Why Are Stainless Steel Screws Also Magnetic?

People often think that magnets attract stainless steel to verify its quality and authenticity. If it does not attract non-magnetism, it is considered to be good, and it is worthy of the name; if it is magnetic, it is considered to be counterfeit. In fact, this is an extremely one-sided and unrealistic method of identifying faults.

There are many kinds of stainless steel, which can be divided into several categories according to the 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.;

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

Most of the stainless steel usually used for decorative tube sheets is austenitic 304 material, which is generally non-magnetic or weakly magnetic, but may also appear magnetic due to fluctuations in chemical composition or different processing conditions caused by smelting, but this cannot be considered as a Counterfeit or substandard, what is the reason for this?

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 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. The magnetic induction is more obvious due to the large deformation of the bending and bending. 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 magnetic force is more obvious.

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

In particular, the magnetism of 304 stainless steel caused by the above reasons is completely different from the magnetism of other materials such as 430 and carbon steel, which means that the magnetism of 304 steel always shows weak magnetism. This tells us that if the stainless steel strip is weakly magnetic or not magnetic at all, it should be judged as 304 or 316 material; if it is the same as carbon steel, it shows strong magnetism, because it is judged as not 304 material. Sometimes, we see that when customers buy stainless steel screws, they will bring a magnet to test the magnetic size of stainless steel. In fact, this approach is unscientific. In fact, stainless steel is non-magnetic in the normal state, slightly magnetic after cold working, and completely non-magnetic only in a vacuum state.

Commonly used materials: 304M is slightly magnetic after cold working (about 1.6u-2.0u); 304HC is magnetic (about 1.01u-1.6u); 316 material is less than 1.01u after cold working. All materials have good ductility, easy to be cold-worked and formed, and the tensile strength and yield strength can all meet the requirements. As long as you choose the product correctly according to the actual use requirements, I believe it will definitely meet your needs. After cold working, the magnetic properties of each material from weak to strong are: 316<304HC<304M.