Hot Forging on Horizontal Multi- Stage · PDF filemeans of hot forging on horizontal multi-stage presses. ... of hot forging and subsequent cold cali-bration. Drive shafts require

verlag moderne industrie

Hot Forging on Horizontal Multi-

Stage Presses

Machine technology, processes and products

Günther MeßmerStephan Huber

++Seissenschmidt_englisch 20.03.2007 7:57 Uhr Seite 1

This book was produced with the technical collaboration of SEISSENSCHMIDT AG.

Translation: Sarah Cory

Editorial team:Siegbert Bauseler, Pablo Guel-López, Stefanus Levermann, Hermann-Josef Müller, Fabian Pingel, Nils Vöpel

© 2007 All rights reserved withsv corporate media GmbH, D-80992 Munich, Germanywww.sv-corporate-media.de

First published in Germany in the seriesDie Bibliothek der TechnikOriginal title: Warmumformung mit horizontalen Mehrstufenpressen© 2006 by sv corporate media GmbH

Illustrations: No. 1 Hip/kpa/SV-Bilderdienst; Nos. 15, 16, 17, 18, 19, 20, 21, 29 Hatebur Umformmaschinen AG, CH-Reinach (www.hatebur.ch); No. 36 pro-beam AG & Co. KGaA, Neukirchen(www.pro-beam.de); all others SEISSENSCHMIDT AG, PlettenbergTypesetting: abavo GmbH, D-86807 BuchloePrinting and binding: Sellier Druck GmbH, D-85354 FreisingPrinted in Germany 889514


ContentsThe significance of forging 4

Spectrum of parts .................................................................................... 5Market situation....................................................................................... 7

Fundamentals of metal forming 10

Plasticity theory....................................................................................... 13Hot-forging technologies......................................................................... 15

Horizontal multi-stage presses 20

Machine concept of horizontal multi-stage presses ................................ 22Principle of operation/machine kinematics ............................................. 24Peripheral equipment............................................................................... 30

Forging process and tools 34

Process design ......................................................................................... 35Tool design .............................................................................................. 42Tool materials .......................................................................................... 44Tool production ....................................................................................... 45Use of pre-assembled tool sets ................................................................ 47

Production process 53

Workpiece materials ................................................................................ 53Heat treatment ......................................................................................... 55Shot blasting............................................................................................ 56Quality properties and tolerances ............................................................ 57Cost considerations ................................................................................. 58Combination with other production processes ........................................ 59Special processes..................................................................................... 62

Examples of products 64

Nuts and cams ......................................................................................... 64Universal joints ....................................................................................... 64Bearing rings ........................................................................................... 65Raw gear parts and bevel gears ............................................................... 66Flanges .................................................................................................... 67Outer races for constant velocity joints................................................... 68

Outlook 69

The company behind this book 71


4

The significance offorgingThe forging of metals is an age-old technol-ogy. As early as 4000 BC, forging was usedto process pure metals such as gold, silverand copper from their natural form into jew-ellery, weapons or objects for daily use. Towork the metal back then, stones were usedas hammers. Throughout the centuries, the artof forging enjoyed a special status. The pro-duction of efficient forged weapons and toolsdecided on the rise or fall of entire civilisa-tions. From this perspective, it is easy to understand why good blacksmiths enjoyedmuch respect in ancient times and in the Middle Ages. Initially, it was mainly individual parts whichwere produced by forging. The production ofcoins (Fig. 1) led to a transition to mass pro-duction very early on. Coinage is known tohave been around as early as 7 BC. Until the16th/17th century, production continued to bemanual, though the tools became increasinglyrefined. Later, pressing was used to producecoins.Current forges no longer enjoy the social esteem they did a few centuries ago. The reason for this may lie in the fact that high-strength forgings (hot-forged parts) aremostly hidden from view when in use. Everymotor vehicle, for example, contains numer-ous hot-forged parts, whether these are in thechassis, the brakes or the entire powertrain.Due to their adapted fibre flow and high duc-tility, hot-forged parts demonstrate safety reserves that render them particularly well-suited to those areas of the vehicle wheresafety is crucial.

Age-old technology

Transition tomass production

Numerousforged parts for vehicles


Spectrum of parts 5

Combined with the machine technology pro-vided by horizontal multi-stage presses, hot-forged parts can be produced cost-efficientlyin large volumes. This book provides informa-tion on this machine technology, as well as onthe types of parts which are produced bymeans of hot forging on horizontal multi-stagepresses. Details on the process combinationswhich are possible during production are alsooutlined.

Spectrum of parts

Hot forging on horizontal multi-stage pressesenables both simple and complex forgedparts to be produced in large volumes and inan extremely economic way. Due to the tooltechnology and the transport system in hori-zontal multi-stage presses, they are mainlyused to produce rotationally symmetric parts,flanged parts or parts with a round envelop-ing contour. The manufacture of asymmetrical

Fig. 1: Gold coin originatingfrom the 13th century

Cost-efficient atlarge volumes

Rotationallysymmetric parts


6 The significance of forging

parts is likewise possible, although it is asso-ciated with greater development efforts andtool complexity. The spectrum of partsranges from small nuts (M16) and cams witha weight of only a few grams through tocomponents such as flanges, wheel hubs,gear wheels and outer races for constant vel-ocity joints weighing up to 7.5 kg. Figure 2

shows some example parts. A large pro-portion of the forgings produced on hori-zontal multi-stage presses are used in motor ve-hicle drive systems and in the chassis (Fig.3). Engine parts made in this way include cams, valve-train gears, balancer shafts andoutput flanges. For various types of gear-boxes, raw parts such as gears, pinions andsynchronising gears are produced. Bevelgears for differential gearboxes can be sup-plied with assembly-ready teeth by means of hot forging and subsequent cold cali-bration. Drive shafts require joints that en-able longitudinal and angular displacement.Such parts are also hot forged, as are wheelhubs.

Fig. 2: Spectrum of hot-forged parts pro-duced on horizontalmulti-stage presses


Market situation 7

Besides the motor vehicle industry, forgedparts produced on horizontal multi-stagepresses are also used in general mechanicalengineering applications as well as in pipe engineering. Many bearing rings for varioustypes of bearings are made using hot-forging operations on this type of machine. Flanges for welding and connecting pipes together are likewise produced in this way.

Market situationApart from a few large forging enterprisesand those forges at car makers, forging com-panies are primarily medium-sized. 77% ofthe forging companies employ less than 200employees. A mere 14% have between 200and 400 members of staff; only 9% have over400. The company structure of the Europeanforges is shown in Figure 4. The statistics donot, however, differentiate between the vari-ous forging processes and the machineryused. It is thus difficult to determine the per-centage of hot-forged parts produced on hori-zontal multi-stage presses. Besides some

Fig. 3: Typical applicationsof hot-forged parts invehicles

Engine

Gearbox Drive shaft Differential

Suspension

Medium-sizedcompanies pre-vail in forging industry


8 The significance of forging

forges which are operated directly on the siteof car makers, the greatest proportion of hot-forging companies is on the open market. Up-wards of a certain size, companies are oftenable to provide comprehensive developmentservices together with forging on horizontalpresses.In general, the larger horizontal multi-stagepresses can be found at car makers or in thelarger medium-sized businesses. This is be-cause the infrastructure necessary for operat-ing such presses incurs high investment costs.In order to be able to provide smaller series inan economic way, the larger forging companiesfrequently offer production on conventionalvertical forging presses.Forged parts produced on horizontal multi-stage presses are generally not in an assembly-ready state following the forging process. Fit-ting surfaces, drilled holes and splines mustoften be produced in subsequent operations.This may involve a cold-forging operationand/or machining process. At the end of theproduction chain, the forging company de-livers assembly-ready forged parts that haveundergone extensive quality inspection. Figure 5 shows the customer structure forforged parts in 2004. It can be seen that the

< 50employees

> 400 employees

50 to 199employees

200 to 400employees

Source: Euroforge

47%30%

14% 9%

Fig. 4: Breakdown of com-pany size within theEuropean forging industry


Market situation 9

majority (53%) of the forged parts went intoautomotive applications, and thus to automo-tive manufacturers and their system and com-ponent producers. 38% were supplied for general mechanical engineering applications.Other markets, with a total share of 9%, included building and agricultural machinery,as well as the railway industry.

Automotiveindustry

Mechanicalengineering

Otherareas

Source: IMU

53%

38%

9%

Fig. 5: Customer structurefor closed-die forgedparts in Europe

Forged partsmainly suppliedto automotivesector


10

Fundamentals ofmetal formingAs defined in DIN 8580 (a standard from theGerman Institute for Standardization), metalforming refers to the targeted alteration of thegeometry, surface and material properties of aworkpiece while maintaining mass and mater-ial coherence. In order to process a material bymeans of metal forming, it must permit plasticdeformation of its geometry under appropriateloads by means of outside forces without los-ing its coherence. This property is a character-istic of metals.The uniaxial tensile test provides a good ex-ample for illustrating the general behaviourof metals which are subjected to force. Dur-ing this test, which is used to characterisematerials, a testpiece is elongated at a slowand continuous rate until necking occurs andthe testpiece breaks. The force applied, Ftensile, and the resulting elongation of thetestpiece, ∆l, are usually related to the initialcross-section, S0, and the initial length, l0, ofthe testpiece. The engineering stress, σ, andthe engineering strain, ε, are then expressedas:

σ = ε =

Figure 6 shows a stress-strain curve for alow-alloy steel plotted during a tensile test. Itmust be pointed out that metals demonstrateboth elastic and plastic behaviour. The elasticphase at the start of the curve is marked by astraight line in the stress-strain curve (Hook’sLaw). If the material is subjected to load upto its yield point during the elastic phase, itwill assume its original form (length) upon

