Why stainless steel is also magnetic

Lead: Why is stainless steel magnetized? People often think that magnets absorb stainless steel, verify its pros and cons and authenticity, do not absorb non-magnetic, think it is good, genuine; suckers have magnetic, it is counterfeit fake.

In fact, this is a very one-sided, imprecise and erroneous method of identification.

A wide variety of stainless steel, at room temperature according to the organizational structure can be divided into several categories: 1. Austenite type: 304, 321, 316, 310, etc.; 2. Martensite or ferrite type: such as 430, 420, 410, etc.; austenite type is non-magnetic or weak magnetic, and martensite or ferrite is magnetic. The most commonly used stainless steel for decorative tube sheets is austenitic type 304. Generally speaking, it is non-magnetic or weakly magnetic. However, magnetic properties may occur due to chemical composition fluctuations or processing conditions caused by smelting. But this can't be considered fake or unqualified. What is the reason?

The above mentioned austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to segregation or improper heat treatment during smelting, it will cause a small amount of martensite or ferrite in austenitic 304 stainless steel. Body organization. In this way, 304 stainless steel will have a weak magnetic. In addition, 304 stainless steel after cold processing, the organization structure will also be transformed to martensite, the greater the cold deformation degree, the more martensite transformation, the greater the magnetic steel. Such as the same batch of steel, production Φ76 tube, no significant sense of magnetic, production Φ9.5 tube. Due to the large bending deformation of the magnetic induction is obvious, the production of rectangular tube due to the deformation of the larger than the round tube, especially the folding part, the deformation is more intense magnetic more obvious.

In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, the stable austenite structure can be recovered by high-temperature solution treatment, thereby eliminating magnetic properties. In particular, the magnetic properties of 304 stainless steel due to the above reasons are not the same as the magnetic properties of stainless steels of other materials such as 430 and carbon steel. That is, the magnetic properties of 304 steel always show weak magnetic properties. This tells us that if stainless steel is weakly magnetic or completely magnetic, it should be judged to be 304 or 316; if it is the same as carbon steel, it shows strong magnetism, because it is not 304 material.

Due to the recent high nickel prices, the production costs of 304 and other Ni-based stainless steels are high. Therefore, the promotion of 400-series stainless steel products with excellent corrosion resistance and attractive prices is becoming increasingly urgent. In order to reduce the production costs of downstream customers and increase the market competitiveness of end products, major stainless steel plants at home and abroad have increased the research, development and promotion of nickel-free or low-nickel stainless steel materials. Unfortunately, the inherent magnetism of the 400 Series stainless steel has deterred many users.

In real life, most people think that stainless steel is not magnetic, and the use of magnets to identify stainless steel, this method is unscientific. First, zinc alloys and copper alloys can generally imitate the appearance color of stainless steel, and they are not magnetic, so they are easily mistaken for stainless steel. Even the most commonly used type of 304 steel that we currently use, after cold processing, will have different degrees of magnetism. . Therefore, we cannot rely on a single magnet to judge the authenticity of stainless steel.

So how does the magnetic stainless steel come from?

According to the study of material physics, the magnetic properties of metals are derived from the structure of electron spins. Electron spins belong to quantum mechanical properties and can either "upward" or "downward." In ferromagnetic metals, the electrons will automatically rotate in the same direction. In antiferromagnetic metal materials, some electrons proceed in a regular pattern, while neighboring electrons spin in opposite directions or antiparallel, but for a triangular crystal. For the electrons in the grid, the spin structure does not exist since the two electrons in each triangle must spin in the same direction.

In general, austenitic stainless steel (represented by 304) is non-magnetic, but it may also have weak magnetic properties, while ferrite (mainly 430, 409L, 439, and 445NF, etc.) and martensite (410) To represent) is generally magnetic.

Some steel grades in stainless steel (such as 304, etc.) are classified as "non-magnetic stainless steel" meaning that the magnetic index is lower than a certain value, that is, stainless steel generally has a certain degree of magnetic properties.

In addition, the above mentioned austenite is non-magnetic or weakly magnetic, while ferrite and martensite are magnetic. Due to improper composition segregation or heat treatment during smelting, a small amount of martensite will appear in austenitic 304 stainless steel. Body or ferrite structure, so weak magnetic properties appear in 304 stainless steel. In addition, 304 stainless steel after cold processing, the organization structure will also be transformed to martensite, the greater the cold deformation, the more martensite transformation, the stronger the magnetic properties. To completely eliminate the magnetic properties of 304 stainless steel, the stable austenite structure can be recovered by high-temperature solution treatment, thereby eliminating the magnetic properties.

Therefore, the magnetic properties of materials are determined by the rules of molecular arrangement and the isotropy of electron spin. We think that it is the physical properties of materials, and the corrosion resistance of materials is determined by the chemical composition of materials. It is the chemistry of materials. Performance is independent of whether the material is magnetic or not.

From the prospect of stainless steel development, the 400 Series can replace the 300 Series. The 200-series material is faced with many difficulties due to the recycling of scrap steel. It is a non-environmentally friendly stainless steel. If the use of this type of steel is increased, it is not conducive to the sustainable development of the entire stainless steel industry. So many large-scale enterprises with foresight are now saying nothing to the 200-series. , and will be looking at more potential development of 400 series stainless steel.

Source: Wuxi Founder Welfare Stainless Steel Products Co., Ltd.