Sanitary grade (food grade) stainless steel pipes are widely used in many fields and industries such as pharmaceuticals, video, beer, drinking water, biological engineering, chemical engineering, air purification, aviation nuclear industry and other national economic construction. There are a lot of imports every year.
1. Surface analysis of stainless steel
Both the AES method and the SPS method can be used to analyze the surface of stainless steel to determine the corrosion ability of the inner and outer surfaces of stainless steel. The analysis diameter issued by AES is very small, which can be less than 20nm. Its original function is to identify elements. The analytical value of XPS method is about 10μm, which is mainly used to determine the chemical state of elements near the surface.
Scanning the mechanically polished surface of 316 stainless steel that has been exposed to the atmosphere with AES and XPS detectors shows that the most typical analysis depth of stainless steel diamond surface is 15nm, and it provides information about the composition and thickness of the passivation layer. The corrosion resistance and so on.
According to the definition, austenitic stainless steel contains high chromium and nickel, and some contain molybdenum, titanium, etc., generally containing 10.5% or more of chromium and has good corrosion resistance. The corrosion resistance is the result of the protective properties of the chromium-rich passivation layer. The passivation layer is usually 3-5nm thick, or equivalent to 15 atoms thick. The passivation layer is formed during the oxidation-reduction reaction process in which chromium and iron are oxidized. If the passivation layer is damaged, a new passivation layer will be formed quickly and electrochemical corrosion will occur immediately, and deep spots of stainless steel will appear. Corrosion and intergranular corrosion. Passivation corrosion resistance is related to the content of chemical components contained in stainless steel, such as high chromium, nickel and molybdenum, etc. can increase the binding energy potential of the passivation layer, and enhance the corrosion resistance of the passivation layer; and use it with the inner surface of the stainless steel pipe. The fluid medium is related.
2. Surface corrosion of stainless steel pipe
(1) The passivation layer on the surface of stainless steel is easily destroyed in the CI-containing medium, because the Ci-oxidation potential is relatively large. If the passivation layer is only on the metal, the printed layer will continue to corrode. In many cases, the passivation layer is only damaged in a local area of the metal surface. The effect of corrosion is to form small holes or pits. The small pits that are randomly distributed on the material surface are called pitting corrosion. The pitting corrosion rate increases with increasing temperature and increases with increasing concentration. The solution is to use ultra-low or low-carbon stainless steel (such as 316L or 304L)
(2) The passive warp layer on the surface of austenitic stainless steel is easily destroyed during manufacturing and welding. When the heating temperature and heating speed during manufacturing and welding are in the stainless steel sensitization temperature region (about 425-815°C), the supersaturated carbon in the material will first precipitate at the grain boundary and combine with chromium to form chromium carbide and lose chromium. As a result, the chromium content of the grain boundary continuously decreases with the continuous precipitation of chromium carbide, forming a so-called chromium-depleted zone, which weakens the potential energy and reduces the corrosion resistance of the passivation layer. When in contact with corrosive media such as Ci- in the medium, it will cause micro-current corrosion. Although the corrosion is only on the surface of the grains, it quickly penetrates into the interior to form intergranular corrosion. Especially the stainless steel pipe is more obvious in the welding treatment part.
(3) Stress corrosion cracking: it is the combined effect of static stress and corrosion that causes cracks and metal embrittlement. The environment for stress corrosion cracking is usually quite complex. Not only the tensile stress, but the combination of this stress and the residual stress in the metal due to fabrication, welding, or heat treatment.
3. Production process of sanitary welded stainless steel pipe
Uncoiling-deburring-forming-welding (gas protection box)-inner leveling-welding seam grinding-pipe cleaning-bright annealing-fine sizing-cutting
It is recommended to use the precision stainless steel sanitary fluid pipe production line of Hangao Tech (SEKO Machinery). Since the steel strip is directly used for welding after forming, the tolerance and ellipticity of the pipeline can be well controlled, and the process of cold drawing can be omitted.
There are several key equipment in production:
(1) Internal leveling equipment: It can be repeatedly pressed back and forth through the roller and the built-in mandrel to flatten the remaining height of the welding seam, so that the welding seam and the base material are more closely aligned and natural transition, making the inner tube wall smoother and reducing the pipeline Residues within. During internal polishing and external polishing, it can also reduce the number and intensity of polishing and reduce loss.
(2) Protective gas bright annealing furnace: It consists of two parts, the bright annealing furnace body and the cooling water jacket.
Bright annealing furnace body: The main structure is a circular section induction heating furnace, which adopts the heating method of induction heating coils, so that the entire pipe section can be heated in all directions. The protective gas not only acts as a barrier to the air, but also serves as a circulating cooling air. Compact structure, safe operation, reliable control and convenient maintenance. The temperature difference in the furnace is controlled within ±1-2℃.
Manufacturers can choose to use ammonia decomposition equipment to make protective gas or directly use canned gas according to their actual conditions.
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Iris Liang
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