Titanium Steel Plate Testing

Titanium steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, dimension and shape inspection, surface quality inspection, non-destructive testing and metallographic structure analysis, 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: Through the methods such as the spectrometry analysis, the contents of the elements such as the titanium, carbon, manganese, silicon, phosphorus and sulphur in the titanium steel are precisely determined. This testing is the basis for verifying the material designation conformity, directly affecting the mechanical properties, corrosion resistance and welding performance of the material. For the specific designations of the titanium steel plates, such as TA1, TC4 or the alloy structural steels, the element contents must be strictly controlled within the ranges specified in the standards.

Mechanical property testing: Evaluates the behavioural performance of the titanium steel plate under the loads, and is the key to judging whether it meets the service requirements. The main testing items include the tensile strength, yield strength, elongation after fracture and reduction of area. For some parts bearing the impact loads, the Charpy V-notch impact testing at the room temperature or low temperatures also needs to be carried out, to determine their impact absorbed energy.

Dimension and shape inspection: Measures the thickness, width, length and their allowable deviations of the titanium steel plate, ensuring that its geometric dimensions meet the provisions of the purchase contracts or the relevant standards (such as GB/T 3621). At the same time, the shape defects such as the flatness, camber and waviness of the plate need to be checked. These defects will affect the subsequent processing and assembly precision.

Surface quality inspection: Under the good lighting, through the visual inspection or with the aid of the tools such as the magnifying glasses and thickness gauges, it is checked whether there are the defects such as the cracks, scars, folds, inclusions, oxide scales, pressed-in iron oxide scales, scratches and corrosion on the plate surface. The surface quality directly affects the corrosion resistance of the titanium steel plate and the final appearance of the product, and is crucial for the applications such as the chemical vessels and offshore engineering.

Non-destructive testing: Under the premise of not damaging the material integrity, the internal or surface defects are detected. The common methods include the ultrasonic testing (UT), used to find the internal delaminations, slag inclusions and cracks; the penetrant testing (PT) or magnetic particle testing (MT, applicable to the ferromagnetic titanium steel), used to detect the fine cracks open to the surface. This is the necessary link for ensuring the safety of the key pressure-bearing parts.

Metallographic structure analysis: By preparing the specimens and observing them under a metallographic microscope or scanning electron microscope, the microstructures of the titanium steel plate are analysed, such as the grain size, phase compositions (α phase and β phase ratio), inclusion morphologies and distributions. The metallographic structure is the essential determinant of the material performance. Through this analysis, the rationality of the heat treatment process can be traced and the material performance can be predicted.

Corrosion resistance testing: Evaluates the stability of the titanium steel plate in the specific corrosive media. The common tests include the salt spray testing, intergranular corrosion testing and pitting potential measurement. For the titanium steel plates applied in the harsh environments such as the chemical industry, seawater desalination and nuclear power, the corrosion resistance is one of the core bases for the material selection.

Testing Scope

Commercially pure titanium plates (such as TA1 and TA2): These plates have the excellent corrosion resistance and good formability, but the relatively low strength. The testing emphasis is on the chemical composition purity, surface quality and corrosion resistance, and they are widely used in the fields such as the chemical equipment, offshore engineering, medical implants and consumer product housings.

Titanium alloy plates (such as TC4 and TA15): Strengthened by adding the elements such as the aluminium and vanadium, they have the higher specific strength. The testing needs to emphatically focus on their complex chemical compositions, mechanical properties after the heat treatment (strength, plasticity and toughness matching) and microstructures. They are mainly applied to the aerospace structural parts, high-performance engine components and military products.

Titanium-steel composite plates: Composed of the titanium cladding layer and steel base layer through the explosive welding or roll bonding, with both the corrosion resistance of the titanium and the structural strength of the steel. The testing scope includes the shear strength of the bonding interface, bonding rate (ultrasonic testing), as well as the chemical compositions and mechanical properties of the cladding layer and base layer respectively. They are commonly used to manufacture the large chemical vessels and flue gas desulphurization equipment.

Plates of the different processing states: The testing scope covers the titanium steel plates of the different processing and heat treatment states such as the hot-rolled state, cold-rolled state, annealed state and solution-aging state. The performance indicators of the plates of the different states are significantly different, and the testing needs to be carried out according to the corresponding product standards and technical agreements. For example, the hot-rolled plates focus more on the surface oxidation and dimensions, while the aged alloy plates focus on the stability of the strength and structures.

Plates of the different specifications and thicknesses: From the foils with the thickness less than 0.3 mm to the heavy thick plates with the thickness over 50 mm, all are within the testing scope. For the thin plates, the dimensional precision, surface finish and formability are the testing emphasis; for the thick plates, the uniformity of the internal quality, anisotropy of the mechanical properties and the reliability of the non-destructive testing at the thick sections are more emphasized.

Titanium steel plates for the special purposes: Including the special titanium steel plates for the nuclear industry (low impurity content control), ships and offshore engineering (seawater corrosion resistance) and medical devices (biocompatibility). In addition to the routine items, the testing scope also involves the special requirements such as the limit analysis of the special impurity elements, long-term corrosion fatigue testing and biological toxicity testing.

