Sciencemadness Discussion Board

Explaining steel

bobliest - 28-8-2007 at 22:49

hey, just out of interest i was wondering if any one could either explain or give me a link to somewhere that will explain what makes steel so much hader than iron just because it has around 1% carbon in it. It may be something simple but im not sure what because i am relativly new to chemistry.

not_important - 28-8-2007 at 23:48

In short the carbon creates obstacles to the slippage of the iron. Cementite, Fe3C, can also play a role due to its intrinsic hardness.

http://en.wikipedia.org/wiki/Steel

Nerro - 29-8-2007 at 04:24

Normally metals are fairy flexible because the layers of metal atoms can slip over eachother, the electrons that bind them together are spread over the whole chunk of metal anyway so the location of the atoms isn't critical. What the carbon does is fill up the intersticial spaces between the metal atoms (in this case Fe atoms). This is why steel is called an intersticial alloy. There is another class of alloys which are called substitutional alloys in which metal atoms are replaced by similarly sized metal atoms of a different kind (like brass in which Sn and Cu are alloyed). The presence of the the carbon atoms makes an obstacle for the metal to have to overcome before the layers of atoms can slip over eachother. It's basically a way to raise the friction between the layers which causes the metal to be stiffer than the pure thing.

12AX7 - 29-8-2007 at 06:25

Quote:
Originally posted by Nerro
Normally metals are fairy flexible because the layers of metal atoms can slip over eachother, the electrons that bind them together are spread over the whole chunk of metal anyway so the location of the atoms isn't critical.


So an amorphous sample will have the same properties as a microcrystalline sample of the same alloy?
Quote:
What the carbon does is fill up the intersticial spaces between the metal atoms (in this case Fe atoms). This is why steel is called an intersticial alloy.


Correct. Of course, the form it occupies -- chunky intergranular deposits, or martensitic precipitate -- makes a dramatic difference in the characteristic of that steel. (Annealed carbon steel is almost as soft as annealed mild steel.)
Quote:
There is another class of alloys which are called substitutional alloys in which metal atoms are replaced by similarly sized metal atoms of a different kind (like brass in which Sn and Cu are alloyed).


Brass is Zn and Cu. Bronze is Cu with any combination of Sn, Pb, Zn, Mn, Al, etc.
Quote:
The presence of the the carbon atoms makes an obstacle for the metal to have to overcome before the layers of atoms can slip over eachother. It's basically a way to raise the friction between the layers which causes the metal to be stiffer than the pure thing.


Alloys are rarely if ever any stiffer than parent metals. They are, however, stronger.

Tim

Nerro - 29-8-2007 at 07:00

First point: Obviously not, the point was that the "sheets" of atoms can slide over eachother without the crystal breaking up because the electrons are diffused in something often referred to as an "electron gas" of electrons that are able to move freely in the crystal lattice.

Third point: Meant bronze, screwup.

Forth point: Meant stronger, English is not my first language. What's the difference?

12AX7 - 29-8-2007 at 10:13

Stiffness is the elastic deformation of a material, corresponding to its elastic modulus. Strength is the pressure at which the threshold of some amount of permanent deformation is reached, typically like 0.1% permanent elongation (tensile yield), or breaking in two (ultimate tensile strength).

Don't feel bad, a lot of people confuse these in everyday language.

Tim