How does rolling the head of a Stick (Ex. Arrow head/Spear) make it Harder?

I have seen videos on Youtube & Instructables about make a Spear/Arrow head stronger, i was wodering if this is due to tempering (Ex. Steel Heating, cooling rapidly for stronger atomic bond) or due to some other method of strengthing I do not know about ( I only know the Steel method due to mass History/Science channel fanatical-ness) Plase help me uderstand this.

Comments

[Gorfram]

Along with tempering, rolling can "work-harden" the steel - the stress of having been slightly deformed in the rolling process strengthens the steel against further deformation.

If the rolling is done in or against a carbon-rich material (charcoal, coke, etc.), there will also be what's called "case-hardening." Steel generally gets harder as more carbon is alloyed with the iron that it's made of, but it also gets more brittle. With case-hardening, you can make a shaft (or tang, or whatever) of lower-carbon steel that is tough enough to withstand blows and impact with shattering; and then heat-treat just the surface of the blade to absorb extra carbon, forming a "case" that is "hard" enough to form and hold a good sharp edge.

(For the difference between "hardness" and "toughness" - a china plate is hard but brittle, a rubber dog-toy is soft but tough.)

[seandogue]

If you're talking about rolling a wooden stick, it would probably be a simple case of compression...that is, by rolling the end of the stick you are compressing the wood fibers, making a denser area where it is rolled. As long as you don't roll it so hard that the fibers essentially delaminate, ie, become separated, a denser area of wood is automatically harder. If metal, well i think Gorfram already covered the subject pretty thoroughly.