Why does qualified stainless steel have magnetism?

Stainless steel is divided into carbon-based stainless steel and alloy-based stainless steel. Carbon-based stainless steel is used for manufacturing household metal products due to its relatively lower requirements; it contains a higher amount of carbon and fewer alloy elements, which gives it magnetic properties, so it can be considered a type of \"rust-resistant iron\". Using a magnet to test it allows one to determine whether it is alloy-based or carbon-based stainless steel. The only criterion for determining whether the material is of good quality is its composition, so it is better to send it to the analysis and testing facilities of large enterprises for testing, where tests for five elements as well as chromium and nickel can be conducted. If it has strong magnetism, it definitely isn’t 304 stainless steel strip. Stainless steel is mainly divided into three types: ferritic, austenitic, and martensitic (with some hybrid types as well). Among them, only austenitic stainless steel is non-magnetic, and 304 is one such type. If further broken down, specialized testing is required. Is it carbon-based or alloy? There is no such way of asking. The two are not concepts within the same scope. In real life, metals are generally alloys; pure metals are rare. Your method works. Stainless steel comes in two types: austenitic and ferritic, and ferritic stainless steel is attracted to magnets more strongly. The 304 stainless steel strip has a relatively weak magnetic attraction; those with a strong attraction are definitely not 304

304 belongs to austenitic stainless steel. Austenitic grades are non-magnetic or weakly magnetic, while martensitic or ferritic grades are magnetic. The stainless steel commonly used for decorative tube sheets is mostly of the austenitic 304 type; it is generally non-magnetic or only weakly magnetic. However, variations in chemical composition due to manufacturing processes or differences in processing conditions can result in magnetism, but this does not mean the material is counterfeit or defective. What is the reason for this? As mentioned above, austenite is non-magnetic or weakly magnetic, whereas martensite or ferrite is magnetic. Due to compositional segregation during manufacturing or improper heat treatment, small amounts of martensite or ferrite structure can form in austenitic 304 stainless steel. In this way, 304 stainless steel will have a slight magnetic property. Furthermore, when 304 stainless steel is cold-worked, its microstructure transforms into martensite; the greater the degree of cold working deformation, the more martensite is formed, and the greater the magnetism of the steel becomes. For steel strips of the same batch, when used to produce Φ76 pipes, there is no significant magnetic response; whereas when used to produce Φ9.5 pipes, such a response does occur. Due to the greater bending deformation, the magnetic susceptibility becomes more noticeable. When manufacturing rectangular tubes, the degree of deformation is higher than that of circular tubes; especially at the corners, where the deformation is more severe and the magnetism is more apparent. To completely eliminate the magnetism of 304 steel caused by the aforementioned factors, it is possible to restore a stable austenitic structure through high-temperature solution treatment, thereby eliminating the magnetism. It should be specifically noted that the magnetism of 304 stainless steel resulting from the reasons mentioned above is not on the same level as that of other types of stainless steel, such as 430 or carbon steel; in other words, 304 steel always exhibits weak magnetism. This tells us that if the stainless steel strip has weak magnetism or no magnetism at all, it should be classified as 304 or 316 material ； If it exhibits strong magnetism similar to that of carbon steel, it is determined not to be 304 material. :)

Here is a useful tool for identifying stainless steel grades; it requires no additional equipment, is easy to use, portable, and allows for testing at any time. Validity period: valid for an extended period. It can be stored at room temperature. Usage: Drop 1–2 drops of the Shuangsheng brand stainless steel model identifier onto the steel surface, wait a few seconds, and the result will be visible immediately. Product types: 304 solution (primarily used to identify 201, 202, 301, 304); 316 solution (primarily used to identify 304, 316). Product specifications: 25ml, with a color chart for reference; packaging available in both Chinese and English. Videos: 304 solution – http://www.56.com/u95/v_MzIyOTg1NDg.html; 316 solution – http://www.56.com/u21/v_MTc0MDU2MTg.html

The method above is one I found on the Internet. I don’t know if it will work. . :P haha

Thank you very much for your reply. Understood!

If unused 304 material is magnetic, it is likely a problem with the material itself. However, it’s not clear what kind of testing was carried out as mentioned by the original poster, nor whether it is accurate and reliable. Additionally, more test samples should be used to ensure the results are representative.

It seems to be a matter of the difference between theory and practice. Many people say that 304 is non-magnetic, but in fact some 304 grades do have magnetism; even without cold working, they possess a certain level of magnetism, though it is relatively weak. As for ferritic and martensitic stainless steels, they naturally possess magnetism. There’s no doubt about this; I’ve conducted many experiments using strong magnets, and the 304 material used is genuine product manufactured by Taiyuan Iron and Steel Company.

