by Motoyasu and WarAngel

What makes one steel tougher than another is by virtue of its metallurgical composition as well as heat-treating.

Certain elements do certain things. You generally want to avoid high-alloy stainless steels if you want a real sword. After all, don't want your sword to smash apart!

Iron - Goes without saying. Swords made according to ancient Japanese tradition - even the serious ones today, are made out of tamahagane (pronounced "ta-mah-hah-gah-neh"). This is basically an iron-bearing sand. Iron, laced with Carbon, form the basis of steel.

Carbon - From the fires of the forge and the finest of charcoal comes carbon, which serves as the hardener of iron to form steel. A higher carbon content means that the resulting steel is harder (but too much carbon means that the steel is so hard it could be brittle!). A lower carbon content gives steel a "tougher" property, but it won't hold an edge as well as higher carbon steel because it's too soft.

Chromium - Avoid at all costs. It is what makes stainless steel corrosion resistant, and in small quantities can act as a grain enhancer and yields a mirror-finish when polished, making stainless steel look very nice. But it also makes the steel brittle. You don't want to fight Kalas with a brittle sword! In addition, chromium - at high amounts - makes it extremely difficult to create a hamon (temper line) in the sword blade.

Silicon - Silicon's presence in traditional steel gave it additional flexibility and strength!