Low Alloy High Strength Steel Plate Testing

Low-alloy high-strength steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, hardness testing, bending property 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 methods such as the spectrometry, ICP-OES or wet chemical analysis are adopted to precisely determine the contents of the five conventional elements carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S), as well as the micro-alloying elements such as the vanadium (V), niobium (Nb), titanium (Ti), nickel (Ni), chromium (Cr) and molybdenum (Mo) in the steel plate. This is the basis for evaluating its metallurgical properties and strengthening mechanisms.

Tensile property testing: Through a tensile testing machine, the yield strength (ReH, ReL), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z) are determined on the specified standard specimens. The yield strength is the key basis of the structural design, while the elongation after fracture reflects the plastic deformation ability of the material.

Impact toughness testing: The Charpy V-notch impact testing is adopted, and the impact absorbed energy (KV2) is determined at the specified temperatures (such as -20 ℃, -40 ℃ and -60 ℃). This indicator is the key to evaluating the ability of the steel plate to resist the brittle fracture under the low temperatures or dynamic loads, especially crucial for the structures in the severe cold regions or bearing the impact loads.

Hardness testing: The Brinell (HBW), Rockwell (HRC/HRB) or Vickers (HV) hardness testers are used to measure the hardness values of the steel plate surface or cross-sections. The hardness is closely related to the strength, wear resistance and processing performance of the material, and is commonly used for the fast evaluation of the material homogeneity and heat treatment effects.

Bending property testing: Through the cold bending testing or repeated bending testing, the ability of the steel plate not to produce the cracks under the bending deformation is evaluated. According to the product standard requirements, the mandrels of the specified diameters are used to bend the specimen to the specified angle, checking whether there are the cracks on the outer surface of the specimen, to verify its process plasticity.

Metallographic structure examination: The transverse or longitudinal cross-section metallographic specimens of the steel plate are prepared. After the grinding, polishing and etching, the microstructures (such as the ferrite, pearlite, bainite and martensite), grain size grades, and the types, sizes and distributions of the non-metallic inclusions are observed under an optical microscope or scanning electron microscope. The structure determines the properties, and it is the core link of the in-depth analysis of the performance causes.

Non-destructive flaw detection: The technologies such as the ultrasonic testing (UT), radiographic testing (RT), magnetic particle testing (MT) or penetrant testing (PT) are applied to carry out the non-destructive testing of the defects such as the cracks, delaminations, slag inclusions and blowholes inside and on the surface of the steel plate. This is the necessary means of guaranteeing the internal quality of the steel plate and meeting the high-standard engineering applications.

Testing Scope

Structural steel plates for the bridges: Mainly used for the key load-bearing parts such as the steel box girders and piers of the railway, highway and cross-sea bridges. The testing focuses on the high strength, excellent low-temperature impact toughness, good welding performance and fatigue resistance, and needs to meet the specific standards such as GB/T 714 and EN 10025-4.

Structural steel plates for the buildings: Applied to the main frames of the steel structures such as the high-rise buildings, large stadiums and industrial plants. The testing core lies in the high yield strength, good seismic performance (low yield ratio and high elongation) and the performance stability meeting the building fire protection requirements.

Steel plates for the pressure vessels: Used to manufacture the pressure-bearing equipment such as the boilers, reactors and storage tanks. The testing requirements are extremely strict. In addition to the routine mechanical properties, the high-temperature creep properties, low-temperature impact toughness, strict chemical composition control (especially the low P and S contents) and the comprehensive non-destructive testing are particularly focused on.

High-strength steels for the construction machinery: Used for the key structural parts such as the boom arms and frames of the excavators, cranes and loaders. The testing emphasis is on the extremely high tensile strength and yield strength, excellent fatigue resistance, good cold formability and wear resistance, to adapt to the complex alternating load working conditions.

Steels for the ships and marine engineering: Used to manufacture the hull structures and offshore platform jackets. The testing needs to meet the classification society specifications (such as the CCS, ABS and DNV), and particularly emphasizes the low-temperature toughness, lamellar tearing resistance (Z-direction properties), seawater corrosion resistance and the performance uniformity of the thick steel plates.

Steel plates for the pipeline transportation: Used to manufacture the pipe bodies of the long-distance oil and natural gas pipelines. The testing core is to meet the standard requirements such as the API Spec 5L, including the high strength, high toughness (often requiring the DWTT drop weight tear testing), good weldability and corrosion resistance, to ensure the safe operation of the pipelines under the high pressures, low temperatures and complex geological conditions.

Wear-resistant steels and wear-resistant lining plates: Used for the wear-prone parts such as the mining dump truck bodies, crusher lining plates and excavator buckets. The testing focuses on the extremely high surface hardness (HB400-HB600), good strength-toughness matching and certain impact wear resistance. The metallographic structures are usually the tempered martensite or bainite.

Testing Methods

Spectrometry analysis method (OES): The spark direct-reading spectrometer is used, and the specimen is used as an electrode to generate the arc-excited atomic spectra, and the characteristic spectral line intensities are measured through the spectroscopic systems for the quantitative analysis. This method has the fast analysis speed and high precision, and is suitable for the fast furnace-front analysis and the precise determination of the finished product chemical compositions. It is the most commonly used method in the production control.

