Forging operations

Basic concepts :

Forging, one of the oldest known metal-forming operations, relies entirely on the application of compressive forces to
effect the change of shape. In its simplest form, forging is used to preform a billet by changing its dimensions (preparatory to
further shaping), and in its more sophisticated form it produces complex shapes to a very high degree of accuracy.
Depending on whether the operation is carried out hot, warm or cold, forging affects the structure and properties of the
forged component to varying degrees. Whilst being essentially simple in concept, forging processes, in their many varied forms, are in fact extremely complex. By far the most common group of processes is concerned with the forging of ferrous alloys and a breakdown of the costs involved in producing the average ferrous forging illustrates the reason for the introduction of new methods, techniques and ideas. The cost can be apportioned as follows: Material in final forging 35% Material wasted in forging 15% Labour 10% Overheads 30% Tools 10% It is clear from these figures that material usage is an area in which savings should be introduced and, consequently, innovative preforming processes of powder forging, transverse rolling and cast preform forging, aimed specifically at reducing material usage, must be considered in addition to the standard techniques normally employed. In this context, the idea of forging is of particular interest. In the hot forging process, the work piece preheat temperature is usually chosen to be as high as possible, consistent with the production of a sound forging, thus exploiting the benefit
of minimum flow stress. Any accompanying consideration of economic and technical feasibility must make reference to the
work piece preheating costs, corresponding handling costs and the effects of surface oxidation and decarburization. Cold
forging of steel, on the other hand, is conducted at room temperature with a consequent saving in heating costs and
material wastage, but against this must be set the higher tooling costs. The fact that some work hardening usually takes
place is only infrequently exploited. It is not surprising, therefore, that the possibility of forging at intermediate warm temperatures is advocated in order to obtain the benefits of both hot and cold forging. In this connection, two approaches are adopted.
The first seeks to improve the accuracy of the hot forging process by selecting preheat temperatures high in the spectrum
between hot and cold forging. If, for a typical mild (low carbon) steel a forging temperature of, say, 800°C is chosen,
then oxidation will be reduced, thus permitting a lower machining allowance and benefit will be gained from the low
flow stress. Tolerances of k0.25 mm are claimed for warm forging in this temperature region. In addition, the small
amount of strain-hardening which occurs may be beneficial in some cases. It is important, however, to realize that the
heating and attendant costs are still present. As the temperature is reduced further, the flow stress begins to increase
rapidly and the process becomes less attractive.

The second approach is from the cold-working end of the spectrum. Due to strain-hardening and strain-ageing effects,
lower grade steels may be taken to higher final strengths. This is also in competition with heat treatment over which it has the
advantage of being more controllable. A further refinement is to start the process with a quenched and tempered structure on top of which warm forging will produce even more strength and ductility.
1. open die,
2. impression die, and
3. closed die.

In addition, the already mentioned modern developments have given rise to a whole series of processes of hot and cold
rotary forging (including tube making), orbital forging, highenergy- rate (dynamic) forging of both shapes and particulate
matter, and cast-preform forging. The objective of the ‘standard’ operations is to produce, in stages, machine parts and components such as gears, wheels, compressor and turbine disks, crankshafts and connecting rods, small tooling, screws and bolt heads, and coins and medals, and to assist in the conduct of other operations by providing simple preforms. Punch piercing of billets is a typical example of the latter application of forging. In its classical form, forging is of three main types:
16.2.5.2 An outline of open-, impression- and closed-die operations Open-die forging, also known as ‘upsetting’, is concerned
mainly with reducing the height of a cylindrical billet. This is generally done between two flat dies, although the dies can be
profiled in a simple manner to impart a specific shape to the ends of the upset specimen. The outcome of an upsetting operation, in terms of the shape of the preform, depends on the frictional effects that develop between the dies and the faces of the billet. With
efficient lubrication, the reduction in height produced by the application of compressive forces is accompanied by an
increase in the diameter of the billet, but with the billet retaining its original sharp edges. In an unlubricated or poorly
lubricated operation, the deformation becomes inhomogenous and barrelling occurs. The amount of barrelling depends on
the value of the width/height (dlh) ratio and the reduction in height r (Figure 16.32). Although open-die forging offers a simple and relatively inexpensive means of producing small components, it calls for a high degree of manipulative skill on the part of the operator, since acquisition of the basic shape can only be achieved by turning the work piece to different positions between successive blows. The deforming forces are applied either mechanically by means of powered hammers or manually. In either
case, the rate of yield of the component is low and the process is unsuitable for mass production. The additional difficulty,
which has an additional cost associated with it, is the requirement to machine the specimen in order to obtain both the
desired final shape and the required dimensional tolerances, neither of which is likely to result from the forging operation
on its own. The machining stage naturally introduces an element of wastage of material and an additional labour
requirement.

