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Hot Forging VS. Cold Forging

2015-7-31      View:

Hot forging and cold forging are two different metal forming processes that deliver similar results. Forging is the process of deforming metal into a predetermined shape using certain tools and equipment—deformation is accomplished using hot, cold, or even warm forging processes. Ultimately, the manufacturer will look at a number of criteria before choosing which type of forging is best for a particular application.
The Hot Forging Process
When a piece of metal is hot forged it must be heated significantly.
The average temperatures necessary for hot forging are:
•Up to 1150 degrees Celsius for Steel
•360 to 520 degrees Celsius for Al-Alloys
•700 to 800 degrees Celsius for Cu-Alloys
During hot forging, the temperature reaches above the recrystallization point of the metal. This kind of extreme heat is necessary in avoiding strain hardening of the metal during deformation. In order to prevent the oxidation of certain metals, like super alloys, a type of hot forging called isothermal forging is a good choice. In isothermal forging, the metal deformation occurs within a highly controlled atmosphere, similar to that of a vacuum.
Hot Forging Considerations
Traditionally, manufacturers choose hot forging for the fabrication of parts that have a greater influence in the technical arena. Hot forging is also recommended for the deformation of metal that features a high formability ratio. Other considerations for hot forging include:
1. Production of discrete parts
2. Low to medium accuracy
3. Scale Formation
4. Low stresses or low work hardening
5. Homorgenized grain structure
6. Increased ductility
7. Eliminiation of chemical incongruities

Possible disadvantages of hot forging include:
•Less precise tolerances
•Possible warping of the material during the cooling process
•Varying metal grain structure
•Possible reactions between the surrounding atmosphere and the metal
Cold Forging
Cold forging deforms metal while it is below its recrystallization point. Cold forging is generally preferred when the metal is already a soft metal, like aluminum. This process is usually less expensive than hot forging and the end product requires little, if any, finishing work. Sometimes, when aluminum is cold forged into a desired shape, it is heat treated to strengthen the piece. This is called "tempering."
The Cold Forging Process
Despite the word "cold," cold forging actually occurs at or near room temperature. The most common metals in cold forging applications are usually standard or carbon alloy steels. One of the most common types of cold forging is a process called impression-die forging, where the metal is placed into a die that is attached to an anvil. The metal is then hit by a descending hammer and forced into the die. Depending on the product, the hammer may actually be dropped on the metal numerous times in a very rapid sequence.
Cold Forging Considerations
Manufacturers may choose cold forging over hot forging for a number of reasons—since cold forged parts require very little or no finishing work, that step of the fabrication process is often dispensable, which saves money. Cold forging is also less susceptible to contamination problems, and the final component features a better overall surface finish. Other benefits of cold forging include:
•Easier to impart directional properties
•Improved interchangeability
•Improved reproducibility
•Increased dimensional control
•Handles high stress and high die loads
•Produces net shape or near-net shape parts

Some possible disadvantages include:
•Easier to impart directional properties
•Improved interchangeability
•Improved reproducibility
•Increased dimensional control
•Handles high stress and high die loads
•Produces net shape or near-net shape parts
•The metal surfaces must be clean and free of scale before forging occurs
•The metal is less ductile
•Residual stress may occur
•Heavier and more powerful equipment is needed
•Stronger tooling is required