Testing Methods

Spectrometry analysis method (OES/ICP): Used for the quantitative analysis of the chemical composition. The spark discharge atomic emission spectrometry (OES) is fast and accurate, and is the mainstream method for the analysis before the furnace and the finished product inspection. The inductively coupled plasma atomic emission spectrometry (ICP) has the lower detection limits and is suitable for the analysis of the trace elements. Before the analysis, a flat and clean sample surface needs to be prepared to ensure the excitation stability.

Tensile testing method: In accordance with the standards GB/T 228.1 or ASTM E8, a universal material testing machine is used to apply the axial tensile loads to the specimens processed to the specified dimensions until the fracture. By recording the load-displacement curves, the core mechanical parameters such as the tensile strength, yield strength (usually referring to Rp0.2) and elongation after fracture can be directly obtained. It is the basic method for evaluating the load-bearing and deformation abilities of the material.

Ultrasonic testing method (UT): The flaw detection is carried out by utilizing the principle that the high-frequency sound waves produce the reflections or attenuations when encountering the defects during the propagation in the material. The single-crystal straight-beam probes or twin-crystal probes are adopted, combined with the suitable couplants, for the scanning on the steel plate surface. By analysing the amplitudes, positions and shapes of the echo signals, the depths, sizes and natures of the internal defects can be judged. It is the key means of the internal quality control of the thick plates.

Metallographic specimen preparation and observation method: Including a series of steps such as the sampling, mounting, grinding, polishing and etching, and finally the structure is observed under an optical microscope or electron microscope. For the titanium and titanium alloys, the Kroll reagent (HF+HNO₃+H₂O) is often used for the etching to reveal the grain boundaries and phase boundaries. This method is the bridge connecting the process, structure and performance, used for the quality judgement and failure analysis.

Salt spray testing method: An accelerated corrosion testing method. In accordance with the standards GB/T 10125 or ASTM B117, the specimens are placed in an enclosed salt spray test chamber, and the sodium chloride solution of a certain concentration is continuously or intermittently sprayed, simulating the marine atmospheric environment. By regularly observing the corrosion signs on the specimen surfaces and recording the time when the red rust appears, the corrosion resistance of the titanium steel plates of the different batches or processes is relatively evaluated.

Dimension measurement method: The measuring tools such as the micrometers, callipers and thickness gauges are used to directly measure the thickness, width and length of the plate. The shape parameters such as the flatness and waviness are mostly measured with the surface plates combined with the feeler gauges or dedicated flatness measuring instruments. All the measurements need to be carried out at the temperatures and measurement point positions specified in the standards, ensuring the comparability and accuracy of the data.

Hardness testing method: The Brinell hardness (HBW) or Rockwell hardness (HRC) testing is commonly used to quickly evaluate the hardness and uniformity of the material. The Brinell hardness has the large indentations and good representativeness, suitable for the softer or non-uniform materials; the Rockwell hardness testing is fast, suitable for the harder materials after the heat treatment. There is a certain conversion relationship between the hardness value and the strength of the material, and it can be used as a means of the on-site fast screening.

Testing Standards

Domestic Standards:

1. GB/T 3621, Titanium and titanium alloy plates and sheets

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 232-2010, Metallic materials — Bend test method

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

6. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

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 optical emission spectrometer: A precision instrument for the fast and simultaneous multi-element analysis. Its core components include the excitation light source, spectroscopic system (grating) and detector (CCD). During the testing, the sample is excited as an electrode in the argon atmosphere to generate the characteristic spectra, which are received by the detector after the dispersion and converted into the electrical signals, and the element contents are calculated through the calibration curves. It is the main equipment of the chemical composition analysis.

Universal material testing machine: The core equipment for carrying out the mechanical tests such as the tensile, compression and bending. It is composed of the loading frames, servo control system, force sensors and extensometers. The modern testing machines can realize the precise closed-loop control of the load, displacement and deformation, automatically draw the stress-strain curves, calculate the various performance indicators, and the data can be directly output and the testing reports can be generated.

Ultrasonic flaw detector: Composed of a pulse generator, receiving amplifier, display unit and probes. The instrument generates the electrical pulses to excite the probes to produce the ultrasonic waves, and receives the echo signals from the defects or the bottom, amplifies them and displays them as the waveforms (A-scans) on the screen. The digital intelligent flaw detectors have the functions such as the automatic calibration, defect assessment, data storage and C-scan imaging, greatly improving the detection efficiency and reliability.

Metallographic microscope and image analysis system: Including the upright or inverted optical microscopes for observing the microstructures, as well as the CCD cameras and computer image analysis software connected to them. This system can not only be used for the observation and photographing, but also automatically measure the quantitative metallographic parameters such as the grain size, phase ratio, inclusion sizes and distributions through the software, making the structure analysis more objective and precise.

Salt spray corrosion test chamber: The dedicated equipment for simulating the corrosion environments, composed of a chamber body, spraying system (including the compressed air purification, salt water storage and conveying, and nozzles), heating and humidity control system and drainage and exhaust system. The temperature, spray amount and sedimentation rate in the chamber all need to be strictly controlled within the ranges specified in the standards, to ensure the reproducibility and comparability of the testing conditions.

Precision dimension measuring equipment: Including the ultrasonic thickness gauges (which can be used for the single-side measurement) and laser thickness gauges for the high-precision thickness measurement; the laser flatness measuring instruments and three-dimensional coordinate measuring machines (CMM) for the shape profile measurement. These equipment usually have the high resolutions and automatic data acquisition and processing functions, and can quickly and accurately obtain the geometric parameters of the plate.