Carbon Structural Steel Plate Testing

Carbon structural steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, bending property testing, hardness testing and metallographic structure examination, 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

Chemical composition analysis: This is the basic item of the carbon structural steel plate testing, aiming to precisely determine the contents of the main elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S) and the residual elements in the steel. The results directly affect the core properties such as the strength, toughness and weldability of the steel plate. Through the spectrometry analysis or wet chemical analysis, it is ensured that the composition meets the specified ranges of the standards such as GB/T 700, and it is the primary basis for judging the material conformity.

Tensile property testing: Used to evaluate the mechanical behaviour of the steel plate under the uniaxial static tensile loads, and is the key indicator for measuring its load-bearing capacity. The testing items mainly include the yield strength (Rp0.2), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z). By drawing the stress-strain curves, the data such as the elastic modulus and yield plateau of the material can be obtained, providing the direct basis for the structural design and safety assessment.

Impact toughness testing: The Charpy V-notch impact testing is mainly adopted. At the specified test temperatures (such as 20 ℃, 0 ℃ and -20 ℃), the ability of the steel plate to resist the impact loads and brittle fracture is determined, expressed in the impact absorbed energy (KV2). This item is crucial for evaluating the use safety of the steel plate under the low-temperature environments or dynamic loads, especially in the key fields such as the bridges, ships and pressure vessels.

Bending property testing: Inspects the plasticity and surface quality of the steel plate during the bending deformation. According to the standard requirements, the 180° or specified angle bending testing is carried out, checking whether the cracks, delaminations or fractures appear on the outer surface of the specimen bend. This testing can effectively reflect the process forming performance of the steel plate and whether there are the internal defects such as the interlayers and bubbles, and is an important means of evaluating its cold working adaptability.

Hardness testing: Through the Brinell hardness (HBW), Rockwell hardness (HRB/HRC) or Vickers hardness (HV) tests, the ability of the steel plate surface to resist the local plastic deformation is measured. The hardness value has a certain correlation with the strength and wear resistance of the material, and the testing has the non-destructive or micro-destructive nature, commonly used for the fast inspection in the production process, finished product acceptance and the verification of the heat treatment effects.

Metallographic structure examination: The microstructures of the steel plate are observed with the optical microscope or scanning electron microscope, including the grain size rating, the morphology and grade assessment of the non-metallic inclusions (such as the types A, B, C and D), and the inspection of the defects such as the banded structures and Widmanstätten structures. The metallographic analysis can fundamentally explain the macro performance of the material, and is the core technical means of controlling the metallurgical quality and process stability.

Dimension and shape inspection: The tools such as the callipers, micrometers, thickness gauges, rulers and straightedges are used to precisely measure the geometric dimensions and shape deviations such as the thickness, width, length, flatness and camber of the steel plate. The strict dimension control is the premise of ensuring the precision and efficiency of the steel plate in the subsequent cutting, assembly and welding processes, and is directly related to the quality and cost of the engineering structures.

Testing Scope

General structural steels: Mainly referring to the carbon structural steels conforming to the GB/T 700 standard, such as the Q195, Q215, Q235 and Q275 series. This type of steel plates is widely used in the buildings, bridges, vehicles, ships and other general metal structural parts, and is the most common type in the testing business. The testing emphasis is on guaranteeing the basic mechanical properties and weldability.

High-quality carbon structural steels: In accordance with the GB/T 699 standard, such as the designations 08F, 45 and 65Mn. The contents of the harmful elements such as the sulphur and phosphorus of this type of steels are more strictly controlled, and they have the more uniform structures and properties. In addition to the routine items, the testing scope more emphasizes the precise control of the chemical compositions, non-metallic inclusion grades and the properties after the special heat treatments required by the designations.

Carbon steels for the boilers and pressure vessels: Such as the Q245R (formerly 20g) in GB 713. Used to manufacture the drums, headers and pressure vessel shells of the medium and low pressure boilers. In addition to the routine mechanical properties, its testing scope particularly emphasizes the high-temperature tensile properties (when necessary), stricter impact toughness requirements (including the aging impact) and the non-destructive testing such as the ultrasonic flaw detection, to ensure the long-term safe operation under the pressure and certain temperatures.

Carbon steels for the ships and marine engineering: The general-grade carbon structural steels for the ships need to meet the specifications of the relevant classification societies (such as the CCS, ABS and DNV). The testing scope not only includes the national standard items, but also needs to meet the special low-temperature impact toughness, Z-direction properties (lamellar tearing resistance), and the stricter chemical composition and internal quality inspection requirements of the ship rules.

Structural steels for the bridges: Although the modern large bridges mostly use the low-alloy high-strength steels, some auxiliary structures still use the carbon structural steels. The testing needs to consider the dynamic loads, fatigue and environmental factors of the bridge structures, and has the higher requirements for the impact toughness, especially the low-temperature impact and aging impact properties, while the appearance quality and internal defect control are strict.

Steels for the cold forming and stamping: Such as the high-quality carbon steel thin plates used to manufacture the complex stamping parts. The testing scope focuses on evaluating their deep drawing properties, including the stricter chemical compositions (low carbon and low silicon), excellent plasticity and the testing of the strain hardening exponent (n value), plastic strain ratio (r value) and surface quality (finish and freedom from the oxide scales).

Testing Methods

Spectrometry analysis method: Mainly used for the fast and precise determination of the chemical compositions. The common ones include the spark discharge atomic emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES method has the high analysis precision, especially suitable for the precise measurement of the elements such as the carbon, sulphur and phosphorus, and is the main means of the fast furnace-front analysis and laboratory arbitration analysis of the iron and steel enterprises. During the testing, the flat sample surfaces need to be prepared, and the corresponding reference materials need to be used for the calibration.

