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Why is stainless steel magnetic?
Jul 31, 2018

Why does stainless steel also carry magnetism? People often think that magnets absorb stainless steel, verify its pros and cons and authenticity, do not attract non-magnetic, think it is good, the goods are genuine; if the attractor is magnetic, it is considered a counterfeit fake.Professional Brass CNC Turning Part Manufacturing

In fact, this is an extremely one-sided, unrealistic way of distinguishing mistakes.


There are many kinds of stainless steel, which can be divided into several categories according to the structure at room temperature: 1. Austenitic 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, martensite or ferrite is magnetic. Most of the stainless steels used for decorative tube sheets are austenitic 304 materials, which are generally non-magnetic or weakly magnetic, but may also be magnetic due to fluctuations in chemical composition or processing conditions due to smelting. But this can't be considered a fake or unqualified. What is the reason?

It is mentioned above that austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic. Due to segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel is caused. Body organization. Professional Brass CNC Milling Manufacturing Thus, 304 stainless steel will have weak magnetic properties. In addition, 304 stainless steel is cold-worked, and the microstructure is also transformed into martensite. The greater the cold work deformation, the more martensite transformation and the greater the magnetic properties of the steel. Like a batch of steel strips, it produces Φ76 tubes with no obvious magnetic induction and produces Φ9.5 tubes. Due to the large deformation of the bending, the magnetic induction is obvious. The rectangular tube produced is larger than the round tube, especially the angled part, and the deformation is more intense and the magnetic is more obvious.

In order to completely eliminate the magnetic properties of the 304 steel caused by the above reasons, the austenite structure can be restored by high-temperature solution treatment 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 magnetic properties. 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 the magnetic steel, it shows strong magnetism, because it is judged not to be 304 material.Professional Titanium CNC Milling Parts Manufacturing 

Due to the recent high nickel prices and the high cost of Ni-based stainless steels such as 304, the promotion of 400-series stainless steel products with excellent corrosion resistance and attractive prices is becoming more and more urgent. In order to reduce the production cost of downstream customers and improve the market competitiveness of end products, major stainless steel plants at home and abroad have increased the research and development and promotion of nickel-free or low-nickel stainless steel materials. Unfortunately, the inherent magnetism of the 400 Series stainless steel has discouraged many users.
In real life, most people think that stainless steel is not magnetic, and using magnets to identify stainless steel is unscientific. First of all, zinc alloys and copper alloys can be similar to the appearance of stainless steel, and they are not magnetic. They are easily mistaken for stainless steel. Even the 304 steels that we use most often have different degrees of magnetic properties after cold working. . Therefore, it is not possible to judge the authenticity of stainless steel by a single magnet.

So how does the magnetic properties of stainless steel come from?

 

According to the study of material physics, Professional Titanium CNC Turning Parts Manufacturing the magnetic properties of metals are derived from the structure of electron spins, which are quantum mechanical properties that can be either "up" or "down". In ferromagnetic metals, electrons automatically rotate in the same direction. In antiferromagnetic metal materials, some electrons follow a regular pattern, while adjacent electrons spin in opposite directions or anti-parallel, but for triangular crystals. For electrons in the lattice, since both electrons in each triangle must spin in the same direction, the spin structure no longer exists.


Generally speaking, austenitic stainless steel (represented by 304) is non-magnetic, but may also have weak magnetic properties, while ferrite (mainly 430, 409L, 439 and 445NF, etc.) and martensite (at 410) As a representative) are generally magnetic.

 

Some steel grades (such as 304) in stainless steel are classified as "non-magnetic stainless steel", which means that the magnetic index is lower than a certain value. That is to say, generally stainless steel has more or less magnetic properties.

In addition, it is mentioned above that austenite is non-magnetic or weakly magnetic, while ferrite and martensite are magnetic. Due to segregation or improper heat treatment during smelting, a small amount of Markov in austenitic 304 stainless steel may occur. Body or ferritic structure, such that weak magnetic properties appear in 304 stainless steel. Professional Copper CNC Milling Parts Manufacturing In addition, 304 stainless steel is cold-worked, and the microstructure is also transformed into martensite. The more cold-formed deformation, the more martensite transformation and the stronger the magnetism. In order to completely eliminate the magnetic properties of 304 stainless steel, it is possible to restore stable austenite structure by high temperature solution treatment, thereby eliminating magnetic properties.

Therefore, the magnetic properties of the material are determined by the regularity of the molecular arrangement and the isotropy of the electron spin. We consider the physical properties of the material, and the corrosion resistance of the material is determined by the chemical composition of the material. Performance, regardless of whether the material is magnetic.

 

From the perspective of the development of stainless steel, the 400 series instead of the 300 series is a general trend. The 200 series materials are facing many difficulties due to scrap steel recycling. They are non-environmental stainless steel. If the use of this steel grade is not conducive to the sustainable development of the entire stainless steel industry, Professional Stainless Steel CNC Turning Parts Manufacturing many large companies with farsightedness now say no to the 200 series. And look to the 400 series stainless steel with more development potential.


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