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Can stainless be magnetic?

Posted by Dave on Apr 28th 2022

Can stainless be magnetic?

Many of the best blog topics come up from conversations we have around the office or customer questions. Today's comes from both so it must be an absolute humdinger!

Stainless steel is generally thought of as non-magnetic - but is it? Simply, the 300 series stainless steels (303, 304, and 316 are ones we use often) are referred to as "austenitic," which in simple terms means that their chromium and nickel contents are such that they are non-magnetic. Other stainless series, like 420 which is in "martensitic" and 430 which is "ferritic" don't have the nickel content that the 300 series stainless steels do, and have higher carbon content than austenitic stainless steels. This makes them magnetic. If your fridge door readily holds onto magnetic stuff, then it's likely either martensitic or ferritic and not austenitic. Great, but what does this have to do with boats?

One of the reasons we use so much stainless on boats is obvious from its name - it's pretty darn resistant to rust. But we also like to use magnetic compasses, and the primary compass on a sailboat is usually in close proximity to a whole bunch of stainless parts that we make. But a customer found that his compass was deviating around some metal in and around his pedestal, and asked about it. 

300 series metals are basically completely non-magnetic when they're formed. But any kind of working can redistribute the components within the metal and cause magnetic "spots." The equipment use to work the metal can also impart small ferrous bits into the stainless, This is why our chains are degaussed - chains are heavily worked in their manufacturing process, and even though we use 300 series stainless chains, they really need to be non-magnetic. Degaussing is the process of removing a magnetic field, though even this can leave a small magnetic field. You can imagine that over the life of a chain, as it's working over the gears and being subject to many different stresses, that degaussing gets unwound and the chain will become slightly magnetic over time. Yet another reason to change your chain and cable at the recommended intervals!