Why Magnet Don't Work On Some Stainless Steel?

STAINLESS steels are iron-based alloys primarilly known for their generally excellent corrosion resistance, which is largely due to the steel's chromium concentration.

There are several different types of stainless steels. The two main types are austenitic and ferritic, each of which exhibits a different atomic arrangement.

Due to this difference, ferritic stainless steels are generally magnetic while austenitic stainless steels usually are not. A ferritic stainless steels owes its magnetism to two factors: its high concentration of iron and its fundamental structure.

The metallic atoms in an austenitic stainless steels are arrangeed on a face-centred cubic (fcc) lattice. The unit cell of an fcc crystal consist of a cube with an atom at each of the cube's eight corners and an atom at the centre of each of the six faces.

In a ferritic stainless steels, however the metalic atoms are located on a body-centred (bcc) lattice. The unit cell of a bcc crystal is a cube with one atom at each of the eight corners and a single atom at the geometric centre of the cube.

Alloying the stainlesss steel with elements such as nickel, manganese, carbon and nitrogen increases the likelihood that the alloy will posses the fcc crystal structure at room temperature.

Chromium, molybdenum and silicon make it more likely that the alloy will exhibit the bcc crystal structure at room temperature.

The most popular stainless teel is Type 304, which contains approximately 18% chromium and 8% nickel. At room temperature, the thermodynamecally stable crystal structure of 304 stainless steel is bcc; nevertheless, the alloy's nickel concentration, as well as the small amounts of manganese (about 1%), carbon (less than 0.08%) and nitrogen (about 0.06%), maintains an fcc structure and therefore the alloy is non-magnetic.

If the alloy is mechanically deformed, i.e. bent, at room temperature, it will partially trasform to the ferritic phase and will be partly magnetic, or ferromagnetic, as it is more precisely termed.

Popular ferritic stainless steels are iron-chromium binary alloys with 13% to 18% chromiun. These alloys are ferromagnetic at room temperature. Like all ferromagnetic alloys, when heated to a high enough temperature - their Curie temperature - the ferritic stainless steels lose their ferromagnetism and become paramagnetic, that is, they do not retain their own magnetic field but continue to be attracted to external ones.

A piece of ferritic stainless steel is typically unmagnetised. When subjected to a magnetic field, however, it will become magnetised and when this applied magnetic field is removed the steel remains magnetised to some degree. This behaviour is a consequence of the steel's microstructure.

Specifically, in its natural state ferritic steel consists of small regions called magnetic domains, which are fully magnetised, but in general the direction of magnetisation is different in each domain.

As a result, the sum total of all the domains gives the piece a zero magnetic moment.

An external magnetic field orients these magnetic domains. Depending on the steel and the applied field, the orrientaion is achieved by a combination of selective growth or shrinking of particular domains and the rotaion of magnetisation within the domains.

If the applied field is sufficiently strong, the steel will retain a significant fraction of its magnetisation as long as the steel has adequate number of imperfections that keep the domains from rotating and growing or shrinking.

Fundamentally, the reason why ferritic stainless steels are ferromagnetic while austenitic stainless steels are not quantum-mechanical in nature.

Suffice it so say a ferromagnetic metal consists of atomes that have an incomplete inner core of electrons and a crystal structure that results in a high density of electron states in the energy bands formed from the incomplete atomic inner core.

It also has an atomic spacing that allows for exchange effects among electrons in the energy bands associated with the incomplete inner-core level.

If the atoms in the metal crystal are too widely spaced, the exchange effects are too small to cause alignment of the magnetic moments of neighbouring atoms and the crystal will not exhibit ferromagnetism.

extracted from Sunday Star –Sci-Tech dated Nov 05 2006