chapter 14 forging

8/18/2019 Chapter 14 Forging

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Chapter 14: Forging of

Metals


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14.1 Introduction

• Forging: a Process in which thework-piece is shaped by compressiveforces applied through various diesand tools

• !ypical forged products: bolts" rivets"connecting rods" shafts for turbines"gears" hand tools" etc

•

Figure 14# shows a part made by #di$erent processes: %a& casting" %b&machining" %c& forging


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14.1 Introduction


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14.1 Introduction

• Cold forged parts have good surface'nish and dimensional accuracy

• (ot forging re)uires smaller forces"but it produces dimensional accuracyand surface 'nish that are not asgood as in cold forging


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14.2 Open die forging

• Most open-die forgings generally weigh 1*-*++ ,g" forgings as heavy as .* tonshave been made

• /i0es may range from very small parts%nails" pins" and bolts& up to # m for shipsshafts

• psetting or 2at-die forging %Fig 14#&: !hedie surfaces in open-die forging may havesimple cavities" to produce relativelysimple forgings


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Figure 14#: %a& 3deal deformation of a solid cylindrical specimencompressed between 2at frictionless dies" an operation known asupsetting %b& eformation in upsetting with friction at the die-workpiece interfaces 5ote barrelling of the billet caused by friction


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• 6arreling is caused primarily by frictional forces at the die-work-piece interfaces that oppose the outward 2ow of thematerials at these interfaces 6arreling can be minimi0ed byusing an e$ective 7ubricant

•

6arreling can also occur in upsetting hot work-piecesbetween cold dies !he material at and near the interfacescools rapidly" while the rest of the work-piece remainsrelatively hot !hus" the material at the ends of the work-piece has higher resistance to deformation than thematerial at its center Conse)uently" the central portion ofthe work-piece e8pands laterally to a greater e8tent than doits ends

• 6arreling from thermal e$ects can be reduced or eliminatedby using heated dies


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14.2 Open die forging -cogging

• Cogging %drawing out&: an open-dieforging operation in which thicknessof a bar is reduced by successive

forging steps at speci'c intervals6ecause contact area per stroke issmall" a long section of a bar can be

reduced in thickness withoutre)uiring large forces or machinery


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Figure 144: %a& /chematic illustration of a cogging operation on arectangular bar 6lacksmiths use a similar procedure to reduce thethickness of parts in small increments by heating the workpiece and

hammering it numerous times along the length of the part %b& 9educingthe diameter of a bar by open-die forging note the movements of the die

14.2 Open die forging -cogging


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• The forging force" F" in an open-die forgingoperation on a solid cylindrical piece:

•

• Y f : 2ow stress of the material" stress re)uired

to continue plastic deformation of the work-piece at a particular true strain

• µ : coe


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• Exampe14.1: = solid cylindrical slugmade of #+4 stainless steel is 1*+ mmin diameter and 1++ mm high 3t is

reduced in height by *+> at roomtemperature by open-die forging with2at dies =ssuming that the coe


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14.! impre""ion-die and co"ed-die forging

• 3n impression-die forging" the work-piece ac)uires the shape of the diecavities while being forged between

two shaped dies %Fig 14*&

• !he thin 2ash cools rapidly" andbecause of its frictional resistance" it

sub?ects the material in the diecavity to high pressures" therebyencouraging the 'lling of the die

cavity


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• F3@9A 34* %a& through %c& /tages in impression-die forging of a solid roundbillet 5ote the formation of 2ash" which is e8cess metal that is subse)uentlytrimmed o$ %d& /tandard terminology for various features of a forging die


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• Pre-forming processes" such as

fuering and edging %Figs14.band c&" are used to distribute thematerial into various regions of theblank

• 3n fuering material is distributed

away from an area• 3n edging" it is gathered into a

locali0ed area

• !he part is then formed into therough shape of a connecting rod bya process called #oc$ing" using

blocker dies• !he 'nal operation is the 'nishing of

the forging in impression dies thatgive the forging its 'nal shape !he2ash is removed later by atrimming operation



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•

!he blank isplaced on thelower die and" asthe upper die

begins to descend"the blankBs shapegradually changes"as is shown for the

forging of aconnecting rod in'g 14.a


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• !he forging force" F" re)uiredto carry out an impression-die forging operation can beestimated from the formula

F ,Df =

where , is a multiplying factorobtained from !able 14" Df isthe 2ow stress of the materialat the forging temperature"and = is the pro?ected area ofthe forging including the 2ash



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• 3n true co"ed-die or %a"h-e""forging "2ash does not form and thework-piece completely 'lls the die

cavity Conse)uently" the forgingpressure is very high"

• &nder"i'e #an$" prevent complete

'lling of die cavity• O(er"i'e #an$" generate e8cessive

pressures and may cause dies to fail



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14.!.1 )reci"ion forging

• 3n order to reduce the number of additional

'nishing operations re)uired-hence the cost-thetrend has been toward greater precision in forgedproducts %net-shape forming&

• /pecial dies produce parts having greater

accuracies than those from impression die forgingand re)uiring much less machining

• Process re)uires higher capacity e)uipmentbecause of greater forces re)uired to obtain 'ne

details on part• Precision forging re)uires special and more

comple8 dies" precise control of billetBs volume andshape" accurate positioning of the billet in die

cavity" and hence higher investment (owever"


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14.4 *ariou" ForgingOperation"- Coining

• !he slug is coined in a completelyclosed die cavity

• 3n order to produce 'ne details thepressures re)uired can be as high as* or E times the strength of thematerial

•

7ubricants cannot be applied incoining" because they can becomeentrapped in the die cavities" andbeing incompressible" prevent the fullreproduction of die surface details

• Coining process is also used withforgings and with other products" toimprove surface 'nish and to impartthe desired dimensional accuracy

!his process" called si0ing" involveshigh pressures" with little change inpart shape during si0ing

Figure 141+: /chematicillustration of the coining process


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• +eading: upsetting operationusually performedat end of round rod

or wire to producea larger 8-section

• !ypical productsare nails" bolt

heads" screws"rivets" and variousother fasteners%Fig1411&

14.4 *ariou" ForgingOperation"- +eading

F3@9A 1411: %a& (eading operation to formheads on fasteners" such as nails and rivets%b& /e)uence of operations used to produce a

typical bolt head by heading


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• ,ue" for up"et forging:

1 Ma8 7ength of unsupported metal thatcan be upset in one blow without

buckling: # times the diameter of bar

lengths of stock greater than # timesdiameter may be upset successfully

such that the diameter of the cavity isno more than 1* the diameter of thebar

14.4 *ariou" ForgingOperation"- +eading

chapter 14 forging

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