Low Alloy Structural Steel Plate Testing

Low-alloy 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: The alloying elements (such as Mn, Si, V, Nb and Ti) and the key elements such as the carbon, sulphur and phosphorus in the low-alloy structural steel plate are precisely determined. This is the basis for judging its designations and properties. Through the analysis results, it can be confirmed whether the steel plate conforms to the composition ranges specified in the standards such as GB/T 1591 and ASTM A572, and the effects on the weldability and cold workability are evaluated.

Tensile property testing: The yield strength (ReH/ReL), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z) of the steel plate are determined at the room temperature. This item directly reflects the load-bearing, deformation resistance and plastic deformation abilities of the material under the static loads, and is the most core mechanical property indicator in the structural design, used to verify whether the strength grade requirements are met.

Impact toughness testing: The Charpy V-notch impact testing is adopted, and the impact absorbed energy (KV2) of the steel plate is determined at the specified temperatures (such as -20 ℃ and -40 ℃). This indicator is used to evaluate the ability of the material to resist the brittle fracture under the low-temperature or dynamic load conditions, and is crucial for the structures in the cold regions or bearing the impact loads. It is the key to guaranteeing the structural safety.

Bending property testing: Through the cold bending or bending testing, the plastic deformation ability of the steel plate under the specified mandrel diameters and bending angles is inspected, to evaluate whether there are the defects such as the cracks and delaminations on the surface and inside. This item is usually used to verify the process adaptability of the material, and has the important guiding significance for the subsequent cold forming processing (such as the plate coiling and bending).

Hardness testing: The Brinell (HBW) or Rockwell (HRB/HRC) hardness testing methods are usually adopted to determine the hardness values of the steel plate surface. The hardness has a certain correlation with the strength, wear resistance and machinability of the material, and can be used for the fast evaluation of the material uniformity and heat treatment effects, and can also be used as the auxiliary indicator of the acceptance.

Metallographic structure examination: The microstructures (such as the ferrite, pearlite and bainite) morphologies, grain size grades, and the types and grades of the non-metallic inclusions of the steel plate are observed with the optical microscope or electron microscope. This testing can deeply reveal the essential causes of the material performance advantages and disadvantages, and evaluate whether its heat treatment process is appropriate and the intrinsic quality levels.

Testing Scope

Material designation coverage: The testing scope covers the various low-alloy high-strength structural steels in the mainstream domestic and international standards, such as the Q345, Q390 and Q420 series of the Chinese standards, ASTM A572 Gr.50 and Gr.60 of the American standards, and S355 and S460 of the European standards. The different designations correspond to the different chemical composition and mechanical property requirements, and the testing needs to be carried out for the specific designations.

Product specification coverage: Applicable to the hot-rolled steel plates, strips and sheared plates of the different thicknesses (usually from 3 mm to above 200 mm), widths and lengths. For the different thickness specifications, the acceptance criteria of some testing items (such as the sampling positions and specimen sizes of the impact testing) may be different, and need to be executed according to the product standards.

Application field coverage: The testing serves the multiple key industrial fields, including the bridge engineering, high-rise buildings, ship manufacturing, pressure vessels, construction machinery, mining equipment and power transmission towers. The performance emphasis of the steel plates is different in the different application scenarios, and the testing items and indicators need to be customized according to the specific uses and relevant industry specifications.

Production process coverage: The testing runs through the whole process of the incoming raw material re-inspection, production process control (such as after the rolling and heat treatment) and finished product ex-factory inspection. Through the testing at the different stages, the whole-process monitoring of the product quality is realized, ensuring that the final products meet the technical agreements or standard requirements.

Special performance coverage: For the steel plates with the special requirements, the testing scope can be extended to the Z-direction (through-thickness) property testing (lamellar tearing resistance), fatigue property testing, weldability assessment testing (such as the carbon equivalent calculation and welding heat-affected zone property testing) and weather resistance testing (for the weathering steels).

Testing Methods

Spectrometry analysis method (chemical composition): The spark discharge atomic emission spectrometry (OES) or X-ray fluorescence spectrometry (XRF) is mainly adopted for the fast, simultaneous multi-element analysis. Among them, the OES has the high precision and is the main method of the furnace-front analysis and laboratory arbitration; the XRF is commonly used for the fast on-site screening and non-destructive analysis. Both need to be calibrated with the standard samples.

