Ultra-high Strength Steel Plate Testing

Ultra-high strength steel plate inspection mainly conducts scientific testing and analysis on the tensile property testing, impact toughness testing, hardness testing, bending property testing, chemical composition analysis 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

Tensile property testing: This is the basic item for evaluating the mechanical properties of the ultra-high strength steel plate. The yield strength, tensile strength, elongation after fracture and reduction of area of the material are mainly determined through the uniaxial tensile testing. For the steel plates with the tensile strength greater than 800 MPa, the test rates need to be precisely controlled to ensure the data accuracy. This testing is the key basis for evaluating the load-bearing capacity and deformation ability of the steel plate.

Impact toughness testing: Aims to evaluate the resistance of the material to the brittle fracture under the dynamic loads and low-temperature conditions. The Charpy V-notch impact testing is usually adopted, and the impact absorbed energy is determined at the specific temperatures (such as -40 ℃ and -60 ℃). For the structures bearing the impact loads such as the construction machinery and bridges, the impact toughness is one of the decisive indicators for measuring their service safety.

Hardness testing: The hardness is closely related to the strength, wear resistance and formability of the material. The Rockwell hardness (HRC), Brinell hardness (HBW) or Vickers hardness (HV) are commonly used for the ultra-high strength steel plates. Through the hardness testing, the uniformity of the mechanical properties of the material surface and core can be quickly and non-destructively evaluated, and its tensile strength can be indirectly estimated.

Bending property testing: Used to evaluate the plastic deformation ability and cracking resistance of the steel plate during the bending forming process. According to the standards, the 180° or 90° cold bending testing with the specified diameters is carried out, observing whether the cracks are produced on the outer surface of the specimen bend. This is an important means of inspecting the forming process adaptability of the steel plate, especially for the complex stamping parts in the automobile manufacturing.

Chemical composition analysis: Precisely determines the contents of the main elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S), as well as the micro-alloying elements such as the niobium (Nb), vanadium (V), titanium (Ti) and boron (B) in the steel plate. The chemical composition is the foundation determining the final strength, toughness and welding performance of the steel plate, and must be strictly controlled to ensure that the performances reach the standards.

Metallographic structure examination: The microstructures of the steel plate, such as the morphologies, sizes and distributions of the martensite, bainite and retained austenite, are observed through the optical microscope or scanning electron microscope. The structure uniformity and grain size grades directly affect the comprehensive mechanical properties of the steel plate. This testing is the core analysis link connecting the material process and final performance.

Fatigue property testing: Evaluates the durability of the steel plate under the alternating cyclic loads. Through a high-frequency fatigue testing machine, the S-N curves (stress-life curves) or fatigue limits of the material are determined at the specific stress ratios (such as R=0.1). For the components bearing the cyclic loads such as the automobile chassis and suspension systems, the fatigue property is the key parameter of the design life.

Testing Scope

Advanced high-strength steels (AHSS): Mainly including the dual-phase steels (DP steels), transformation-induced plasticity steels (TRIP steels), complex-phase steels (CP steels) and martensitic steels (MS steels). Their tensile strengths usually range from 500 to 1500 MPa. The testing emphasis is on their excellent product of strength and elongation, work hardening ability and collision energy absorption characteristics.

Press-hardened steels (PHS): Represented by 22MnB5, after the hot stamping forming and martensitic transformation, the tensile strength can reach above 1500 MPa. The testing scope covers their original austenitizing states, the structure and properties after the hot forming processes, as well as the effects of the possible decarburized layers and oxide scales on the properties.

Quenching and partitioning steels (QP steels): A new generation of ultra-high strength steels, obtaining the complex structures of the martensite and carbon-enriched retained austenite through the quenching and carbon partitioning processes. The testing needs to focus on their extremely high product of strength and elongation (usually greater than 30 GPa%), as well as the effects of the stability of the retained austenite on the formability and delayed fracture properties.

Ultra-high strength steels for the construction machinery: Mainly used for the excavator boom arms, crane jibs, etc., with the strength grades of 700-1100 MPa. The testing needs to particularly emphasize their high yield ratios, excellent low-temperature impact toughness (usually requiring the -40 ℃ impact energy) and the fatigue and stress corrosion resistance.

Wear-resistant ultra-high strength steels: Such as the NM series wear-resistant steel plates. While ensuring the high strength, their surface hardness can reach above HBW400. The testing scope includes the Brinell hardness, wear testing (such as the three-body abrasive wear), as well as the depths and uniformity of their hardened layers.

Steels for the aerospace and military industries: The performance requirements of this type of steels are extremely stringent, such as the 300M steel (tensile strength ≥1860 MPa). The testing scope far exceeds the routine ones, and needs to cover the fracture toughness (KIC), stress corrosion cracking threshold (KISCC), very high cycle fatigue properties and the strict non-destructive testing (such as the ultrasonic and magnetic particle).

Testing Methods

Room-temperature tensile testing (GB/T 228.1, ISO 6892-1): This is the most core mechanical property testing method. For the ultra-high strength steels, the strain-controlled mode is usually adopted, and the extensometers are used to precisely measure the yield points. During the testing, attention needs to be paid to the machining quality of the specimens to avoid the stress concentrations, and the testing machine with the suitable range is selected according to the expected strength grades of the material.