To introduce a high degree of dimensional accuracy and better material usage, impression and closed-die forging operations
are used. These operations are shown in their most basic form in Figure 16.33. The fundamental requirement is
that the die cavity, whether that of an impression die (Figure 16.33(a)) or a closed die (Figure 16.33(b)), is completely filled
with the forged material and, most importantly, that the material remains structurally sound. The more complex the
shape to be forged, the more involved the production line becomes since, clearly, it is impossible to convert a starting
cylindrical or square billet to the desired shape in a single blow. Not only are preforming operations required, but
several actual forging stages may also be needed.

The basic characteristics of impression dies is the fact that,when activated by a mechanical hammer or hydraulic press,
they do not close completely, and thus some metal is allowed to escape and forms a flash (Figure 16.33(a)) between the flat
surfaces. The presence and magnitude of the flash are of considerable importance in impression-die forging because
they influence the mechanics of the operation. By the nature of its geometry, the flash is subjected to high pressures and,
consequently, it experiences a high degree of frictional resistance as it propagates radially outwards. This constrains the
tendency of the bulk of the material to flow between the dies and, therefore, creates better conditions for the filling of the
die cavity. This action is further assisted by rapid cooling of  the flash (as compared with the cooling of the material in the
die), which results in a further increase in the resistance to flow. These characteristics form the basis of die design.
In the normal sequence of operations, preforming will be necessary and may consist of the following operations:
1. simple upsetting,
2. blocking, and
3. rolling.
Upsetting and blocking in blocker dies are the more usual routines, although rolling (see previous sections) of profiled
shapes is also used.

A blocker die belongs to the impression-die group, but provides only a general outline of the work piece. The final
stage is not achieved using a blocker die, but the material is distributed in a way that ensures a more acceptable uniformity.
On trimming the flash, the work piece can undergo further processing in a finishing die that will impart to it its final
shape.

A set of impression dies will produce a parting line in the specimen and, unless the die design takes into account the
most likely response of the material, in terms of its pattern of flow and formation of fibres in its structure, problems may
arise because, on trimming, the fibres are severed. To facilitate the removal of the forging on completion of the
operation, dies are provided with draft angles (a situation similar to that in casting). Reduction in height, piercing and bulging are the basic operations associated with impression-die forging. However, their sequence and intensity depend on the required shape of the forging. Figure 16.34 illustrates a selection of the possibilities arising in these operations. If the required forging is of
simple shape, the arrangement shown in Figure 16.34(a) may

be sufficient. If one- or two-sided indentations are needed (Figure 16.34(b) and (c)), the final shape is obtained by height
reduction and piercing in the die. Formation of more complex shapes with side bulges, etc., depends on the dimensions of
the starting material. If the diameter of the billet is less than that of the elongated part of the axisymmetric forging, then
the cavity is filled simply by reducing the billet height (Figure 16.34(d)). On the other hand, when the starting diameter is
larger than that of the forging, height reduction and flow into the die cavities are required (Figure 16.34(e)). A more
complex operation is required (Figure 16.34(f)) when the forging has both indentations and side bulges, and the starting
billet diameter is larger than that of the side of the forging. Height reduction, piercing and bulging must occur to give the
final impression.

