We usually subconsciously believe that 304 and 316 stainless steel bolts are non-magnetic. Many users even judge the quality of stainless steel bolts by their magnetism, thinking that non-magnetic bolts are genuine and magnetic ones are of poor quality. Here, we responsibly clarify that this judgment is completely incorrect!
Why is this judgment wrong? First, it is necessary to clarify the material classification of stainless steel bolts: the core materials used to produce 304 and 316 stainless steel bolts are austenitic stainless steels (grades starting with "3", such as SUS304 and SUS316); while the materials used to produce self-drilling self-tapping stainless steel bolts are mostly martensitic stainless steels (grades starting with "4", such as SUS410). Martensitic stainless steel (such as SUS410) is inherently magnetic due to its relatively high carbon content. This inherent magnetism is associated with the material's properties-carbon is added to increase the hardness of the self-tapping screws, ensuring they can cut threads in workpieces during installation. Without sufficient hardness, they would fail to tap effectively. Therefore, it is normal for SUS410 stainless steel bolts to be magnetic, and this has no bearing on their quality.
Non-Magnetic and Magnetic Austenitic Stainless Steel Bolts
Theoretically, austenitic stainless steels (SUS304, SUS316) are non-magnetic or weakly magnetic. However, after undergoing cold working processes such as cold heading and thread rolling, both 304 and 316 stainless steel bolts will exhibit weak magnetism. This magnetism does not stem from material quality issues but from martensitic transformation caused by cold working-cold processing causes part of the austenitic structure to transform into martensitic structure, which generates weak magnetism. Importantly, this weak magnetism has no impact on the corrosion resistance, mechanical properties, or functional performance of the stainless steel bolts.
If specific application scenarios (such as electronic circuit board installation) require completely non-magnetic stainless steel bolts, demagnetization treatment can be applied to achieve a fully non-magnetic state. Currently, there are two mainstream demagnetization methods:
Physical Demagnetization (Electromagnetic Demagnetization): This method reduces the magnetism of bolts through an alternating magnetic field. Contrary to the misconception that it is similar to centrifugal dehydration, its principle relies on electromagnetic induction. However, the effect of physical demagnetization is not permanent, and magnetism may recover over time.
Solution Annealing: This is a permanent demagnetization method. It involves heating the bolts to 1050-1150℃, holding them at that temperature for a certain period, and then rapidly cooling them. This process completely eliminates the martensitic transformation caused by cold working and restores the pure austenitic structure. Bolts treated with solution annealing will be completely non-magnetic and will not regain magnetism.
In addition, stainless steel bolts produced by turning (such as precision bolts) typically use SUS303 material. SUS303 is suitable for turning because it has a slightly higher carbon content than SUS304 and added sulfur to improve machinability-not because turning increases the carbon content. Such bolts will inevitably exhibit weak magnetism. Moreover, due to the addition of sulfur, their corrosion resistance is slightly inferior to that of SUS304. In practical applications, material composition adjustments are made to optimize the machinability of bolts for turning processes. This is a targeted design choice to meet processing requirements, not a quality defect.
In summary, the magnetism of 304 and 316 stainless steel bolts mainly results from martensitic transformation after cold working, which is unrelated to material authenticity or quality. The magnetism of martensitic stainless steel bolts (such as SUS410) is an inherent material property, designed to meet the hardness requirements of self-tapping functions. When evaluating the quality of stainless steel bolts, core indicators such as material certification, mechanical performance grades (e.g., strength class), and surface treatment quality should be prioritized, rather than relying solely on the presence or absence of magnetism.
