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Magnetic properties of stainless steel screws

People often think that magnets attract stainless steel screws to verify their quality and authenticity. If they do not attract and have no magnetism, they think they are good and live up to the name. If they have magnetism, they think they are fake. In fact, this is an extremely one-sided and unrealistic method of distinguishing faults. The variety of stainless steel screws is complex, and can be divided into several categories according to the arrangement at room temperature:

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

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

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

Most of the stainless steels usually used for decorating tube sheets are austenitic 304 raw materials, which are generally non-magnetic or weakly magnetic, but can also show magnetic properties due to fluctuations in chemical composition or different processing conditions due to tempering, but this cannot be considered Is it fake or unqualified, what is the reason for this?

It is mentioned above that austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Because of component segregation or improper heat treatment during tempering, a small number of martensite or martensite in austenitic 304 stainless steel will be formed. Ferritic arrangement. In this way, 304 stainless steel will have weak magnetism.

In addition, 304 stainless steel will be transformed into martensite through cold working, and the layout will also be transformed into martensite. Like a batch of steel strips, Φ76 tubes are produced without significant magnetic induction, and Φ9.5 tubes are produced. The magnetic induction is more pronounced due to the larger deformation of the ling bend, and the deformation of the square rectangular tube is larger than that of the round tube, especially with some folded corners, and the deformation is more severe and the magnetic force is more significant.

In order to completely eliminate the magnetic properties of the 304 steel formed by the above reasons, the stable austenite structure can be restored by high-temperature solution treatment, and then the magnetic properties can be eliminated.

In particular, the magnetic properties of 304 stainless steel formed by the above reasons are completely different from the magnetic properties of other raw materials such as 430 and carbon steel. In addition, the magnetic properties of 304 steel have always shown weak magnetic properties.

This tells us that if the stainless steel strip is weakly magnetic or completely non-magnetic, it should be judged as 304 or 316 material; if it has the same magnetic properties as carbon steel, it shows strong magnetism, so it is judged as not 304 material.

Stainless steel screws are widely used in machinery, petrochemicals, communications, chemicals, construction, electricity, textiles, sports equipment, papermaking, sewage treatment, ships, automobiles and other industries. With the great improvement of domestic economic and living standards, the use of stainless steel screws will become more extensive. .