In true closed-die forging, the operation produces a completely flashless forging (Figure 16.33(b)). To achieve this
highly desirable situation, the diebillet relationship must be carefully worked out, since incorrect dimensions of the billet
lead to either incomplete filling of the die cavity or damage to the dies and punches. If the forging system is correctly
designed and the die (or dies) and the punch are machined to the required degree of accuracy, a precision, or near-netshaped
forging will be obtained.

 Special applications of closed-die forging

A number of special operations associated with closed-die forging are carried out cold, i.e. at a temperature below the
recrystallization temperature of the metal in question.However, some difficulty is experienced in the case of steels,
since these alloys recrystallize at temperatures above 60&700°C, temperatures that are too high to be called ‘cold’.
Consequently, forging at room temperature is often referred to as ‘cold’, and forging at elevated, but below recrystallization,
temperatures is called ‘warm’. Generally, the purpose of cold forging is to produce a finished part with high dimensional
accuracy.

Highly ductile materials, such as aluminium, lead and tin, have been cold forged for a long time, mainly in the form of
extrudates, but ‘proper’ cold forging is normally limited to either small parts in low and medium carbon steels (such as
bolts and nuts) or larger parts of up to 10 kg in mass which require good dimensional tolerances.
Of the better known applications in this area are the coining and embossing operations (Figure 16.35) used for the production
of coins and medals, and for the improvement in dimensional accuracy of other preforms. The coining operation is
one that is actually carried out in a closed-die system, sometimes with embossing forming, a stage that includes an opendie
system. In these processes, three-dimensional details are reproduced in the material giving not only a faithful impression
of the punch and/or die surface, but also good surface finish. It is not surprising that the forging loads used are high.
The design of the dies is critical since lubrication cannot be used in coining operations because of the danger of entrapment
and the consequent damage of the forging.A completely different type of application of closed-die hot forging is that of powder forming, also known as 'sinter forging'. This process, an offshoot of conventional powder metallurgy processes, involves the following sequence of operations.

Cold powder is compacted in a press to produce apowder preform. The preform is subsequently sintered in a controlled-atmosphere furnace but, instead of being allowed to cool in that atmosphere (as in conventional powder metallurgy processes), the preform is removed from the furnace whilst still hot and forged in closed dies to produce the final shape. Sin'ce only the exact amount of powder required to make the final shape is actually used, there is an obvious elimination of waste. The same final shape couid also be made in the first compaction operation, but the introduction of the forging stage produces a superior product. Improvements in strength, ductility anid density are expected, with accuracy increasing to some 0.025 mm. Against these advantages must be balanced the drawbacks of additional processing and the possibility of deleterious, oxidation occurring immediately before and after the forging, operation.
Commercial applications of powder forging range from forged-powder connecting rods, through materia! properties
approaching those of commercial forged steels. to valve spring cage components. In the latter; for instance, the density (as
compared with normally sintered material) increases by some 30%, the elongation quadruples, and the ultimate tensile
strength almost trebles.

More recentiy. the process has been extended to the cold forging of polymeric powders which, although they show some
post-forging elastic recovery, can be quite successfully compacted in this way.

Subsidiary forging operations

These operations are often associated with preforming and comprise:
1. cogging,
2. fulleriing,
3. heading, and
4. hubbing.
Cogging or drawing (Figure 16.36) of a rectangular section specimen involves reduction in height by successive blows or
bites, leading to a gradual elongation of the original blank. The successive bites reduce the force requirement, but they
must be grouped closely together to produce an even surface. The operation will not cause plastic deformation of the bulk
material to' occur, unless bites of sufficient length are initiated. To avoid possible buckling of the specimen, the height/width
(hlw) ratio should be kept below 2.5.

