i read the wiki page on Carburizing but want more info, specifically on this part. why not just have the whole thing hard? or am i missing the point?

"...the higher carbon content on the outer surface becomes hard, while the core remains soft and tough." http://en.wikipedia.org/wiki/Carburization

 

Toughness is the key word there. High hardness on the surface allows for improved wear resistance, while a tough core will allow deform plastically before failure instead of cracking and failing catastrophically. (think of an aluminum cylinder vs a glass one).

There are whole texts and courses on this stuff if you are interested.

 

Yeah that would be great but from I know of carburizing,and i've done it a few times, is that it is a surface treatment only. Basically, heat the steel to the draw temp and introduce carbon to it in some way. Getting the carbon deeper into the steel is fairly difficult from what I've been told.

 

carburizing is the process of adding carbon to a metal by surrounding it with a gas or fluid high in carbon content. The carbon does penetrate through the surface but absorption gets much slower the deeper from the surface you get, after a couple of mm there is very little carbon because most of it is bonded to molecules nearer the surface.

 

It doesn't have to be done with fluid or gas. We performed the process using graphite powder to submerge the red-hot metal. That's the "old skool" way.

 

No way dude. the old school way is to use leather, bone charcoal or wood charcoal.