Forging Defects


Though forging process give generally prior quality product as compared to the other manufacturing processes. There are some defects that are lightly to come a proper care is not taken in forging process design.
Forging Defects can be categorized into two broad categories:
Geometrical Defects
Non Geometrical Defects

Geometrical Defects: The main types of geometrical defects are

1. Laps and folds
2. Underfills
3. Overfills

There are a number of other different geometrical defects that can occur during forging. These include:
Piping
Forging shape does not match design
Die deflection, yielding or wear
Eccentricity or buckling

A) Unfilled Section: In this some section of the die cavity are not completely filled by the flowing metal. The causes of this defects are improper design of the forging die or using forging techniques.

B) Cold Shut: This appears as a small cracks at the corners of the forging. This is caused mainly by the improper design of die. Where in the corner and the fillet radii are small as a result of which metal does not flow properly into the corner and the ends up as a cold shut.

C) Laps and Folds: This is caused by the improper die design, making the laps created onto the final part which is very much undesirable as they distort the surface finish and also tend to weaken the product due to internal or external cracks.
Less-than-optimum process and preform design is the principal cause of most geometrical defects. By understanding the process issues, the forge is better able to design its processes to minimize the occurrence of such defects.
When the press or hammer dies close, the workpiece will move in a path of least resistance. It is imperative that the die and pre-form design create this least resistant path so that the net result is a sound forging. On occasion, the die design may create a situation in which the path of least resistance is the one that results in a defect during forging. By examining the various types of geometrical defects, the fundamental cause can be understood and the die designer can produce a die that creates a sound, defect-free product

Non Geometrical Defects: The main type of Non geometrical defects are:

A) Flakes: These are basically internal ruptures caused by the improper cooling of the large forging. Rapid cooling causes the exterior to cool quickly causing internal fractures. This can be remedied by following proper cooling practices.

B) Scale Pits: This is seen as irregular deputations on the surface of the forging. This is primarily caused because of improper cleaning of the stock used for forging. The oxide and scale gets embedded into the finish forging surface. When the forging is cleaned by pickling, these are seen as deputations on the forging surface.