Why is the stainless steel tube magnetic

People often think that magnets absorb stainless steel pipe, and verify its quality and authenticity. If it does not attract and has no magnetism, it is considered good and genuine; if it attracts magnetism, it is considered a counterfeit. This is an extremely one-sided, impractical, and wrong way to distinguish.

There are many kinds of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
1. Austenitic type: such as 304, 321, 316, 310, etc.;
2. Martensite or ferrite type: such as 430, 420, 410, etc.;
Austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
Most of the stainless steel used for decorative tube sheets is austenitic 304 material, generally speaking, it is non-magnetic or weakly magnetic, but it may also appear magnetic 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?

As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to composition segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel will be formed. body tissue. In this way, 304 stainless steel will have weak magnetism.
In addition, after the cold working of 304 stainless steel, the microstructure will also transform into martensite. The greater the degree of cold working deformation, the more martensite transformation, and the greater the magnetic properties of the steel. Like a batch of steel strips, Φ76 tubes are produced, without obvious magnetic induction, and Φ9.5 tubes are produced. The magnetic induction is more obvious due to the larger cold bending deformation. The deformation of square and rectangular pipes is larger than that of round pipes, especially at the corners, and the deformation is more intense and the magnetism is more obvious.
To eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be recovered by high-temperature solution treatment, to eliminate the magnetism.
In particular, the magnetism of 304 stainless steel caused by the above reasons is not at the same level as that of other stainless steels, such as 430 and carbon steel, that is to say, the magnetism of 304 steel always shows weak magnetism.

This tells us that if stainless steel has weak magnetism or no magnetism at all, it should be identified as 304 or 316 material; if it is the same as carbon steel, it shows strong magnetism, because it is not 304 material. We suggest that when buying stainless steel products, you should choose products from reputable manufacturers, don't be greedy for cheap, and beware of being fooled.