Detailed explanation of magnetism of stainless steel screws

People often think that magnets absorb stainless steel to verify its quality and authenticity. If it does not attract non-magnetic products, it is considered to be good and genuine; if it attracts magnets, it is considered to be a fake. In fact, this is an extremely one-sided, unrealistic and wrong method of identification. There are many types of stainless steel screws, which can be divided into several categories according to their organizational structure at room temperature:

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

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

Austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic.

Most of the stainless steel usually used for decorative tube sheets is austenitic 304 material. Generally speaking, it is non-magnetic or weakly magnetic. However, due to fluctuations in chemical composition due to smelting or different processing conditions, magnetism may also occur, but this cannot be considered 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 magnetism.

In addition, after cold working of 304 stainless steel, the organizational structure will also transform to martensite. The greater the degree of cold working deformation, the more martensitic transformation, and the greater the magnetism of the steel. Just like a batch number of steel strips, Φ76 pipes are produced. There is no obvious magnetic induction, and Φ9.5 pipes are produced. Because the bending deformation is larger, the magnetic induction will be more obvious. 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.

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

In particular, it should be pointed out that the magnetism of 304 stainless steel due to the above reasons is not at the same level as the magnetism of other stainless steel materials, such as 430 and carbon steel. In other words, the magnetism of 304 steel always shows weak magnetism.

This tells us that if stainless steel has weak magnetism or no magnetism at all, it should be identified as 304 or 316 material; if it has the same magnetism as carbon steel and shows strong magnetism, it should be identified as not 304 material.

Application areas

Stainless steel screws are widely used in machinery, petrochemicals, communications, chemicals, construction, electric power, textiles, sports equipment, papermaking, sewage treatment, ships, automobiles and other industries. As the domestic economic and living standards greatly improve, the application scope of stainless steel screws will become more widely.