∆ll0

Ftensile

S0

Definition ofmetal forming

Engineeringstress/strain


Fundamentals of metal forming 11

removal of the load. It is only when subjectedto a load above the yield point that the mater-ial undergoes permanent, i.e. plastic, deform-ation.Metals barely change in volume during plasticdeformation. This means in the case of the ten-sile test that the cross-section of the testpiecedecreases with increasing elongation. If the ef-fective force, Ftensile, is divided by the currentcross-section surface, S, “true stress” can becalculated as follows:

kf =

Plastic flow is referred to as flow stress, kf.The flow stress of a material refers to thestress that is necessary for introducing ormaintaining permanent deformation in the uni-axial stress state. In a diagram, flow stress isusually depicted over the logarithmic principalstrain, ϕ, also known as plastic strain. In orderto calculate plastic strain, the change in length,dl, is divided by the current length, l, as follows:

Ftensile

S

Rm

ReHReL

Str

ess

σ

Strain ε

Fig. 6: Stress-strain dia-gram of a low-alloysteel during a tensiletestReH Upper yield

pointReL Lower yield

pointRm Tensile strength

Flow stress

True stress

Plastic straincalculation


12 Fundamentals of metal forming

dϕ =

In each forming increment the logarithmicprincipal strain is:

The plastic strain is more suited to describ-ing high levels of plastic deformation than theengineering strain, ε, which is related to thestarting length (unfortunately, English nota-tion does not differentiate between elastic andplastic strain. The symbol ε is used in bothcases. German notation differentiates betweenelastic (ε) and plastic strain (ϕ). Therefore,German notation is used in this book). Duringmultiple-stage forming processes, the plasticstrain values may simply be added up, andthus the total plastic strain of a formingprocess can be calculated.The representation of flow stress over plasticstrain is plotted as a flow curve. In the area offorging, these curves are recorded in upsettingtests for various parameters such as forgingtemperature, T, and strain rate, ϕ. (= dϕ/dt).This is because upsetting tests can reachgreater plastic strains than can be achievedduring tensile tests. As the temperature in-creases, the flow stress, and thus the (press)force needed for forging, generally decrease(Fig. 7, above). At elevated temperatures (i.e. temperatures above which recrystallisationoccurs), the flow stress is also dependent onthe strain rate. As the strain rate increases, theforce necessary for metal forming increases,too (Fig. 7, below). Taking into account thematerial state (grain structure, hardness, etc.),

l1

l0

= dll

=> = lnl1l0

dll

Values influen-cing flow stress


Plasticity theory 13

the flow stress for a material is thus a functionwhich depends on the plastic strain, ϕ, themetal-forming temperature, T, and the strainrate ϕ. .

Plasticity theoryTensile tests, just like compression and up-setting tests, represent a special case inwhich the material is subjected to load in onedirection only. Generally, however, the ma-terial must withstand load in several direc-tions, both in the form of tensile and com-pressive stresses as well as shear stresses.Figure 8 shows a rectangular solid (removedfrom the workpiece for illustration purposes)with the stresses that occur under multiple-axis load.

250

200

150

100

50

0

Flo

w s

tres

s k f

in N

/mm

2

0.0 0.1 0.2 0.3 0.4

Plastic strain ϕ0.5 0.6 0.7

T = 900°C

T = 1000°C

T = 1100°C

T = 1200°C

150

100

50

0

Flo

w s

tres

s k f

in N

/mm

2

0.0 0.1 0.2 0.3 0.4

Plastic strain ϕ

0.5 0.6 0.7

ϕ = 0.1 s-1

ϕ = 1 s-1

ϕ = 10 s-1

ϕ = 100 s-1

Fig. 7: Above: Dependenceof the flow stress onthe testpiece tempera-ture using the ex-ample of 20MnCr5(ϕ. = 10s–1)Below: Dependenceof the flow stress onthe strain rate, ϕ. , atelevated tempera-tures (1200°C) usingthe example of20MnCr5

Multiple-axisload



z

x y

τxz

τzx τzy

τyz

τyxτxyσx

σy

σz

Fig. 8: Multiple-axis load onthe material duringforgingx, y, z directions inspaceσx, σy, σz stressesalong the directionsin spaceτxy, τxz, τyx, τyz, τzx, τzyshear stresses

There are various hypotheses which assist inattributing the multiple-axis stress state to theuniaxial one. In these hypotheses, the effect-ive stress, σv, is defined and calculated. Veryoften, the effective stress hypotheses accord-ing to Tresca and von Mises are used. The effective stress according to Tresca (shearstress hypothesis) is as follows:

σv = σ1 – σ3

The effective stress according to von Mises(distortion energy theory) is as follows:

with the mean stress, σm:

σm = (σ1 + σ2 + σ3)

The principal stresses in the principal-stress dir-ections are σ1, σ2 and σ3. It has been proventhat, for every possible stress state, the coord-inate system can be turned in such a way thatthree directions perpendicular to each other

13

v = – m)2 + ( m)2 + ( m)23 – 2 – ( 1

32 ( )

Effective stresshypotheses


Hot-forging technologies 15

can be found in whose planes no shear stressesact, but only the three principal stresses. Thestress which is most tensile is designated asσ1. The stress which is most compressive istermed σ3. Between these two stresses lies σ2.

Hot-forging technologiesA principle distinction is made in metal-forming processes between forging and sheet-metal forming (Fig. 9). Forging is frequentlysub-divided into cold forging, warm forgingand hot forging, depending on the workpiecetemperature. The temperature at which a ma-terial is forged has a significant influence on

Fig. 9: Sub-division of forging processes

Hydroforming

Sheet-metal forming Forging

Cold forging Warm forging

Extrusion

Closed-die forging

Open-die forging

Rolling

Coining

Hot forgingDeep drawing

Superplastic forming

Metal-forming technologies

……

……

Stretch forming

Sub-division of forgingprocesses

the required forces, the plastic strain and thusthe complexity of the workpieces to be pro-duced. The forging temperature likewise af-fects the precision and tool life that may beachieved. Hot forging allows highly sophisti-cated parts to be produced which have a fibreflow that is adapted to the component geom-etry and which thus demonstrate good strengthproperties. The most important hot-forgingprocesses are:

Important hot-forgingprocesses



• open-die forging• closed-die forging with flash • closed-die forging without flash.

In open-die forging, tools with a very simpledesign – often flat upsetting punches – areused, and only part of the material is forged ata time. Open-die forging involves moving theworkpiece between the individual forgingstages so that after several steps the desiredgeometry is achieved. The process encom-passes forging carried out manually with ahammer to produce art objects as well as thatundertaken on hydraulic presses to producevery large shafts and discs for energy andpower plant engineering applications. Sincethe entire open-die forging process is rela-tively time-consuming, it is only used forsmall volumes.Closed-die forging with flash uses toolswhich correspond to the geometry of theworkpiece to be produced. Once the materialhas been heated to forging temperature, it ispressed into the desired shape between theupper and lower dies. Excess material,known as flash, flows out of the thin seamsbetween the two halves of the tool (Fig. 10,left). This flash is removed in a separateprocess step (trimming). Frequently, theclosed-die forging process is divided intoseveral stages. The first stages involve mater-ial distribution. During the subsequent stages,

Fig. 10: Left: Principle ofclosed-die forgingwith flashRight: Principle ofclosed-die forgingwithout flash (pre-cision forging)

Workpiece

Upper die

Lower die

Upper die

Flash

Lower die

Open-die forging

Closed-die forging withflash



the main forging operations take place stepby step. Closed-die forging without flash, also knownas precision forging, is characterised by thefact that the tools create a completely closedhollow form in which the workpiece is shaped.Thus, no material is pressed into a flash duringthe forging operation. This means that thestarting weight of the slugs corresponds exact-ly to the weight of the finished forged parts(Fig. 10, right). The need to continuously optimise forgingprocesses, both from a technological andfrom an economic point of view, has led tothe development of special horizontal ma-chines for closed-die forging without flashthat operate with a high stroke frequencyand that are equipped to carry out severalforging steps. The individual steps aremostly based on some elementary types offorging or combinations thereof (Fig. 11).These include:

• Upsetting and squaring upThese operations involve reducing the heightof the workpiece so that its diameter is en-larged. If this is achieved without a limit to theradius, it is referred to as upsetting. If the cir-cumferential surface of the workpiece comesinto contact with the tool, it is referred to assquaring up.

• Forward extrusionIn forward extrusion, the workpiece is intro-duced into a die and pressed through a smallerdie opening. This leads to a reduction in theworkpiece diameter and to an increase in thetotal length.

• Forward can extrusionIn forward can extrusion, the workpiece isformed on a die-side punch. The material

Types of forging

Closed-die forging with-out flash



Upsetting

Flat upsettingtool

Flat upsetting tool

Workpiece

Squaring up

Die

Workpiece

Ejector

Punch

Forwardextrusion

Die

Workpiece

Punch

Forward can extrusion

Die

Counterpunch

Workpiece

Ejector sleeve

Punch

Lateralextrusion

Punch

Workpiece

Lower die

Punch

Upper die

Trimming

Die

Tool-holdingfixture

Trimming punch

Flash

WorkpieceSlug

Die

Piercingpunch

Piercing

WorkpieceTool-holdingfixture

Ejector

Die

Backward canextrusion

Workpiece

Punch



flows forwards around the punch and a cavityis produced. This causes the total length of theworkpiece to increase, while its outer diameterremains unchanged.

• Backward can extrusionBackward can extrusion involves a punch being pushed into the end face of the work-piece. This causes the material to rise betweenthe punch and the die cavity. The total lengthof the workpiece thereby increases, while itsouter diameter remains unchanged.

• Lateral extrusionIn lateral extrusion, the material flows trans-versely to the main direction of the machinemovement into a tool opening with the desiredshape. The tool is designed with split dies inorder to be able to remove the workpiece afterit has been formed.

• Shearing/piercing and trimmingDuring shearing or piercing operations, thematerial is separated by cutting. If the materialis separated from the inner part of the work-piece, it is referred to as piercing. Trimmingdescribes the removal of material from theoutside of the workpiece.

• Hot sizing/finish forgingHot sizing or finish forging refers to a processin which individual surfaces of the workpieceare formed again following the main forgingprocess. This is carried out in order to meetgreater demands on geometrical, positionaland dimensional tolerances.

