1WARM AND COLD FORGING

PRECISION FORGINGCOLD HEADING

COLD EXTRUSION

2Precision forging

Main characteristics: Closed die forging Temperature is below the hot range, it

is warm forging (T~0,5*Tm oK) Close tolerances, acceptable surface

finish near net shaping process Good material yield Good mechanical properties

3Economic considerations

Precision forging is more costly thanconventional forging, but!

Savings in material and machiningcosts are significant

Forming complex shapes is possible Precision forging represents a higher

value product than a conventionalforging (higher added value)

4Temperature of forging

Low alloy steels:

950 1150 oCHot forging

540 815 oCWarm forging

< 250 oCCold forging

Controlled cooling may be necessary afterforging to avoid distorsion and to control themicrostructure of the workoiece

5Tooling (1)

Dimensions Allowances (thermal contraction,

machining) Draft angles, no sharp corners good

material flow

Workability High deformation levels needs good

formability Try to avoid cracking!

6Tooling (2)

Precision: Tolerance bands of tool: 1030% of the

workpiece High-precision machining (EDM) Rigid alignment Preform considerations (volume, weighting,

appropriate shape)

Conditions: good lubrication, removecontaminants, good control of billet and tooltemperature

7Forging equipment

Billet separation: shearing or sawing Heating:

Furnace oxide formation Induction heating Resistance heating

Presses: Hammers Crank presses Hydraulic presses

8Comparison of forgings

9Cold forging

Processes: Upsetting Cold forging of components

Effect of cold working on material properties the ductility of the material drops, strengths and

hardness increases (because of higher dislocation density - strain hardening)

the microstructure changes, crystals (grains) become elongated in the direction of major deformation

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Cold heading

To upset the metal in a portionof wire or rod blank

The cross-sectional area ofthe initial material is increasedas the height of the workpieceis decreased

variants: Free (head formed between flat

punch and die) Closed (head formed in punch

and die)

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Limits of deformation

Material dependent:

7.2...2 dD

3...2 kD

3,2dL

Material independent:

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Calculation of force

Where:

kf flow stress

A cross-sectional area

R radius of head

h height of head

coefficient of friction

)Rh321(AfkF+=

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Two-stroke upsetting

If L/d > 2,3 then twooperations areneeded: Preforming Finish heading

Preforming head: conic-cylindrical

Finish: closedheading punch

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Example: screw

head heading reduction shearingshearing

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Example: screws and bolts

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Example: heading

Closed-die cold heading

Die can be opened for feeding and removing theworkpiece

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Head shearing

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Cold extrusion of parts

Principle: a punch applies pressure tothe preform or billet, causing the workmetal to flow in the required direction

Process variants: Direct indirect or forward backward Extrusion of rod, can and hollow part Single or combined operations

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Forward extrusion of rod

billetpunch

dieworkpiece

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Backward extrusion of can

billet

punch

workpiece

diecounter punch

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Combined : forward andbackward extrusion of can

punch

die

counter punchworkpiece

billet

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Combined: forward extrusion ofrod, backward extrusion of can

die

workpiece

punch

counter punch

billet

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Cold extrusions

Forward extrusion of rod

Back extrusion of can

Forward extrusion of can

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Extrusion pressure and force

Extrusion pressure:

where:kfm mean flow stress logarithmic strain: ln(Ao/A1) coefficient of extrusion (0,40,7)

fmkp =

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Extrusion pressure and force

Punch load (force)F = p A

Work (energy)W = c F h(c=0,60,8)

where: A cross sectional

area h punch travel

Load

Punchtravel

h

Fmax

W

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Materials and lubrication

Good formability: mild steel, copper, aluminium

Medium formability:low alloy steels, Zn

Preparation for cold forging: Surface treatment Lubrication Forging

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Equipment

Machines: Crank presses Knuckle-joint presses Hydraulic presses Special purpose cold forging and heading

machines Requirements:

Sufficient flywheel energy and loadcapacity

Rigid frame

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Presses

crank press knuckle-joint press

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Automated press

Coining andsizing

Coldheading

Preformingof head

Feeding, shearing

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Cold Forging Machines

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Cold forged parts

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Example: multiple-step forging

1 2 3

Forward extrusionof rod, back

extrusion of canand rod Forward

extrusionof hollow

part radialextrusion

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Example: multiple-step forging

Radialextrusion

PiercingBackextrusion

of can

Billet

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Example: production of nut

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Comparison of material yield andenergy consumation

66824050Cutting

46497580Hot forging

4185Warm forging

4185Cold forging

Energy, 106 J/kg

Materialyield, %

Process