In everyday life, we often use the presence of magnetism to determine the quality of stainless steel, but this method is actually unscientific. Common materials like zinc tube alloys and copper tube alloys can often mimic the appearance and color of stainless steel and generally do not exhibit magnetism, making them easily mistaken for stainless steel. Even the commonly used 304 steel grade, after cold working, will show varying degrees of magnetism. Therefore, one cannot rely on magnetism to judge the quality of stainless steel. So, why does 304 stainless steel tube material have magnetism?
Under normal circumstances, stainless steel pipes can be categorized into two types based on their microstructure at room temperature. The first type is Austenitic stainless steel pipes, such as 304, 321, 316, and 310 pipes, etc. The second type is Martensitic or Ferritic stainless steel pipes, like 430, 420, and 410 pipes, etc. Austenitic stainless steel pipes are non-magnetic or weakly magnetic, while Martensitic or Ferritic stainless steel pipes are magnetic. Therefore, 304 stainless steel pipes are non-magnetic or slightly magnetic.
We often refer to 304 stainless steel as non-magnetic, with "tube" indicating its magnetic properties are below a certain threshold. In other words, most stainless steels have varying degrees of magnetic tube properties. The previously mentioned austenitic stainless steel is non-magnetic or weakly magnetic, whereas ferritic and martensitic stainless steels are magnetic. This is due to component segregation or improper heat treatment during smelting, which can result in trace amounts of martensite or ferrite in the austenitic 304 stainless steel, leading to weak magnetic tube properties. Furthermore, as 304 stainless steel is cold worked, its microstructure can transform towards martensite, and with greater cold working deformation, more martensite is formed, increasing the magnetic properties.
Therefore, the magnetism of the material is determined by the regularity of the molecular arrangement and the alignment of the electron spin, which can be considered as the material's physical properties. The corrosion resistance of the material, on the other hand, is determined by the chemical composition of the material, representing its chemical properties, and is unrelated to whether the material has magnetism.
