Stainless Steel Plate Testing

Stainless steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, corrosion resistance testing, macro and micro structure examination, dimension and shape inspection 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: This testing aims to determine the contents of the various alloying elements in the stainless steel plate, such as the chromium (Cr), nickel (Ni), molybdenum (Mo), carbon (C) and manganese (Mn). Through the precise chemical analysis, it can be verified whether the material designation conforms to the standard requirements, and it directly affects its corrosion resistance, mechanical properties and welding performance. It is the core basis of the raw material acceptance and material determination.

Mechanical property testing: This testing is used to evaluate the behaviour characteristics of the stainless steel plate under the loads. The main indicators include the tensile strength, yield strength, elongation after fracture and hardness (Brinell, Rockwell or Vickers). These data are crucial for evaluating the load-bearing capacity, forming processing performance and the ability to resist the deformation and failure of the steel plate in the structural applications.

Corrosion resistance testing: The corrosion resistance is the core value of the stainless steel. The testing items include the intergranular corrosion testing (such as the sulphuric acid-copper sulfate method), pitting potential determination, salt spray testing and the uniform corrosion rate determination under the specific media. These tests simulate the ability of the material to resist the various types of corrosion (such as the pitting, crevice corrosion and stress corrosion cracking) in the actual service environments.

Macro and micro structure examination: The macro examination mainly observes whether there are the macro defects such as the cracks, delaminations, inclusions and shrinkage cavities on the surface and cross-sections of the steel plate. The micro structure examination (metallographic examination) uses the microscopes to observe the grain sizes, inclusion types and grades, phase compositions (such as the austenite and ferrite proportions) and whether there are the σ phase precipitations, to reveal its internal quality and heat treatment states.

Dimension and shape inspection: This testing ensures that the geometric dimensions of the steel plate meet the ordering requirements. Specifically including the measurement of the thickness, width, length and their allowable deviations, as well as the judgement of the plate shape defects such as the flatness, camber and waviness. The precise dimension control is the basis of the subsequent processing and assembly.

Surface quality inspection: For the different surface finish grades (such as 2B, BA, No.1 and No.4), the surface conditions are inspected. The main inspection items include whether there are the defects such as the residual oxide scales, scratches, pits, roll marks, rust and over-pickling, and the surface roughness and gloss are evaluated, ensuring that the aesthetic or specific process requirements are met.

Testing Scope

Austenitic stainless steel plates: Such as the 304 and 316 series, and are the most widely used types. The testing emphasis is on the verification of their high nickel and high chromium contents, excellent corrosion resistance and non-magnetism confirmation, as well as the structure uniformity after the solution treatment, preventing the increase of the intergranular corrosion tendency caused by the sensitization treatment.

Ferritic stainless steel plates: Such as the 430 series, with the magnetism and low nickel contents. The testing needs to focus on their chromium contents, lower ductile-brittle transition temperatures, and the possible grain coarsening and 475 ℃ brittleness after the welding or high-temperature heating, ensuring their applicability under the specific environments.

Martensitic stainless steel plates: Such as the 410 and 420 series, which can be strengthened through the heat treatments. The testing core lies in their carbon content control, hardness and strength distributions after the quenching and tempering, as well as the corresponding corrosion resistance (relatively weaker) assessment. Commonly used for the cutting tools and structural parts with the strength and hardness requirements.

Duplex stainless steel plates: Such as the 2205, with both the austenite and ferrite phases. The difficulty and emphasis of the testing lie in the precise control of the two-phase proportions (usually about 50% each), the verification of the extremely high strength and excellent resistance to the chloride stress corrosion cracking, as well as the screening of the harmful intermetallic phase precipitations.

Precipitation hardening stainless steel plates: Such as the 17-4PH, with the strengthening phases precipitated through the aging treatments. The testing needs to precisely analyse the contents of the precipitation strengthening elements such as the copper and niobium, evaluate the mechanical properties (ultra-high strength) after the solution treatment and aging treatment, and inspect their work hardening characteristics.

Steel plates of the different processing states: The testing scope covers the hot-rolled plates, cold-rolled plates, heat-treated state (annealed, solution treated, quenched and tempered) plates and the plates of the different surface finish states (polished, pickled and sandblasted). The testing emphases are different in the different states, for example, the cold-rolled plates pay more attention to the surface quality and formability.

Testing Methods

Spectrometry analysis method: Used for the fast and accurate chemical composition determination. The common ones include the spark discharge atomic emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES has the high precision, suitable for the furnace-front analysis and precise laboratory determination; the XRF, as a non-destructive testing means, is commonly used for the fast on-site designation identification and screening.

