It is common belief that a simple test for stainless steel is to check with a magnet. If it attracts, the product is not stainless steel. However, this assumption is incorrect.
“The stainless steel fasteners I received stick to a magnet.” This is one of the more frequently heard complaints among users. Stainless steel fasteners being nonmagnetic is also one of the largest misconceptions.
This post attempts to explain why most stainless steel fasteners are at least slightly magnetic and why many are so magnetic they are attracted to even weak household magnets.
The grades of stainless steel commonly used for fasteners are the Austenitic grades such as AISI-304 and AISI-316. When supplied in sheet or coil form they are essentially non-magnetic.
When these forms are worked – bent, deep drawn, formed into a tube – they become magnetic.
Try a fridge magnet around the bowl of your kitchen sink, which is grade 304 – you might be surprised.
The strength of the magnetism depends on how much the metal has been deformed. Even when these grades are cut (cold, by shearing) the deformation in the edge of the metal causes magnetism.
Stainless steel bolts are made by cold forging the head, and cold rolling or machining the thread. They are often quite strongly magnetic.
How Does an Austenitic Stainless Steel Become Magnetic?
The micro structure of the metal is what gives the steel its magnetic properties. If the stainless steel chosen was austenitic, e.g. type 316, and a portion of the micro structure were changed to any one of the other four classes then the material would have some magnetic permeability, i.e. magnetism, built into the steel.
The micro structure of austenitic stainless steel also changes by a process called martensitic stress induced transformation (MSIT). This is a micro structural change from austenite to martensite and the transformation can occur due to cold working (the process by which many fasteners are made) as well as slow cooling from austenitizing temperatures. Due to martensite being magnetic, the once nonmagnetic austenitic stainless steel will now have a degree of magnetism.
Though it may not seem like it, all fasteners can go through quite a bit of cold working prior to seeing service in the field. Cold working fasteners occurs in the wire drawing, forming, and thread rolling processes. Each of these processes will, typically, create enough martensite to produce a measurable degree of magnetism.
Does Magnetism affect the Corrosion resistance of Stainless Steel?
Thankfully, Magnetism and corrosion resistance are not connected. Corrosion resistance depends on how much chromium and (sometimes) molybdenum is in the stainless steel. The higher the chromium and molybdenum, the better corrosion resistance.
Therefore, the main purpose for which Stainless Steel Fasteners are used – Corrosion Resistance is unaffected by any magnetism and is safe to use.
It is only for specialized applications such are use in MRI machines where there is a strong magnetic field that the user/designer needs to consider de-magnetising stainless steel fasteners.