Toughness

Toughness is the property which enables a metal to withstand shock loading or impact. Metal which have poor qualities of toughness are said to be brittle. Cast iron is a brittle metal and has poor resistance to shock load such as hammer blows.

Hardness is the property which enables a material to resist abrasion and indentation. Like the glass of your watch.

Low carbon steel 0.25%
Medium carbon steel 0.25 - 0.65 %
High carbon steel 0.65 - 1.4 % carbon