Stainless Steel Plate Testing

Stainless steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, hardness testing, corrosion resistance testing, metallographic structure analysis and surface quality inspection, 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 key alloying elements and impurity contents in the stainless steel plate, such as the carbon (C), chromium (Cr), nickel (Ni), molybdenum (Mo), manganese (Mn), silicon (Si), phosphorus (P) and sulphur (S), are quantitatively determined. This is the cornerstone for judging whether the stainless steel designation (such as 304, 316 and 430) is accurate, directly affecting the corrosion resistance, mechanical properties and welding performance of the material.

Tensile property testing: Evaluates the mechanical behaviour of the stainless steel plate under the static tensile loads. The main determination indicators include the yield strength (Rp0.2), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z). These data are the key design bases for the strength, plasticity and formability of the plate.

Hardness testing: Measures the ability of the stainless steel plate to resist the local plastic deformation. The common methods include the Brinell hardness (HBW), Rockwell hardness (HRB/HRC) and Vickers hardness (HV). The hardness value can indirectly reflect the strength, wear resistance and heat treatment state of the material, and the testing is fast with the little damage to the specimen.

Corrosion resistance testing: Examines the ability of the stainless steel plate to resist the erosion of the environmental media. The common tests include the neutral salt spray testing (NSS), copper-accelerated acetic acid salt spray testing (CASS), intergranular corrosion testing (such as the sulfuric acid-copper sulfate method) and pitting potential measurement. This is the core item for verifying the "stainless" core function of the stainless steel.

Metallographic structure analysis: The microstructure of the plate is observed through an optical microscope or scanning electron microscope, including the grain size rating, inclusion types and grade analysis, phase compositions (such as the austenite, ferrite and martensite) and distributions. The structure determines the macroscopic performance of the material, and is an important means for judging whether the heat treatment process meets the standard.

Surface quality inspection: Checks whether there are the unallowed defects on the plate surface, such as the cracks, scars, folds, pitting, oxide scale residues, scratches and colour differences. According to the relevant standards (such as GB/T 3280), the surface finish grades (such as No.1, 2B, BA and HL) are judged.

Testing Scope

Austenitic stainless steel plates: Such as the 304 and 316 series. The testing emphasis is on ensuring that the nickel and chromium contents meet the standards to maintain the stable austenitic structures, as well as verifying the sensitization resistance through the intergranular corrosion testing. Their excellent corrosion resistance and formability are the key assessment points.

Ferritic stainless steel plates: Such as the 430 and 443 series. The testing core is to control the lower carbon and nitrogen contents, evaluating their plasticity, toughness and intergranular corrosion resistance. They have the good thermal conductivity and low expansion coefficients, but the poor toughness, and their cold working performance and welding heat-affected zones need to be emphatically focused on.

Martensitic stainless steel plates: Such as the 410 and 420 series. The testing is carried out around the performance after the heat treatment, including the hardness, strength and corrosion resistance after the quenching and tempering. The decisive influence of the carbon content on the hardenability and hardness needs to be analysed.

Duplex stainless steel plates: Such as the 2205 and 2507 series. The testing key is to verify the phase ratio of the austenite and ferrite (usually about 50/50), and evaluate their extremely high strength, excellent resistance to the chloride stress corrosion and pitting. The phase ratio imbalance will seriously affect the performance.

Plates of the different processing states: Covering the hot-rolled plates (hot rolling, heat treatment and pickling), cold-rolled plates (cold rolling, annealing and temper rolling) and the plates after the surface treatments (such as the polishing, grinding and titanium plating). The unique dimensional tolerances, surface roughness and performance indicators of the different states need to be tested correspondingly.

Plates of the different specifications: From the ultra-thin precision strips (thickness < 0.1 mm) to the medium and thick plates (thickness > 20 mm), the testing items have different emphases. The thin plates focus on the dimensional precision, surface finish and formability; the thick plates focus more on the internal macroscopic structures, uniformity of the mechanical properties and the flaw detection results.

