Professional Answer: Why do stainless steel screws have magnetism?

Previously, the dirtiest way we could distinguish stainless steel screws from iron screws was with a magnet. Suck this screw, if it can suck, it means it is an iron screw, because it is magnetic. What cannot be sucked up is stainless steel screws, because they are not magnetic. In fact, it is wrong to say that stainless steel screws are not magnetic. Here is a detailed introduction to why stainless steel screws are also magnetic. There are many kinds of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
one Austenitic type: such as 304, 321, 316, 310, etc. 2. Martensitic or ferritic type: such as 430, 420, 410, etc.; Austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic. Usually used as decorative tube plate stainless steel is mostly Austenitic type 304 material, generally speaking, is non-magnetic or weak magnetic, but because of smelting caused by chemical composition fluctuations or processing state may also appear magnetic, but this can not 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 improper segregation or heat treatment during smelting, it will cause a small amount of martensite or ferrite structure in austenitic 304 stainless steel. In this way, 304 stainless steel will have a weak magnetic. In addition, 304 stainless steel after cold processing, the structure will also be transformed to martensite, the greater the deformation of cold processing, the more martensite transformation, the greater the magnetic steel. Like a batch of steel strips, the production of Φ76 tubes, no obvious magnetic induction, the production of Φ9.5 tubes. Because the bending deformation is larger, the magnetic sense is obvious, and the production of square rectangular tube is larger than the round tube because of the deformation, especially the corner part, the deformation is more intense and the magnetism 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, so as to eliminate the magnetic properties.
In particular, the magnetic properties of 304 stainless steel caused by the above reasons are not at the same level as the magnetic properties of other materials such as 430 and carbon steel, that is to say, the magnetic properties of 304 steel always show weak magnetism. This tells us that if the stainless steel has weak magnetism or no magnetism at all, it should be judged as 304 or 316 material; If it is the same as the magnetism of carbon steel, it shows strong magnetism, because it is not 304 material. Sometimes, we see customers in the purchase of stainless steel screws, will bring a piece of magnet to experiment with the magnetic size of stainless steel, in fact, this approach is not scientific. In fact, stainless steel is non-magnetic under normal conditions, slightly magnetic after cold processing, and it is only possible to be completely non-magnetic in a vacuum state. Commonly used material: 304M slightly magnetic after cold processing (about 1.6u-2.0u); 304HC magnetic (1.01u-1.6u); 316 material after cold working magnetic less than 1.01u. All materials have good ductility, easy cold processing and forming, resistant Tensile strength, yield strength, can meet the requirements. As long as you choose the product correctly according to the actual use requirements, I believe it can meet your needs. After cold working, the magnetic properties of each material are 316< from weak to strong. 304 hc & lt; It's 304M. Originally, stainless steel screws must be produced in different stainless steel materials. Each stainless steel screw has different magnetic properties. With magnetic weakness, with magnetic strength. But nothing is non-magnetic. Because stainless steel is non-magnetic under normal conditions, but after cold processing becomes a screw with a point of magnetism. Contact with air, but stainless steel screws can be completely non-magnetic if they are in a vacuum.