FORGING PROCESS
When a sufficient load(force) is applied to a metal, it will cause the material to change shape. This change in shape is called deformation.
There are 2 (Two) types of deformation1.Elastic Deformation2.Plastic Deformation
A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation.
This type of deformation involves stretching of the bonds, but the atoms do not slip past each other.
When the force is sufficient to permanently deform the metal, it is called plastic deformation.
Plastic deformation involves the breaking of a limited number of atomic bonds by the movement of dislocations.
Forging is defined as the
controlled plastic deformation
of metal into predetermined
shapes by applying pressure
or impact blows, or
combination of both.
A forged component has the ability to withstand higher load during service. A forged components can be produced to close tolerance. It reduces the machining time, material and labor considerably. It increase the strength and toughness of the metals by producing directional
grains. It refines the structure of the metal and thus renders it more dense. The internal defects like segregation, cracks and porosity are eliminated.
WHY FORGING ? ? ?
TEMPERATURE
COLD FORGING
WARM FORGING
HOT FORGING
THIXO FORGING
EXTRUSION
COINING
DIE
OPEN DIE FORGING
IMPRESSION / CLOSED DIE
FORGING
UPSET FORGING
ROLLING
PUNCHING
PEARCING
BLANKING
HEADING
MACHINE
HAMMER FORGING
PRESS FORGING
RING ROLLING
ORBITAL FORGING
Cold forging refers to working metal at room temperature.
The cold forging process was developed in Germany just before the end of World War II. It was used to produce artillery shells and other ordinance items for the war.
RAW MATERIAL
HEAT TREATME
NT
SURFACE TREATME
NT
COLD FORGING
Receipt, Inspection & Billet cutting etc
Annealing to remove strain hardening to set the desired mechanical properties to normalize the microstructureSurface treatment to remove dust & oil cleaning by acids and
degreasers Pre-Coating - Phosphating Soaps – Sodium / Calcium stearates
Single or multi stage forgingSecondary Forming – Thread rolling / Machining
COLD FORGING
EXTRUSION
COINING
ROLLING
PUNCHING
PEARCING
BLANKING
HEADING
FORWARD EXTRUSIONBACKWARD EXTRUSION
Extrusion is the process of squeezing metal in a closed cavity through a tool, known as a die using either a
mechanical or hydraulic press.
Forward Extrusion Forward extrusion reduces slug diameter and increases its length to produce parts such as stepped shafts and cylinders.
Backward ExtrusionIn backward extrusion, the steel flows back and around the descending punch to form cup-shaped pieces.
This process is used to produce coins, medallions and other similar products on flat stock with relief features. Very fine detail can be reproduced.
Coining is the squeezing of metal while it is confined in a closed set of dies.
• The first blow combines coning with
shank extrusion.
• Coning is a partial head upset.
• The second blow finishes the head
shape.
1-Die/2-punch method in producing headed fasteners.
Piercing and blanking are essentially the same process, involving the stamping of shapes out of sheet metal or metal strip.
In piercing a shaped hole is made in the metal, Whereas in blanking a shape is stamped out of the metal and
then used.
Better accuracy, Closer tolerances, Better surface finish.
Strain hardening increases strength and hardness.
Grain flow during deformation can cause desirable
directional properties in product.
An ideal method for increasing hardness of those metals which do not respond to the heat treatment.
A cold forging can generally achieve tolerance of ±0.005”.
• Cold forging improves the finished
part’s grain structure by making it
conform to the flow of the design.
• The machined diagram shows how
the grain structure is weakened by
cutting operations.
Higher forces and power required for deformation. Surfaces of starting work must be free of scale and
dirt. Ductility and strain hardening limit the amount of
forming that can be done.
Metal must be annealed and pre-coated before
further deformation can be accomplished.
In other cases, metal is simply not ductile enough to
be cold worked.
Microscopic structure of a
crystal.
Purple is sodium ion
Green is chlorine ion
Arrangement of atoms is called the crystal structure
• Close packed directions are face diagonals.
FACE CENTERED CUBIC STRUCTURE (FCC)
BODY CENTERED CUBIC STRUCTURE (BCC)
• Close packed directions are cube diagonals.
The change in the crystal structure of a substance that takes
place upon heating or cooling, without a changing the chemistry
of metals.
The minimum temperature at which destroyed grains
of a crystal structure are replaced by the new strain
free grains.
Recrystallization is usually accompanied by a
1.Reduction in the strength.
2.Reduction in hardness of a material.
3.Simultaneous increase in the ductility.
Above the recrystallization temperature the kinetic energy of atoms increases and therefore they are able to attach themselves to the newly formed nuclei which in turn begin to grow into crystals. This process continues until all the distorted crystals have been transformed.
