Open die forging actually serves two purposes, hammering the metal into a desired shape while at the same time refining the grain structure which improves the strength properties of the material.
Open die forging is named such because the metal is never totally confined by the die. The material is heated to a temperature above its recrystallization temperature. (For steels this is usually between 1900 and 2500 degrees Fahrenheit.) The die strikes the metal as it is manipulated to result in the desired shape. The person managing the manipulation of the material as it is struck is still to this day known as a blacksmith. Often the material must be reheated numerous times before the desired shape is reached. Open die forging is not a net shape process. Machining allowance should be considered when ordering forgings.