Marine Steel Plate Testing

Ship 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 macro and micro metallographic 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: Through the methods such as the spectrometry analysis and chemical titration, the contents of the main elements such as the carbon (C), manganese (Mn), silicon (Si), phosphorus (P) and sulphur (S), as well as the micro-alloying elements such as the nickel (Ni), chromium (Cr), molybdenum (Mo) and niobium (Nb) in the steel plate are precisely determined. This is the fundamental basis of the material grading, directly related to the strength, toughness, weldability and corrosion resistance of the steel plate, and is the basic testing for meeting the classification society specification requirements.

Tensile property testing: The axial tensile testing of the standard specimens is carried out on a universal material testing machine, to determine the key mechanical indicators such as the yield strength (usually referring to the lower yield strength ReL), tensile strength (Rm) and elongation after fracture (A) of the steel plate. These data are the core for evaluating the load-bearing capacity, deformation resistance and safety margin of the hull structures, and must conform to the mandatory requirements of the corresponding classification society specifications for the steels of the different grades.

Impact toughness testing: The Charpy V-notch impact testing is adopted, and the impact absorbed energy (KV2) of the steel plate is measured at the specified temperatures (such as -20 ℃, -40 ℃ and -60 ℃). This is the key indicator for evaluating the ability of the steel plate to resist the brittle fracture when the ship is in the low-temperature sea areas or under the dynamic loads (such as the wave impacts and collisions), and is crucial for guaranteeing the navigation safety of the ship in the harsh environments.

Bending property testing: Through the cold bending testing or guided bending testing, the process properties of the steel plate under the bending deformation are inspected. The specimen is bent to the specified angle (usually 180°), checking whether there are the defects such as the cracks and delaminations on its outer surface. This testing aims to verify the adaptability and quality stability of the steel plate when it undergoes the bending and forming processing in the shipbuilding.

Hardness testing: The Brinell hardness (HBW) or Vickers hardness (HV) methods are usually adopted to measure the hardness values of the steel plate surface. The hardness reflects the ability of the material to resist the local plastic deformation, and has a certain correlation with the strength. It can be used for the fast evaluation of the material uniformity, heat treatment effects and wear resistance, and is the auxiliary testing item of the material acceptance.

Macro and micro metallographic examination: The macro examination checks the defects such as the segregation, porosity and flakes in the acid-etched macrostructure of the steel plate; the micro metallographic examination observes the grain sizes, types and grades of the non-metallic inclusions, and the microstructures (such as the ferrite and pearlite proportions) of the steel under the microscope. These examinations are used to evaluate the metallurgical quality and the internal structure uniformity of the steel plate.

Non-destructive testing: Mainly carries out the non-destructive inspection of the internal and surface defects of the steel plate, including the ultrasonic flaw detection (UT) for detecting the internal defects such as the delaminations and inclusions; and the magnetic particle flaw detection (MT) or penetrant flaw detection (PT) for detecting the defects such as the cracks and folds on the surface and near-surfaces. This is an important link for ensuring that the steel plate has no major quality hazards before leaving the factory.

Testing Scope

Steels for the hull structures: Mainly including the general-strength hull structural steels (grades A, B, D and E) and high-strength hull structural steels (AH32/36/40, DH32/36/40, EH32/36/40 and FH32/36/40). This is the core object of the testing, covering the main load-bearing structures from the bottoms, sides, decks to the bulkheads, with the most comprehensive and strict testing items.

Low-temperature toughness steels: Specially referring to the steel plates used to build the cargo containment systems of the liquefied gas (LNG/LPG) carriers, such as the nickel-based low-temperature steels (such as the 9% Ni steel) or high-manganese austenitic low-temperature steels. The testing emphasis is on the impact toughness, fracture toughness at the extremely low temperatures (-165 ℃ or even lower) and the special welding procedure qualification.

Seawater corrosion-resistant steels: Applied to the steel plates of the parts with the harsh corrosive environments such as the ship waterline areas and ballast tanks, and the corrosion resistance is often improved by adding the elements such as the copper, chromium and nickel. In addition to the routine items, the testing needs to emphatically focus on the corrosion resistance tests, such as the salt spray testing and electrochemical corrosion testing.

High-strength thin plates: Mainly used for the non-main load-bearing structures such as the ship superstructures and interior fittings. The testing focuses on their forming properties (such as the bending and stamping), welding deformation control and surface quality, and the matching requirements of the strength and toughness are different from those of the main hull steels.

Clad steel plates: Referring to the plates composed of the base layers (ordinary ship steels) and cladding layers (corrosion-resistant materials such as the stainless steel and titanium) through the explosive or rolling cladding, used for the cargo holds of the chemical carriers. The testing needs to consider both the mechanical properties of the base layers and the corrosion resistance of the cladding layers, and particularly focus on the shear strength and bonding rates of the bonding interfaces.