Fig. 11 (opposite): Important types offorging in overview


20

Horizontal multi-stage pressesProducing hot-forged parts in a cost-efficientway depends on a number of different factors.One decisive point is the availability of a suit-able forging unit. In determining which for-ging unit is best, certain criteria need to beconsidered such as the production volume, therequired press force, the number of forgingstages and the number of strokes.In the case of small batch sizes (approx. 100to 5000 parts per annum), several individualtools are produced and then used on hy-draulic presses or hammers for generatinghot-forged parts (Fig. 12). The starting mater-ial is firstly cut to length by means of sawingor shearing. Subsequent to this, it is thenheated to forging temperature in a gas fur-nace or induction-heating facility. Parts hand-ling is mostly carried out manually. Formedium-sized overall production volumes(up to 20 000 parts per annum), linked indi-vidual presses or vertical multi-stage presses

Fig. 12: Closed-die forgingusing a forging hammer

Criteria for se-lecting optimumforging unit


Horizontal multi-stage presses 21

with an automated transfer system are used(Fig. 13). The presses used can be hydraul-ically or mechanically driven. Such pressescan achieve up to 40 strokes per minute, de-pending on the length and thickness of thepart. The forging slugs produced by sawingor shearing are introduced one at a time intoan induction-heating facility. After this, theyare removed by a mechanically or electron-ically controlled transfer beam system andtransported to the relevant forging stages.For medium-sized to large total productionvolumes (from 20 000 parts per annum), hori-zontal multi-stage presses can be used (Fig.14). These forging units are characterised bya very high parts output, which no other typeof press is able to match. Depending on thesize of the parts, 50 to 200 strokes per minuteare possible. Horizontal multi-stage presseshave an integrated shearing facility, whichenables the induction-heated bars to besheared to the required slug length. The crosstransfer, which is mechanically controlled viacam discs, takes up the slugs and transportsthem from one forging stage to the next. Gen-erally, precision forging processes are carried

Fig. 13: Linked individualpresses

More than 20 000 parts per annum


22 Horizontal multi-stage presses

out on such machines so that parts with greatlevels of accuracy and without flash can beproduced at a high degree of material util-isation.

Machine concept of horizontalmulti-stage pressesHorizontal multi-stage presses can be usedfor cold-, warm- and hot-forging operations.The presses used for cold forging draw theirmaterial mostly from coiled wire, and are de-signed for the greater flow stress of such ma-terials with respect to stage forces, the toolassembly space and the shearing forces. Oilsare usually used as lubricoolants. By con-trast, horizontal multi-stage presses for hotforging – and for warm forging with certainworkpiece geometries – generally work withbar stock. The lubricoolant used during hotforging is either pure water or water mixedwith corrosion inhibitors and wetting addi-tives. The following text focuses in particular on theuse of horizontal multi-stage presses for hot

Fig. 14: Horizontal multi-stage press

Suitable for allforging areas


Machine concept of horizontal multi-stage presses 23

forging. Horizontal multi-stage presses consistof a machine base that has usually been cast inone piece. The press ram is driven within themachine base by means of a crankshaft and aconnecting rod, and it executes an oscillating,horizontal movement. The tools fixed to thepress ram are mostly punches and are held bya suitable tool-holding fixture. These work to-gether with the tool parts mounted on the pressbase, usually the dies, in order to form the partduring several forging stages (Fig. 15). Threeto four forging stages are customary on hot-forging presses. Depending on the givengeometry of the part, the following forgingstages are required:

• upsetting• preforming• finish forming• trimming and/or piercing and hot sizing.

Fig. 15: Structure of a horizontal multi-stage press

Press structure

Cross-transfer system

Press ram

Drive systemShearing system

Materialfeedingsystem

Forging stages



As the press ram is arranged horizontally, thelarge volumes of lubricoolant used for cool-ing the tools are able to flow off downwardsafter use. Even bar ends and piercing slugsor separated flash gravitate downwards, andare thus removed easily. The good parts arealso routed downwards following the lastforging stage.

Horizontal multi-stage presses used in hotforging are linked with peripheral units toform a fully automated forging line (Fig.16). This forging line mainly consists of a bar-stock feeding facility, an induction-heating unit, the horizontal multi-stage press itself and an unloading belt for controlledcooling of the workpieces. As a rule, the en-tire facility can be operated by one or twoemployees.

Principle of operation/machinekinematicsThe machine kinematics of horizontal multi-stage presses corresponds to those of conven-tional (vertical) crank presses. Both types ofpresses demonstrate a small ratio between thelength of the connecting rod and the ramstroke. This leads to a very rapid forgingprocess. Furthermore, due to the brief contacttime, the thermal load on the tools is low. The

Fig. 16: Horizontal multi-stage press with peripheral unitsa) Material cart and

bar magazineb) Induction-heating

facilityc) Forging unitd) Unloading/cooling

belt

a)b)

c)d)

Small connectingrod length/ramstroke ratio


Principle of operation/machine kinematics 25

rapid forging process also generates a rela-tively large time gap for transferring the work-pieces.

Machine driveThe electric motor for the press drive systemis located outside of the actual machine base.It drives the flywheel, which is equipped witha clutch-brake combination. The task of thebrake is to bring the press ram to a standstillquickly should there be a fault with the pressor the forging process. This is intended to pre-vent damage occurring to the machine or tothe tools. A gear transfers the energy from themotor to the crankshaft which, in turn, drivesthe press ram via a connecting rod (Fig. 17).All other movements of the multi-stage pressare derived directly from the crankshaft bymeans of power take-offs via auxiliary shafts.The synchronised, positively controlled move-ments of the shearing blade, the transfer sys-tem, the punch, the punch monitoring system,the punch-side holding pin and the ejector onthe die side are thus always guaranteed.

Fig. 17: Drive system of ahorizontal multi-stage press

Motor

FlywheelClutch/Brake

Crankshaft



Bar-stock feeding systemA bar-stock feeding system is used to draw theraw material bars directly out of the induction-heating facility. This feeding device, which isusually designed with four feed rolls, conveysthe bar stock up to the bar stop of the shearingstation within the work cycle of the machine.The feed rolls are adapted to the diameter ofthe bar stock. Due to the pressure which theysubject the bars to, they remove a large pro-portion of the adhering scale that arises in theambient atmosphere during induction heating.

Shearing stationThe sheared slugs required for forging theworkpieces are produced directly in the multi-stage press by means of hot shearing. Theshearing facility consists of a fixed blade at-tached to the die block, a clamping piece, aswell as a movable shearing slide with a shearblade and a slug holding pin (Fig. 18). Theshearing slide is driven by a radial cam.Firstly, the bar is pushed until it reaches thebar stop, upon which it is then fixed by theclamping piece. The shearing slide moves for-wards, shears the material and transports it tothe first forging station. During the transportphase, the slug continues to be held by the cut-off holding pin until it is taken up by thepunch-side holder pin of the first station.Decisive factors for the quality of the finishedparts include the highest possible consistencyin slug length, and thus also slug weight, aswell as the lowest possible slug deformation.Also of importance is the quality of the shear-ing surface on the end faces of the slugs.

Transfer systemIn order to achieve a high number of strokes,a reliable transfer system for transporting theworkpieces from one forging stage to the

Fig. 18: Shearing station

Feeding processwithin machinework cycle



next is an absolute prerequisite. The transferfacility consists of a rail with several ad-justable upper and lower grippers, which inturn are mounted on gripper supports (Fig.19). The grippers are arranged in front of the

Fig. 19: Transfer facility

Transfer-systemcomponents

dies. Their purpose is to hold the workpiecein a perfectly centred way at the particularforging stage until the workpiece is pushedinto the die by the punch. While the pressram moves into the forward dead-centre pos-ition (forging), the cam-controlled grippersare driven back by one forging stage, readyfor taking up the next workpiece. During thebackward stroke of the press ram, the work-piece is brought into a transport position infront of the die block by means of a die-sideejector and a punch-side holding pin. It re-mains fixed there until the gripper takes upthe workpiece or the workpiece is pushedinto the gripper. A cross-transfer systemtransports all workpieces forward by oneforging stage.

Transport of the workpiece



Ejectors and holding pinsThe ejectors and the holding pins integrated inthe tool fix the workpiece in a defined positionin front of the die-block face following com-pletion of a forging operation so that it maythen be taken up by the transfer facility. Theworkpiece is held between a punch-side hold-ing pin and a die-side ejector. The time ofejection can be individually adapted at eachforging stage to the part being produced bymeans of adjustable and exchangeable radialcams (Fig. 20). Prior to the subsequent forging

Fig. 20: Kinematics of thepunch and die sidesa) Kinematics of the

punch sideb) Kinematics of the

punch sensorsc) Kinematics of the

die side

a)

b) c)

Positioning andtransfer function

stage, the workpiece is clamped between theejector and the holding pin while the grippersopen. Following this, the workpieces arepushed into the die by the punch or the hold-ing pin and are then forged. An additional taskof the die-side ejector lies in introducing cool-ing water precisely at those sites of the die thatare subjected to strong abrasive wear due toelevated temperature loads.

Cooling systemDue to the high workpiece temperatures andthe high number of strokes of the presses, agreat deal of thermal energy is transferred tothe tools during the forging operations. Thisthermal transfer arises from the heated barslug itself as well as from the forging energy,



90% of which is converted into heat. In orderto prevent high levels of abrasive wear on thetools, a cooling system is required that alsoserves as a carrying medium for the variouslubricant additives. What is known as a cool-ing bridge is used to introduce the coolingmedium into the forging process. From thisbridge, nozzles and water outlets direct thecooling water onto the tools. On the die side,the tools are also cooled from the inside. Thedie and the ejector need to withstand particu-larly high thermal loads. Drilled holes in themachine base allow the cooling water to berouted to the ejector, which directs the wateronto the die upon ejecting the part.

