Small knowledge of stainless steel pipes

First, surface processing grades, characteristics, and uses
- Original surface: NO.1 The surface is treated with heat and pickling after hot rolling. Generally used for cold-rolled materials, industrial tanks, chemical industrial equipment, etc., with a thicker thickness of 2.0MM-8.0MM.
- Blunt surface: NO.2D After cold rolling, heat treatment, and pickling, the material is soft and the surface is silvery white. It is used for deep stamping, such as automotive components, water pipes, etc.
- Matt surface: NO.2B After cold rolling, heat treatment, pickling, and then fine rolling to make the surface moderately bright. Because the surface is smooth, it is easy to re-grind, making the surface brighter, and it has a wide range of uses, such as tableware, building materials, etc. After surface treatment to improve mechanical properties, it can meet almost all uses.
- Coarse sand: NO.3 Products ground with 100-120 grinding belts. It has better gloss and discontinuous coarse grains. It is used for building interior and exterior decoration materials, electrical products kitchen equipment, etc.
- Fine sand: NO.4 is a product ground with a grinding belt of 150-180 grit. It has better gloss, discontinuous coarse grain, and finer stripes than NO.3. It is used for baths, interior and exterior decoration materials of buildings, electrical products, kitchen equipment food equipment, etc.
- #320 is a product ground with a grinding belt of 320. It has better gloss, discontinuous coarse grain, and finer stripes than NO.4. It is used for baths, interior and exterior decoration materials of buildings, electrical products, kitchen equipment food equipment, etc.
- Hairline: HL NO.4 is a product with grinding patterns generated by continuous grinding with a polishing belt of appropriate grit (subdivided into 150-320). It is mainly used for building decoration, elevators, doors panels of buildings, etc.
- Bright surface: BA is a product obtained by cold rolling, bright annealing, and flattening. The surface gloss is excellent and has a high reflectivity. It is like a mirror surface. It is used for home appliances, mirrors, kitchen equipment, decorative materials, etc.
- Corrosion resistance: The corrosion resistance is better than that of 304 stainless steel, and it has good corrosion resistance in the production process of pulp and papermaking. Moreover, 316 stainless steel is also resistant to corrosion in marine and corrosive industrial atmospheres.
- Heat resistance: 316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees: It is best not to use 316 stainless steel continuously in the range of 800-1575 degrees, but it has good heat resistance when used continuously outside this temperature range. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used in the above temperature range.
- Heat treatment: Annealing in the temperature range of 1850-2050 degrees, then rapid annealing, and then rapid cooling. 316 stainless steel cannot be hardened by overheating.
- Welding: 316 stainless steel has good welding performance. It can be welded by all standard welding methods. When welding, 316Cb, 316L, or 309Cb stainless steel filler rods or electrodes can be used for welding according to the application. For the best corrosion resistance, the welded section of 316 stainless steel needs to be annealed after welding. If 316L stainless steel is used, post-weld annealing is not required.
- Typical uses: heat exchangers for pulp and papermaking equipment, dyeing equipment, film washing equipment, pipelines, and materials for the exterior of buildings in coastal areas.

Why does stainless steel rust?
Why does stainless steel rust? When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: they think "Stainless steel does not rust, and rust is not stainless steel, and there may be problems with the steel quality." This is a one-sided and wrong view of stainless steel. Stainless steel will also rust under certain conditions.
- The surface of stainless steel accumulates dust containing other metal elements or foreign metal particles. In humid air, the condensed water between the attachments and the stainless steel connects the two into a micro-battery, triggering an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.
- Organic juice (such as vegetables, noodle soup, sputum, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, it forms organic acid, which corrodes the metal surface for a long time.
- The surface of stainless steel adheres to substances containing acid, alkali, and salt (such as alkaline water and lime water splashing from wall decoration), causing local corrosion.
- In polluted air (such as an atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), it encounters condensed water to form sulfuric acid, nitric acid, and acetic acid liquid spots, causing chemical corrosion.

The above situations can damage the protective film on the surface of stainless steel and cause rust. Therefore, to ensure that the metal surface is permanently bright and not corroded, we recommend:
- The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause decoration.
- 316 stainless steel should be used in coastal areas. 316 material can resist seawater corrosion.
- The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the requirements of 304 materials. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers. Why does stainless steel also carry magnets? People often think that magnets absorb stainless steel to verify its quality and authenticity. If it does not absorb magnetism, it is considered to be good and genuine; if it absorbs magnetism, it is considered to be a fake. This is an extremely one-sided, impractical, and wrong identification method.

There are many types of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
- Austenite type: such as 304, 321, 316, 310, etc.;
- Martensite or ferrite type: such as 430, 420, 410, etc.;
- Austenite type is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic.

Most of the stainless steel used for decorative tube sheets is austenite 304 material, which is generally non-magnetic or weakly magnetic, but magnetism may also occur due to chemical composition fluctuations or different processing conditions caused by smelting, but this cannot be considered as counterfeit or unqualified. What is the reason?
It is mentioned above that austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to component segregation or improper heat treatment during smelting, a small amount of martensite or ferrite organization will be caused in austenite 304 stainless steel. In this way, 304 stainless steel will have weak magnetism. In addition, after cold working, the organization structure of 304 stainless steel will also transform into martensite. The greater the cold working deformation, the more martensite transformation, and the greater the magnetism of the steel. For example, for a batch of steel strips, there is no obvious magnetic induction when producing Φ76 tubes, and there is no obvious magnetic induction when producing Φ9.5 tubes. The magnetic induction is more obvious due to the larger cold bending deformation. The deformation of square and rectangular tubes is larger than that of round tubes, especially the corners, where the deformation is more intense and the magnetism is more obvious. To eliminate the magnetism of 304 steel caused by the above reasons, high-temperature solid solution treatment can be used to restore the stable austenite organization, thereby eliminating the magnetism.

It should be pointed out in particular that the magnetism of 304 stainless steel caused by the above reasons is completely different from the magnetism of other materials such as 430 and carbon steel. In other words, the magnetism of 304 steel always shows weak magnetism. This tells us that if the stainless steel strip is weakly magnetic or not magnetic at all, it should be judged as 304 or 316 material; if it is the same as the magnetism of carbon steel, it shows strong magnetism, so it is judged as not 304 material.