Performed at temperatures above room temperature,
below recrystallization temperature
HEATING AND COOLING OF
IRON
Temperature, C
BCC Stable
FCC Stable
914
1391
1536
shorter
longer!shorter!
longer
Tc 768 magnet falls off
BCC Stable
Liquid
heat up
cool down
WARM FORGING
BAND 550 0 C
950 0 C
Reduced tooling loads, as compared to cold forging.
Reduced press loads, as compared to cold forging.
Increased steel ductility.
Elimination of annealing prior to forging.
Favorable as-forged properties that can eliminate
heat
treatment.
Transmission Gears
Transmission Shafts
Hot forging is the plastic deformation of metal at a
temperature and strain rate such that recrystallization
occurs simultaneously with deformation, thus avoiding
strain hardening.
For this to occur, high work piece temperature (matching the metal's recrystallization temperature) must be attained throughout the process.
- Decrease in yield strength, therefore it is easier to work
and takes less energy (force).
- Increase in ductility.
- Elevated temperatures increase diffusion which can
remove or reduce chemical inhomogeneity.
- Pores may reduce in size or close completely during
deformation.
Lower dimensional accuracy due to thermal contraction & wrapping from uneven cooling.
Grain structure may vary through out the metal for various reasons.
Higher total energy required, which is the sum of The thermal energy needed to heat the work piece Energy to deform the metal
Work surface oxidation (scale) Thus, poorer surface finish
Shorter tool life Dies and rolls in bulk deformation
Two dies are brought together and the work piece
undergoes plastic deformation until its enlarged sides touch
the die side walls.
Some material begins to flow outside the die impression, forming flash.
Impression die forgings may be produced
on a horizontal forging machine (Up
setter) in a process referred to as
upsetting.
A work piece may be forged by a series of punch and die operations (or by several cavities in the same die) to gradually change its shape.
Amount of energy is imparted by impulsive load,
Close tolerances are not much important
1. Starting stock cut to size by weight is first rounded, then upset to achieve structural integrity and directional grain flow.
2. Work piece is punched, then pierced to achieve starting "donut" shape needed for ring rolling process.
3. Completed pre-form ready for placement on ring mill for
rolling.
4. Ring rolling process begins with the idler roll applying pressure to the pre-form against the drive roll.
5. Ring diameters are increased as the continuous pressure reduces the wall thickness. The axial rolls control the height of the ring as it is being rolled.
6. The process continues until the desired size is achieved.
Forming Type Cold Warm Hot
Temperature
Room
550 - 950oC 950 - 1250oC
1020 - 1740oF 1740 - 2300oF
Accuracy High Good Low
Formability Restricted Good Good
Material Restricted Large variety Large variety
Energy Costs Low Moderate High
Surface Quality High Good Low
Tolerances Close Closer Low
Grain Structure Good Good Variable
Heat Treatments Few Few Necessary
Machining Least Less Necessary
Forged components are shaped either by a hammer or press.
Forging on the hammer is carried out in a succession of die impressions using
repeated blows.
The quality of the forging, and the economy and productivity of the hammer
process depend upon the tooling and the skill of the operator.
In a press, the stock is usually hit only once in each die impression, and the
design of each impression becomes more important while operator skill
is less critical.
In the most basic example of impression die forging, which accounts for the majority of forging production, two dies are brought together and the work piece undergoes plastic deformation until its enlarged sides touch the die side walls
Then, some material begins to flow outside the die impression, forming flash. The flash cools rapidly and presents increased resistance to deformation, effectively becoming a part of the tool. This builds pressure inside the bulk of the work piece, aiding material flow into unfilled impressions.
Impression die forging
Grain Structure
• Parts have good strength
• High toughness
• Forgings require additional heat treating
grain flow (a) casting (b) machining (c) forging
Forging Defects• Unfilled Section: In this some section of the die
cavity are not completely filled by the flowing metal.• Cold Shut: This appears as a small cracks at the
corners of the forging.• Scale Pits: This is seen as irregular depurations on
the surface of the forging. • Die Shift: This is caused by the miss alignment of
the die halve, making the two halve of the forging to be improper shape.
• Flakes: These are basically internal ruptures caused by the improper cooling of the large forging.
• 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.
Forging Defects
Crack:Cracks are two types –
Surface Crack
Micro Crack
Reason –
Ingot Cracks
Improper Heating and Forging @ Low temperatures
Incorrect cooling of Alloy steel forgings
Incorrect Forging methods
Lap:Reason –
Due to folding of metal over itself during forging, laps are usually found where vertical and horizontal section intersect, when fillet radius is less.
Unfilling:Reason –
Inadequate Forging pressure
Chilled die or forge metal
Short supply of feed metal from either web or blocker
Inadequate Preform Design
Pitting:Depression on component surface due to scale imbedded during forging
Reason-Incomplete cleaning of dies & scales.
Dents:
Reason-Improper positioning of stock on the die.Hot forgings are thrown from place to place.
MismatchReason –Improper alignment of dies
Buckling:Reason-In upsetting forging. Subject to high compressive stress.
Thank You
Top Related