Why are stainless steel screws magnetic

Update:10 Nov 2020
Summary:

Reasons for the magnetism of stainless steel screws: Pe […]

Reasons for the magnetism of stainless steel screws:

People often think that magnets absorb stainless steel materials to verify their quality and authenticity. They are not attractive and non-magnetic, and they are considered to be good and genuine; if they are magnetic, they are considered to be counterfeit products. In fact, this is an extremely one-sided and impractical method of identifying errors.

There are many types of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:

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

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

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

The stainless steel screws usually used as decorative tube plates are mostly austenitic 304 materials. Generally speaking, they are non-magnetic or weakly magnetic. However, due to chemical composition fluctuations or different processing conditions caused by smelting, magnetism may also appear, but this cannot be considered Is it counterfeit or unqualified, what is the reason?
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 magnetic properties.

After 304 stainless steel is cold worked, the structure will also transform 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. Because the bending deformation is larger, the magnetic induction is more obvious, and the deformation of the square rectangular tube is larger than that of the round tube, especially the corner part, the deformation is more intense and the magnetism is more obvious.

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