Through-thickness Performance Steel Plate Testing

Through-thickness performance steel plate inspection mainly conducts scientific testing and analysis on the through-thickness reduction of area, sulphur content determination, non-metallic inclusion rating, macroscopic fracture examination, ultrasonic flaw detection, and tensile performance and impact toughness verification, 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

Through-thickness reduction of area: This is the most core testing item for evaluating the through-thickness performance (Z-direction performance) steel plate. Through the tensile testing, the maximum reduction percentage of the cross-sectional area (Ψz) after the plastic deformation of the specimen in the thickness direction until the fracture is determined. This indicator directly reflects the ability of the steel plate to resist the lamellar tearing. The higher the value, the better the lamellar tearing resistance.

Sulphur content determination: Sulphur is the main element in the steel that easily forms the sulphide inclusions (such as the MnS). These inclusions will extend along the rolling direction during the rolling process, seriously weakening the plasticity of the steel plate in the thickness direction. The infrared carbon-sulphur analyser or spectroscopy analysis method is usually adopted for the precise determination. It is the prerequisite chemical analysis item for controlling the Z-direction performance.

Non-metallic inclusion rating: In accordance with the standards such as GB/T 10561 or ASTM E45, a metallographic microscope is used to rate the types, sizes, morphologies and distributions of the non-metallic inclusions such as the oxides and sulphides on the thickness cross-section of the steel plate. The B-type (alumina) and D-type (spherical oxide) inclusions are emphatically evaluated, and their quantities and morphologies have the significant influence on the Z-direction performance.

Macroscopic fracture examination: The steel plate specimen is broken in the direction perpendicular to the rolling surface or turned into a specific shape and then pulled to fracture, and the fracture morphology is directly observed. Through the naked eye or low-power microscope, it is checked whether there are the "flakes", delaminations, slag inclusions or wood-grain-like lamellar tearing characteristics on the fracture. This is a fast and intuitive qualitative evaluation method.

Ultrasonic flaw detection: Utilizing the principle that the ultrasonic waves produce the reflections when they encounter the defects (such as the delaminations and slag inclusions) during the propagation inside the steel plate, the non-destructive scanning of the whole steel plate is carried out. This testing is used to find the large-area delamination defects parallel to the rolling surface inside the steel plate, and is the batch rapid screening means that must be inspected before the ex-factory of the through-thickness performance steel plate.

Tensile performance and impact toughness verification: Although the Z-direction performance is the core, the routine longitudinal (L-direction) and transverse (T-direction) mechanical properties of the steel plate still need to meet the requirements. Its tensile strength, yield strength, elongation and the Charpy V-notch impact energy at the different temperatures need to be detected, ensuring that the comprehensive performance of the steel plate reaches the standard.

Testing Scope

High-strength structural steels for the buildings: Mainly used for the key load-bearing node parts such as the high-rise and super high-rise buildings, large-span stadiums and airport terminals. These parts are easy to produce the stresses along the plate thickness direction under the welding and complex loading conditions, and the Z-direction steels must be used to prevent the lamellar tearing and ensure the building safety.

Offshore engineering and shipbuilding: Applicable to the welded structures bearing the high restraint stresses such as the offshore oil platforms, hatch coamings and sheer strakes of the large ships. The superposition of the marine environment corrosion and alternating loads makes the requirements for the crack resistance in the thickness direction of the steel plate extremely strict. It is the key application field of the testing.

Pressure vessels and boilers: Especially the shells of the large spherical tanks, reactors and power station boiler drums, whose welded joints have the large thickness and high restraint. The use of the Z-direction steel plates can effectively prevent the dangerous lamellar tearing failures during the manufacturing and hydrostatic testing processes.

Bridges and heavy machinery: Applied to the key load-bearing components such as the box girder nodes of the large-span bridges and the welding connections of the heavy crane booms. These components bear the dynamic loads and impacts, and have the clear testing requirements for the through-thickness ductility and toughness of the material.

Pipeline steels and storage tanks: For the large-diameter and high-grade oil and gas transmission pipelines, during the pipe forming process, the plate edges may bear the larger through-thickness strains. The T-shaped welded joints of the large storage tanks also need to evaluate the Z-direction performance, to prevent the cracking.

Steel plates of the different thickness specifications: The testing scope covers from the thicker medium plates (such as above 40 mm) to the ultra-thick plates (above 100 mm). Usually, the larger the plate thickness, the higher the welding restraint stress, and the requirements for the through-thickness performance are correspondingly increased, and the testing standards are more stringent.

Testing Methods

Z-direction tensile testing method: This is a direct quantitative testing method carried out in accordance with the standards such as GB/T 5313 "Steel plates with through-thickness characteristics" or ASTM A770. The cylindrical specimens are prepared from the thickness direction of the steel plate, pulled to fracture on a tensile testing machine, and the reduction of area (Ψz) is precisely measured and calculated. According to the Ψz value, the steel plate can be divided into the multiple grades such as Z15, Z25 and Z35.

