Are All Stainless Steels Magnetic?

Some stainless steels are magnetic and some are not. It depends on the grade. Grade 430 is strongly magnetic. Grades 304 and 316 are weakly magnetic or not magnetic at all.

This matters in food, pharmaceutical and chemical plants, where most equipment is made from stainless steel and wear particles from that equipment can end up in the product.

What stainless steel is

Stainless steel is an alloy. It is mostly iron, with at least 10.5 percent chromium. The chromium forms a thin layer on the surface that stops rust. Nickel, molybdenum, carbon and other elements are added to change the strength and the corrosion resistance.

The mix of elements changes the crystal structure of the steel. The crystal structure decides whether the steel is magnetic.

The main families of stainless steel

There are five families. Three of them matter for magnets.

Ferritic stainless steels are magnetic. They contain chromium and little or no nickel. Grades 409, 430 and 439 are ferritic. They are used in car exhausts, kitchen appliances and cheaper fittings.

Martensitic stainless steels are magnetic. They contain more carbon and can be hardened. Grades 410, 420 and 440 are martensitic. Knife blades, valve parts and shafts are often made from them.

Austenitic stainless steels are not magnetic in their normal state. They contain nickel, which changes the crystal structure. Grades 304 and 316 are austenitic. Most food and pharmaceutical equipment, tanks, pipes and screens are made from these grades.

Duplex stainless steels are a mix of ferritic and austenitic. They are magnetic and very corrosion resistant. They are used in chemical plants and in oil and gas.

Precipitation hardened stainless steels are magnetic. They are used in aerospace and in high strength parts.

Why 304 and 316 can still be caught

Austenitic steel becomes partly magnetic when it is worked. Cutting, bending, grinding and rubbing change the crystal structure in the worked area. A screen that has been vibrating for years, a blade that has been sharpened, or a pipe that has been bent will all show a weak pull.

The fine particles that wear off stainless equipment are work hardened by the wear itself. This is why a high strength magnet can remove them, even though the parent metal seems to ignore a magnet.

The pull is weak, so the magnet must be strong and the particle must pass close to it. Standard strength rods will miss most of these particles. Rods of 9,500 gauss and above hold them.

A simple test

Hold a small permanent magnet against the metal.

• A firm pull means a ferritic, martensitic or duplex grade.
• A weak pull, or a pull only at cut edges and welds, points to a worked austenitic grade such as 304.
• No pull at all means an austenitic grade in its soft state.

A gauss meter gives a more useful measurement when you need to check the strength of a separator itself.

Choosing a separator for stainless contamination

• Use the highest practical strength. SMAG rods are made in four tiers, up to 13,500 gauss.
• Keep the product path close to the magnet. Grids and drawer housings space the rods so that every particle passes within a few millimetres of a rod.
• Clean the rods often. A rod covered in collected iron has a weaker surface field.
• Check the strength once a year with a gauss meter against the certificate supplied with the unit.

The SMAG range

Santosh Magnetic Works makes neodymium magnetic rods in 304 stainless tubes, in standard, high, super high and extreme high strength. Every rod is supplied with a tested certificate of magnetic strength. Contact us with a sample and we will test how well it is held.