Cast steel and forged steel parts: Including the key ship parts such as the sternposts, rudder stocks and mooring equipment. In addition to the properties of the materials themselves, the testing scope more focuses on the ultrasonic flaw detection and magnetic particle flaw detection of the cast/forged blanks, to find the internal shrinkage cavities, porosity or surface cracks caused by the casting or forging defects.

Testing Methods

Spectrometry analysis method: The spark discharge atomic emission spectrometry (OES) is the mainstream, which can quickly and accurately quantitatively analyse the contents of more than twenty elements in the steel within dozens of seconds. This method has the simple sample preparation, fast analysis speed and high precision, and is the main means of the fast furnace-front analysis and finished product chemical composition arbitration of the steel mills.

Static tensile testing method: In accordance with the standards such as ISO 6892-1 or ASTM A370, carried out on a universal testing machine equipped with the electronic extensometers. During the testing, the loading rates need to be precisely controlled. Especially when determining the yield strength, the upper and lower yield points need to be distinguished, and the load-displacement curves need to be accurately recorded, to obtain the true and reliable strength and plasticity data.

Charpy pendulum impact testing method: Strictly carried out in accordance with the standards such as ISO 148-1 or ASTM A370. The key lies in the precise machining of the specimens (the root radii of the V-notches need to be strictly controlled), the precise maintenance of the test temperatures (using the low-temperature baths) and the calibration of the pendulum energies. The test results are assessed with the average value of a group of three specimens and the individual minimum value.

Ultrasonic flaw detection method: The pulse reflection method is mainly adopted, using the straight-beam probes or twin-crystal probes, with the water or couplants as the media, to carry out the full-plate scanning or grid-line scanning of the steel plate. By observing the attenuation conditions of the back-wall echoes on the oscilloscope screens or the positions and amplitudes of the defect echoes, the positions, sizes and natures of the internal defects are judged. It is the most effective method of detecting the delamination-type defects.

Metallographic microscope analysis method: After the sampling, mounting, grinding, polishing and etching, the observation is carried out under an optical microscope or scanning electron microscope. The grain size rating refers to the standard diagram comparison method or intercept method of ASTM E112; the non-metallic inclusion rating refers to the standard diagram method of ASTM E45 or ISO 4967, carrying out the grade assessment of the various inclusions A (sulphides), B (aluminas), C (silicates) and D (globular oxides).

Hardness testing method: The Brinell hardness (HBW) has the large test forces and large indentation areas, with the good result representativeness, commonly used for the thicker plates; the Vickers hardness (HV) has the wide selectable test force ranges and small indentations, suitable for the thin plates or hardened layer measurement. Before the testing, the tested surfaces of the specimens need to be smooth and flat, and the indentation spacings need to conform to the standard provisions, to avoid the effects of the work hardening.

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 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

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

International Standards:

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

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

3. ASTM E112, Standard test methods for determining average grain size

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

5. ISO 4967, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standard diagrams

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

7. 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 precision instrument commonly configured in the modern shipyards and laboratories, such as the bench-type or mobile spark direct-reading spectrometers. Its core is the spectroscopic system and photomultiplier tubes or CCD detector arrays, which can realize the simultaneous multi-element analysis, and have the built-in calibration curves and matrix effect correction programs, ensuring the high efficiency and accuracy of the chemical composition testing.

Microcomputer-controlled electronic universal testing machine: The core equipment for the tensile and bending testing. The system consists of the high-stiffness main frame, precision servo motor driving system, high-precision load cells and multi-functional testing software. The software can automatically calculate and output all the parameters such as the yield strength, tensile strength, elongation after fracture and reduction of area, and generate the test reports conforming to the standards.

Fully automatic impact testing machine: The modern equipment integrating the low-temperature devices, sample feeding mechanisms, pendulum impact and data acquisition in one. It can realize the automatic low-temperature immersion, automatic grabbing and conveying, automatic impact and automatic reading and recording of the energy values of the specimens, minimizing the human operation errors and ensuring the reliability and repeatability of the low-temperature impact testing data.

Digital ultrasonic flaw detector: The modern UT equipment is mostly digital, with the high-resolution colour LCD displays, supports the A/B/C scan displays, and can store a large number of waveforms and data. Combined with the probes of the multiple frequencies and types (such as the straight-beam probes, angle-beam probes and twin-crystal probes), as well as the scanning devices or immersion tanks, the automated and graphical non-destructive testing of the steel plates can be realized.

Research-grade metallographic microscope and image analysis system: Including the inverted or upright optical microscopes, equipped with the various observation modes such as the bright field, dark field, polarization and differential interference contrast (DIC). Connected to the high-resolution digital cameras and professional image analysis software, it can automatically measure the grain sizes, phase proportions, inclusion sizes and distributions, realizing the quantification and intelligence of the metallographic examination.

Full Rockwell/Brinell/Vickers hardness tester: The highly automated hardness testers are usually equipped with the motorized loading and unloading mechanisms, automatic turrets and automatic indentation measurement systems. Some models integrate the testing functions of the three hardness scales, can automatically convert the hardness values according to the standards and directly output the statistical results, greatly improving the efficiency and consistency of the batch testing.