Chemical analysis method: The high-frequency infrared carbon-sulphur analyser is mainly adopted to determine the sulphur content in the steel. To ensure the good Z-direction performance, there is a strict upper limit control for the sulphur content (usually required ≤0.010%, or even ≤0.005%). Sometimes, the photoelectric direct-reading optical emission spectrometer also needs to be combined to analyse the contents of the other harmful elements such as the phosphorus.

Metallographic examination method: The transverse (perpendicular to the rolling direction) metallographic specimens of the steel plate are prepared, and after grinding, polishing and etching, observed under a metallographic microscope. According to the relevant standard atlas, the types and grades of the non-metallic inclusions are rated, providing the microstructural basis for the Z-direction performance.

Ultrasonic testing method: The multi-channel or automatic scanning ultrasonic flaw detectors are usually adopted, and the straight-beam probes or twin-crystal probes are selected, with the water or couplant as the medium, for the full-coverage scanning of the steel plate. By analysing the amplitude and positions of the reflected echoes, the internal delamination-type defects are judged and positioned, and graded according to the acceptance standards.

Macroscopic fracture method: An auxiliary qualitative testing method. The steel plate can be processed into the specific shapes (such as the square specimens with the notches) and then broken, or the tensile specimens prepared in the thickness direction are directly pulled to fracture, and then the fracture is observed. If the fracture is fibrous, the performance is good; if there is a flat and reflective separation surface, it indicates that there are the delamination defects.

Welding heat-affected zone (HAZ) toughness testing method: Although it is not the direct testing of the steel plate body, in order to evaluate the performance of the Z-direction steel plate after the actual welding, the welding thermal cycles are often simulated, and then the Charpy impact specimens are prepared in the heat-affected zone for the testing, ensuring that the thickness direction still has the sufficient brittle fracture resistance after the welding.

Testing Standards

Domestic Standards:

1. GB/T 5313, Steel plates with through-thickness characteristics

2. GB/T 10561-2023, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

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

International Standards:

1. ASTM A770, Standard specification for through-thickness tension testing of steel plates for special applications

2. ASTM E45, Standard test methods for determining the inclusion content of steel

3. 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

Electronic universal material testing machine: The core equipment for carrying out the Z-direction tensile testing. It needs to be equipped with the high-precision load cells and extensometers, can precisely control the loading rate, automatically record the load-displacement curves, thereby accurately calculating the through-thickness reduction of area. The range needs to cover the expected tensile strength of the tested steel plate.

High-frequency infrared carbon-sulphur analyser: A dedicated instrument for precisely determining the sulphur and carbon element contents in the steel. Its principle is to burn the sample in the high-temperature oxygen flow, and the generated SO₂ gas is detected at the specific infrared wavelength, thereby calculating the sulphur content. Its detection limit can reach the ppm level, and it is the key equipment for controlling the purity of the steel.

Metallographic microscope and image analysis system: Including the inverted or upright metallographic microscopes, equipped with the observation modes such as the bright field, dark field and polarization, as well as the high-resolution digital cameras. Combined with the professional image analysis software, the automatic identification, counting, measurement and rating of the non-metallic inclusions can be realized, improving the objectivity and efficiency of the detection.

Multi-channel digital ultrasonic flaw detector: The main equipment for the non-destructive detection of the internal defects of the steel plate. The modern digital flaw detectors have the functions such as the multi-channel synchronous scanning, high sampling rate, real-time A/B/C scan imaging, data storage and playback, and combined with the automatic scanning devices, can realize the large-area fast and precise detection of the steel plate.

Photoelectric direct-reading optical emission spectrometer: Used for the fast, multi-element qualitative and quantitative analysis of the steel plate. The sample is excited by the arc or spark to generate the characteristic spectra, which are dispersed by the grating and received by the CCD detector for the analysis. It can simultaneously determine the contents of the dozens of elements such as the C, Si, Mn, P and S within a few minutes, and is an important tool for the smelting process control and incoming material inspection.

Impact testing machine: The equipment for determining the Charpy V-notch impact toughness of the steel plate (including the base material and welding heat-affected zone). It is usually the pendulum type, and needs to be equipped with the low-temperature baths or high-temperature furnaces meeting the different temperature requirements, to obtain the impact energy (KV2) and shear area of the material at the series of temperatures, evaluating its ductile-brittle transition characteristics.

Specimen processing equipment: Including the band saws, lathes, milling machines, wire cutting machines, as well as the dedicated metallographic specimen cutting machines, mounting machines and grinding and polishing machines. It is ensured that the various specimens such as the tensile, impact and metallographic specimens meeting the standard dimensions and surface finish requirements can be precisely prepared from the large plate blanks. This is the basis for obtaining the accurate testing data.