304 Stainless Steel Material Testing

304 material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, metallographic structure examination, corrosion resistance testing, dimension and appearance inspection and non-destructive testing, 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 is the basis of verifying the material identity, mainly according to the standards such as GB/T 222 and ASTM A751, determining the contents of the key elements such as the carbon (C), chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), phosphorus (P) and sulfur (S) in the 304 stainless steel. Through the precise analysis, ensuring that the alloy compositions meet the standard requirements, and it is the primary basis of judging whether the material is the qualified 304 stainless steel.

Mechanical property testing: This testing aims to evaluate the load-bearing capacity and deformation characteristics of the material, and is the important guarantee of the material service safety. The core testing indicators include the tensile strength (≥515MPa), specified plastic extension strength (≥205MPa), elongation after fracture (≥40%) and hardness (usually the Brinell hardness ≤201HBW). The testing methods follow the standards such as GB/T 228.1 and ASTM A370.

Metallographic structure examination: Through the observation and analysis of the micro-structures of the material, its heat treatment states, grain sizes, non-metallic inclusions, as well as whether there are the defects such as the overheating, overburning and σ phase precipitation are judged. According to the standards such as GB/T 13298 and ASTM E112, the typical austenitic structures are observed with the metallographic microscopes, and it is the key means of evaluating the internal quality and process stability of the material.

Corrosion resistance testing: This is the key item of evaluating the "stainless" characteristic, the core property of the 304 stainless steel. The common testing methods include: the copper-copper sulfate-sulfuric acid intergranular corrosion testing (method C) according to GB/T 4334, the oxalic acid electrolytic etching testing (method A) according to ASTM A262, and the salt spray testing. Through these tests, the stability of the material in the specific corrosive media is verified.

Dimension and appearance inspection: For the 304 materials of the different forms such as the plates, pipes and bars, the geometric dimensions (thicknesses, outer diameters and lengths, etc.) and permissible deviations are measured according to the corresponding product standards (such as GB/T 3280 and GB/T 14976). At the same time, whether the surfaces have the defects such as the cracks, folds, scars, pits and scratches is checked through the visual inspection or low-power magnifying glasses.

Non-destructive testing: Under the premise of not destroying the material, its internal and surface defects are probed. The common methods include: liquid penetrant testing (PT, for the surface open defects), ultrasonic testing (UT, for the internal defect flaw detection) and eddy current testing (ET, for the fast detection of the surface and near-surface defects of the conductive materials), according to the standards such as GB/T 18851 and GB/T 6402.

Testing Scope

Various rolled products: The testing scope covers the common product forms of the 304 stainless steel, including the hot-rolled/cold-rolled plates, strips and coils, whose thicknesses range from the ultra-thin precision strips to the thick plates. The testing needs to be carried out according to the standards corresponding to the different thickness specifications, focusing on the conformity of their mechanical properties, surface qualities and dimension tolerances, ensuring that the material meets the subsequent processing requirements.

Pipes and pipe fittings: Covering the seamless steel pipes (GB/T 14976), welded steel pipes (GB/T 12771) and the matching fittings such as the elbows and tees. In addition to the chemical compositions and mechanical properties, the testing emphases also include the hydrostatic testing or non-destructive flaw detection to ensure the pressure-bearing sealing, as well as the precise measurement of the pipe diameters and wall thicknesses.

Bars, wires and sections: Including the hot-rolled, forged and cold-drawn round steels, square steels, angle steels, channel steels and other sections, as well as the wire rods and steel wires. The testing needs to focus on the cross-section dimension precision such as the diameters and side lengths, straightness, as well as the special properties required by the uses, such as the upsetting testing of the cold heading wires.

Castings and forgings: For the 304 stainless steel parts formed through the casting or forging processes, the testing scope focuses on the defects that may be introduced by the specific processes, such as the shrinkage cavities, porosity and segregation of the castings, and the flow lines and folds of the forgings. The testing items usually include the stricter ultrasonic flaw detection and comprehensive mechanical property testing.

Welded joints and welds: For the welded structures of the 304 material, the testing scope focuses on the welded joint areas. The testing items include the chemical compositions of the welds and heat-affected zones, metallographic structures (observing whether the sensitization or harmful phases occur), mechanical properties (such as the weld tensile and bending testing) and non-destructive testing (RT, UT and PT), ensuring the welding quality.

In-service equipment and components: The periodic inspection or failure analysis of the 304 stainless steel equipment in use also belongs to the important testing scope. Mainly through the on-site non-destructive testing (such as the ultrasonic thickness measurement and penetrant testing), the corrosion thinning, stress corrosion cracking and pitting are evaluated, and combined with the metallographic and composition analysis, the failure causes are traced.

Testing Methods

Spark atomic emission spectrometry (OES): This is the mainstream method of carrying out the fast quantitative analysis of the chemical compositions. The sample is used as the electrode, and under the excitation light sources, the characteristic spectra are generated, and the contents are determined by detecting the spectral line intensities of the various elements through the dispersion systems. This method has the fast analysis speed and high precision, suitable for the fast furnace front analysis and precise laboratory testing.

