1 February 2016

What is forging? | Types | Characteristics | Applications | Defects

Introduction of forging:

Forging is the oldest metal-working process where the metal is heated then force applied to manipulate the metal in such a way that the required final shape obtained is known to mankind since copper age. 

In a forging process two types of operation in order to arrive at the final object configuration.
They are as follows :

Drawing Out is the operation in which the metal gets elongated with a reduction in the cross-sectional area.

Upsetting is applied to increase the cross-sectional area of the stock at the expense of its length. To achieve upsetting force is applied in a direction parallel to the length of the axis.

Length change OR Change in cross-section resulting in favourable grain flow for strong sections.  

Points you need to remember to selecting the right forging process:

  • Understand the forged material's flow behaviour under the processing condition. 
  • Knowledge of the die geometry and die material. 
  • Condition of environmental considerations. 
  • Evaluating the stress and strain mechanics of the deformation process.
  • Lubricating and friction process. 
  • Nature of the forging equipment. 

The reason why computer-aided technology such as CAD, CAM, CAE and FEA finite element analysis based computer simulation is used to select the right forging process is to remain competitive and cost-effective. 

Types of forging process:

It is useful to classify this process systematically in order to understand and optimize forging operations. There are various types of forging processes which can be classified as follows

Types according to the temperature of workpiece following. 

Cold Forging 

This process is carried out below the recrystallization or near room temperature of the metal. It is generally preferred when the metal is already soft, like aluminium. This is a less expensive process than hot forging and the end products require less or no finishing work so better overall surface finish. Carbon and standard alloy steels are the most commonly used metal for this type of process. Cold forging could be used in case of fasteners.

Warm Forging 

This process is carried out the temperature range from above room temperature to below the recrystallization temperature. Warm forging has the advantage of reduced tooling loads, reduced press loads and increased steel ductility. In warm, the billet is heated below the recrystallization temperature up to 700 to 800 0C, in order to lower the flow stress and the forging pressures. 

Hot Forging 

This process is carried out at a temperature above the recrystallization temperature of the metal at which the new grains are formed in the metal. This kind of extreme heat is necessary for avoiding strain hardening of the metal during deformation. 

Hot impression die forging process used to produce parts like connecting rods, shafts etc. 

Types according to arrangements of dies are following.

Open Die Forging 

This process in which the flat dies of simple shape are used to allow the material to freely deformed in lateral directions of an applied load. 

Open die forging is only suitable for simple shapes for its less dimensional accuracy, and high skill operators require, the dies of open die forging are simple and less expensive, which is simplest of all the forging operations. 

Closed Die or Impression Forging 

This process in which the material is fully constrained in the cavity created by the upper and lower die halves allows more accurately shaped parts to be formed, higher interface pressures and very accurate control of material volume required, and also proper die design. 

Closed die forging is a form of impression-die forging, which does not depend on the flash formation to achieve complete filling of the die. 

It is used to make cutlery, automotive parts, and parts for aircraft engines. 

Types according to forging equipment are following.

Hammer Forging 

The most common type of equipment is the hammer. It is the most versatile and least expensive type of equipment for generating load and energy. Hammer forging is also called a drop forging where a hammer hits the workpiece repeatedly to deform it.

It has greater forging capacity and can produce forgings ranging from a few kgs to several tonnes. It is preferred in closed-die forging. 

Press Forging 

Press forging is a method of forging that involves applying slow and continuous pressure on the workpiece. This approach differs greatly from the strong and swift blows used in the forging of fall and hammer.

There are two main types of press forging such as mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and toggles to produce a preset and reproducible stroke. The hydraulic press is a load restricted machine has more of squeezing action than hammering action. Hence dies can be smaller and have a longer life than with a hammer. 

The advantages of a hydraulic press over a mechanical press is flexibility and greater capacity while the disadvantages are a slower, larger, and costlier machine to operate.

Characteristics of Forgings: 

Higher-strength provided by grain flow.
Fine structures and no internal defects resulting from the application of pressure.
Machining is required is less or none after completion of the process.
Lighter weight and material savings made it possible to make sections thinner without reducing strength.

Most popular forging application: 

  • Aerospace forging
  • Oilfield forging
  • Mechanical power forging
  • Transportation forging
  • Off-highway forging
  • Defence forging
  • Power generation forging
  • Pumps & compressors forging

Defects in forging:

Forging process gives superior quality products compared to other manufacturing process but still, there are some defects that are likely to come if proper care is not taken in forging process design. 

Different types of forging defects are listed below. let us see one by one.
  • Unfilled section :
Some sections of the die cavity are not completely filled by the flowing metal.
The cause of this defect is the improper design of forging die or using faulty forging techniques.
  • Cold shut :
A cold shut appears as a small crack at the corners of the forging. 
The cause of this defect are an improper design of the die wherein the corner and fillet radius are small as a result the metal does not flow properly into the corner and ends up as a cold-shut.
  • Scale pits :
This defect is seen as irregular depressions on the surface of the forging.
The cause of this defect is improper cleaning of the stock used for forging. Those are seen as depressions on the forging surface when the forging is cleaned by pickling.
  • Die shift :
Die shift caused by the misalignment of two die halves.
  • Flakes :
Flakes are internal ruptures caused by the improper cooling of the large forging. 
This can be improved by following the proper cooling practice.
  • Improper grain flow :
This defect caused by the improper design of die which makes the flow of metal not following the final intended directions.

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