There are many kinds of stainless steel, which can be divided into several categories according to the organisational structure at room temperature:
1. Austenite type: such as 201, 202, 301, 304, 316, etc.;
2. Martensitic or ferrite type: such as 430, 420, 410, etc.;
The austenite type is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
The stainless steel commonly used as a decorative pipe plate is mostly austenitic 304 material, which is generally non-magnetic or weakly magnetic, but it may also be magnetic due to chemical composition fluctuations or different processing states caused by smelting, but this cannot be regarded as fake or unqualified. What's the reason for this?
As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to component segregation or improper heat treatment during smelting, it will cause a small amount of martensite or ferrite tissue in austenite 304 stainless steel. In this way, there will be a weak magnetism in 304 stainless steel.
In addition, after cold processing, the tissue structure of 304 stainless steel will also transform into martensite. The greater the deformation degree of cold processing, the more martenite conversion, and the greater the magnetism of the steel. Like the same batch of steel strips, Φ76 tubes are produced without obvious magnetic induction, and Φ9.5 tubes are produced. Due to the large magnetic sense of cold bending deformation, the magneticity of the production square rectangular tube is more obvious because the deformation amount is larger than that of the round tube, especially the angled part.
To completely eliminate the magnetism of 304 steel caused by the above reasons, the stability of austenite tissue can be restored through high-temperature solid solution treatment, thus eliminating magnetism.
In particular, the magnetism of 304 stainless steel due to the above reasons is completely different from that of other materials, 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 is weakly magnetic or completely non-magnetised, it should be judged as 304 or 316 materials; if it is the same as the magnetism of carbon steel, it shows strong magnetism, because it is judged that it is not 304 material.
