Defects in Forging and Its Remedies
- Cold shuts or laps: It has short cracks at the corners and at right angles to the surface. It is caused by metal surface folding against itself during forging. Sharp corners in dies can result in hindered metal flow which can produce laps. It can be eliminated by proper design of die; the crack on the surface is removed by machining or grinding.
- Pitting: Pitting of the forging surface is caused by scale, which if not removed thoroughly from the die cavity is worked into the surface of forging. When this scale is cleaned from the forging depression remains which are known as ‘scale pits’. Pitting may be avoided as much as possible during manufacture of forging by proper control of furnace and frequent cleaning of dies.
- Die shift: Die shift is caused by misalignment between the top and bottom forging dies. This may be caused due to lose wedges.
- Incomplete filling dies: This defect may occur due to insufficient number of blows during forging, wrong amount of metal, forging at too low temperature, poor forging, die design, etc.
- Burnt and overheated metal: This defect is caused by improper heating conditions and soaking the metal too long.
- Fins: Fins are small projections or loose metal driven into the surface of the forging.
- Ruptured fibre structure: This is a discontinuity in the flow lines of the forging which is revealed only when observing the microstructure. This defect is caused by working some of the alloys too rapidly during the forging operation, inadequate stock size, or improper die design.
- Cracks: It occurs on the forging surface which may be longitudinal or transverse. Their occurrence may be due to poor quality of ingot, improper heating, forging at low temperature, or incorrect cooling of alloy steel forging.
- Decarburization: If the raw stock is subjected to too high temperature for too long period, it can produce decarburized surface on the forgings, particularly in high carbon steels.
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