“Is Stainless Steel Magnetic?” when asked this question to material engineers, a mixed response is obtained. Some say Stainless steel is non-magnetic and some say it is magnetic. Let’s explore more on this subject to find out if Stainless Steel is Magnetic or not.
What is Stainless Steel?
Stainless Steel is a group of ferrous alloy materials containing more than 11% chromium to prevent it from rusting. Other elements of stainless steel are carbon, molybdenum, nitrogen, aluminum, sulfur, silicon, nickel, copper, niobium, selenium, etc. The chromium present in stainless steel creates a passive film to protect the underlying material from corrosion.
What makes a material magnetic?
For a material to be magnetic
- it must contain a ferromagnetic material like iron, nickel, cobalt, terbium, gadolinium, dysprosium, etc.
- crystal structure should be such that it has partially-filled electron shells (the incomplete inner core of electrons) that create a magnetic dipole moment. The Body-centered cubic lattice of ferritic microstructure shows this property.
Is Stainless Steel Magnetic?
As mentioned above, the magnetic property of Stainless Steel will depend on the crystal structure of the alloy. Stainless Steel, being a ferrous material it contains iron, a ferromagnetic material. Now to possess magnetism, it needs to satisfy the second criteria mentioned above.
Stainless steels available in the market are classified into the following four groups:
- Ferritic Stainless Steels
- Austenitic Stainless Steel
- Martensitic Stainless Steels, and
- Duplex stainless steels
Is Ferritic Stainless Steel Magnetic?
Ferritic Stainless Steels have a ferritic structure in their microstructure. At room temperature they are arranged as BCC structure, meaning eight atoms are arranged at the corners of the unit cell, connecting to one lattice point in the middle of the structure. Such crystal arrangement shows magnetic properties. So, Ferritic stainless steels are magnetic. However, the magnetic strength of ferritic stainless steel is not as strong as conventional irons. Hence, they can be used as soft magnets. This is why stainless steel grades 409, 430, and 439 shows magnetic properties.
Note that, at temperatures above Currie point temperature the ferritic BCC structure is converted into FCC and the magnetic property is lost.
Is Austenitic Stainless Steel Magnetic?
Austenitic Stainless Steels consist of a high amount of austenite. Their crystal structure is arranged as an FCC structure, meaning a cube with an atom at each of the cube’s eight corners and an atom at the center of each of the six faces. Such grades of stainless steel are paramagnetic and do not show any magnetic properties. This is the reason that most popular austenitic stainless steel grades like SS 304, SS 316, etc are non-magnetic.
Is Martensitic Stainless Steel Magnetic?
All martensitic (body-centered tetragonal crystal structure) and most precipitation-hardenable stainless steels exhibit ferromagnetic properties and hence, they are magnetic. Because of the induced stresses by the hardening transformation, these stainless steel grades show permanent magnetic properties when magnetized in hardened conditions. Because of this martensitic stainless steel grades like grades 410, 420, 440 shows magnetic properties. The table in the following figure (Fig. 1) provides the magnetic properties of some ferritic and martensitic stainless steels.
Is Duplex Stainless Steel Magnetic?
Duplex stainless steels (DSS) consist of a mixture of ferrite and austenite and are usually magnetic. The ferritic structure contributes to this magnetism. But, due to the fact that most DSS contains more austenite than ferrite, their magnetic strength is normally less.
So from the above discussions, it is clear that stainless steel can be magnetic or non-magnetic and it fully depends on the crystal structure and composition. But, The magnetic behavior of stainless steel varies considerably from paramagnetic in fully austenitic stainless steel grades to permanent magnetic behavior in the hardened martensitic stainless steel grades.
Why does the magnetic property of stainless steel matter?
The magnetic capabilities of stainless steel material are always inferior to the conventional magnetic material. Hence, Stainless steels are usually not used based on their magnetic behaviour. However, the magnetic or nonmagnetic capabilities can sometimes influence the fabrication and use of these alloys. Also, in magnetic materials electric current behaves differently.