It has a slight degree of magnetism, and this indeed differs from what theory predicts. People often assume that magnets can be used to test whether stainless steel is of good quality or genuine – if a magnet doesn’t attract it, meaning there’s no magnetism, then it’s considered to be of good quality and authentic； If the inhaler has magnetism, it is considered to be a fake. In fact, this is an extremely one-sided and impractical wrong method of judgment. There are many types of stainless steel, and based on their microstructural characteristics at room temperature, they can be classified into several categories: 1. Austenitic type: such as 304, 321, 316, 310, etc ； 2. Martensitic or ferritic types: such as 430, 420, 410, etc ； Austenitic grades are non-magnetic or weakly magnetic, while martensitic or ferritic grades are magnetic. The stainless steel commonly used for decorative tube sheets is mostly of the austenitic 304 type; it is generally non-magnetic or only weakly magnetic. However, variations in chemical composition due to manufacturing processes or differences in processing conditions can result in magnetism, but this does not mean the material is counterfeit or defective. What is the reason for this? As mentioned above, austenite is non-magnetic or weakly magnetic, whereas martensite or ferrite is magnetic. Due to compositional segregation during manufacturing or improper heat treatment, small amounts of martensite or ferrite structure can form in austenitic 304 stainless steel. In this way, 304 stainless steel will have a slight magnetic property. Furthermore, when 304 stainless steel is cold-worked, its microstructure transforms into martensite; the greater the degree of cold working deformation, the more martensite is formed, and the greater the magnetism of the steel becomes. For steel strips of the same batch, when used to produce Φ76 pipes, there is no significant magnetic response; whereas when used to produce Φ9.5 pipes, such a response does occur. Due to the greater bending deformation, the magnetic susceptibility becomes more noticeable. When manufacturing rectangular tubes, the degree of deformation is higher than that of circular tubes; especially at the corners, where the deformation is more severe and the magnetism is more apparent. To completely eliminate the magnetism of 304 steel caused by the aforementioned factors, it is possible to restore a stable austenitic structure through high-temperature solution treatment, thereby eliminating the magnetism. It should be specifically noted that the magnetism of 304 stainless steel resulting from the reasons mentioned above is not on the same level as that of other types of stainless steel, such as 430 or carbon steel; in other words, 304 steel always exhibits weak magnetism. This tells us that if the stainless steel strip has weak magnetism or no magnetism at all, it should be classified as 304 or 316 material ； If it exhibits strong magnetism, similar to that of carbon steel, it should be determined not to be 304 material

I’ve seen people use special solutions to test stainless steel before; I conducted tests on 304 and 316L stainless steel tubes, and in both cases the results indicated issues with the material. I then tried testing Alfa Laval tubes as well, and there were still problems. Therefore, to obtain accurate results, it’s necessary to go to a qualified testing laboratory. When stainless steel pipes come into contact with or rub against iron, it can cause the stainless steel to develop a weak magnetic property or even rust; therefore, proper storage methods should be followed during transportation and storage.

Pure stainless steel definitely has no magnetism, so this testing method is reasonable

Previously, I worked in a factory where stainless steel was checked using magnets for a rough assessment. But most of them failed, and in the end, it was decided based on expert opinions. The elbow had to be replaced; it took three attempts before it was satisfactory. Have professionals monitor it.

Using a magnet to test whether stainless steel is genuine is highly unscientific, and in some cases it can even lead to opposite conclusions. An example can illustrate this: genuine 304 cold-rolled steel generally exhibits a weak magnetic property when tested with a strong magnet, but 201 steel, or even 201 steel of low quality made from scrap material, can be completely non-magnetic. Therefore, manufacturers who judge the quality of materials based on whether they are magnetic or not should pay attention to this. As for stainless steel solutions, don’t trust such nonsense; one can only simply distinguish between steel grades that differ significantly in their elemental composition, and these are generally used by companies that recycle stainless steel waste.

Is the issue you’re referring to this phenomenon? Stainless steel pipes should not be magnetic, but fittings such as elbows and tees do have magnetism, with the curved parts of the elbows having the strongest magnetism

Something that is magnetic is not necessarily not stainless steel. Both 304 and 321 are magnetic, whereas grades such as 301, which are commonly used in construction, may not be magnetic, as some manufacturers subject them to demagnetization treatment. Therefore, magnets cannot be used to determine whether it is made of 304 material; 316L has no magnetism. To find out the material, it’s best to conduct a test.