Many people believe that using a magnet to test stainless steel is an effective way to determine its quality and authenticity. If the metal doesn't stick, it's considered genuine and high-quality. However, if it does stick, it's often labeled as fake or low-grade. This method is actually misleading and inaccurate.
Stainless steel comes in various types, and they can be categorized based on their microstructure at room temperature. The two main categories are:
- 1. Austenitic: such as 304, 321, 316, and 310.
- 2. Martensitic or Ferritic: like 430, 420, and 410.
Austenitic stainless steels are typically non-magnetic or only weakly magnetic, while martensitic or ferritic grades are generally magnetic. Most decorative stainless steel tubes are made from austenitic 304, which is usually non-magnetic or only slightly so. However, under certain conditions, 304 can develop slight magnetic properties.
Why might this happen? During the smelting process, small amounts of martensite or ferrite can form due to segregation or improper heat treatment. These changes can cause 304 to show weak magnetic characteristics. Additionally, cold working—like bending or rolling—can also lead to a transformation of the microstructure into martensite, increasing the magnetic response. The more deformation the material undergoes, the stronger the magnetic effect becomes.
For example, when producing stainless steel tubes, a larger diameter tube like Φ76 may not show any noticeable magnetism, while smaller tubes like Φ9.5 may become more magnetic due to greater deformation. Square or rectangular tubes, especially at the corners where deformation is more intense, tend to show stronger magnetic properties than round ones.
To eliminate these magnetic properties caused by structural changes, a high-temperature solution treatment can restore the original austenitic structure, making the material non-magnetic again.
It’s important to note that the magnetism seen in 304 stainless steel is always weak compared to that of materials like 430 or carbon steel. So, if a stainless steel strip is not magnetic or only slightly so, it’s likely 304 or 316. But if it shows strong magnetism similar to carbon steel, it’s probably not 304.
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