Charpy pendulum impact testing (GB/T 229, ISO 148-1): The standard specimen size is 10 mm × 10 mm × 55 mm with the V-notch. The testing needs to be carried out in a temperature-controllable low-temperature bath, to evaluate the toughness transition behaviour of the material at the low temperatures. The results are expressed in the impact absorbed energy (KV2), which is an important criterion of the material toughness.

Hardness testing: The suitable method is selected according to the thickness and state of the steel plate. The Vickers hardness (HV) is applicable to the testing of the thin plates and surface hardened layers; the Brinell hardness (HBW) has the large indentations and good representativeness, commonly used for the medium and thick plates; the Rockwell hardness (HRC) has the fast operation, suitable for the high-hardness materials after the heat treatments. The various hardness values can be approximately converted through the standard conversion tables.

Bending testing (GB/T 232, ISO 7438): The mandrels of the specified diameters are adopted to bend the specimen to the specified angle. The ratio of the mandrel diameter to the specimen thickness (d/a) is the key parameter measuring the bending severity. After the testing, it is checked whether there are the cracks, splits or delaminations longer than the specified lengths on the outer surface of the specimen bend.

Spectrochemical analysis: The spark discharge atomic emission spectrometry (OES) or X-ray fluorescence spectrometry (XRF) is adopted for the fast composition analysis. For the trace elements or more precise analysis, the dedicated equipment such as the inductively coupled plasma atomic emission spectrometry (ICP-OES) or carbon-sulphur analysers needs to be used.

Microstructure analysis: The metallographic specimens are prepared according to GB/T 13298 (cutting, mounting, grinding, polishing and etching), and observed under an optical microscope. For the more precise analysis, the scanning electron microscope (SEM) needs to be used to observe the structure morphologies, combined with the electron backscatter diffraction (EBSD) to analyse the grain orientations, or the transmission electron microscope (TEM) is used to observe the nanoscale precipitated phases.

Fatigue testing (GB/T 3075, ISO 1099): Usually carried out on a hydraulic servo fatigue testing machine with the axial loading mode. The testing can carry out the high-cycle fatigue (stress-controlled, number of cycles >10^5) or low-cycle fatigue (strain-controlled, number of cycles <10^5) tests, obtaining the fatigue limits or strain-life curves of the material, providing the data for the structural life prediction.

Testing Standards

Domestic Standards:

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

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

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

4. GB/T 3075, Metallic materials — Fatigue testing — Axial force-controlled method

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

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

International Standards:

1. ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

2. ISO 7438, Metallic materials — Bend test

3. ISO 1099, Metallic materials — Fatigue testing — Axial force-controlled method

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

Electronic universal material testing machine: The core equipment for carrying out the static mechanical tests such as the tensile, compression and bending. For the ultra-high strength steel testing, the testing machine is required to have the high stiffness, high-precision load cells (precision grade not lower than 0.5) and all-digital closed-loop control systems, and the maximum load capacity usually needs to reach 300 kN or above, to ensure the accuracy and stability of the testing.

Impact testing machine: Used to complete the Charpy impact testing. The equipment needs to have the automatic sample feeding, pendulum lifting and automatic energy reading functions, and be equipped with the low-temperature cooling devices meeting the standards (such as the alcohol + liquid nitrogen cooling baths), to ensure that the specimens can be tested at any set temperature in the range from -196 ℃ to the room temperature.

Fully automatic hardness tester: The modern hardness testing equipment integrating the image recognition, automatic positioning and indentation measurement functions. It can programmably carry out the Vickers, Brinell or Rockwell hardness testing, and automatically generate the hardness distribution maps (Mapping), efficiently evaluating the hardness uniformity of the material and greatly reducing the human errors.

Direct-reading optical emission spectrometer (OES): Used for the fast non-destructive chemical composition analysis. The equipment excites the sample through the spark discharge and measures the intensities of the characteristic spectral lines of the various elements for the quantitative analysis. The modern direct-reading spectrometers can simultaneously analyse dozens of elements, and analysing one sample only takes dozens of seconds. It is the key equipment of the steel mill process control and incoming material inspection.

Metallographic microscope and scanning electron microscope (SEM): The metallographic microscope is used for the routine structure observation and grain size rating, usually equipped with the bright field, dark field, polarization and digital image acquisition systems. The scanning electron microscope provides the higher resolution and depth of field, used to observe the finer structure characteristics and the fracture morphology analysis (ductile fracture, cleavage fracture, etc.), and can be integrated with the energy dispersive spectrometer (EDS) for the micro-area composition analysis.

High-frequency hydraulic servo fatigue testing machine: The dedicated equipment for the material fatigue property testing. The hydraulic servo actuators are adopted to provide the cyclic loads, with the frequency range from several Hz to hundreds of Hz. The system is equipped with the precise load and displacement sensors, as well as the powerful control and data acquisition software, and can complete the complex load spectrum testing.

Ultrasonic flaw detector: Used for the non-destructive testing of the internal defects (such as the delaminations, inclusions and flakes) of the steel plate. The ultrasonic waves are transmitted into the steel plate through the probes, and the sizes and positions of the defects are judged according to the positions and amplitudes of the reflected echoes. For the ultra-high strength steel plates for the important uses, the fully automatic ultrasonic testing (UT) is the mandatory ex-factory inspection item, ensuring the intact internal quality.