metalman, I know this is unrelated but this is perfect timing. How exactly does tempering make steel harder? I get that steel contains carbon and iron, but on a molecular scale, why is it stronger. Does it also make the metal more brittle?
Well technically tempering makes steel less hard... tempering occurs after the hardening process to remove excess hardness. As far as i understand it reduces the stress on the steel. I'm not near as adept on carbon steel as i am on stainless steels so I may be off...
Okay, now that I'm on break i can elaborate more on how i understood it from school. This will probably be multipart as it's actually really complex and for me to explain it all requires me to start from the beginning.
Basically at the atomic level all atoms for very tight bonds with the other atoms surrounding them. Now what differs from other atomic structures is that the electrons in the outer layers of the electron fields (valence electrons) aren't tied to one specific atom but to all atoms. In essence all the valence electrons move about freely within the metal.
As all things do metals have a metling point, at which point the metal is a liquid. As the metal begins to cool a crystaline structure is formed. Within this crystaline structure each individual crystal has it's own grain. Each grain acts independently of eachother.
Now after the initial cooling and crystalization often times defects/gaps are formed between crystal structures. To remove these defects the metal is then heated to the point of nearly melting and cooled almost instantly. This freezes all the atoms/crystaline structures that filled these gaps in place. This is the point in which the metal is hardest and most brittle. This occurs because there is no gaps and no room for the crystaline structure to move about. Meaning that if brunt force were to be used the metal would break as there is no where for the metal to give.
Now this is where tempering comes in as the metal is reheated, tempering very rarely reaches the point of almost melting, to a temperature lower than the melting point and air cooled. This lack of sudden cooling allows the crystaline structure to move about more freely which in turn lessens the hardness of the material.
I hope this made sense as without a reference to pictures or physical objects it's not easily explained in an understandable manner. I learned much of this throgh youtube/internet as for me i have to learn through example and seeing something done. I also hope i answered your initial question... i tend to ramble on at times
Basically at the atomic level all atoms for very tight bonds with the other atoms surrounding them. Now what differs from other atomic structures is that the electrons in the outer layers of the electron fields (valence electrons) aren't tied to one specific atom but to all atoms. In essence all the valence electrons move about freely within the metal.