14-01-2013, 02:41 PM
Cold Forging
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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 aluminium. This process is usually less expensive than hot forging and the end product requires little, if any, finishing work. Sometimes, when aluminium 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 interchange ability
• Improved reproducibility
• Increased dimensional control
• Handles high stress and high die loads
• Produces net shape or near-net shape parts
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
1. Cold working is defined as plastic deformation of metal below the recrysallisation temperature,
2. During cold working the metal temperature is approximately nearer to room temperature and oxidation is negligible which will result into clean scale free surfaces.
3. Cold working causes distortion of lattice due to which tensile strength, yield strength and hardness increase.
4. After a metal has been subjected to severe cold working operation further cold deformation may cause a decrease in strength due to internal failure of metal structure.
5. The defects depends upon composition, environment, and the extent and uniformity of cold work. thus cold working operation, which involves higher forces then hot working, offer following advantage;
i-close dimensional control
ii-Better surface finish
iii-grain refinement
iiii-increase mechanical properties.
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 draft less, 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.
Some Cold Working Processes:
Cold rolling
Cold forging
Cold extrusion
Bending
Drawing
Shearing
Reason for Cold Working:
Provides better surface finish and dimensional precision