Testing Methods

Spectrometry analysis method: The spark direct-reading optical emission spectrometer (OES) or X-ray fluorescence spectrometer (XRF) is adopted for the fast and accurate chemical composition analysis. The OES is suitable for the metal samples with the high precision; the XRF can carry out the non-destructive testing, commonly used for the on-site fast screening and designation identification.

Tensile testing method: In accordance with the standards GB/T 228.1 or ISO 6892-1, a universal material testing machine is used to apply the axial tensile forces to the rectangular or circular specimens prepared according to the standards, recording the force-displacement curves, thereby calculating the various tensile property parameters.

Hardness testing method: The Brinell hardness (GB/T 231.1) uses the cemented carbide ball indenter, suitable for the softer or non-uniform plates; the Rockwell hardness (GB/T 230.1) has the simple operation, and the HRB is commonly used for the annealed stainless steel; the Vickers hardness (GB/T 4340.1) has the small indentations, suitable for the thin plates or the measurement of the surface hardened layers.

Salt spray testing method: In accordance with the standards GB/T 10125 or ASTM B117, the specimens are placed in an enclosed salt spray chamber, and the 5% sodium chloride solution is sprayed, simulating the marine or industrial atmospheric environments. The corrosion resistance grade is evaluated by observing the surface corrosion conditions within the specified time.

Metallographic examination method: After the sampling, mounting, grinding, polishing and etching, the specimens are observed under a microscope. The grain size is rated according to GB/T 6394, the non-metallic inclusions are rated according to GB/T 10561, and the structure types and phase ratios are judged according to the relevant standards.

Non-destructive testing method: Including the ultrasonic testing (UT, for the internal defect flaw detection), eddy current testing (ET, for the surface and near-surface defect detection) and penetrant testing (PT, for the detection of the defects open to the surface). These methods do not damage the material and are suitable for the batch or online testing.

Testing Standards

Domestic Standards:

1. GB/T 3280, Cold rolled stainless steel plate, sheet and strip

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

3. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: 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

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

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

8. 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: The core equipment of the chemical composition analysis. The arc or spark excites the sample to generate the characteristic spectra, which are detected by the photomultiplier tubes through the grating dispersion, and the contents of up to dozens of elements in the sample can be simultaneously determined within tens of seconds, with the high precision. It is the essential instrument for the rapid analysis before the furnace and the finished product re-inspection.

Universal material testing machine: The key equipment for carrying out the mechanical property tests such as the tensile, compression and bending. Equipped with the high-precision sensors and extensometers, the loading rate is precisely controlled by the computer control system, and the data are automatically collected and the testing reports are generated, ensuring the accuracy and reliability of the mechanical property data.

Hardness tester: According to the different testing principles, it is divided into the Brinell, Rockwell, Vickers and micro-hardness testers. The modern digital hardness testers have the functions of the automatic loading, load holding, measurement and conversion, and can directly read and store the hardness values, greatly improving the testing efficiency and the objectivity of the results.

Salt spray test chamber: The dedicated equipment for simulating the accelerated corrosion environments. It has the precise temperature and humidity control, salt water spraying and saturated air barrel systems, and the chamber body is made of the corrosion-resistant materials. It can carry out the continuous or cyclic spraying tests, and is the standardized equipment for evaluating the corrosion resistance of the stainless steel.

Metallographic microscope and sample preparation equipment: Including the cutting machines, mounting machines, grinding and polishing machines, etching devices and optical/digital microscopes. The high-magnification metallographic microscope (with the image analysis system) can carry out the quantitative analysis of the structures, such as the grain size measurement and phase area percentage calculation.

Non-destructive testing equipment: The ultrasonic flaw detector (with the probes of the multiple frequencies) is used to detect the internal defects; the eddy current flaw detector is used for the online surface detection of the pipes, bars and strips; the penetrant testing kits (cleaners, penetrants and developers) are used to find the surface open defects. These are the important tools for ensuring the internal and surface quality integrity of the plate.