Universal material testing machine method: The core equipment for the mechanical property tests such as the tensile, bending and compression. In accordance with the standards such as GB/T 228.1, the axial tensile forces are applied to the standard specimens through the hydraulic or motor-driven grips, and the loads and deformations are precisely measured by the force sensors and extensometers, automatically calculating and outputting the key data such as the yield strength, tensile strength and elongation after fracture. The test rate control and specimen alignment are the keys to ensuring the result accuracy.

Pendulum impact testing machine method: The standard method for the Charpy impact testing. In accordance with GB/T 229, the standard specimen with the V-notch is placed on the supports of the testing machine, and the pendulum of the specified height is released to break it in one blow. Through measuring the potential energy difference of the pendulum before and after breaking the specimen, the impact absorbed energy value is directly read. The testing must be carried out in a temperature-controllable low-temperature bath, to accurately evaluate the toughness of the material at the different temperatures.

Hardness tester indentation method: Divided into the Brinell, Rockwell and Vickers methods according to the different principles. The Brinell hardness (HBW) uses the cemented carbide ball indenters of the larger diameters, with the large indentation areas and good representativeness, suitable for the materials with the uneven structures. The Rockwell hardness (HRB/HRC) has the simple operation and high efficiency, and can directly read the values. The Vickers hardness (HV) uses the diamond square pyramid indenters, suitable for the precise measurement of the hardness of the thin layers or small areas.

Metallographic microscopic analysis method: The standard method of studying the microstructures of the material. The testing process includes a series of sample preparation steps such as the sampling, mounting, grinding, polishing and etching (such as using the 4% nitric acid alcohol solution), and then the observation and photographing are carried out under an optical metallographic microscope, and according to the standards such as GB/T 13298 and GB/T 10561, the grain sizes and inclusions are quantitatively or semi-quantitatively rated, analysing the relationship between the structures and properties.

Ultrasonic flaw detection method: An important non-destructive testing method, used to detect the defects such as the delaminations, inclusions and flakes inside the steel plate. The probes transmit the high-frequency ultrasonic waves into the steel plate, and the positions, sizes and natures of the defects are judged by receiving the reflected echoes. This method has the strong penetration ability and high sensitivity, especially suitable for the internal quality inspection of the thick plates. It is one of the mandatory inspection items of the key materials such as the boiler plates and ship plates.

Testing Standards

Domestic Standards:

1. GB/T 700, Carbon structural steels

2. GB/T 699, Quality carbon structural steels

3. GB 713, Steel plates for boilers and pressure vessels

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

5. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

6. GB/T 13298, Inspection methods of microstructure for metals

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

8. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

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

2. 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

Direct-reading spectrometer: The core equipment of the modern steel testing laboratories, used for the fast quantitative analysis of the chemical compositions. Its working principle is that the sample generates the characteristic spectra under the spark discharge excitation, which are received by the photomultiplier tubes or CCD detectors after the grating dispersion, and the contents of the various elements are calculated through the calibration curves. It has the advantages of the fast analysis speed (measuring the multiple elements within dozens of seconds), high precision and high automation, and is the first choice of the production control and incoming material inspection.

Microcomputer-controlled electronic universal testing machine: A high-precision mechanical testing equipment integrating the mechanical, electronic, sensor and computer technologies. Driven by the servo motors or servo hydraulics, combined with the high-precision load cells and electronic extensometers, it can realize the precise closed-loop control of the load, displacement and deformation. The computer system automatically acquires the data, draws the curves and generates the test reports, and the testing process fully conforms to the rate control requirements of the international and national standards.

Fully automatic impact testing machine: A new generation of impact testing equipment, realizing the full automation of the sample feeding, positioning, impact, data acquisition and sample recovery. Equipped with the automatic low-temperature devices, it can precisely control the test temperatures. The equipment has the built-in energy calibration and friction loss compensation functions, and can directly transmit the results such as the impact absorbed energy and brittle fracture percentages to the laboratory information management systems (LIMS), improving the test efficiency and the reliability of the results.

Metallographic microscope system: Composed of an inverted or upright metallographic microscope, high-resolution digital camera and professional image analysis software. The microscope provides the various observation modes such as the bright field, dark field and polarization. The digital image system can capture and store the microstructure images, and realize the quantitative metallographic analysis such as the automatic grain size rating, automatic inclusion identification and statistics, and phase area fraction measurement through the software, greatly improving the objectivity and efficiency of the testing.

Ultrasonic flaw detector: Usually composed of a pulse generator, receiving amplifier, probes (transducers) and display unit. The digital intelligent flaw detectors have the various display modes such as the A-scan, B-scan and C-scan, can automatically set and store the detection parameters (such as the DAC/TCG curves), and have the automatic defect alarm and recording functions. Combined with the probes of the various angles and scanning devices, they can adapt to the steel plate flaw detection tasks of the different specifications and detection requirements.

Brinell/Rockwell/Vickers hardness tester: The basic equipment of the hardness testing. The modern hardness testers mostly adopt the digital display and automatic loading mechanisms, and the test results are directly displayed in the digital form, reducing the human reading errors. Some high-end models have the functions of the automatic turret switching of the indenters and objectives, motorized loading, test process programming and automatic data transmission, and can meet the fast and accurate testing requirements of the materials of the different standards and hardness ranges.