Universal material testing machine method: In accordance with the standards such as GB/T 228.1 (ISO 6892-1), the standard tensile specimens are clamped on the testing machine, and the axial tensile forces are applied at the specified rates until the fracture. The force-displacement curves are recorded in real time through the sensors and software, and the various tensile property parameters are automatically calculated and output. It is the benchmark method of the mechanical property testing.

Pendulum impact testing method: In accordance with the standard GB/T 229 (ISO 148-1), the Charpy V-notch specimen is placed on the supports of the impact testing machine, and the pendulum of the specified energy is released to break it in one blow. The impact absorbed energy is read through the dial or digital display. The testing needs to be carried out in a temperature-controllable low-temperature bath, to evaluate the ductile-brittle transition characteristics of the material.

Hardness indentation method: The Brinell hardness (GB/T 231.1) uses the cemented carbide ball indenters to measure the indentation diameters; the Rockwell hardness (GB/T 230.1) measures the indentation depth differences of the indenters under the initial test forces and total test forces; the Vickers hardness (GB/T 4340.1) uses the diamond square pyramid indenters to measure the indentation diagonal lengths. The suitable method is selected according to the steel plate thickness and expected hardness ranges.

Metallographic microscope observation method: In accordance with the standards such as GB/T 13298, the prepared specimens are observed and rated under an optical microscope. The grain size rating refers to GB/T 6394 (comparison method or intercept method), and the non-metallic inclusion rating refers to GB/T 10561 (ASTM E45). This is a bridging testing method connecting the process, structure and properties.

Ultrasonic pulse reflection method: In accordance with the standard GB/T 2970, the probes are used to transmit the high-frequency ultrasonic pulses into the steel plate. When the sound waves encounter the defects or the bottom surface, they will reflect, and by receiving and analysing the amplitudes, positions and shapes of the reflected echoes, the sizes, positions and natures of the defects are judged. This method has the high detection sensitivity to the planar defects (such as the delaminations and cracks).

Bending testing method: In accordance with the standard GB/T 232, the rectangular cross-section specimen is bent around the mandrels of the specified diameters to the specified angles on a bending device. During the testing, the steady pressures are applied. After the bending, it can be visually inspected or checked with the magnifying glasses whether there are the cracks on the outer surface of the specimen bend, to evaluate the ductility and processing forming ability of the material.

Testing Standards

Domestic Standards:

1. GB/T 714, Structural steel for bridge

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

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

4. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

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

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

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

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

9. GB/T 2970, Thick steel plates — Method for ultrasonic testing

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

International Standards:

1. EN 10025-4, Hot rolled products of structural steels — Part 4: Technical delivery conditions for thermomechanical rolled weldable fine grain structural steels

2. API 5L, Specification for line pipe

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

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

5. 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

Spark direct-reading spectrometer: The core components include the excitation light sources, spectroscopic systems (gratings or prisms), detectors (photomultiplier tubes or CCD) and computer control systems. It has the low detection limits and high analysis precision, and can simultaneously analyse dozens of elements. It is the key equipment for the fast and accurate quantitative analysis of the chemical compositions.

Microcomputer-controlled electro-hydraulic servo universal testing machine: This system consists of the main frame (loading frame), servo actuators, high-precision load cells, electronic extensometers, all-digital controllers and testing software. It can realize the closed-loop precise control of the force, displacement and deformation, and complete the various static mechanical property tests such as the tensile, compression and bending.

Fully automatic impact testing machine: The equipment includes the automatic sample feeding devices, low-temperature baths, impact main bodies and energy measurement systems. It can realize the automatic grabbing, positioning, impact and residual sample recovery of the specimens. Some models also integrate the infrared temperature measurement, ensuring the precise specimen temperatures and improving the efficiency and result reliability of the low-temperature impact testing.

Metallographic specimen preparation and analysis system: This system usually includes the automatic grinding and polishing machines, mounting machines, metallographic cutting machines, optical microscopes and image analysis software. The automatic grinding and polishing machines can ensure the flat and scratch-free specimen surfaces, and the digital microscopes combined with the professional software can realize the quantitative metallographic work such as the structure image acquisition, automatic grain size rating and inclusion statistical analysis.

Ultrasonic flaw detector: Mainly composed of the pulse generators, receiving amplifiers, probes (transducers) and display units. The digital flaw detectors have the A/B/C scan displays, data storage, automatic alarm and defect quantitative analysis functions. Combined with the probes of the different frequencies and types (straight-beam probes and angle-beam probes), they can adapt to the steel plate flaw detection of the different thicknesses and detection requirements.

Brinell-Rockwell-Vickers hardness tester: The three-in-one hardness tester can realize the measurement of the three hardness scales of the Brinell, Rockwell and Vickers. The equipment is equipped with the automatic turrets, can quickly switch the different indenters and test forces, automatically measure the indentation sizes through the optical systems or depth sensors and directly display the hardness values, with the convenient operation and wide applicability.

Bending testing machine: Also known as the bending machine or press, used to carry out the cold bending testing. The equipment has the rigid frames, movable pressure heads and a group of mandrels of the different diameters. During the testing, the specimen is placed on the two support rollers, and the pressure head presses down to make the specimen bend around the mandrel. The whole process needs to be loaded steadily, to conform to the bending conditions specified in the standards.