Stainless Steel Channel Steel Testing

Stainless steel channel inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, dimension and shape inspection, surface quality inspection, corrosion resistance 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: This testing aims to determine the precise contents of the alloying elements such as the carbon (C), chromium (Cr), nickel (Ni), molybdenum (Mo) and manganese (Mn) in the stainless steel channels. This is the basis of judging whether their designations (such as 304, 316 and 201) match and the corrosion resistance. The spectroscopic analysis method is usually adopted, which can quickly and accurately provide the full-element reports, ensuring that the material compositions meet the requirements of the relevant standards (such as GB/T 20878 and ASTM A276).

Mechanical property testing: Mainly evaluating the performance of the channels under the stressed states, including the tensile testing (determining the yield strengths, tensile strengths and elongations after fracture), hardness testing (Brinell, Rockwell and Vickers hardness) and impact toughness testing. These data are directly related to the safety and reliability of the channels in the applications such as the load-bearing structures and mechanical frames, and are the key link of verifying whether they meet the standards such as GB/T 1220 or ASTM A484.

Dimension and shape inspection: The tools such as the callipers, micrometers, templates and laser scanners are used to precisely measure the geometric parameters such as the heights, flange widths, web thicknesses, flange thicknesses, web bending deflections and flange edge slopes of the channels. The strict dimension tolerance control (according to GB/T 706 or JIS G3192) is crucial for ensuring the assembly precision with the connectors and guaranteeing the overall structural stability, avoiding the installation difficulties or stress concentrations caused by the dimension deviations.

Surface quality inspection: Through the visual inspection or with the help of the tools such as the magnifying glasses and endoscopes, the surface smoothness of the channels and whether the defects such as the cracks, laps, scars, pits, scratches and rust exist are evaluated. For the stainless steel materials, the surface states not only affect the appearance, but are also the direct reflection of the integrity and corrosion resistance of their passivation films. The depths, lengths and distributions of the defects need to be judged according to the standards.

Corrosion resistance testing: This is one of the core testing items of the stainless steel channels. The commonly used methods include the neutral salt spray testing (NSS), copper-accelerated acetic acid salt spray testing (CASS), as well as the pitting potential measurement and intergranular corrosion testing (such as the sulfuric acid-copper sulfate method) for the specific environments. Through these accelerated corrosion tests, the corrosion resistance of the channels of the different materials in the harsh environments can be evaluated and compared, providing the basis for the material selection.

Metallographic structure analysis: Through the sampling, mounting, grinding, polishing and etching, the micro-structures of the material are observed under the optical or electron microscopes. The phase compositions such as the austenite, ferrite and martensite, grain size grades, morphologies and grades of the non-metallic inclusions (such as the sulfides and oxides), as well as whether the abnormal structures such as the overheating, overburning and σ phase precipitation exist can be checked, thereby relating them to the mechanical properties and corrosion resistance.

Testing Scope

By material designations: The testing scope covers the common and special purpose stainless steel channel designations, such as the austenitic series 304/304L and 316/316L; the ferritic series 430; the martensitic series 410; as well as the duplex steels 2205, etc. The testing emphases of the different designations vary, for example, for 304 the intergranular corrosion is emphasized, and for the duplex steels the two-phase ratio needs to be paid attention to.

By production processes: Including the hot-rolled stainless steel channels and cold-formed stainless steel channels. For the hot-rolled channels, the structure uniformity and decarburized layers under the rolling states need to be emphatically tested; for the cold-formed channels, the thinning rates at the bends, surface scratches, as well as the hardness increases and residual stresses caused by the cold working need to be paid attention to.

By section specifications: All the series from the small specifications (such as 50*37*4.5) to the large specifications (such as 400*102*12.5) are within the testing scope. For the large specification channels, the mechanical property uniformity of the various parts of the sections, as well as the shape defects such as the warping and twisting which are more likely to occur due to the large dimensions need to be paid attention to.

By application fields: Including the channels used in the different fields such as the building structures, curtain wall supports, mechanical equipment frames, food and pharmaceutical equipment, offshore platforms and chemical container internals. The testing needs to be combined with the specific service environments, for example, the chloride ion corrosion resistance testing needs to be tightened for the marine environments, and the surface cleanliness and passivation effects need to be paid attention to for the food industry.

By delivery states: Covering the products of the various surface treatment states such as the solution treatment state, pickling and passivation state and polishing state. The testing needs to verify the treatment effects, such as whether the solution treatment is sufficient to eliminate the carbide precipitation, and whether the passivation films are complete and uniform.

By standard systems: The testing needs to be carried out according to the specific product standards under the different systems such as the national standards (GB), international standards (ISO), American Society for Testing and Materials standards (ASTM), Japanese Industrial Standards (JIS) and European standards (EN), depending on the customer requirements or target markets.

