Hot Forming

Kalpakjian

Hot Working:

Forging

Paul Berenson, www.paulb.com T.Green, WIT

www.smeedwerkunica.nl

Open Die Forging

Forging: Heat Loss Metal near die surfaces are coolest, flow less

www.freedomalloysusa.com

Temperature in Metal Forming

• For any metal, K and n in the flow curve

depend on temperature

• Both strength and strain hardening are

reduced at higher temperatures

• In addition, ductility is increased at higher

temperatures

• Tm is melting temperature +273

6

Cold Working Hot Working

Carried below recrystallization temp. Carried above recrystallization temp.

Hardening is not eliminated, rather

accompanied by strain hardening

Strain hardening is completely

eliminated

Elongation, area reduction, ductility

and impact strength are reduced

Ductility, elongation, area reduction

and impact strength are increased

Crystallization does not occur Crystallization occurs again

Uniformity of material is lost and

properties are greatly affected

Grain refinement occurs

Chances of crack propagation is

more

Cracks and defects like blow holes,

cavities are welded

Increases ultimate tensile strength,

yield point, hardness, fatigue

strength

Does not affect ultimate tensile

strength, yield point, hardness,

fatigue strength

Comparison

7

Resistance to corrosion decreases

due to presence of residual stresses

Resistance to corrosion is not

affected

Internal and residual stresses are

produced

Internal and residual stresses are not

produced

Load and energy requirements are

high

Load and energy requirements are

low

Less tool wear High tool wear

Less costly High cost operation

No oxidation of metal occurs High scaling and oxidation occurs

No metal loss High metal loss

Surface decarbonization of steel

does not occur

Surface decarbonization of steel

occurs

Good surface finish Surface finish is not good

High dimensional accurcy low dimensional accurcy

Easy to control dimensions within

tolerance limits

Difficult to control dimensions within

tolerance limits

Material handling is easy Material handling is difficult

FORGING

• Forging is term applied to a family of processes where

deformation is induced by localized compressive forces.

• The equipment can be manual or power hammers, presses, or

special forging machines.

• The hot forging done above the recrystaIlization temperature.

• The cold forging done below the recrystaIlization temperature.

FORGING • The forging material may be subjected to:

1-Drawn out to increase its length and

decrease its cross section,

– 2-Upset to decrease the length and increase

the cross section,

– 3-Squeezed in closed impression dies to

produce multidirectional flow.

Different types of Forging processes

• Open Die Forgings or Hand forgings.

•Impression Die Forgings or Precision Forgings or

closed die forging.

• Upset Forgings

1- Open Die Forgings or Hand forgings.

In this process the forgings are made with the help of

repeated blows in an open die. Here the operator

manipulates the work piece in the die during the ‘blow’

process, similar to the traditional manufacturing process

used by a blacksmith.

Open Die Forgings or Hand forgings

• Open-die forging is the same type of forging done by

blacksmith. Metal is first heated to proper temperature by gas,

oil, or electric furnaces.

• Impact delivered by some type of hammer like pneumatic

hammer or board hammer.

Open die forging

Blacksmiths use this process to reduce the thickness of bars by hammering the part on an

anvil. Reduction in thickness is accompanied by barreling, as in Fig. 14.3c. (b) Reducing the

diameter of a bar by open-die forging; note the movements of the dies and the workpiece. (c)

The thickness of a ring being reduced by open-die forging.

Stages in Open-Die Forging

(a) forge hot billet to max diameter

(b) “fuller: tool to mark step-locations

(c) forge right side

(d) reverse part, forge left side

(e) finish (dimension control)

[source:www.scotforge.com]

Open-Die Drop-Hammer Forging

• Steam or air hammers(pneumatic hammers) use pressure to :

give higher striking velocities,

– more control of striking force,

– easier automation,

– the ability to shape pieces up to several tons.

pneumatic hammer

Open-Die Drop-Hammer Forging

• Open-die forging is usually employed to pre-shape metal for

further manufacturing operations for example consider such

massive parts turbine rotors and generator shafts which may

be 70 ft in length and up to 3 ft in diameter.

• Open-die forging is used to minimize the amount of subsequent

machining.

• Open die forging is considered as a primary forging process.

Impression Die Forgings or Precision Forgings

These forgings are the refined form of blocker forgings. The

finished metal part much more identical to the die impression.

• Impression-die or closed-die forging overcomes these

difficulties of obtaining the final shape by using shaped dies to

control the flow of metal.

