In daily life, we often judge the quality of stainless steel by whether it has magnetism, but in fact, this method is very unscientific. For example, our common zinc alloys, copper alloys, etc. can generally imitate the appearance and color of stainless steel, and there is no magnetism. It is easy to be mistaken for a stainless steel pipe. Even the most commonly used 304 stainless tubings will have different degrees of magnetism after cold working, it is impossible to judge the quality of stainless steel by virtue of whether it has magnetism. So why is the 304 stainless tubing magnetic?
Under normal circumstances, stainless steel can be divided into two categories according to the structure at room temperature, one is austenitic stainless steel, such as 304, 321, 316, 310, etc., and the other is martensitic or ferritic stainless steel, such as 430, 420, 410, etc., while austenitic stainless steel is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic, so the 304 stainless tubing is non-magnetic or weakly magnetic.
We often say that 304 stainless steel is non-magnetic stainless steel, which means that its magnetic index is lower than a certain value. That is to say, generally, stainless steel is more or less magnetic. The aforementioned austenitic stainless steel is non-magnetic or weakly magnetic, while ferritic and martensitic stainless steels are magnetic, this is due to the segregation of components or improper heat treatment during smelting, resulting in a small amount of martensite or ferrite in austenitic 304 stainless steel, so that weak magnetism will appear in the stainless steel 304 pipes. And in addition, 304 stainless steel is cold worked, the microstructure will also transform to martensite, and with the greater the cold working deformation, the more martensite transforms, the stronger the magnetism will be.
The magnetism of the stainless steel 304 pipes material is determined by the regularity of the molecular arrangement and the isotropy of the electron spins, which can be considered as the physical properties of the material, while the corrosion resistance of the material is determined by the chemical composition of the material. The chemical properties of a material have nothing to do with whether the material is magnetic or not.