Fullering (Figure 16.37) is another preform operation in which the 'original bar is shaped by profiled open dies into an
outline suitable for more detailed, further forging. Thus the basic function of fullering is to redistribute the bar material
along its length, prior to cropping of the individual preforms. Heading, is an operation that combines forging and extrusion.
In the forging part an upsetting operation carried out between flat dies will flatten (by bulging) the end of a
cylindrical billet, thus forming a bolt head. The remaining portion of the billed can then be extruded to form the bolt.
Hubbing is used as a means of forming die cavities by indenting the material of the blank with suitably profiled
punch heads. The operation is thus very similar to in-die punch piercing (discussed above). It generally calls for punch

Rotary forging
Rotary forging, also known as 'transverse rolling', is used as a preform operation in which profiied tools are positioned on
driven rolls (see Figure 16.38).

The bar stock is located transversely between three rolls. The rolls make one revolution and the bar makes several. The
wedge impressions on the roll surface (Figure 16.39) are then progressively imparted to the work piece to give the required
preform shape. Subsequent flash discards of 15-20% are usually expected.

 Rotary tube forging
Of very considerable industrial importance is the use of the rotary-forging concept in hot and cold seamless tube manufacture.
The operation is carried out on a pilger mill. In hot pilgering, the already rotary-pierced bloom is threaded onto a cylindrical floating mandrel and is steadily advanced forward and retracted in a reciprocating motion in which the forward stroke always exceeds the length of the backward one (hence the name of the operation, which is derived from the movement of the chorus of pilgrims in an opera) and is executed by two profiled, driven rolls. Each successive movement is accompanied by a 90" turn of the
tube-mandrel assembly.

Figure 16.40 shows a sequence of successive 'bites' culminating in a finished tube. Irrespective of whether the working
roll surfaces are parabolic, hyperbolic or form logarithmic

each roll (being basically a cam) is characterized by three distinct zones. The working zone AB consists of the reducing
portion that produces the highest degree of wall reduction. It is this deformation which, in turn, converts a bloom to a tube
and is accomplished by a change in roll radius. The finishing zone BC imparts the required dimensions to the tube; the roll
radius remaining constant along this part of the profile. The third zone is the idle intercept CA during which reciprocating
and rotational motions of the tube-mandrel assembly are performed. To obtain the final geometry of the tube up to six
passes may be required, with each successive bite reducing the wall thickness of the bloom further. To obtain even thinner walled seamless tubing displaying high dimensional accuracy and good mechanical properties, cold pilgering (also known as 'cold reducing') is employed.

The cold-reducing process is a step-by-step operation which uses tapered grooved rolls (Figure 16.41). Several dissimilarities
exist between hot and cold pilgering. The most obvious of which is that, whilst in the hot process the rolls revolve
completely and continuously, in cold reducing they rotate backwards and forwards through approximately 180". In the
hot process the rolls are mounted on a fixed stand, and the work piece reciprocates, whilst in cold reducing the roll stand
reciprocates and the work piece remains stationary. The machine operates as follows. Each time the saddle is at the end of the stroke, so that the large ends of the die grooves are presented to the in-going hollow, the cross-head pushes the hollow forward by a fixed amount (or the feed). The dies then roll the material thus inserted down the cone, simulta

neously thinning the wall and reducing the diameter, with the consequent elongation of the tube. At the far end of the saddle
stroke, the tube and mandrel are rotated through approximately 60°, and the saddle returns. This is repeated at
frequencies of 1-2 Hz, depending on the size and type of machine. At each end of the saddle stroke, because of the clearance in
the dies ;and also because the rolls turn through slightly more than BO", there is a short period of time when the dies are not
in contact with the tube being rolled. It is during this time that feed has to occur at one end of the stroke and turning of the
tube at the other.
In general, this cold forging process is associated with deformation of tubing in the 1G230 mm wall thickness range,
and is employed whenever the specified wall-thickness tolerances of the finished product are required to be below f lo%,and the quality of the tubing, with regard to surface or structure, calls for cold deformation. Tube obtained in this manner can either be used directly, in industrial applications, or can form an intermediate stage of production requiring a cold-drawing operation.

