The 300-series stainless steels contain sufficient nickel to render them non-magnetic at room temperature. Most people are familiar with 316 and 304 as these are the most common stainless steel grades.

All stainless steels from the 300-series are non-magnetic, which has resulted in a common misconception that all stainless steels are non-magnetic. However, stainless steels from the other series, such as 400, PH, duplex, etc., have higher percentages of ferrite and are, therefore, magnetic. Some of these “magnetic” stainless steels have superior strength and corrosion resistance to those of the 300 series.

It is usual for your “316 equivalent” casting to exhibit some magnetic properties.

Castings said to be from the 300-series (e.g., a casting specified as 316) can be, and most often are, at least slightly magnetic. This is because 316 is a designation for hot-rolled or forged stainless steel, known as a “wrought” designation. The equivalent casting specification is CF-8M, which is similar but not identical in composition and properties of the wrought grade.

The wrought grade has its chemistry optimised to be fully austenitic; this is an advantage for a wrought (worked) grade as the austenitic microstructure requires less energy to squeeze or roll the material into shape.

This is not an advantage for a casting as it does not have to be squeezed into shape. A casting forms its shape by solidifying the liquid metal into the shape of the mould in which it is contained.

The chemistry of a CF-8M casting is balanced to be predominantly austenite with usually 5 to 15 % ferrite.

This enhances hot strength and reduces the possibility of complex cast shapes tearing as they cool in the mould. Since the casting will not be rolled, the increased hot strength doesn’t cause any inconvenience. However, the presence of some ferrite renders the casting slightly magnetic.

Although the chemistries and microstructures of the wrought and cast grades vary, the physical and corrosion properties are closely matched.

The hot rolling imparts some strength to the wrought grade, which is countered by the ferrite phase, which imparts additional strength to the cast grade. The hot rolling imparts good corrosion resistance to the wrought grade by refining the structure, and this is countered by the increased corrosion resistance in the cast grade due to slightly higher chromium content.

Now that you understand some metallurgical factors, you can accept that it is normal for your “316 equivalent” casting to exhibit some magnetic properties.

If you need a non-magnetic stainless-steel casting, please specify this so we can select a chemical composition that will result in a fully austenitic structure.

If you need any advice on your casting requirements, please contact: admin@malabou.com