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Forging defects and residual stresses in forging

Metal forging is a metal forming process that involves applying compressive forces to a work piece to deformed it, and create a desired geometric change to the material. The forging process in very important in industrial metal manufacture, particularly in the extensive iron and steel manufacturing industry.

Metal forging

Though forging process give generally prior quality product compared other manufacturing processes. There are some defects that are lightly to come a proper care is not taken in forging process design.

Forging defect can be categorized into two broad categories:

Geometrical defect Non geometrical defect Forging defects

Geometrical defectThe main types of geometrical defect are:Laps and foldUnderfillsOverfillsThere are number of other diffrent geometrical defect that can occur during forging. These include:PipingForging shape does not match designDie deflection, yielding or wearEccentricity or bucking

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. A cold shut is a discontinuity produced when two surfaces of metal fold against each other without welding completely. A cold shut can occur when a flash or fin produced by one forging operation is pressed into the metal surface during a subsequent operation.Cold Shut

UnderfillsUnderfills are another geometrical defect commonly caused by inadequate press force, energy and/or power. 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.

Underfills

Laps and FoldsThis 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.

Buckling

Shearing

Less-than-optimum process and perform 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 die close, the workplace will move in a path of least resistance. It is imperative that the die 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.

FlakesThese 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.

Cracking at the flash of closed-die forgings is another surface defect, since the crack generally penetrates into the body of the forging when the flash is trimmed of his type of cracking is more prevalent the thinner the flash in relation to the original thickness of the metal. Flash cracking can be avoided by increasing the flash thickness or by relocating the flash to a less critical region of the forging.Cracking

Scale PitsThis 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.

Die Shift:

This is caused by the miss alignment of the die halve, making the two halves of the forging to be improper shape.

Improper Grain Flow:

This is caused by the improper design of the die, which makes the flow of the metal not flowing the final interred direction.

In general, defects in parts manufactured by metal forging can be controlled first by careful consideration of work stock volume, and by good design of both the forging die, and the process. The main principle is to enact the right material distributions, and the right material flow to accomplish these distributions.

Residual Stresses in Forgings The residual stressproduced in forgings as a results of inhomogeneous deformation are generally small because the deformation is normally carried out well into the hot-working region. However, appreciable residual stresses and warpingcan occur on the quenching of steel forgings in heat treatment.

Large forgings are subjected to the formation of small cracks, or flakes at the centre of the cross section. This is associated with the high hydrogen content usually present in steel ingots of large size, coupled with the presence of residual stresses. Large forgings therefore have to be slowly cooled from the working temperature. Examples: burying the forging in ashes for a period of time or using a controlled cooling furnace.Finite element analysis is used to predict residual stresses in forgings.

Why are residual stresses important?Quenching of closed die forgings which have thin cross sections can cause distortion during the quenching operation, leading to expensive hand finishing.Forgings that are machined after the quenching operation will distort from their intended shape if the internal stress pattern is not relieved.Tensile stresses which are revealed during the machining operation will enhance stress corrosion cracking.The unaccounted for stress pattern may lead to premature failure of forged parts.

References:

Abbaschian, Reed-Hill. Physical Metallurgy Principles. 4th edition. 2009 Beer & Johnston (2006). Mechanics of Materials (5th edition). McGraw Hill. Robert S Williams Metallurgy and metallurgical engineering series. McGraw-Hill Book Co; 5th edition (1948).