Stainless steel is typically considered a non-magnetic alloy however, that is not always the case. Stainless steel is an alloy primarily composed of iron, chromium (at least 10.5%), and other elements such as nickel, molybdenum, and manganese. The addition of chromium provides corrosion resistance by forming a passive layer of chromium oxide on the surface.
There are several types of stainless steel, and each is classified based on its microstructure and composition: austenitic, ferritic, martensitic, duplex, and precipitation-hardening. Some grades of stainless steel may include small amounts of other alloys that are magnetic. Even a small amount of magnetism can be detrimental in sensitive environments such as aerospace, medical, and electronic sectors. The magnetism can disrupt the functionality of sensitive instruments, potentially leading to malfunctions or inaccurate readings. Non-magnetic grades of stainless steel can become magnetized during processes such as rolling, drawing, bending, pressing, or hammering that deforms the metal at temperatures below its recrystallization temperature; commonly known as cold working.
How does cold working transform stainless steel?
Stainless steel becomes magnetic after cold working due to the transformation of its microstructure. Austenitic stainless steels, such as types 304 and 316, are generally non-magnetic in their annealed state because of their Face-Centered Cubic (FCC) crystal structure. However, during cold working processes, the severe deformation increases dislocation density and introduces stress, leading to a phase transformation. This deformation can cause the non-magnetic austenitic phase to partially transform into the magnetic martensitic phase, which has a Body-Centered Cubic (BCC) or body-centered tetragonal structure. The martensitic phase supports the alignment of magnetic domains, making the steel magnetic. The degree of magnetism depends on the extent of the cold working and the resultant amount of martensitic phase formed. Alloys with higher carbon content or other alloying elements may be more prone to martensitic transformation.
Can this process be reversed?
The magnetic properties induced by cold working can be reversed by annealing the steel. Heating it to a high enough temperature allows for the reversion of martensite back to austenite, restoring its non-magnetic properties.
Does this mean that stainless screws are all magnetic?
Not necessarily, it is best to consult the manufacturer if you are uncertain. Stainless steel screws are often made from austenitic grades like 304 or 316, which are typically non-magnetic. However, if the screws undergo significant cold working during manufacturing (such as through thread rolling), they can exhibit mild magnetic properties. The extent of magnetism will vary based on the degree of cold working and the specific alloy composition.
How can fasteners from UC Components, Inc. help you?
UC Components, Inc. has been the world leader in clean-critical hardware since 1974. We offer a variety of fasteners and O-rings, including, but not limited to, standard and vented screws, washers, and hex nuts in multiple materials. We offer numerous finish options, specialized venting designs, complete custom product development, and Class 100/ISO Class 5 Cleanroom cleaning & packaging. View our parts catalog online to find the components you need, request a quote, or contact us for more assistance or additional information. If you do not see the fastener, material, or O-ring that you need listed in our standard products, please give us a call! We may not stock it, but we are happy to acquire or manufacture just about any fastener or O-ring you need.