Tool-changing systemsIn order to achieve a high level of efficiencywithin the forging process, it is essential thatthe tool change is carried out rapidly. To dothis, the tools from all forging stages can be removed together as one block from thepress via a tool-changing facility, and then

Coolant distribution via bridge

Fig. 21: Tool-changing system



transferred to a rack that has been placednearby. The tools to be changed are removedfrom another rack and transported into the machine (Fig. 21). There, the tool block isclamped manually or automatically via a hy-draulic tool clamping system. If necessary, thecross-transfer system with grippers for eachstage can also be completely changed. If it is only individual tool elements which need to be changed, the process can be carried outmanually.

Monitoring equipmentBesides the safety equipment that is incorp-orated into multi-stage presses, as prescribedby occupational health and safety regulations,comprehensive monitoring systems are alsointegrated in order to prevent injury to peopleas well as damage to tools and the machine it-self. Proximity switches are thus used to moni-tor the opening and closing of the grippers, forexample. Workpieces which remain adhered tothe punch are not taken up by the transfer sys-tem, and are thus not transported further. Dur-ing the forging operation on the next work-piece, the tool would then have two work-pieces to deal with, thereby leading to an over-load for tool and machine. For this reason,punch sensors are moved in front of the punchupon each stroke in order to detect any work-pieces adhering to it and, if necessary, to bringthe machine to a standstill prior to the nextstroke.

Peripheral equipmentThe high yield of horizontal multi-stagepresses requires that they be linked with per-ipheral equipment further up and down thechain. Examples of such peripheral units in-clude the bar magazine, the heating facility

Automatic ormanual toolchange

Proximityswitches

Linking to aforging line


Peripheral equipment 31

and the cooling belt. These peripheral unitsneed to be designed to precisely meet the vol-ume output, the maximum bar diameter aswell as the maximum permissible startingweight of the multi-stage press. Equipment forheat-treating and shot-blasting the parts mayeither be assigned directly in line with theforging machine in continuous operation or beused for several presses in batch operation.

Bar magazine/stock reelThe raw material is usually in the form ofround rolled bar stocks. Where the diameter ofthe raw material is smaller, rod wire from acoil can be used. Following the incoming in-spection, the raw material is held in intermedi-ate storage before being introduced into thefeed unit. When using rod wire, a vertical orhorizontal stock reel is used. Bar stocks, usu-ally with lengths of between 6 m and 9 m, arelaid in bundles on a material cart where theyare then separated. From there, they are trans-ported to the heating facility.

Heating systemThe raw material is heated to temperatures ofbetween 1100°C and 1250°C in the heatingsystem. At these temperatures, the materialdemonstrates considerably reduced flow stressand higher ductility compared with its behav-iour at room temperature, and thus can beformed into a complex part over several for-ging stages. Heating is carried out by inductionunder normal atmospheric conditions. The rawmaterial is transported through several induc-tion coils which are adapted in size and powerto the material to be processed (bar stock diameter) and to the material throughput. Thealternating current flowing through the induc-tion coils generates an electromagnetic alter-nating field in the insides of the coils. This in

Bars orcoiled wire

Induction heating



turn induces circular currents that decrease ex-ponentially towards the centre of the barstocks. This leads to the outer parts of the barbeing heated in particular (skin effect). Thebar stocks are heated through to the inside bymeans of thermal conduction (Fig. 22).

2

1j

dδ

3

Fig. 22: Principle of induc-tion heatingAbove: Workpiece(1) with inductioncoil (3) and magneticfield (2)Below: Current distribution in theworkpiece along thediameterj Induced currentδ Current penetra-

tion depthd Diameter

Low levels of scale

The consistency of the final temperature isseen as a significant quality criterion of heat-ing systems. Decisive for this is the tempera-ture difference both in the bar stock cross-section as well as across the entire bar length.One advantage of induction heating is thatscaling of the material may be kept low due to the rapid heating process.

Unloading and cooling lineFollowing the last forging stage, the work-pieces fall downwards and are transported outof the multi-stage press by means of a con-veyor belt. If suitable steel types are used (i.e.quenched and tempered steels or micro-alloyed dispersion-hardening steels), or if suit-able cooling units such as furnaces or covering


Peripheral equipment 33

hoods (with case-hardening steels) are used, it is possible to carry out heat-treatmentprocesses in a cost-efficient way directly afterthe forging operations have been completed.This has the advantage that the workpieces donot need to be heated again. To achieve this, itis necessary to cool the workpieces in a con-trolled way and relatively quickly from theforging temperature to temperatures of approx.600°C to 750°C and, if necessary, to holdthem within this temperature range for a cer-tain time. This endows the workpieces withthe desired strength and hardness, as well aswith a certain toughness, depending on thegrain structure. Cooling is carried out either onspecial cooling belts, which may be equippedwith ventilating fans or covering hoods, orelse in suitable continuous furnaces. In the lat-ter case, the workpieces may either be cooledusing an adjustable air stream or held within acertain temperature range. It is important thatthe workpieces are evenly distributed on theconveyor belt so that they are all cooled uni-formly.

Heat treatmentsubsequent toforging …

… requires con-trolled cooling


34

Forging process and toolsThe movement of the press ram enables thetools to form the workpiece into its finalgeometry. The various forging stages deter-mine the grain flow and plastic strain gener-ated in the workpiece. Thus, the forging toolswhich are used have a considerable influenceon the quality of the parts produced. On hori-zontal multi-stage presses, the tool set-up forone forging stage consists of several individu-al forging tools which are stacked. The toolsets for the single forging stages are in turngrouped together to form a complete tool set(Fig. 23). Each tool set is designed for one

Fig. 23: Tool set in assembly block

particular forging operation. As a rule, the fol-lowing forging stages are involved: upsetting,material distribution, finish forging (precisionforging without flash) and separating (piercingand/or trimming). Tool life and thus costs are greatly influencedby the design of the individual forging stages,

Effect on partquality


Process design 35

known as the sequence of forging stages, aswell as by the design and production of the in-dividual tools. Besides the forging unit used,tools thus also play a key role in producinghigh-quality hot-forged parts in series produc-tion in a cost-efficient way. When designingtools, experienced tool designers as well asprocess and material specialists are required.Besides this, an absolute must is the continu-ous support provided by computer-aided en-gineering tools such as FEM, CAD, CAM andCAQ. It is not only the three to four tool setsfor the forging stages which need to beadapted to the particular slug diameter or tothe workpiece, but also the feed rolls, theshear blades, the grippers, the punch sensorsand, in some cases, the cooling water jets, too.

Process designProcess design involves determining the indi-vidual forging stages needed for producing thefinished part. These are summarised in a se-quence of forging stages. Depending on theworkpiece geometry to be produced, the fol-lowing forging operations may be carried out:upsetting or squaring up, forward extrusion,forward and backward can extrusion, lateralextrusion, piercing or trimming, and in somecases hot sizing, too.

Sequence of forging stagesThe sequence of forging stages details whichgeometry the workpiece must demonstrate after each forging stage has been completed. Asequence of forging stages involving fourstages is shown in Figure 24. The geometryand the tolerance requirements which the fin-ished part needs to fulfil form the startingpoint for determining the surfaces to be ma-chined and for providing them with a machin-

Considerablecost-efficiencyfactor

Determining theforging stages


36 Forging process and tools

ing allowance. In collaboration with the cus-tomer, the geometry of the part is modified inorder to take process limitations into account.These include, for example, draught angles,minimum wall thicknesses, minimum radiiand holes, as well as any necessary materialoverflow. The following boundary conditionsare taken into account to outline the individualforging stages:

• bar diameter• material• surface requirements• strength requirements• number of pieces• number of stages• interval between stages• press force• machine transfer.

Process limits and volume consistency have tobe taken into account as well. Process limitsare set by material-related values such as themaximum plastic strain and degree of upset-ting, by tool-related values such as the internalpressure, as well as the sliding movement between tool and workpiece. All these valuesgreatly influence tool life. The sequence offorging stages is used as a basis for designingthe individual forging tools.

Fig. 24: Sequence of forgingstages for an outerrace of a constantvelocity joint

Squaring up Preforming Finish forming Hot sizing

Boundary conditions


Process design 37

FEM process simulationThe sequence of forging stages is verified bymeans of FEM process simulations (FEM; Finite Element Method). FEM simulations areused to determine at a very early stage of thedevelopment and planning process whetherproblems will arise when forging the material.Furthermore, they allow substantiated state-ments to be made on possible forging defectsthat may occur, such as folds, overlapping, aswell as underfilling of the die, or material sep-arations (Fig. 25). Besides this, the simulationalso serves to estimate the press forces andthus the loads that the tool and machine mustwithstand.The material volume is divided into individualworkpiece elements for the FEM process simu-lation. These workpiece elements are describedby means of a material model in terms of yield conditions and flow law. The materialmodel can describe the material either as elastic-plastic or rigid-plastic. In the case of hotforging, the elastic elongation of the materialcompared to the plastic deformation can be neg-lected. Thus it is generally rigid-plastic ma-terial models which are used. The yield condi-tion describes at which stress state the materialdeforms plastically. Flow curves, as shown inFigure 7 (see p. 13), serve as the basis for this.The flow law provides information on how thematerial will deform at the given stress state.The simulation program calculates the mater-ial flow on the basis of a balance between deformations and boundary conditions such astool geometry and tool movement. The degreeto which the die is filled, the fibre flow andother parameters such as the temperature andstresses in the workpiece can be derived fromthis.The selected boundary conditions such as thefriction law and the values of the friction co-

Fig. 25: Formation and inclusion of a foldduring a forgingprocess (simulation)



efficients between the workpiece and the in-dividual tool elements also have a significantinfluence on the simulation results.Besides simulating the actual forging process,modern FEM programs also have the cap-acity to calculate the grain structure which willbe generated by the forging process. This pro-vides the opportunity to optimise the work-piece properties by means of suitable processdesign prior to actual production operations.It is also possible for the simulation to model the load which the tools must withstandwith respect to prevailing stresses, the materialfatigue arising as a result of mechanical andthermal influences, as well as the abrasive loadon the tool surface.Through simulation it is possible to check thefeasibility of producing a part in advance in arelatively simple and cost-effective way. Ifforging defects happen to be revealed, it is stillpossible at this point to undertake modifica-tions to the geometry of the tool and to alterthe sequence of forging stages in order to en-sure that the parts are subsequently producedwith zero defects. Increased computing power and high-performance software packages providereadily accessible results. They are also re-sponsible for having ensured that FEMprocess simulations are nowadays an indis-pensable part in the development of forgingprocesses on horizontal multi-stage presses.

