Forging Defects
Theory
Though the forging process generally gives superior quality products compared to other manufacturing processes, certain defects are likely to occur if proper care is not taken during forging process design and operation.
Forging defects can be broadly classified into two categories:
- Geometrical Defects
- Non-Geometrical Defects
Geometrical Defects
Geometrical defects arise due to improper die design, poor process parameters, or incorrect material flow during forging.
Main Types of Geometrical Defects
- Laps and folds
- Underfills
- Overfills
Other geometrical defects include:
- Piping
- Forging shape not matching the design
- Die deflection, yielding, or wear
- Eccentricity or buckling
A) Unfilled Section
In this defect, some sections of the die cavity are not completely filled by the flowing metal. The main causes of unfilled sections are improper forging die design and incorrect selection of forging techniques.
B) Cold Shut
Cold shut appears as small cracks at the corners of the forging. This defect is mainly caused by improper die design, where corner radii or fillet radii are too small. As a result, the metal does not flow properly into the corners and forms a cold shut.
C) Laps and Folds
Laps and folds are caused by improper die design and incorrect material flow. These defects appear as overlapping metal layers on the forged surface. Laps distort the surface finish and weaken the product due to the presence of internal or external cracks.
Less-than-optimum process and preform design is the principal cause of most geometrical defects. When press or hammer dies close, the workpiece moves along the path of least resistance. If the die or preform design creates an undesirable flow path, defects are formed.
By understanding these process issues, the die designer can modify the die and preform design to ensure proper metal flow and produce a sound, defect-free forging.
Non-Geometrical Defects
Non-geometrical defects mainly affect the internal structure or surface quality of the forged component.
A) Flakes
Flakes are internal ruptures caused by improper cooling of large forgings. Rapid cooling causes the exterior to cool and contract quickly while the interior remains hot, leading to internal stresses and fractures. This defect can be avoided by following proper cooling and heat treatment practices.
B) Scale Pits
Scale pits appear as irregular depressions on the surface of the forging. They are caused by improper cleaning of the stock material before forging. Oxide and scale present on the surface get embedded into the forging during deformation. After pickling or cleaning, these defects become visible as pits on the surface.