Cold, Warm, and Hot Forging
Cold, Warm, and Hot Forging -- What's the Difference?
Cold forging involves either impression die forging or true closed die forging with lubricant and circular dies at or near room temperature. Carbon and standard alloy steel forgings are most commonly cold-forged. Parts are generally symmetrical and rarely exceed 25 lb. The primary advantage is the material savings achieved through precision shapes that require little finishing. Completely contained impressions and extrusion-type metal flow yield draftless, close-tolerance components. Production rates are very high with exceptional die life. While cold forging usually improves mechanical properties, the improvement is not useful in many common forging applications and economic advantages remain the primary interest. Tool design and manufacture are critical.
Warm forging has a number of cost-saving advantages which underscore its increasing use as a manufacturing method. The temperature range for the warm forging of steel runs from above room temperature to below the recrystallization temperature, or from about 800 to 1,800 degrees Fahrenheit. However, the narrower range of from 1,000 to 1,330 degrees Fahrenheit is emerging as the range of perhaps the greatest commercial potential for warm forging. Compared with cold forging, warm forging has the potential advantages of reduced tooling loads, reduced forging press loads, increased steel ductility, elimination of need to anneal prior to forging, and favorable as-forged properties that can eliminate heat treatment.
Hot forging is the plastic deformation of metal at a temperature and strain rate such that recrystallization occurs simultaneously with deformation, thus avoiding strain hardening. For this to occur, high workpiece temperature (matching the metal's recrystallization temperature) must be attained throughout the process. A form of hot forging is isothermal forging, where materials and dies are heated to the same temperature. In nearly all cases, isothermal forging is conducted on superalloys in a vacuum or highly controlled atmosphere to prevent oxidation.