magnetic knives question

[hayshaker]

since knives have to be heat treated to a non magnetic state right?
then how come i see so many knives stick to a magnet.
like kershaw,gerber, schrade ect... what am i missing here
please understand i'm very new to this.

i joined the forum so as to learn from those who know.

 

[weo]

Steel becomes non-magnetic at around 1420-1425 degrees, below that temp it is magnetized.

At that temp there is what is called a phase shift or decalescence. Do a search and you'll see a lot of neat information, for instance in a dark room, you can actually see a significant change in brightness of the metal at this point. Also, there is a molecular change in the orientation of the carbon atoms in the steel at this point.

The importance for knife makers is that you need to have your knife steel reach this temperature point and the resultant change in the molecular arrangement and then cool (quench) the steel quickly to make the steel hard to hold an edge, but the steel is very brittle and fragile at this time.

Others will most likely chime in with more detailed information.
~billyO

 

[Callum.D]

You have to understand what causes ferromagnetism.

Ferromagnetism is due to the arrangement of delocalised electrons in the metal lattice. These electrons aren't fixed into the bonding layer of the metal atoms and are free to move around, these free moving electrons give metals their conductive and malleable properties. However it should be noted that not all metals display ferromagnetism, this is because in most metals these delocalised electrons arrange themselves in pairs so that their spins cancel each other out (electron spin causes an electron's magnetic moment). However in iron and some other ferromagnetic elements these electrons arrange themselves with their spins parallel to each other to maintain a more stable, lower energy state, this means that their spins don't cancel out and they will display ferromagnetism.

All ferromagnetic metals have a curie temperature which is the temperature where the material loses its ferromagnetic properties. This is due to the addition of thermal energy which overwhelms the energy lowering of ferromagnetic order, thus the relative spin directions of the electrons become chaotic and the material losses its ferromagnetism. In the case of iron it is 770 Celsius.

However at any point below this temperature there isn't enough thermal energy to maintain a this chaotic state ferromagnetic order kicks back in, this is what weo is talking about when he mentions decalescence. Since the ferromagnetic parallel spin arrangement is of lower energy, when the iron transitions (from its high energy chaotic state to a lower energy ferromagnetic state) energy is lost from the system into the environment, this is responsible for the brief rise in temperature and luminescence as the steel cools past its curie point.

Basically above 770 Celsius the delocalised electron arrangement of Iron is chaotic so the steel won't be ferromagnetic, below that the steel prefers to exist in a more stable, ferromagnetic state so it will stick to a magnet.

Hopefully you understood at least a little of that. We physics majors are notoriously bad at explaining ourselves and magnetism is an awfully tricky thing to wrap your head around.