Inductively coupled plasma optical emission spectrometry (ICP-OES): The sample is dissolved into the solution, and after the atomization, excited by the argon plasma, and the quantitative analysis is carried out by measuring the characteristic spectral line intensities. This method has the low detection limits, wide linear ranges and strong multi-element simultaneous analysis abilities, especially suitable for the precise analysis of the trace elements (such as P and S).

X-ray fluorescence spectrometry (XRF): A non-destructive surface composition analysis method. The X-rays are used to irradiate the sample, and the wavelengths and intensities of the excited secondary X-rays (fluorescence) are measured for the qualitative and quantitative element analysis. It is suitable for the fast non-destructive screening of the finished or semi-finished products, but the ability to analyse the light elements (such as C) is weak.

Tensile testing method: The core method of determining the mechanical properties of the material. The standard specimens are slowly loaded on the tensile testing machines until the fracture, and the tensile strength, yield strength, elongation after fracture and reduction of area are calculated through the recorded force-displacement curves. The testing strictly follows the GB/T 228.1 standard, which has the clear provisions for the specimen shapes, dimensions and loading rates.

Hardness testing method: The indenters of the specific shapes are pressed into the material surfaces under the standard test forces, and the hardness of the material is evaluated by measuring the indentation sizes. The 304 stainless steel commonly uses the Brinell hardness (HBW, the indenters are the cemented carbide balls) and Rockwell hardness (HRB) testing, respectively carried out according to the GB/T 231.1 and GB/T 230.1 standards.

Metallographic analysis method: Through a series of sample preparation steps such as the sampling, mounting, grinding and polishing and etching (commonly using the aqua regia or oxalic acid electrolysis), the specimens observable with the microscopes are prepared. The micro-structures, grain sizes and inclusion morphologies and distributions are observed under the optical metallographic microscopes or scanning electron microscopes, and it is the important bridge connecting the material processes, structures and properties.

Testing Standards

Domestic Standards:

1. GB/T 222, Permissible tolerances for chemical composition of steel products

2. GB/T 223, Methods for chemical analysis of iron, steel and alloy

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

4. GB/T 14976, Seamless stainless steel pipes for fluid transport

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

6. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

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

8. GB/T 4334, Method of copper sulfate-sulfuric acid test for intergranular corrosion of stainless steels

9. GB/T 13298, Inspection method of microstructure for metals

10. GB/T 6402, Steel forgings — Method for ultrasonic testing

International Standards:

1. ASTM A751, Standard test methods, practices, and terminology for chemical analysis of steel products

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

3. ASTM A262, Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels

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

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

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

Spectrometer: The core equipment of the chemical composition analysis. Mainly including the bench-type/mobile direct-reading spectrometers (OES) and X-ray fluorescence spectrometers (XRF). The former has the high analysis precision and is often used in the laboratories; the latter does not need to destroy the samples and is suitable for the fast on-site screening. The high-precision laboratories are also equipped with the ICP-OES or carbon-sulfur analysers for the precise determination of the elements such as C and S.

Universal material testing machine: The key equipment of carrying out the mechanical property testing such as the tensile, compression and bending. The modern testing machines are mostly the microcomputer-controlled electro-hydraulic servo or electronic universal testing machines, which can precisely control the loading rates, automatically collect and process the data, and output the complete stress-strain curves and test reports, ensuring that the results meet the requirements of the standards such as ASTM, ISO and GB.

Hardness tester: According to the different testing principles, mainly divided into the Brinell hardness testers, Rockwell hardness testers, Vickers hardness testers and portable Leeb hardness testers. The laboratories commonly use the former three for the precise measurement, while the portable hardness testers are suitable for the large workpieces or on-site online testing, and the necessary conversion and calibration of the testing results can be carried out.

Metallographic microscope and sample preparation equipment: The systematic equipment of the metallographic analysis. Including the cutting machines and mounting machines for the sampling, the automatic or manual grinding and polishing machines for the grinding and polishing, and the electrolytic etching instruments or etching dishes for the etching. The core is the metallographic microscopes, equipped with the observation modes such as the bright field, dark field and polarization, and often connected with the image analysis systems for the quantitative measurement such as the grain sizes and phase proportions.

Non-destructive testing equipment: Including the various professional instruments. The penetrant testing uses the colour or fluorescent penetrants, cleaners and developer kits; the ultrasonic testing requires the ultrasonic flaw detectors, probes of the different frequencies and angles, and standard test blocks; the eddy current testing requires the eddy current flaw detectors and probes. These equipment are used to probe the material defects from the different dimensions.

Corrosion testing equipment: The dedicated devices of evaluating the corrosion resistance of the material. The intergranular corrosion testing needs to be equipped with the controllable-temperature heating devices (such as the constant-temperature water baths or glycerol baths) and the containers for the corrosion solutions. The salt spray test chambers are the accelerated corrosion equipment simulating the marine atmospheric environments, and the salt spray corrosion resistance of the material is assessed by the continuous spraying of the sodium chloride salt mist.