Letter 12589

Magnetic character of stainless steel explained  

I would like to know the chemistry explanation of why when Nickel is removed from stainless steel, it loses its magnetic characteristic. For example, stainless steel 430 is magnetic but 304,304L,316 and 316L are not? It is important to know whether this has to do with the atomic rearrangement that affects the polarity of the iron.

Adoga Kiharangwa
poultry farm - Delhi, California, U.S.A

Adoga,

I will attempt to answer this question without getting too deep into the metallurgy.

The difference in magnetic properties is due to the structure of the material at room temperature. The structure is due to (in most cases) the chemistry and thermal processing of the steel.

Nickel is added to stainless steel to stabilize the austenitic phase. Most 300 series stainless steels contain sufficient amounts of nickel so the structure is austenitic at room temperatures.

300 series stainless steel (also know as Austenitic stainless steel) has "higher" nickel contents and consists of a single phase, (austenite) which is non-magnetic.

Most 400 stainless steels have "lower" nickel contents and have a martensitic or ferritic structure (both of which are magnetic).

As with anything in metallurgy, there are lots of exceptions. As an example, there are actually five types of stainless steel (martensitic, austenitic, ferritic, precipitation-hardening and duplex). To make matters more interesting, 300 series stainless steel can be made magnetic with sufficient amounts of cold working. The ASM Metals Handbooks are a great source of additional information.

Matthew Horton
- St. Paul, Minnesota

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