A Complete Guide to Metal Materials Inspection, Testing and Certification Process

April 17, 2026

Metal materials fall under the category of metallurgical products. The production, ordering, transportation, use, storage and inspection of metal materials must be carried out in accordance with unified technical standards - metallurgical product standards. Staff engaged in metal material-related work must be familiar with the relevant contents of these standards. The standards used in China's metallurgical products are divided into three levels: national standards (denoted by "GB"), departmental standards (metallurgical industry department "YB", Ministry of Machinery Industry "JB", etc.) and enterprise standards. These standards are also important bases for factory audits and product certifications.
1. Packaging Inspection
Based on the type, shape, size, precision and anti-corrosion requirements of the metal materials, the packaging methods are classified as follows:
Bulk: Unpackaged, such as ingots, blocks (not prone to corrosion and not valuable), large steel products (large steel, thick steel plates, railway tracks), pig iron, etc.
Bundles: Materials with smaller dimensions and those that are not significantly affected by corrosion in terms of usage, such as medium-sized steel, steel pipes, wire rods, thin plates, etc.
Cases (Buckets): Products with high anti-corrosion requirements, small size and thin thickness, such as tinplate, silicon steel sheets, magnesium ingots, etc.
Axial components: wire rods, steel cables, steel strands, etc.
When conducting the inspection, the inspectors should first check the completeness of the packaging for bundled, boxed, and axle-packaged products, as well as whether there are any damages or deformations. The quality of the packaging directly affects the protection of the products during transportation and storage, and it is also an assessment point for the packaging process during the factory inspection.
II. Marking Inspection (Certification Mark)
The marking serves as an identification for the material's quality and specifications, mainly including the supplier's name, brand, inspection batch number, specifications, dimensions, grade, net weight, etc. The marking forms include:
Coloring: Apply different colored paint on the end face or end part of the metal material. This method is mainly used for steel, pig iron, and colored raw materials.
Printing: At the designated positions (such as the end face or end), apply a steel stamp or spray paint to indicate the brand, specification, standard number, etc. This is mainly used for medium and thick plates, profiles, and colored materials, etc.
Labeling: Metal materials in bundles, boxes, or rolls are labeled externally on the signboards, indicating the model number, size, weight, standard number, supplier, etc.
During the inspection process, carefully identify the labels and ensure they are consistent with the order requirements. The labels of certified products (such as export materials) should comply with the certification standards of the target market (such as CE, ISO, etc.). Proper protection of the labels should be ensured during transportation, storage, and other processes.
III. Inspection of Specifications and Dimensions
Specifications and dimensions refer to the nominal dimensions of the main parts of the metal material (length, width, thickness, diameter, etc.).
Nominal dimension (nominal size): The ideal size, which differs from the actual size.
Dimension deviation: The difference between the actual size and the nominal size. A positive deviation occurs when the actual size is greater than the nominal size, while a negative deviation occurs when the actual size is smaller. Within the specified standard range, it is considered a permissible deviation. Any deviation beyond this range is classified as a dimension exceeding limit and is regarded as a non-conforming product. During inspection, it should be determined according to the AQL standard.
Accuracy grade: A classification based on the size of allowable deviations, such as ordinary, higher, and superior. The accuracy grade should match the certification requirements.
Delivery length (width): Main delivery dimensions.
Typical length (unfixed length): Within the specified length range, no fixed requirements are imposed.
Short length (narrow length): Less than the lower limit of the usual length but not less than the minimum allowable length.
Fixed measurement length: The length specified by the buyer in the contract (with a normal positive deviation).
Length per piece: The required length must be an integer multiple of the specified length (including the saw cut and positive deviation).
When conducting the inspection, pay attention to measuring the specific areas and choosing the appropriate measuring tools (such as calipers, tape measures, micrometers, etc.). These tools need to be calibrated regularly, and during the factory inspection, the calibration records should be checked.
IV. Quantity Inspection
The quantity of metal materials generally refers to weight (except for pads, fish tail plates, etc., which are counted by pieces). The method of quantity inspection is:
By actual weight measurement: Generally, all items undergo weighing inspection. For items with sturdy packaging (such as boxes, containers, barrels, etc.), the gross weight, net weight and tare weight are indicated on the packaging. For thin steel plates, silicon steel sheets, and iron alloys, at least 5% of the batch can be randomly inspected; if the weight of the randomly inspected items differs significantly from the marked weight, all the packages must be opened for weighing.
According to theoretical conversion calculation: Calculate the weight based on the nominal size (actual size) and specific gravity. This method is applicable to materials such as standard-length plates. When converting, pay attention to the formula and the actual specific gravity.
The inspectors should verify that the quantity matches the order and record it in the inspection report. During the factory inspection, it is necessary to check the calibration certificates of weighing scales, scales, and other measuring equipment.