Tensile testing method: In accordance with the standards such as GB/T 228.1 or ASTM A370, carried out on a universal material testing machine. By preparing the standard tensile specimens and recording the load-displacement curves, the key mechanical property parameters such as the tensile strength, yield strength (usually adopting Rp0.2) and elongation after fracture are calculated.

Metallographic examination method: This is the standard method of observing the micro-structures of the material. The process includes the sampling, mounting, grinding, polishing and etching (such as using the aqua regia and ferric chloride hydrochloric acid solutions), and finally the structure morphologies are observed and photographed under an optical microscope or scanning electron microscope, carrying out the grain size rating, phase analysis and defect judgement.

Corrosion testing method: The methods are various. The intergranular corrosion commonly uses GB/T 4334 (sulphuric acid-copper sulfate corrosion method); the pitting evaluation uses the electrochemical potentiodynamic reactivation method; the salt spray testing is carried out in the neutral salt spray chambers according to the GB/T 10125 standard, evaluating its long-term atmospheric corrosion resistance; the uniform corrosion is calculated with the corrosion rates through the immersion weight loss method.

Non-destructive testing method: Detects the internal and surface defects without damaging the material. The ultrasonic testing (UT) is used to detect the internal delaminations and inclusions; the penetrant testing (PT) is used to detect the surface open defects; the eddy current testing (ET) is suitable for the fast detection of the surface and near-surface defects of the conductive materials, commonly used for the thin plates.

Dimension and morphology measurement method: The thickness is commonly measured with the ultrasonic thickness gauges or micrometers; the length and width are measured with the steel tape measures or laser range finders; for the plate shape indicators such as the flatness, the steel plate is placed on a platform, and the maximum clearances between it and the platform are measured with the feeler gauges. The surface roughness is quantitatively evaluated with the stylus-type roughness testers.

Testing Standards

Domestic Standards:

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

2. GB/T 4237, Hot rolled stainless steel plate, sheet and strip

3. GB/T 4334, Corrosion of metals and alloys — Corrosion test method for intergranular corrosion tendency of austenitic stainless steel

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

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

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

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

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

International Standards:

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

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

3. 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: As the main equipment of the chemical composition analysis, it can simultaneously determine the contents of more than ten elements in the steel within dozens of seconds, with the high precision and good repeatability. The equipment needs to be regularly calibrated with the standard samples, ensuring the accuracy of the analysis results. It is the standard instrument of the iron and steel enterprise laboratories.

Universal material testing machine: Used for the mechanical property tests such as the tensile, bending and compression. The modern testing machines are all equipped with the computer control systems, which can automatically acquire the data, draw the curves and calculate the various performance indicators. The extensometers need to be used for the precise deformation measurement, and the test environment temperatures and loading rates need to conform to the standard provisions.

Metallographic microscope system: Including the inverted or upright optical microscopes, image acquisition systems and analysis software. Used for the micro-structure observation, photographing and quantitative analysis, such as the automatic grain size rating and phase area percentage calculations. Equipped with the high-power objectives and differential interference contrast (DIC) functions, the structure details can be better detected and observed.

Electrochemical workstation: The precision instrument used to evaluate the corrosion resistance of the material. Through the tests such as the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), the key electrochemical parameters such as the free corrosion potentials, pitting potentials and re-passivation potentials of the material can be determined, scientifically evaluating its corrosion behaviour and mechanisms in the electrolytes.

Salt spray test chamber: The accelerated corrosion testing equipment simulating the marine or industrial atmospheric environments. By atomizing the sodium chloride solution, a constant corrosive environment is created in the enclosed chamber body, used to assess the long-term corrosion resistance of the coatings or substrates of the stainless steel plates. The test cycles can range from dozens of hours to thousands of hours.

Ultrasonic flaw detector and thickness gauge: The ultrasonic flaw detector transmits the high-frequency sound waves into the material through the probes and receives the echoes, thereby locating and evaluating the positions and sizes of the internal defects. The ultrasonic thickness gauge has the similar principles, specially used for the precise measurement of the plate thicknesses, especially suitable for the occasions where only one side is accessible.

Surface profilometer/roughness tester: Used for the quantitative measurement of the surface roughness parameters (such as Ra and Rz) of the steel plate. The diamond styluses of the stylus-type profilometers move across the surface, and the vertical displacements are converted into the electrical signals through the sensors, obtaining the surface profile curves after the processing and calculating the various roughness values, evaluating the surface processing quality.