Magnetoconductivity of Stainless Steel
Why stainless steel cannot be attracted by permanent magnet?
In daily life, austenitic stainless steel (also known as Ni stainless steel) and martensitic stainless steel (also referred to as stainless steel, but such name is not scientific, easy to misunderstand and should be avoided) are the two types of stainless steel which people are most familiar with. The reason why austenitic stainless steel is not attractable to magnets is that there is high percentage of Ni and Cr in it(Cr about 18% and Ni ≥4%), which makes the internal structure of it presents a so called austenite microstructure. The austenite structure is non-magnetic, so austenitic stainless steel cannot be attracted to any strong permanent magnets like neodymium magnets , ferrite magnets.
The austenitic microstructure of steel will lose its ferromagnetism at temperature above 770 degrees Celsius. Austenitic stainless steel with large proportion of alloy is also not ferromagnetic. Except for the austenitic steel, other types of stainless steel are ferromagnetic such as ferritic stainless steel, martensitic stainless steel and so on. Stainless steels that can be attracted by strong magnets are called Cr13, and stainless iron 1Cr18Ni9Ti and 0Cr18Ni9Ti, etc..
People often test the quality and genuineness of stainless steel using a neodymium permanent magnet .If the stainless steel is not attractive to magnets, it is thought to be qualified. Actually, such test method is not realistic. Anyone who has rich experience in producing steel material will never apply this confusing test method to verify stainless steel.
According to manufacturers of stainless steels, there is a variety of stainless steel, which can be categorized into austenitic stainless steel and martensitic stainless steel based on their structure at room temperature. Austenitic steel is no magnetic or weakly magnetic, while martensite or ferrite steel is magnetic. However, it is not necessarily the case. For an example of austenitic 304 stainless steels that are commonly used as tube plates in decoration, they are nonmagnetic, but they might appear magnetic due to fluctuations of chemical composition in melting or different processing status. So such magnetism of stainless steel is not a proof of inferior quality.
Beside, the inner structure of 304stainless steel may transform to be martensitic after cold processing. The greater the degree of cold processing, the more the martensitic transformation is, and the stronger the magnetism of the steel will be. On the contrary, the 200 series stainless steel with inferior quality might probably be non-magnetic. Therefore, verifying the quality and genuineness of stainless steel by testing its magnetism is terribly wrong.