I. The main reasons for screws to be magnetic
Material's inherent ferromagnetism
Most screws are made of carbon steel or stainless steel (such as 304, 316), among which carbon steel contains ferromagnetic elements like iron, nickel, and cobalt. The atomic structure of these materials has unpaired electrons, which can spontaneously form magnetic domains (tiny magnetic areas). When the magnetic domains are aligned, the material exhibits macroscopic magnetism.
Magnetization during processing
Cold processing deformation: During cold working processes like forging and cutting, the crystal structure of the screws is distorted due to mechanical stress, causing the magnetic domains to rearrange. Experimental data shows that cold processing can increase the residual magnetic strength of low-carbon steel by 30%-50%.
Friction generates magnetism: During high-speed cutting, friction between the tool and the screw generates local high temperature, which may trigger the thermomagnetic effect. For example, when the drilling speed exceeds 50m/min, some stainless steel screws may develop weak magnetism due to frictional heat.
External magnetic field interference
If screws are transported or stored near strong magnetic fields (such as motors, magnetic separation equipment), they may be magnetized. The long-term effect of the Earth's magnetic field may also cause weak magnetism, but it is usually negligible.
II. How to avoid or eliminate the magnetism of screws
Material selection optimization
Choose non-magnetic or low-magnetic materials, such as austenitic stainless steel (316L has a magnetic permeability of <1.05) or titanium alloys. However, note that some "non-magnetic" stainless steel (such as 304) may still have weak magnetism due to processing.
Annealing treatment
Heat the screws above the Curie temperature (iron at 770°C) and then cool slowly to eliminate processing stress and disrupt the magnetic domain arrangement.
Application of demagnetizers
For magnetized screws, a alternating magnetic field demagnetizer can be used.
III. The actual impact and response of magnetic screws
Risk to precision equipment: Magnetic screws may interfere with sensors.
Assembly convenience: Moderate magnetism can attract screwdrivers, improving assembly efficiency. This characteristic is often utilized in automotive production lines.
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