Many people believe that testing whether a magnet sticks to stainless steel can determine its quality, authenticity, and grade. If the magnet doesn’t stick, it’s often assumed to be genuine or high-quality. But if it does stick, it's seen as fake or low quality. This is a common misunderstanding and not an accurate method of identification.
Stainless steel comes in various types, and they can be classified based on their microstructure at room temperature:
1. Austenitic: such as 304, 321, 316, and 310;
2. Martensitic or Ferritic: such as 430, 420, and 410.
Austenitic stainless steels are typically non-magnetic or weakly magnetic, while martensitic and ferritic grades are usually magnetic. Most decorative stainless steel tubes are made from austenitic 304, which is generally non-magnetic or only slightly so. However, under certain conditions, 304 may show some magnetic properties, but this doesn't mean it's fake or substandard.
The reason for this is that during the smelting process, small amounts of martensite or ferrite can form due to segregation or improper heat treatment. This can cause 304 stainless steel to have weak magnetic properties. Additionally, cold working—such as bending or rolling—can transform the microstructure into martensite, increasing the magnetic response. The more deformation, the stronger the magnetism.
For example, when producing a Φ76 tube, there may be little to no magnetic effect, but when making a Φ9.5 tube, the increased deformation leads to more noticeable magnetism. Square or rectangular tubes, especially at the corners, undergo even greater deformation, resulting in stronger magnetic characteristics.
To eliminate these magnetic properties caused by structural changes, a high-temperature solution treatment can restore the original austenitic structure and remove the magnetism.
It’s important to note that the magnetic properties of 304 stainless steel are always weak compared to those of materials like 430 or carbon steel. So, if a stainless steel strip shows no magnetism or only a very slight one, it’s likely 304 or 316. But if it strongly attracts a magnet, it's probably not 304.
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