Process steps during hot forging on horizontal multi-stage pressesThe procedure for a four-stage forging process,including the typical steps involved, will beoutlined in the following text. In the case ofthree-stage forming processes, the first twostages – upsetting and material distribution –are combined, or else one stage is omitted.

Feasibilitychecks


Process design 39

ShearingPrior to the actual forging process, theheated raw material bar stock is conveyedfrom the feed rolls into the shearing stage.Here, the bar stocks run on an adjustable barstop, which ensures that a consistent sluglength is achieved. The actual shearing sys-tem consists of the movable shear blade, thefixed blade, the carrier, the cut-off holdingpin and the clamping piece. The shear blade,driven by a radial cam, cuts the shearingslugs. The blades must be adapted to the par-ticular bar diameter in each case in order toavoid deformation of the slugs. The cuttingedges of the blades are frequently armoured(welded) with special heat-resistant mater-ials, and they have a recess in order to guar-antee an appropriate blade clearance. Theshearing operation is aimed at producingslugs that are as cylindrical as possible, thatdemonstrate a consistent weight as well asgood end-face qualities.

1st forging stage – upsettingDuring the first forging stage, the slug is usu-ally only upset. A distinction is made betweenupsetting that is carried out between two flatupsetting punches and squaring up or closed-die upsetting. Generally speaking, free upset-ting serves to remove the oxide layer on theoutside of the slug. The slight deformationwhich occurs during the upsetting operationcauses the brittle oxide layer to flake off andfall downwards away from the slug. This en-ables good surface qualities, low machiningallowances and a longer tool life to beachieved. In the case of workpieces with more complexgeometries, process steps such as squaring upin a die or forward extrusion may be used inthe first forging stage if these are no longer

Shearing systemstructure

Free upsettingor squaring up



possible in the following stages, as laid downby the sequence of forging stages. This means,however, that the oxide layers cannot be re-moved, and thus greater machining allowancesgenerally need to be taken into account. Certain systems allow a combination of bothupsetting and squaring up to be carried out(Fig. 26).

2nd forging stage – material distributionThe second forging stage involves preforgingthe workpiece. It is during this stage that thegreatest levels of plastic strain are achieved.Depending on the course of the subsequentstages, the workpiece geometry after the pre-forging operation either already roughly cor-responds to the workpiece geometry to be pro-duced, or it is initially pressed into a simplepreform which demonstrates optimum volumedistribution for the forging stages that follow.Usually, preforging is an operation withoutflash, i.e. the header punch arranged on theram side enters the die and thereby forms theworkpiece.

3rd forging stage – finish forging During the third forging stage, the workpieceis given its final shape, except for those areaswhich are to be separated during the subse-

a)

EjectorDie

Slug

Punch b) c) d)

Fig. 26: Combined upsettingand squaring upa) The slug is trans-

ferred to the firststage and ispushed against the ejector by the punch.

b) The ejector re-mains stationaryso that the slugcan be upset andthe scale therebyremoved.

c) The ejector ispushed into the diesubsequent to thedescaling stroke.

d) The descaled slugis squared up inthe die.

Production of final geometry


Process design 41

quent stage. The levels of plastic strain aregenerally significantly lower than those duringthe preforging stage. Small radii and areaswith thin wall thicknesses are shaped. Gen-erally, the highest press forces need to be applied during the finish-forging stage.

4th forging stage – piercing and trimmingDuring the final forging stage, the parts areusually pierced or trimmed. In the piercingprocess, the workpiece is supported bymeans of a die with an appropriate recess. Apiercing punch presses the floor of a cavitythat was preformed out of the workpiece dur-ing the previous stages. The workpiece isthen drawn out of the die upon the returnstroke of the punch and is stripped off thepunch by a stripper. It then falls onto theconveyor belt which transports it out of themachine. The piercing slug is pushedthrough the die and falls through a passageonto another conveyor belt that removes ma-terial waste (piercing slugs, flash, under-length slugs from bar ends). In the case ofparts with complex outer geometries such asspiders or multiple-arm flanges, productionwith flash (similar to closed-die forging withflash) may be necessary. This flash is like-wise separated from the workpiece duringthe last stage.Furthermore, the fourth forging stage may be used to hot-size the parts. Hot sizing is

Fig. 27: Stage plan for a convoluted speed gear

Material distribution Preforming Finish forging Piercing



used to form certain surfaces to meet tightertolerances.Figure 27 shows the sequence of forgingstages which is necessary for producing a gearwheel with six waves.

Tool designTools represent the heart of forging oper-ations and are of great significance with re-spect to the cost-efficiency of the productionprocess. Tool precision, reliability and ser-vice life are all considerable cost factors, andthus tool development, design and engineer-ing are among the core competences requiredin operations using horizontal multi-stagepresses.

Tool structureThe tools are stacked for the individual for-ging stages and arranged in a basic tool block(Fig. 28). This fixes the precise position ofthe punch holder and die to each other. Thepunch holder, which enters the die, is usuallyfixed to the press ram. Holding pins are inte-

Punch holder Sleeve Punch

Holding pin

Base plate

Fig. 28: Principle of astacked tool

Core compe-tence: tool engineering


Tool design 43

grated into the punch side to fix the work-piece prior to the forging operation and tohold it in front of the punch during the returnstroke. Pusher rods, which extend through the tool structure, activate the holding pins.These pusher rods can be moved via radialcams that are integrated in the machine. Onthe die side, forging tool elements are locatedin a die holder. Depending on the load andthe forging pressures which arise, the toolsare pre-stressed using shrinkage rings. Eject-ors are arranged on the die side to move theworkpiece out of the die into the transportposition.It is important to cool all those tool elementsthat are in contact with the workpiece. Toachieve this, the die or the ejector is oftenprovided with drilled holes and ducts in order to be able to apply coolant to the areasthat are subjected to high loads. At the sametime, it is also important to have ducts to allow the water vapour generated during theforging operations to escape from the diecavity.

CAE (Computer-Aided Engineering)Nowadays, it is not only 2D CAD systems(CAD; Computer-Aided Design) which areused for designing forging tool elements, butalso 3D CAD systems, too. These enable acontinuous data flow to be achieved, rightfrom the design phase and simulation stagethrough to production using CAM systems(CAM; Computer-Aided Manufacturing).CAM systems are used during the prepar-ation stage to generate the programs for theCNC machine tools (CNC; ComputerisedNumerical Control). The geometry dataforms the basis for quality assurance surveil-lance using CAQ systems (CAQ; Computer-Aided Quality Assurance). Using this geom-

Tools pre-stressed withshrinkage rings

Cooling ducts

Continuous data flow



etry data, testing programs are developed onthe computer and the permissible tolerancesare determined both for the forging tools andfor the workpieces. In order to minimise the standstill times ofthe presses, the movement of the transfer system used to transport the workpieces isalso modelled on the computer. This enablespossible problems such as collisions or workpieces falling off due to the grippers re-leasing them too soon to be monitored in advance.

Tool materialsThe tools which come into contact with theworkpiece during hot-forging processes onhorizontal multi-stage presses need to with-stand high thermal and mechanical loads andsignificant levels of abrasive wear. In add-ition, the intensive use of cooling watercauses a very pronounced repeated change intemperature. This leads to the generation ofthermal-shock cracks. For this reason, high-alloy tool steels (hot-forging tool steels) areused as tool materials. These are usually alloyed with chromium, nickel, molybdenumor vanadium in order to achieve adequatetoughness at the highest possible hardnessvalues and a great level of wear resistance(Table 1). For those tool parts which do notcome into direct contact with the hot work-piece, it is possible to use more reasonablypriced conventional tool steels, as here it isonly good strength and hardness propertiesthat are decisive.Depending on where and how the tools will beused, a compromise must be made betweenhardness and toughness. Hardness ranges ofbetween >40 HRC and 60 HRC are usuallychosen. In order to increase wear resistance,

High-alloy tool steels


Tool production 45

the forging tools may be additionally hardenedby means of carburisation or nitriding. Thechoice of parameters with respect to the heattreatment can have a considerable influence ontool life.

Tool productionA complete tool set consists of the stack oftools needed for each of the forging stages.These are known as stage tool stacks. Eachstage tool stack comprises approx. 15 to 25 in-dividual parts, depending on complexity. Theyare machined from sawn slugs of rolled steelbars or from rolled steel slabs.

Soft machiningThe tool components are machined in the softstate. Due to the low strength values prevail-ing during this phase, the machining tools areable to achieve high metal-removal rates and along tool life. Tool parts with low dimensionaltolerances are provided with an allowance inorder to compensate for the distortion that occurs during the hardening process. Thechanges to the dimensions that arise during thehardening process are subsequently compen-sated for when the surfaces undergo hard machining.

