The role of steel shot in container pretreatment

　　Steel shot is a widely used man-made metal abrasive. It is a common material for metal surface pretreatment. The characteristics of steel shot are relatively stable. Smaller steel shot has a place in the surface polishing of fine metal parts.

　　Steel shot is used in manufacturing industries such as machinery, automobiles, ships, machine tools, containers, locomotives, surface treatment of steel structures, precision casting, oil pipelines, wind power equipment, spring steel and bearings, among which container pretreatment process is the most widely used in this industry very critical.

　　1. Query the amount of steel raw materials before shot blasting

　　Before the steel shot treatment, in order to make the quality of the pretreatment process meet the grade requirements of the regulations, the quality of the steel that has not entered the pretreatment process is checked. The items to be inquired include: steel type, raw material, serial number, manufacturer, relevant clauses in the quality assurance book and technical specification, which shall be checked one by one and registered and recorded. In addition, the surface corrosion of the steel plate before painting should also be investigated. According to the ISO8501-1 and GB50205-1992 standards, the degree of corrosion on the surface of the steel plate is divided into four grades: A, B, C, and D. According to the standard model and photos, visually compare and identify the grades. D-grade steel plates are steel plates that cannot be used in the production of containers, and C-grade steel plates are not allowed to be used in door panels, side panels, front panels, etc.

　　In order to improve the paint film quality of shot blasting and workshop primer, some auxiliary measures can be taken before shot blasting, such as pre-coating rust with a wire brush for serious rust spots; steel surfaces, especially section steel, should be scrubbed with xylene and other solvents for oil stains .

　　2. Surface cleanliness of steel after steel shot

　　In the technical documents drawn up by the box owner, it is stipulated that the cleanliness of the steel surface treatment must meet the Sa2.5 standard of ISO8501-1, or the Sa2.5 level of SIS05590-1967; or the SSPC-SP10 close to the white level.

　　3. Roughness after shot blasting

　　Most container owners specify in their technical documents that the surface roughness of container steel after shot blasting is Rz35-50um, that is to say, the average height of the roughness peak-to-valley value is 35-50um. In some foreign technical documents, the Ra roughness parameter may be quoted, which is the average distance between the assumed center line between the peak and valley values, and the theoretical sandblasting (shot blasting) roughness Rz: 4-6 times Ra.

　　When actually evaluating the roughness, the surface of the steel to be tested should be cleaned first to remove dust and debris; then, according to the type of abrasive, select an appropriate comparison sample (G or S sample), and compare it with a certain surface on the surface to be tested. The area constitutes a comparison, and the grade and approximate value of the roughness of the measured surface can be confirmed by visually comparing the measured surface with the 4 parts on the sample block in turn.

　　In the actual use process, other adjustments can be made according to the actual cleaning effect of the steel shot. During the application process, the loss of the steel shot should also be calculated and replaced regularly, otherwise the surface cleaning will not be clean.