Universal material testing machine method (tensile/bending): The tensile testing is carried out in accordance with GB/T 228.1 or ASTM A370. A universal testing machine is used to apply the axial tensile forces to the standard specimens until the fracture, automatically recording the load-displacement curves and calculating the various strength and plasticity indicators. The bending testing is carried out in accordance with GB/T 232 or the relevant standards, placing the specimen on the specified supports and applying the concentrated loads through the punches to complete the bending.

Pendulum impact testing method (impact toughness): In accordance with GB/T 229 or ASTM A370, the standard Charpy V-notch specimen is placed on the supports of the impact testing machine, and the pendulum is released to break it in one blow. By measuring the potential energy difference of the pendulum before and after breaking the specimen, the impact absorbed energy of the specimen is directly read, expressed in the unit of joule (J).

Hardness tester indentation method (hardness): The Brinell hardness (HBW) testing uses the cemented carbide ball indenters. Under the specified test forces and after maintaining for a certain period, the indentation diameters are measured to calculate the hardness values, applicable to the softer to medium-hardness materials. The Rockwell hardness (HR) testing calibrates the hardness by measuring the indentation depth differences of the indenters under the initial test forces and total test forces, with the faster operation.

Metallographic microscope observation method (microstructures): In accordance with the standards such as GB/T 13298, the specimens are cut from the specific parts of the steel plate, and after the grinding, polishing and etching (commonly using the 4% nitric acid alcohol solution), the metallographic specimens are made. Under an optical metallographic microscope, the microstructure types and grain sizes (in accordance with GB/T 6394) and the non-metallic inclusion grades (in accordance with GB/T 10561) are observed and assessed.

Testing Standards

Domestic Standards:

1. GB/T 1591, High strength low alloy structural steels

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

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

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

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

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

7. GB/T 6394-2017, Determination of estimating the average grain size of metal

International Standards:

1. ASTM A572/A572M, Standard specification for high-strength low-alloy columbium-vanadium structural steel

2. ASTM A370, Standard test methods and definitions for mechanical testing of steel products

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

Direct-reading spectrometer: As the core equipment of the chemical composition analysis, the sample is excited to generate the characteristic spectra, which are converted into the electrical signals through the spectroscopic systems and detectors for the quantitative analysis. The modern direct-reading spectrometers have the characteristics of the fast analysis speed (results within dozens of seconds), high precision and simultaneous multi-element determination, and are the essential instruments of the iron and steel enterprise laboratories.

Microcomputer-controlled electronic universal testing machine: This equipment integrates the mechanical, electronic and computer technologies, precisely controls the loading rates through the servo motors or hydraulic systems, and is equipped with the high-precision sensors to measure the force values and displacements. Combined with the professional testing software, it can automatically complete the tests such as the tensile, bending and compression, and generate the test reports conforming to the standards, with the objective and reliable data.

Fully automatic impact testing machine: Adopting the microcomputer control, it can realize the automatic sample feeding, positioning, impact, data acquisition and sample recovery. Equipped with the low-temperature baths, it can precisely control the test temperatures (such as from -196 ℃ to the room temperature). Its measurement and control systems can precisely calculate the impact energies and automatically judge the validity of the test results, greatly improving the testing efficiency and accuracy.

Brinell/Rockwell hardness tester: The Brinell hardness testers are usually loaded hydraulically or electrically, equipped with the optical measurement systems or automatic indentation measuring devices. The Rockwell hardness testers have the digital display and electromechanical integration models, which can preset the test forces and load holding times and directly display the hardness values in the digital form. Some advanced models can also be connected to the computers for the data management and statistical process control.

Upright/inverted metallographic microscope: The upright microscopes have the simple optical path structures and intuitive operations; the inverted microscopes have the stable specimen placements, particularly suitable for observing the large specimens that are not easy to move. The modern metallographic microscopes are mostly equipped with the high-resolution CCD cameras and image analysis software, which can realize the acquisition, measurement, rating and report generation of the structure morphologies, improving the scientificity and efficiency of the analysis.

Specimen processing equipment: Including the metallographic cutting machines, mounting machines, pre-grinding machines and polishing machines, used to prepare the testing specimens conforming to the standard requirements. The precise specimen processing is the premise of obtaining the accurate testing results. For example, the notches of the impact specimens need to be machined with the dedicated broaching machines or milling machines, to ensure that the notch root radii and surface roughnesses conform to the standard provisions.