Material DIN-designation AISi Chemical composition in %number (DIN = German Institute reference (typical values)

C Cr Mo V Co

1.2344 X 40 CrMoV 5 1 H13 0.40 5.20 1.30 1.00 –

1.2365 X 32 CrMoV 3 3 H10 0.32 3.00 2.80 0.50 –

1.2367 X 38 CrMoV 5 3 H11 0.37 5.00 3.00 0.60 –

1.2885 X 32 CrMoCoV 3 3 3 H10A 0.32 3.00 2.80 0.50 3.00

Table 1:Tool materials forhot forging

Machining

for Standardization)



Heat treatmentFollowing soft machining, the tools are heat-treated in order to adapt the material propertiesto the subsequent operating conditions. Thetool components which come into contact withthe workpiece are usually hardened. This in-volves initially bringing the tool to a tempera-ture which causes the carbon to completelydissolve in the iron crystals (austenitic range),and which is dependent on the material qual-ity. The heating process is not continuous, butis carried out in temperature stages. Continu-ous heating poses the hazard of stress cracks,as the steels demonstrate reduced heat conduc-tivity due to the high proportion of alloys. Inorder to achieve even temperature distributionthroughout the tool, it is thus necessary to holdthe material for a certain time at each tempera-ture stage and also upon reaching the hardnesstemperature. This ensures that the tool isevenly heated through to the core. The lengthof time it takes to achieve this even tempera-ture distribution depends on the geometry andwall thickness of the tool. After an even temperature has been reached,accelerated cooling is carried out in hardeningoils, salt baths or using a stream of nitrogen(quenching). The characteristic critical coolingrate for each steel needs to be reached in orderto ensure that the austenite completely trans-forms into martensite (tempered martensiticstructure). Due to its high degree of supersat-uration in the atomic lattice, the dissolved car-bon leads to atomic strains in the grain struc-ture. Following austenitizing and quenching,the tool is as hard as glass and brittle, andwould lead to failure by fracturing under thegiven load cases. Repeated heating to tempera-tures of 450°C to 650°C (tempering) causesthese atomic strains in the grain structure to berelieved. A tempered grain structure and an

Heating in tem-perature stages

Acceleratedcooling

Repeated tempering


Use of pre-assembled tool sets 47

optimum balance between surface hardnessand toughness are reached in order to achievethe best possible wear-resistance properties. Inso doing, it is necessary to allow the tools tocool to room temperature following each tem-pering operation. High-quality tool steels areusually tempered at least three times. In orderto increase the surface hardness and the wearresistance, the tools can undergo nitriding orcarbonitriding in addition to the initial harden-ing operation.

Hard machiningBesides conventional hard-machining pro-cesses involving grinding, the past few yearshave seen an increase in turning and HSCmilling (HSC; High Speed Cutting) oper-ations. This development is a result of themodern cutting materials available today suchas CBN (CBN; Cubic Boron Nitride) and fine-grain cemented carbides. These processes en-able the distortion caused by heat treatment to be compensated for in a simple and cost-effective way. Hard machining at high cuttingspeeds and low feeds leads to surface qualitieswhich lie in the range of Ra = 0.8 to 3.2 µm. Itis thus sometimes possible to put the tools intooperation without the need for manual finish-ing operations such as polishing.

Use of pre-assembled tool sets

Pre-assemblyDue to the high hourly machine costs of hori-zontal multi-stage presses, the running time ofthese facilities needs to be maximised for fi-nancial reasons. In order to reduce the stand-still times during tool changes, the tools undergo pre-assembly outside of the machine.This involves stacking the tools for the indi-

Additional nitriding orcarbonitriding

Machining by turning orHSC milling

Tool stacking



vidual forging stages and then mounting andadjusting them in a holding block (Fig. 29).This facilitates any necessary adjustment workto the individual tool parts. During the tool change, the entire block con-taining the used tools is removed from the ma-chine by means of a crane. A crane is likewiseused to introduce the new tool set into the ma-chine where it is then clamped in position bymeans of clamping bolts.

Tool lubrication and coolingTools used during the hot forging of steel aresubjected to high loads. Besides the mech-anical loads which arise during the forgingprocess, the tools also need to withstand highthermal alternating stresses. Furthermore, thecontact with the forged part leads to abrasiveload on the tool surface.If tools are permanently used at temperaturesof above approx. 600°C, the tool material undergoes a continual reduction in strength dueto tempering effects. This means that the hardmartensitic grain structure produced during thehardening operation transforms into softercomponents such as bainite, ferrite and

Fig. 29: Complete toolchange

Reduction instrength from600°C onwards



pearlite. The tools are thus cooled in order toprevent this (Fig. 30). Via spray nozzles whichare mounted near the tools, the active tool elements, in particular the punch, are sprayedwith cooling water. Frequently a nozzle ring ismounted in front of the die in order to cool thewalls of the die, too. Furthermore, in the caseof deep die cavities, it is necessary to providethe ejectors with ducts through which the die wall can be cooled during ejection of theworkpiece. When designing the tools, it is necessary to also provide openings in the dieto allow the water vapour generated during the forging process to escape. This preventsthe punch and die from becoming overloadedand the die cavity from not being filled completely.The coolant used is water with salts, corrosioninhibitors and stabilizers added to it. The addi-tives serve to improve wettability between thecoolant and the tools, while the corrosion inhibitors protect the tool and machine. Thecoolant not only cools the tool surface but alsoensures that the heat is distributed evenly

Fig. 30: Cooling of the tools

Cooling measures

Cooling medium:water with additives



within the entire tool assembly space. Uneventhermal expansion of tool blocks on the dieand punch sides would otherwise lead to devi-ations in part accuracy.The tool surface is heated during the actualforging process when it comes into contactwith the forging material, which has a tem-perature of approx. 1200°C. This leads to highcompressive stresses occurring on the toolsurface. Subsequent cooling, by contrast,leads to tensile stresses on the tool surface.This frequent alternating stress can cause fa-tigue on the tool surface, resulting in thermalcracks.

Methods for increasing tool lifeDepending on geometrical factors, the forgingmaterial, the temperature and the configur-ation of the sequence of forging stages, highpartial contact pressures can occur during theforging process. These can amount to levelsseveral times greater than the flow stress ofthe forging material if the material slides alongthe tool during the forging process. Abrasivewear results particularly if scale, i.e. oxidisedhard material, is introduced into the tool. Thiswear load can only be counteracted to a limit-ed extent with the use of lubricants. In order toreduce the level of abrasive wear, it is thusnecessary to increase the hardness of the toolsurface, for example by means of nitriding.During the nitriding process, the tools undergoconventional hardening before being treated innitrogen-emitting gases (temperatures of be-tween 480°C and 540°C) or in salt baths (tem-peratures of between 520°C and 570°C). Ni-trogen diffuses into the tool surface and strainsthe dislocation gliding planes on an atomiclevel, thereby increasing the material hardness.A very high nitriding depth of up to severaltenths of a millimetre may be achieved by

Heat distributionfunction

Hardening bynitriding



treating the tools in a salt bath. The nitridingdepth depends on the duration of the particulartreatment.If increased levels of scale are drawn into theindividual forging stages, it is important topay particular attention to the upsetting oper-ation when optimising the forging process. Insuch a case, it is necessary to vary the degreeof upsetting, i.e. the ratio of the startinglength of the slug to the length of the upsetpart, in order to improve spalling of the scalearound the circumference of the slug. Anothersimple option is to reduce the forging tem-perature so that scale formation is minimisedfrom the outset. To do this, however, the for-ging process needs to permit a lower forgingtemperature.An optimally designed sequence of forgingstages ensures that those forging operationswhich lead to high abrasive loads on the dieand punch are distributed across several for-ging stages so that the tools used for each par-ticular forging stage achieve a similar tool life.The efforts involved in the tool change arethereby reduced, and the operating time of themachine can be increased. A briefer contact time between the tool andworkpiece always leads to lower heat inputinto the tool surface. The effect of the tempera-ture on the tool surface is thereby reduced. For example, there are fewer burns on the surface structure, and the levels of thermal alternating stress are lower (Fig. 31). Due tothis, the tool life is correspondingly higher. In order to achieve a brief contact time betweentool and workpiece, a high press-ram speed atthe front dead centre needs to be aimed for.However, as this is mainly predetermined bythe press kinematics, it can only be varied bythe frequency of the strokes. One drawback ofshortening the contact time is that the time

Variation in the degree of upsetting

Optimised designof the sequenceof forging stages

Brief contact time



interval needed for cooling the tools is also reduced. It is necessary here to find the bestpossible compromise.One way of improving the wear resistance ofthe tools and of the shear blades in particularis to apply high-alloy welding filler materials,known as stellites. These are applied usinglaser welding or tungsten inert-gas surfacing atcoating thicknesses of between 2 and 3 mm. Insome cases, even coating with PVD or CVDcoatings (PVD stands for Physical Vapour Deposition, CVD for Chemical Vapour Deposition) may be advisable. These very thin(< 10 µm) and extremely hard (~ 3000 HV)coatings are unable to significantly reduce thethermal load of the tool material, however.Furthermore, the tool material must alsodemonstrate an appropriate basic hardness inorder to prevent the PVD and CVD coatingsfrom spalling.

Fig. 31: Worn tools

Stellites or PVD/CVD coatings


53

Production processDue to the high stroke frequency achieved withhorizontal multi-stage presses, and to the factthat the forging processes carried out on thesemachines are designed in such a way that everystage is occupied, no other machine typeachieves such a high performance. Thus, de-pending on the machine size, it is possible toproduce up to 12 000 parts per hour or, theoret-ically, a throughput of up to 36 tons of steel perhour. In order to achieve these values, sophisti-cated logistics systems must be in place with re-spect to material supply and transport as well asto the further processing of the workpieces.

