributions possibly caused by the different rigidities.

Dissimilar steel welded joints: Referring to the welding between the steels of the different chemical compositions or grades. In addition to the routine items, the testing scope needs to emphatically focus on the composition dilution of the weld metal, carbon migration near the fusion lines, and the additional stresses and potential cracks caused by the differences of the thermal expansion coefficients.

Repair weld areas: The areas after the excavation and repair of the defective welds. This area is the top priority of the testing, and the testing scope needs to be expanded, ensuring that the defects are completely removed, and the properties of the repaired welds are not lower than the original weld requirements, and the possible secondary thermal effects are noted.

Testing Methods

Visual testing: The most basic and widely applied non-destructive testing method. Before, during and after the welding, the naked eyes or with the aid of the tools such as the magnifying glasses and endoscopes are used to check the steel plate bevel preparation, weld appearance formation and surface defects. This method is fast and low-cost, but depends on the experience of the inspectors, and can only find the surface open defects.

Radiographic testing: The X-rays or γ-rays are used to penetrate the workpiece, and the imaging is carried out through the films or digital detectors to detect the internal defects. It displays the volumetric defects (such as the blowholes and slag inclusions) intuitively, and has a certain detection ability for the planar defects (such as the cracks and lack of fusion). The testing results can be permanently recorded, but there are the radiation safety protection requirements, and it is insensitive to the thin-layer defects perpendicular to the ray directions.

Ultrasonic testing: Utilizes the principle that the high-frequency sound waves reflect and refract when encountering the defects during the propagation in the material for the flaw detection. Particularly suitable for detecting the planar defects (such as the cracks and lack of fusion) inside the thick plate welds, with the accurate positioning, high sensitivity and no harm to the human body. However, it has the high technical requirements for the operators, and the testing results have no intuitive image records (except the A-scan).

Magnetic particle testing: Applicable to the detection of the surface and near-surface defects of the ferromagnetic material welded steel plates. After the magnetization of the workpiece, the leakage magnetic fields will be produced at the defects to attract the magnetic particles to form the magnetic indications. This method is extremely sensitive to the linear defects such as the surface cracks, with the portable equipment, but is only applicable to the ferromagnetic materials, and the workpiece surfaces need to be cleaned.

Penetrant testing: Utilizes the capillary action principle. The coloured or fluorescent penetrants penetrate into the surface open defects of the workpiece, and the defect indications are observed after the development. It is applicable to all the non-porous metal materials, and can detect the fine cracks open to the surfaces. The operation is simple, but it cannot detect the non-open and near-surface defects, and has the requirements for the surface finish of the workpiece.

Eddy current testing: Utilizes the electromagnetic induction principle to detect the surface and near-surface defects of the conductive materials. It is applicable to detecting the surface cracks of the welds, evaluating the coating thicknesses or the material conductivity changes. The detection speed is fast, no couplants are needed, and the automated scanning can be realized, but it is mainly used for the thin-wall or surface detection, and the defect shape judgement is difficult.

Acoustic emission testing: A dynamic non-destructive testing method. By monitoring the stress wave signals released during the defect propagation of the specimen under the loads, the defects are located and evaluated for their activities. Commonly used for the pressure testing monitoring of the welded vessels and the structural health monitoring, it can find the active defects such as the crack growth in real time.

Testing Standards

Domestic Standards:

1. GB/T 3323, Non-destructive testing of welds — Radiographic testing

2. GB/T 11345, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment

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

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

International Standards:

1. ISO 17636-1, Non-destructive testing of welds — Radiographic testing — Part 1: X- and gamma-ray techniques with film

2. ISO 17640, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment

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

Digital radiographic flaw detector: A detection system combining the X-ray tubes or γ-ray sources with the digital flat panel detectors (DR) or imaging plates (CR). Compared with the traditional films, it has the advantages of the fast imaging, wide dynamic ranges, digital storage and processing, and environmental friendliness (no chemical processing), greatly improving the detection efficiency and image quality.

Ultrasonic flaw detector: The core equipment, divided into the analogue and digital types, and the modern all-digital intelligent ultrasonic flaw detectors are mostly adopted. Equipped with the probes of the multiple angles (straight-beam probes, angle-beam probes and twin-crystal probes), with the A-scan display, B-scan/C-scan imaging, DAC/TCG curve preparation, automatic defect assessment and data storage functions, it is the main equipment of the internal defect detection of the welds.

Magnetic particle flaw detector: Including the fixed, mobile and portable yokes. The equipment can generate the circumferential, longitudinal or composite magnetic fields for the magnetization of the workpiece, used with the fluorescent or non-fluorescent magnetic suspension liquids. The high-end equipment integrates the ultraviolet light sources, illuminance meters and demagnetization functions, ensuring that the detection sensitivity meets the standard requirements.

Penetrant testing kit: Usually including the cleaners, penetrants, emulsifiers and developers. There are the colouring method and fluorescent method, and the fluorescent method needs to be equipped with the black light lamps (ultraviolet lamps) and darkroom environments. The kits need to meet the sensitivity grade requirements of the relevant standards, and ensure the compatibility between the various reagents.

Welding inspection gauge: A multi-functional mechanical measuring tool, specially used to measure the weld dimensions. It can measure the weld reinforcements, widths, misalignments, bevel angles, clearances, and the fillet weld leg sizes and throat heights. It is the simple and efficient tool necessary for the on-site welding inspectors.

Hardness tester: Used to measure the hardness of the various areas of the welded joints (weld, heat-affected zone and base material), evaluating the effects of the welding thermal processes on the material properties and whether there are the hardening or softening. The common types include the Leeb hardness testers (portable), Vickers hardness testers and Brinell hardness testers, and the suitable methods and loads need to be selected according to the standards.

Metallographic microscope and specimen preparation equipment: Used for the micro-structure analysis of the welded joints. The equipment system includes the cutting machines, mounting machines, grinding and polishing machines, etching devices and optical or electron microscopes. Through the preparation of the metallographic specimens, the weld crystallization morphologies, heat-affected zone structure gradients and micro defects can be detected and observed, providing the direct evidence for the process optimization and failure analysis.