Testing Methods

Spectroscopic analysis method: Mainly used for the fast quantitative analysis of the chemical compositions. The common ones include the spark direct-reading spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES has the high precision, suitable for the furnace-front and laboratory precise analysis; the XRF is the non-destructive testing, convenient for the on-site material designation identification. Both methods need to be calibrated with the standard samples to ensure the data accuracy.

Tensile testing method: According to GB/T 228.1 or ASTM A370, the standard tensile specimens are made from the legs or webs of the channels, and are loaded at the constant rates on the universal testing machines until the fracture. Through the recorded force-displacement curves, the key mechanical indicators such as the proof strengths at the plastic extension (Rp0.2), tensile strengths (Rm) and elongations after fracture (A) can be calculated.

Salt spray testing method: According to GB/T 10125 or ASTM B117, the channel specimens are placed in the closed salt spray test chambers, and the sodium chloride solutions of the specific concentrations are continuously or intermittently sprayed, simulating the marine or industrial atmospheric environments. Through the observation of the time of the corrosion product appearance on the specimen surfaces, their corrosion resistance grades are evaluated. The test cycles can range from 24 hours to thousands of hours.

Ultrasonic flaw detection method: A non-destructive testing method, used to detect whether the channels have the internal defects such as the cracks, lamination and slag inclusions. The probes emit the high-frequency ultrasonic waves into the workpieces, and the positions and sizes of the defects are judged through the reception of the reflected echo times and intensities. This method is particularly effective for detecting the internal defects parallel to the surfaces.

Penetrant flaw detection method: Suitable for detecting the fine open defects (such as the cracks and blowholes) on the surfaces of the stainless steel channels. The penetrant containing the fluorescent or red dyes is applied to the surfaces, making it seep into the defects. After removing the excess penetrant, the developer is applied, and the penetrant in the defects is sucked out to display the indications. This method has the simple equipment and convenient operation.

Metallographic microscope method: The classic method of the micro-structure observation. After the cutting, mounting, coarse grinding, fine grinding and polishing of the specimens, the specific etchants (such as the aqua regia and oxalic acid electrolysis) are used to etch the surfaces, making the grain boundaries and phase structures appear, and then the observation and photographing are carried out under the metallographic microscopes, and the grain sizes are rated according to the standards such as GB/T 6394.

Testing Standards

Domestic Standards:

1. GB/T 706, Hot-rolled section steel — Dimensions, shape, weight and allowable deviations

2. GB/T 20878, Stainless and heat-resisting steels — Designation and chemical composition

3. GB/T 1220, Stainless steel bars

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 6394-2017, Determination of estimating the average grain size of metal

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

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

4. ASTM A276, Standard specification for stainless steel bars and shapes

5. ASTM A484, Standard specification for general requirements for stainless steel bars, billets, and forgings

6. ASTM B117, Standard practice for operating salt spray (fog) apparatus

7. JIS G3192, Dimensions, mass and permissible variations of hot rolled steel sections

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 core equipment of the chemical analysis. Through the excitation of the samples to generate the characteristic spectra, the photomultiplier tubes or CCD detectors receive them after the grating dispersion, and the contents of the various elements are calculated by comparing with the standard curves. The modern spectrometers have the high automation degree and fast analysis speeds, and can complete the simultaneous determination of the multiple elements including the trace elements such as the C, S and P within tens of seconds.

Universal material testing machine: The key equipment of carrying out the mechanical testing such as the tensile, compression and bending. It consists of the loading frames, servo hydraulic or electric drive systems, high-precision force transducers and displacement measurement systems, as well as the computer control systems. It can precisely control the loading rates, collect and process the data in real time, and output the complete test reports and curves.

Salt spray corrosion test chamber: The dedicated equipment used to simulate the accelerated corrosion environments. The chambers are made of the corrosion-resistant materials, equipped with the spraying systems, heating systems, saturated air barrels and control systems. The chamber temperatures, salt spray sedimentation amounts and spraying/stopping cycles can be precisely controlled, to meet the testing requirements of the different standards.

Ultrasonic flaw detector: Consisting of the pulse generators, probes, receiving amplifiers and display units. The digital flaw detectors have the functions such as the A-scan waveform display, B-scan imaging, automatic defect alarm, data storage and transmission. Combined with the probes of the different frequencies and angles, they can adapt to the testing needs of the channels of the various specifications.

Metallographic specimen preparation system: Including the cutting machines, mounting machines, pre-grinding machines and polishing machines. The automatic polishing machines can set the pressures, speeds and times, and combined with the diamond polishing agents or suspensions of the different particle sizes, can efficiently prepare the deformation-free specimen surfaces without the scratches and comets, laying the foundation for the high-quality metallographic observation.

Three-dimensional coordinate measuring machine: Used for the high-precision dimension and geometric tolerance testing. Through the probe movement on the three mutually perpendicular guide rails, the spatial coordinates of the various characteristic points of the channels can be precisely measured, and the geometric parameters such as the lengths, angles, roundness, straightness and parallelism can be automatically calculated, with the efficiency far higher than the traditional manual measurement.