• Consist of set of dies, one half of which attaches to hammer

and other half to anvil.

• Impression die may be flash and flash less

2-Impression-Die Drop-Hammer Forging

• Heated metal is positioned in lower cavity and struck one or

more blows by upper die.

• Hammering causes the metal to flow to completely fill die

cavity.

• Excess metal is squeezed out around the periphery of the

cavity to form a flash.

• When final forging is completed, flash is trimmed off by

trimming die.

Impression-Die Drop-Hammer Forging

Stages in Impression-die (Closed-Die) Forging

[source:Kalpakjian & Schmid]

Stages in Impression-die (Closed-Die) Forging( flash)

Stages in Impression-die (Closed-Die)

Forging(flash)

Impression-Die Forging( flash shape)

(a) through (c) Stages in impression-die forging of a solid round billet. Note the formation

of flash, which is excess metal that is subsequently trimmed off (d) Standard terminology

for various features of a forging die.

Flash less forging

• Final shape and size are set by additional forging in the final or

finisher impression

• The shape of the various cavities forces the metal to flow in the

desired direction.

• After forging, the flash is trimmed

• The part is quenched to room temperature.

Impression-Die Drop-Hammer Forging

Impression-Die Drop-Hammer Forging

Advantages

1- complicated shapes

2- accurate dimensions.

3- better surface properties.

4- can be used in mass production scale.

5- mechanical properties is achieved by thermal

treatment.

6- reduce scraps by using flash.

Trimming Flash After Forging

Trimming flash from a forged part. Note that the thin

material at the center is removed by punching.

Press Forging

• Hydraulic presses with capacities up to 50,000 tons (445 MN)

are used.

• Press forgings have higher dimensional accuracy and can often

be completed in a single closing of dies and the process can be

readily automated.

• 10000T10010000 ton

29

Upset Forging

Upsetting or Upset Forging is the simplest case of open-die

forging involving compression of a workpiece between two

flat dies. Upset forging reduces the height of the workpiece

but increases its cross-sectional area. We will consider

upsetting of a round billet.

Under ideal conditions where there is no friction between the

work piece and the dies, the billet deforms homogeneously

(the cylindrical shape of the billet remains cylindrical

throughout the process). But in practical conditions the billet

tends to barrel since there is some friction.

30

Homogeneous

Practical upsetting of a cylindrical billet (with friction &

barreling)

Homogeneous upsetting of a cylindrical billet (without friction)

Practical upsetting of a cylindrical billet (with friction & barreling)

31

Terminology

In homogeneous upsetting / no friction:

ho = starting height of workpiece (before deformation)

h = instantaneous height of the work piece (at an

intermediate press stroke)

F = instantaneous upsetting force

A = instantaneous cross sectional area of the workpiece

h

hStrainTrue oln,

A

FStressTrue

(1)

(2)

32

Terminology

In homogeneous upsetting:

K = strength coefficient, n = strain hardening coefficient

F increases with deformation (press stroke) since Yf and

A both increase with deformation and strain (Eqs. (1), (3)

& (4))

AYFForceUpsetting f

n

f KstressflowY

(3)

(4)

33

Terminology

Practical Upsetting of a cylindrical workpiece (with friction &

barreling):

Where = coefficient of friction (0.05 – 0.3)

D = instantaneous workpiece diameter, mm (in),

h = instantaneous workpiece height, mm (in)

AYKF ff (5)

h

DKFactorShapeForging f

4.01 (6)

34

4

02hDvolumebilletV o

Specimen before and after the compression:

Do D1 (avg)

h1

ho

1

4.1

h

VD

D1 (average) can be calculated from volume constancy, i.e.,

35

Specimen before and after the compression:

Original Specimen

Barreled Specimen

after compression

36

Comparison of cold and hot forging:

Original steel specimen Steel specimen

after hot forging

Steel specimen

after cold forging

(5 steps/hits)

Swaging

• Swaging generally involves the hammering of a rod or tube to

reduce its diameter where the die itself acts as the hammer

• Term swaging is also applied to processes where material is

forced into a confining die to reduce its diameter.

Swaging

Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a

tubular workpiece by swaging. (c) A die-closing swaging machine showing forming of a

stepped shaft. (d) Typical parts made by swaging. Source: Courtesy of J. Richard

Industries.

Coining

• Closed-die forging process

• Closed-die forging process

• Used for minting coins, medallions, & jewelry

• Lubricants can not be used in coining

• Can be used to improve surface finish