V. Surface Quality Inspection (Appearance Defects)
Surface quality inspection mainly examines the appearance, shape and surface defects of the materials. Common defects include:
Ellipticity: The phenomenon where the diameters in different directions on the same circular cross-section are not equal. It is expressed by the difference between the maximum and minimum diameters.
Bending, bending degree: The material is not straight in the length or width direction and takes on a curved shape. The degree of this curvature is expressed by a bending degree value.
Twist: A strip material is twisted along the longitudinal axis to form a spiral shape.
Sickle bend (side bend): The bending of metal plates, strips, and rectangular section profiles along the narrow side, indicated by the depth of the concave part.
Warp curvature: The board or strip exhibits wave-like undulations in both the length and width directions. The degree of this curvature is expressed by a warp curvature value.
Surface cracks: Cracks on the metal surface.
Ear: A protrusion extending in the direction of rolling, caused by improper alignment of the rolling rolls.
Scratch: Linear or arc-shaped grooves on the surface, with the bottom of the grooves visible.
Scar: A thin layer with a tongue-like, nail-like or fish-scale-like appearance.
Bonding: Intermolecular adhesion between metal plates, foils, and strips during the rolling and annealing process. After being lifted, traces remain.
Iron oxide scale: The metal oxide that forms on the surface during the heating, rolling and cooling processes.
Folding: The surface duplex metal layer formed by hot rolling (or forging) overlaps.
Blemish: A rough surface with unevenness on its surface.
Subcutaneous bubbles: Small raised bumps or cracked chicken-foot-shaped fissures distributed irregularly on the surface.
Surface defects are mainly caused by improper operations during production, transportation, handling, and storage. During inspection, it is necessary to determine whether the defects are allowed to exist or exceed the limit according to the standards. Certified products have stricter requirements for surface quality.
VI. Assurance Conditions for Internal Quality Inspection of Metal Materials
The internal quality inspection of metal materials is based on the material's suitability for different requirements. The assurance conditions are as follows:
Basic guarantee conditions: Minimum requirements that must be met regardless of whether they are explicitly stated or not, such as chemical composition, basic mechanical properties, etc. These are the foundation of the certification.
Additional guarantee condition: The supplier will conduct the inspection only after the demander specifies the requirements in the purchase order contract, and will guarantee that the results meet the regulations.
Agreement guarantee conditions: The items that are negotiated by both the supplier and the buyer and guaranteed in the contract.
Reference conditions: The inspection will be conducted through mutual agreement, but it is for reference only and shall not be used as a basis for assessment.
Internal quality inspection mainly includes mechanical properties, physical properties, chemical properties, process performance, chemical composition and internal structure inspection. During factory audits, it is necessary to verify the inspection records of the suppliers and their laboratory capabilities.
VII. Chemical Composition Testing (Laboratory Certification)
Chemical composition is the primary factor determining the performance and quality of metal materials. The testing methods mainly include:
Chemical analysis method: Determine the composition by chemical reactions. It is divided into qualitative analysis (identifying the types of elements) and quantitative analysis (measuring the content). Quantitative analysis commonly uses gravimetric analysis and volumetric analysis methods.
Weight analysis method: By using separation techniques, the measured element is separated from other components, and then the content is calculated through weighing.
Spectral analysis method: Utilizing the characteristic spectra of elements for qualitative and quantitative analysis, it is fast and highly accurate, and is commonly used by certification laboratories.
Spark analysis method: It is simple in equipment and fast in operation, and is suitable for the simple identification of steel components.
During the inspection process, the supplier can be requested to provide a third-party inspection report or certification document to prove that the chemical composition complies with the standards. During the factory inspection, the calibration of laboratory equipment and the qualifications of personnel should be verified.
VIII. Link between Factory Audit and Certification
Supplier Audit: It is necessary to verify the process control of the smelting, rolling, and heat treatment procedures in metal material production enterprises; the calibration records of testing equipment (universal testing machines, hardness testers, spectrometers, calipers, etc.); the qualifications of inspection personnel; and the certification certificates of raw materials and finished products (such as ISO 9001, CE, PED, etc.).
Product Certification: For the export of metal materials, they must comply with the certification requirements of the target market (such as the EU CE mark, the ASTM standard in the US, the JIS standard in Japan, etc.). During the inspection process, verify the consistency between the product labels, quality assurance documents and the certifications.
Continuous improvement: For the non-conformities identified during the inspection, a rectification report should be prepared. The inspection process should be used to close the loop, and a quality traceability file should be established.
Through the above inspection, testing, factory verification and certification processes, it is ensured that the metal materials meet all the requirements in terms of chemical composition, dimensional accuracy, surface quality and internal performance, thus meeting the market access standards in both domestic and international markets.