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Why is rust steel also magnetic?

Mar 25, 2020

People often think that magnets absorb stainless steel, verify its pros and cons and authenticity, do not attract non-magnetic, think it is good, genuine goods; if the sucker is magnetic, consider it to be fake. In fact, this is an extremely one-sided, impractical method of identifying errors. There are many types of stainless steel, which can be divided into several types according to the organizational structure at room temperature:

1. Austenite type: such as 304, 321, 316, 310, etc .;

2. Martensite or ferrite type: such as 430, 420, 410, etc .;

The austenite type is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.

Most of the stainless steel commonly used as decorative tube sheet is austenitic 304 material, which is generally 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 What is the reason for counterfeit or unqualified?

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, a small amount of martensite or ferrite in austenitic 304 stainless steel will be caused. Body tissue. In this way, 304 stainless steel will have weak magnetism.

In addition, after cold working of 304 stainless steel, the structure will also be transformed to 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. Due to the large bending deformation, the magnetic induction is obvious. Due to the larger amount of deformation in the production of rectangular tubes than round tubes, especially the corners, the deformation is more intense and the magnetic is more obvious.

In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, a stable austenite structure can be restored by high temperature solution treatment, thereby eliminating the magnetic properties.

In particular, the magnetic properties of 304 stainless steel due to the above reasons are completely different from the magnetic properties of other materials such as 430 and carbon steel, which means that the magnetic properties of 304 steel always show weak magnetism.

This tells us that if stainless steel is weakly magnetic or completely non-magnetic, it should be judged as 304 or 316; if it is as strong as carbon steel, it should be judged as not 304.