Workpiece materialsHot forging is suitable for almost all metallicmaterials. It is primarily steels that areprocessed. These can be subdivided into un-alloyed and low-alloy steels. Insofar as the re-quirements placed on the part demand that an-other material be used, it is also possible to em-ploy high-alloy steels, too. For the plumbingand pipe-fitting industry, various brass alloysare processed on horizontal multi-stage presses.Even though the forging of aluminium alloys isalso possible in principle, it is not currently car-ried out due to the unfavourable behaviour ofaluminium during the hot-shearing process.Frequently used steels are listed in Table 2.Besides unalloyed and low-alloy steels, case-hardening steels, quenched and temperedsteels and dispersion-hardening steels are alsoused. Case-hardening steels are generally easyto machine following hot forging and subse-quent isothermal cooling or subsequent an-nealing. The surface-hardening treatment bymeans of carburising, hardening and temper-

High performance

Primarily steels

Case-hardeningsteels …


54 Production process

ing at approx. 200°C is carried out only afterthe parts have been machined. Gears, for ex-ample, are frequently produced from case-hardening steels. These steels are charac-terised by tooth flanks which are extremelyhard and which have a high load-bearing cap-acity, as well as by a tooth base that can with-stand impact loads.In the case of quenched and tempered steels, itis possible to use certain heat treatments toadapt their mechanical and technological prop-erties (such as tensile strength, elongation atfailure, toughness, and even machinability,too) within a wide range to the particular ap-plication. Drawbacks of these heat-treatmentprocesses, however, are the efforts and coststhat they involve. Using steels with micro-alloying elements such as vanadium, titanium or niobium makes it possible to achieve theproperties of quenched and tempered steels by means of controlled cooling from the hot-forging temperature. Such steels are referred to as dispersion-hardening steels. In spite of their

Material DIN- Excerpt of the chemical Mechanical Properties Applicationnumber designation composition properties

(typical values) (heat-treated,in bars

< Ø 16 mm)

C Mn Cr Mo V Rm Rp0.2 A5

1.0037 S235JR 0.20 370 235 25 Unalloyed Parts with no spe-(St37-2) structural steel cial requirements

1.1191 Ck45 0.45 0.65 700 490 14 Quenched and Nuts,tempered steel fasteners

1.7218 25CrMo4 0.25 0.70 1.00 0.25 900 700 12 Quenched and Transmission tempered steel shafts, chassis

1.7227 42CrMoS4 0.42 0.75 1.10 0.22 1100 900 10 Quenched and Transmissiontempered steel shafts, chassis,

gears

1.5231 38MnSiVS5 0.38 1.40 20.3 0.10 820 550 12 Dispersion- Chassis,hardening steel wheel hubs

1.7131 16MnCr5 0.16 1.2 0.90 780 590 10 Case- Gears,hardening steel camshafts

1.3505 100Cr6 1.0 0.35 1.5 Bearing Cams,steel bearing rings

Table 2:Frequently usedsteels for producinghot-forged parts

… quenchedand temperedsteels …

… dispersion-hardening steels

(DIN =German

Institute forStandardi-

zation)


Heat treatment 55

high price, they nevertheless represent a cost-effective alternative to conventional quenchedand tempered steels, as the heat-treatmentprocess can be omitted.Parts that need to withstand high contact pres-sures and a high level of sliding wear such asbearing rings or cams are produced from spe-cial materials alloyed with chromium. Theseare known as bearing steels (e.g. 100Cr6).

Heat treatmentHot forging endows workpieces with theirgeometry. The final component propertiessuch as strength and toughness are onlyachieved, however, by a combination of hotforging and subsequent heat treatment. Thereare many different types of heat treatment tochoose from in order to achieve an optimumcombination of properties as required for theparticular application. Common heat-treatmentprocesses include:

• NormalisingThis is used to achieve an even and finegrain structure.

• Hardening and temperingThis is a heat-treatment process which in-volves hardening and tempering in order toachieve a predetermined strength valuewhile attaining high levels of toughness.

• Quenching and tempering from forging heatUnder certain circumstances, quenching andtempering are possible using hot-forgingtemperatures. Direct quenching of the work-pieces from the forging heat allows thehardening process in a furnace to be omit-ted. The subsequent conventional temperingoperation ultimately serves to set the de-fined strength and toughness properties.

Commonprocesses



• Soft annealing This refers to the process of reducing thehardness of a workpiece to below a givenvalue, e.g. for subsequent cold forging.

• Treating for strengthAnnealing to achieve a certain tensilestrength.

• Treating for ferrite-pearlite microstructureAnnealing to achieve a certain grain struc-ture. Besides improving machinability, thisheat treatment causes a reduction in the dis-tortion behaviour of ferrite-pearlite grainstructures during case hardening subsequentto machining.

• Controlled cooling from the forging heat For certain parts, the required properties canbe directly achieved by means of controlledcooling from hot-forging temperatures. Micro-alloy dispersion-hardening steels, forexample, are particularly suited to this typeof heat treatment.

Shot blasting

Generally, all workpieces are shot-blastedprior to final inspection and dispatch. This re-moves any scale generated during the previousoperations as well as any traces of flash mater-ial. The blasting agent consists of steel grainsof various sizes (from 0.1 mm) and shapes(round or angular). In order to be able to usethe most suitable steel mix for each particularapplication, it is common to work with com-positions consisting of various grain sizes andto vary the duration of the shot-blasting oper-ation. The parts can be shot-blasted on a con-tinuous conveyor, on a troughed belt or on anoverhead conveyor.

Removal of scale


Quality properties and tolerances 57

Quality properties and tolerancesA significant advantage of forged workpieceslies in the grain flow of the material, as it fol-lows the workpiece geometry and is thus opti-mally adapted to the particular load case. Incontrast to machined workpieces, the grain flowin forged parts is not interrupted. The grainflow can be made visible in metallographiccross-section by means of etching. Figure 32

shows the fibre flow in a universal joint. Thestrength and toughness properties in the direc-tion of the grain structure are better than thosetransverse to it. An uninterrupted grain flow en-ables a higher fatigue strength to be achievedthan is possible with machined workpieces.As a rule, forged parts achieve accuracies ofbetween IT 14 and IT 16 according to the ISOtolerance classifications. When carrying outhot-forging operations on horizontal multi-stage presses, smaller tolerances within therange of between IT 11 and IT 13 are achievedand, under certain circumstances, even IT 10is not impossible. Such tolerances cannot beobtained, however, without considerable tech-nological efforts. These involve achievinglower tolerances in the raw material, ensuringthat the heating temperatures are kept within anarrow range, providing thermally stable con-ditions in the machine and within the forgingprocess, as well as using precision formingtools which demonstrate the lowest possible

Fig. 32: Grain flow of a universal joint

Uninterruptedgrain flow

Tolerances ofbetween IT 10and IT 13



wear. Such considerations generally lead to in-creased production costs. In general, though, itis only certain workpiece dimensions that arecritical to function which are produced to suchtight tolerance values. The applicable dimen-sions must be agreed upon by the manufac-turer and the customer in each individual case.If particularly tight tolerances are not required,allowances of between 0.5 and 2.0 mm arecustomary on the surfaces in question. An add-itional advantage is that any skin decarbur-isation of the forged parts, which is caused byinduction heating of the bars in air, may beminimised. The demands placed on the work-pieces with respect to material and dimensionsare determined by the customer. Meeting thesedemands is guaranteed by means of a qualitymanagement system. The criteria which needto be monitored are established in close col-laboration with the customer in advance of the project. The scope of the quality inspec-tion may include not only geometrical values(Fig. 33), but also strength properties andcrack detection.

Cost considerationsDue to the high investment costs of horizontalmulti-stage presses and all the associated peripheral equipment, such facilities have high

Fig. 33: Inspection of geo-metrical values offorged parts using aprojection laser

Quality requirements


Combination with other production processes 59

hourly machine cost rates. Due to the fact thatthese machines are often only operated by oneor two employees, it is clear that the key toachieving cost-efficient operation does not liein reducing labour costs but in consistentlyutilising the machine operating time to thefull. Irrespective of the wage level, horizontalmulti-stage presses only operate efficiently incompanies where it is possible, from a tech-nical and organisational point of view, to maxi-mise the operating time of the machine. Greatefforts are thus made to minimise scheduledstandstills for tool changes, maintenance andrepair as well as unscheduled ones arisingfrom tool failure or organisational problems,for example.As it is very time-consuming to adapt hori-zontal multi-stage presses and the entireforging process to a different part, produc-tion is only feasible from an economic per-spective with batch sizes of 20 000 parts andabove. In the case of smaller batch sizes,production on manual or automated hammersor presses often represents the more cost-effective solution.

Combination with otherproduction processesIn general, hot-forged workpieces undergo sub-sequent operations to make them into assem-bly-ready components. In designing produc-tion processes and process chains, varioustypes of processes need to be weighed up, tak-ing technological and economic aspects intoaccount. The advances being made in indi-vidual processes such as machining or coldforging result in the need to periodically revise and, where necessary, adapt existingprocess chains. Examples of this include theproduction of outer races for constant velocity

Optimisation of machine running time

Cost-effectivefrom a batch sizeof 20 000 parts

Checking andadaptingprocesses



joints by means of hot forging and machining,or the subsequent direct heat treatment of cer-tain steels from hot-forging temperatures.

MachiningMachining enables very tight tolerances andoptimum surface qualities to be achieved (Fig.34). A distinction is made between machiningin the soft, unhardened state and machining inthe hardened or quenched and tempered state.The development of modern cutting materialshas led to the possibility of turning, millingand broaching workpieces with hardness values of up to 60 HRC and above with a cost-effective tool life.

In the case of milling and turning, HSC tech-nology is used. This is characterised by highcutting speeds and low cutting depth in eachcut. The advantage of hard machining lies inthe fact that it is a cost-effective way of com-pensating for the workpiece distortion thatarises during the hardening process.

Cold forgingHot-forged parts can be further processed bymeans of conventional cold forging. If highlevels of accuracy are demanded of particular

Fig. 34: Parts machined subsequent to hot forging

Hard or soft machining


Combination with other production processes 61

component surfaces, it is possible to cold-cali-brate the workpieces (Fig. 35). This involvesforging the workpieces which have been previ-ously coated with molykote, for example, withhigh press forces in a tool that is pre-stressedwith several shrinkage rings. In general, onlysmall levels of plastic strain occur, i.e. thechanges to the workpiece geometry are onlynegligible, while the accuracy and surfacequality are greatly enhanced. Bevel gears fordifferential gearboxes are typical parts whichcan be produced by hot forging with subse-quent cold calibrating. By using both forging

processes, the geometry of the tooth flanks canbe produced ready for assembly. This is due tothe fact that the levelling of roughness peakson the workpiece surface leads to a high per-centage of contact area.

