How to ensure the quality of materials for polishing storage tanks
Information summary：To ensure the quality of polishing tank materials, the core is to select the right materials, control incoming materials, strictly manufacture, and conduct precise testing, covering the entire process from raw materials to finished products. The following are the key points that can be implemented:
To ensure the quality of polishing tank materials, the core is to select the right materials, control incoming materials, strictly manufacture, and conduct precise testing, covering the entire process from raw materials to finished products. The following are the key points that can be implemented:
1、 Material selection: Matching working conditions, compliance priority
It is necessary to select materials correctly based on storage medium, pressure, and temperature, while strictly meeting national and industry standards. Among them, in the fields of food and medicine, due to the requirements of sterility and low adsorption, 304 or 316L stainless steel (corresponding to grades S30408 and S31603) is preferred. The inner surface roughness Ra is required to be ≤ 0.8 μ m, the carbon content of 316L material should be ≤ 0.03%, the chromium content of 304 material should be ≥ 18%, and the nickel content should be ≥ 8%, in accordance with GB 4806.9 and GB/T 3280 standards; For chemical and highly corrosive environments (such as chlorine containing and acid-base media), 316L stainless steel or 2205 duplex steel is recommended. 316L material has a molybdenum content of ≥ 2.0% and must have good resistance to pitting corrosion and intergranular corrosion. 2205 duplex steel has a tensile strength of ≥ 620MPa and complies with GB/T 21433 and GB/T 24511 standards; For pressure vessels and high-pressure working conditions, stainless steel composite plates or specialized forgings should be selected, and the wall thickness should be determined according to design calculations. Key parts such as flange necks should undergo ultrasonic testing and comply with relevant standards.
2、 Raw materials entering the factory: triple verification to eliminate hidden dangers
Firstly, certificate verification is required. The supplier must provide a Material Certificate (MTC) and carefully check the furnace batch number, grade, specifications, chemical composition (including carbon, chromium, nickel, molybdenum, phosphorus, sulfur), and mechanical properties (tensile strength, yield strength, elongation) to ensure that the certificate information is completely consistent with the actual product. Next, check the appearance and dimensions. The surface of the raw material should be free of defects such as cracks, bubbles, scars, and delamination, with uniform wall thickness and deviations that meet standard requirements. For example, the wall thickness of the atmospheric pressure cylinder should not be less than 3mm, and the thickness of the head should be 1-2mm thicker than the cylinder. Sampling and retesting should be carried out, and key components such as the cylinder, head, and flange should be sampled in batches. The composition should be tested using a spectrometer, and tensile and impact tests should be carried out. For thick wall panels with a thickness of ≥ 50mm, additional ultrasonic testing (UT) should be added to ensure that the performance of the raw material meets the.
3、 Manufacturing process: dual control of polishing and welding
（1） Polishing control
The polishing process adopts a combination of rough polishing, semi precision polishing, precision polishing, and electrolytic polishing (EP). The sand belt needs to transition step by step, upgrading from 180 # to 320 #, 600 #, and 1000 # in sequence. Jumping operation is strictly prohibited. During the process, it is necessary to strictly control the polishing pressure and temperature, with a temperature not exceeding 0.8kJ/mm, to prevent material overheating and deformation; Process validation is required before electrolytic polishing to ensure that there is no mechanical damage or residual impurities on the surface after polishing. The polishing target should meet the requirement of inner surface Ra ≤ 0.6-0.8 μ m (in the fields of food and medicine), and the surface should be free of dead corners and scratches to ensure compliance with usage requirements.
（2） Welding quality
Welding materials must be of the same grade as the base material and undergo strict drying treatment before use; The welding process is protected by argon gas to reduce the risk of oxidation and intergranular corrosion. Welds must undergo 100% non-destructive testing, including ultrasonic testing (UT) or radiographic testing (RT), with additional penetration testing (PT) required for critical areas to ensure that there are no defects such as incomplete penetration, slag inclusion, porosity, etc. After welding is completed, it is necessary to remove welding slag and polish the weld seam in a timely manner. If necessary, solution treatment and acid pickling passivation should be carried out to restore the corrosion resistance of the material and avoid the welding area becoming a weak point in quality.
4、 Finished product testing: full coverage, data tracking
The surface roughness detection adopts a roughness meter, which is executed according to ISO 4287 standard. The qualified standard must meet the design requirements, such as Ra ≤ 0.8 μ m; The appearance quality is inspected by visual inspection with a magnifying glass or endoscope at a magnification of ≥ 10x, and the surface is required to be free of pits, scratches, pitting, and polishing blind spots; The corrosion resistance has been verified through salt spray test and copper sulfate drop test, and the qualified judgment is that there is no pitting, rusting, or discoloration phenomenon; The intergranular corrosion detection shall be carried out in accordance with GB/T 21433 (for stainless steel pressure vessels), and it is qualified if there are no intergranular cracks or signs of corrosion; The size and sealing detection are measured using calipers and level gauges, combined with air tightness tests to ensure compliance with the design drawings and no leakage; Cleanliness is measured by wiping or particle counting, with no polishing agent or abrasive residue, and meets the hygiene requirements of corresponding fields (such as food and medicine). All testing data must be recorded in detail to ensure traceability.
5、 Compliance and traceability: traceable throughout the entire lifecycle
The manufacturing and acceptance process must follow core standards to ensure compliance at every step of the process. Establish a full process traceability system, covering raw material furnace batch numbers, manufacturing process parameters, all inspection reports, and factory numbers, to ensure that each polishing tank can achieve full lifecycle traceability. If it is a special equipment (pressure vessel), the corresponding manufacturing license must be obtained, and a supervision and inspection certificate must be attached when leaving the factory to ensure that the product leaves the factory in compliance.
6、 Key Avoiding Pits Guide
Attention should be paid not to blindly believe in surface "brightness", brightness does not equal roughness, and the Ra value detection result must be used as the standard to avoid material residue or corrosion caused by micro roughness of the surface; Strictly control the content of impurity elements. Excessive phosphorus and sulfur content (such as phosphorus>0.035%) can easily lead to material hot and cold brittleness, affecting welding quality and corrosion resistance; During the electrolytic polishing process, it is necessary to strictly control the voltage, time, and temperature to avoid surface corrosion and the formation of micro pits, which may leave quality hazards.
Summary: The quality of polished storage tank materials is the result of "selecting correctly+verifying accurately+doing fine+measuring comprehensively", with the core being based on standards, using testing as a means, and using traceability as a guarantee. It is recommended to require suppliers to provide complete material certification (MTC), re inspection reports, and third-party testing certificates during the procurement and manufacturing stages to ensure that the material quality of each storage tank is compliant and reliable.
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