Orbital forging

The orbital forging process is intended to allow smaller machines to be used for suitable forging operations. This
process is based on the simple concept that the axial force required to effect a desired deforming zone between the
platens is confined to a small region. The plastic zone is then moved through the work piece, thus resulting in progressive
deformation.

The plastically deforming region is formed by initially indenting with a conical platen into the work piece which is
supported by a lower platen capable of axial movement only. The plastic region is then moved through the material by the
upper conical platen which is capable of rotation about the central axis of the machine and, also, about its own geometric
axis. In order to produce a flat top surface on the work piece, the geometric axis of the upper conical platen is inclined to the
central axis of the machine.

The process, known also as 'Rotaform', can be applied successfully to the hot, warm and cold forging of a wide range
of materials and axisymmetric shapes. The Rotaform is a completely automatic machine provided with automatic feeder
and ejection equipment. Figure 16.42 illustrates the principle involved.

The die uses a pair of cooperating dies, one of which is adapted to perform a wobbling motion, relative to the other,
about a centre at or near the axial centre-line of the dies. The wobbling die, which has a circular rocking motion without
actual rotation about its axis. is actuated at high frequency. The complementary half-die is secured to a hydraulic ram
which conveys the work piece to, and presses against, the wobbling die. The cycle of operations commences with the
introduction of a billet into the receiving station of the automatic feeder which places the billet accurately into the
non-wobbling die. The hydraulic ram raises the non-wobbling die at a speed programmed for the most suitable deformation
rate throughout the forging stroke. When the forging has assumed its final shape, the ram retracts rapidly and the
forging is automatically ejected and blown clear of the dies.

The cycle time is very short and outputs of 15 forgings per minute have been achieved. 16.2.5.8 Cast-preform forging The material of all metal components usually originates from a cast and subsequently undergoes a considerable degree of processing before being finally transformed by, say, forging into a component. Since all tile intermediate mechanical working improves the properties of the structure. this improvement, to a level necessary for most components, can be achieved by cast-preform forging. From a technological point of view, the structure of a reasonably homogeneous casting can be transformed to a
forged structure by quite small amounts of hot working, amounts which could quite easily be achieved in the forging
process itself. Internal cavities can be closed up and welded, providing they are located in regions of high deformation. Due
to oxidation, however, cavities extending to the surface are likely to remain as faults. The economics of such a process
require some consideration, but it becomes obvious that the operation is viable when the casting can be produced without a
lot of waste material in the form of runners and risers. In addition, shapes normally forged from a cropped billet in
single impression dies, where material usage is high, are not amenable to this process. Major economic advantages can be gained if the casting and forging operations are combined. Several techniques are available for this; for example, a conventional steel-casting plant is situated alongside the forging press and reheating is done between the two operations. This technique has been developed as the ‘Auto-forge’ process which consists of a rotary table with a number of stations at which the various operations
are performed. Casting is carried out at the first station and the moulds are water cooled while the table rotates to the next
station where the forging operation is performed. The work piece then rotates to the clipping station. Another type of
process, the ‘Auto Forcast’, consists of a melting plant coupled with a continuous casting plant. The various sections of cast
steel produced are straightened and hot cropped into billets before being fed into the forging press.

Isothermal forging
A more expensive and complicated process is that of isothermal forging in which the dies are heated to the temperature
of the blank prior to the operation itself. The advantages of this technique include low stress fields and, consequently, improved flow of the material which, combined with the absence of a temperature gradient at the tool-work piece interface, reduces the inherent difference in the bulk forging material.