JoiningOften, process limitations mean that parts can-not be produced from one workpiece. In suchcases, the parts can be produced by joining twoworkpieces together which may have been pro-duced by different processes. Common joiningoperations include welding processes such asfriction welding and flash-butt welding as well

Fig. 35: Parts which havebeen cold-forgedsubsequent to hot forging

Cold calibration



as laser welding or electron-beam welding.Very widespread is the joining of synchronis-ing gear rings with hot-forged speed gears forvehicle gearboxes. This is because such partshave a functional undercut that makes it ex-tremely difficult to produce them with assem-bly-ready dog teeth from one workpiece (Fig.36). Hot-forged parts are also frequently joinedwith tubular parts. Input shafts for commercialvehicles, for example, consist of hot-forgedflanged parts which are joined with tubes bymeans of friction welding. Likewise, in orderto achieve weight savings, a great number of camshafts are produced from forged and machined cams as well as a tube by means ofvarious joining processes (assembled camshaft).

Special processes

Double-part productionDouble-part production refers to the manufac-ture of two parts made of the same materialfrom one bar slug. Particularly in the case of

Fig. 36: Speed gear withjoined synchronisa-tion (front) consist-ing of a hot-forgedand machined gearwheel (back left),and a hot-forged andmachined synchro-nising gear (backright)


Special processes 63

annular or tubular parts, it is possible to pro-duce an inner and an outer ring from one slug,for example. The parts are separated fromeach other during the penultimate or the finalforging stage by means of piercing and by using a special stripping device. Double-partproduction is used in particular for producinginner and outer races for all types of bearings.Besides increasing productivity (two parts perstroke), this process also achieves better mater-ial utilisation. In addition, it is very cost-effective if the production volumes are thesame for both parts.

Rotation during transferBy means of a special rotating device on thegrippers, it is possible to turn the slug by 90°during the transfer. After the slug has been rotated in this way, its longitudinal axis is nolonger parallel to the press-ram movement butis in the direction of the cross transfer. Thisenables workpieces to be produced whichdemonstrate an undercut related to the longi-tudinal axis or which require a fibre flow thatis now parallel to the ram. Furthermore, therotating device allows workpiece geometriesto be produced which would otherwise only be possible with complex tool technologies orhigh material use.

Two parts fromone bar slug

90° rotation of the slug


64

Examples of productsNuts and camsFigure 37 shows a selection of nuts and camsproduced on horizontal multi-stage presses bymeans of hot forging. Besides standard nuts,special nuts such as locknuts, collar nuts andweld nuts are frequently produced, as well as

Fig. 37: Nuts and cams

nuts for particular applications such as hubaxle nuts and gearbox nuts. In order to pro-duce these parts, three-stage presses are oftenused. The production sequence consists of freeupsetting, main forging into the desired shapeand piercing. A similar production sequence isalso used to produce raw cam parts for assem-bled camshafts.

Universal jointsDue to the high volumes possible on hori-zontal multi-stage presses, tripods and spiders (Fig. 38) for universal joints as wellas for propeller and drive shafts are typicalparts produced on this type of forging ma-chine. These are mainly produced with flash.


Bearing rings 65

As the flash does not surround the entire cir-cumferential surface, but is only located be-tween the individual journals, the end facesof the journals can sometimes be producedready for installation. The removal of thissmall amount of flash as well as any pier-cing of the parts that may be necessary are carried out at the same time during the thirdforging stage of the process. Subsequent cold calibration enables tight geometrical,dimensional and positional tolerances to begenerated.

Bearing ringsFigure 39 shows bearing rings made from100Cr6. The inner and outer rings are pro-duced together in a double-part productionprocess. During the fourth forging stage, theyare separated and the smaller of the two ringsis pierced in the centre. Both can be producedwithout flash on their outer contour. This isbeneficial with respect to subsequent oper-ations. These parts frequently serve as preformsfor additional processes such as ring rolling,

Fig. 38: Tripods and spiders

Assembly-readyend faces

Without flash


66 Examples of products

during which they are then made into largerparts with greater diameters.

Raw gear parts and bevel gearsRaw parts for gears used in the engines of pas-senger and commercial vehicles and gearboxes(Fig. 40) are very often produced by means of

Fig. 39: Bearing rings madeof 100Cr6

Fig. 40: Hot-forged raw partsfor gears and bevelgears


Flanges 67

hot forging with subsequent soft machining.This is followed by the induction hardening offunctional surfaces (teeth). Depending on therequirements with respect to running noise andservice life, they subsequently undergo hardfinishing by means of shaving or lapping.Bevel gears are generally preforged by meansof hot forging so that the material distributionof the teeth is optimally prepared for the cold-forging process. The teeth can then be cold-forged until they are assembly-ready, with ac-curacies of between IT 6 and IT 8. Followingthis, the part is then finished mechanically inthe concave region and, where appropriate, inthe drilled hole.

FlangesAmong the parts belonging to the large familyof flanges are three-armed flanges, wheel hubsand flange shafts. Flange shafts are used for

Fig. 41: Various flanges:three-armed flanges,flanged shaft withand without grooveand wheel flange

the output of manual and automatic gearboxes,as well as for the input and output of differen-tial gearboxes. Besides highly accurate con-centric running properties, adequate balancingof the shafts is also important in order to pre-vent vibrations in the powertrain. During the

Subsequenthard finishing


68 Examples of products

hot-forging process, a surrounding groove canbe introduced in order to accommodate a seal(Fig. 41).

Outer races for constant velocity jointsOuter races for constant velocity joints, asshown in Figure 42, serve to transmit torquefrom the input shaft to the wheels. Balls roll inthe ball tracks to ensure that torque is trans-mitted evenly at variable angles between theinput and output shafts. The outer races forconstant velocity joints are hot-forged during a

Fig. 42: Hot-forged raw partfor an outer race of aconstant velocityjoint (left), and anouter race of a con-stant velocity jointwith a soft-machinedinner contour (right)

four-stage forging process. Following coolingand heat treatment, the parts are soft-machined. The races then undergo inductionsurface-hardening treatment. The distortionwhich arises in the races during this process ismachined away by hard-finishing operations.Another solution is to pre-correct the distor-tions during soft machining, thereby omittingthe hard-finishing operation subsequent tohardening.

Induction surface-hardening treatment


69

OutlookContinuous developments in the hot forging ofmetallic workpieces – a process which has itsroots in the age-old tradition of hammer for-ging – will secure the future of products whichare competitive both from a technological andeconomic point of view. It is not only new,even more efficient forging machines that willplay a significant role in this, but also the con-tinuing process optimisations and ambitiousproduct innovations of hot-forging companies.Great development potential lies in the on-going optimised planning and implementationof process combinations. This involves exploit-ing and combining the particular process-specific advantages of hot forging, warm for-ging and cold forging as well as the benefits of machining operations in order to achieve an optimum production process for the de-sired part.New, more cost-efficient materials which nevertheless fulfil the demands placed on theparts are currently being developed. Materialswhich involve lower process costs due to theomission of heat-treatment processes and add-itional operational steps are also the focus ofcurrent research and development efforts. Fur-thermore, in view of the trend towards light-weight construction in vehicles and the asso-ciated increase in production volumes, the for-ging of certain aluminium alloys on horizontalmulti-stage presses can be reckoned with inthe future.Enhanced machine and tool technologies to-gether with the use of advanced planning toolsshould enable ever more sophisticated geom-etries to be produced and significant reductionsin part tolerances, and thus in machining al-lowances, to be achieved. To reach this object-

Process combinations

New materials

Enhanced machine technology


70 Outlook

ive, a close working relationship is necessarybetween the forging company and the cus-tomer in the sense of a development partner-ship. Only then it is possible to meet customerrequirements while at the same time allowingfor an optimised cost structure.The high strengths and safety reserves that areassociated with the optimum grain flow of hot-forged parts enable these to be transformedinto precision lightweight components. Thushot-forged precision parts produced on hori-zontal multi-stage presses will continue tomake an important contribution to weight optimisation in future, too, and will therebyplay a key role in reducing automotive fuelconsumption.

Trend towardsprecision lightweight components


The company behind this book

SEISSENSCHMIDT AGPrecision ComponentsDaimlerstraße 11D-58840 Plettenberg, GermanyPhone: + 49 2391 915 0Fax: + 49 2391 915 196E-mail: [email protected]: www.seissenschmidt.de

SEISSENSCHMIDT AG, headquartered in Plettenberg, Germany, wasfounded in 1846 and has established itself as an outsourcing partner tothe international automotive industry.Previously a producer of parts for the railway industry, for heating sys-tems and for cement construction, SEISSENSCHMIDT has been activeas a supplier in the automotive sector for four decades now. The com-pany’s success story as an automotive supplier began in 1965 with itsinvestment in the first fully automatic horizontal multi-stage hot-forgingpress.For large-series production, SEISSENSCHMIDT employs a broadrange of fully automatic horizontal multi-stage presses that is un-matched worldwide and with which it achieves an output of up to 9000forged parts per hour. The forging enterprise accompanies its customersfrom the design stage right through to the production of the assembly-ready part. State-of-the-art technology is used during each developmentphase.The range of services provided by SEISSENSCHMIDT also encom-passes conventional hot forging and heat treatments. In addition, coldforging is offered for producing parts with extremely low allowances andtolerances from hot-forged raw parts. SEISSENSCHMIDT ComponentsProcessing GmbH + Co. KG, a subsidiary of the SEISSENSCHMIDTAG, provides comprehensive capacities for carrying out various machin-ing processes. The entire service spectrum is rounded off with a highlevel of competence in the area of logistics.The subsidiaries, SEISSENSCHMIDT Corporation in the US and SEISSENSCHMIDT Precision Components Kft. in Hungary, as well as numerous representations abroad, ensure that the company has an excellent international presence.SEISSENSCHMIDT delivers precision components for drive and chas-sis applications to almost all the renowned automotive manufacturersand leading system suppliers.The production programme includes precision components for gear-boxes, engines, differentials, suspensions, drive shafts and other appli-cation areas within the drive system and the chassis.



Documents

Hot Forging on Horizontal Multi- Stage · PDF filemeans of hot forging on horizontal multi-stage presses. ... of hot forging and subsequent cold cali-bration. Drive shafts require