Metal forming

Metal forming, i.e. changing the shape of the material without actually removing any part of it. was practised at least 3000
years ago in Egypt, where hammer forging to produce gold sheet, cut subsequently to make wire, is recorded in the Bible
to have taken place. Rolling in wooden mills was employed to manufacture papyrus. Manual swaging and wire drawing were
well established in the Middle Ages but. naturally, were limited in scope by the power available. It was only with the
advent of the Industrial Revolution that progress was made and processes like extrusion and cross and longitudinal rolling
became available. But even here, the restrictions imposed by the low quality of tool materials, lubrication problems and the
lack of understanding of the basic precepts of plasticity impeded progress until, in some cases, well into the twentieth
century.

The ever-increasing demand for high quality productsoften of sophisticated shape in difficult to process materials--
economically produced, fabricated or semi-fabricated, combined with the rising cost of metallic engineering alloys has
focused attention on metal-forming processes and techniques. The emphasis here lies on the ‘chipless’ approach to shaping.
This provides an economical. direct means of converting a cast ingot to slab, plate, billet or bloom and then-in another
chipless operation-of changing these basic shapes into profiled finished or semi-finished products. The avoidance of the
removal of the material during a forming operation enhances the economics of the process by reducing wastage associated
with the swarf-producing machining. Whereas the latter has, of course, a very considerable and necessary role to play in the
range of manufacturing activities, its indiscriminate use (a feature of the early years of plentiful supply of cheap labour
and materials) is no longer acceptable when high tonnage of accurately manufactured product can be obtained at a much
lower cost.

In the most simplistic terms, the desired change in shape is effected either in the cold, warm or hot state (the latter below
the melting point of the material) by the application of external forces, pressures or torques of sufficient magnitude to
induce plastic flow. and thus a permanent set, of the material through the forming pass. Depending on the operation, the
material is forced to flow between driven rolls, through (or into) open or closed dies, or between sets of dies and rolls.
Solid or hollow sections are thus produced from the initially solid blocks of metal.

The standard basic operations are:
1. rolling (flat, oblique or longitudinal),
2. extrusion (axisymmetrical or asymmetrical).
3. drawing (solid or hollow components),
4. sheet forming (deep drawing, bending, pressing or bulging),
5. forging (solid and hollow sections). and
6. cropping (shearing and piercing).

Within the compass of any of these operations, a number of variants exists which reflects not only a variety of manufacturing
routes and subroutes, but also the nature. properties and characteristic responses of the processed materials. Modern metal-forming technology makes use of solid and semi-solid (‘mashy’ state), and superplastic, as well as explosively prewelded
metallic composites and dynamically compacted particulate matter. Mixtures of metallic and/or ceramic and polymeric
materials are formed to manufacture composites of very specific properties. The problem of forming these into desirable
shapes presents the engineer with new and often difficult situations to solve. Selection of the appropriate forming
process, the tool design, the effects of the pass geometry on the final physical and mechanical properties of the product,
the dimensional accuracy, and the achievement of the as near as possible final shape in the minimum of operational stages
have to be faced.

The apparently simple sequence of ingot-slab-semifabricate- finished product becomes complex unless there is good
understanding of the basic characteristics of the individual processes and an appreciation of the principles of the theory of
plasticity, as well as that of the concepts of tool and process design. The bases for and fundamentals of the major processes
and technological developments are discussed in the following sections, but detailed treatment of the individual topics is only
indicated by reference to the appropriate literature.

 Classification of processes

For a given application, the selection of the correct process necessitates the introduction of a criterion of process classification.
Since hot working homogenizes and refines the crystallographic structure of the material and thus, ultimately,
improves its strength and toughness, whereas cold working increases strength, hardness, dimensional tolerances and improves
surface finish, these temperature-induced effects are often used to differentiate between the various manufacturing
methods.

Important as the processing temperature is, in some circumstances other criteria of classifying metal-forming processes
may well be more appropriate. From a purely manufacturing point of view, quantity and shape may have to be
considered, while the likely response of the processed material to the level and/or rate of stressing, as well as the manner of
application of the forming load system, may offer a better clue to the desirability or otherwise of using a particular technique
or operation.

The parameters that characterize forming operations give rise to the following possible classification systems:

1. operational temperature (hot, warm or cold forming),
2. shape effect (bulk or sheet forming),
3. operational stress system,
4. operational strain rate,
5. starting material (ingot. slab, billet, bloom, slurry, or
powder).

 Operatiotial-temperature criterion

The idea behind the subdivision into hot, warm and cold processing of materials is not only to indicate the nature of the
operation, but also to draw attention to the plant and ancillary equipment needed, to the level of force parameters required,
and to the likely metallurgical response of the processed material. An outline of this classification scheme, including only the
basic operations, is given in Table 16.1.

Starting with a cast ingot, the primary hot operations of flat, billet and slab rolling, and slab forging will produce the
starting stage for the secondary, further processing of the slab into plate, billet or a large forging. These, in turn, will form
the first step in the manufacturing route of a more sophisticated, profiled product. Hot operations are carried out at
elevated temperatures exceeding annealing and normalizing ranges and, consequently, yield a hot-finished product showing
a relatively Bow level of flow stress. However, the force parameters required match the mechanicai properties of the
material and are also relatively low. It follows that the rate of wear of the tooling can be kept at an economical level
especially if the lubrication problems are well under control. To improve the mechanical properties of the product, while
at the same time keeping the loading at a moderate level, warm processing is used. Here, the temperatures are well
above ambient but, equally, well below the hot-processing range, ansd usually slightly less than for recrystallization. The
increased material ductility is sufficient to reduce the power requirement of the plant. Cold-working conditions are confined
to ambient temperature and are characterized by a high energy requirement-necessitated by large operational forces
and/or torques-but result in very high quality final product displaying both good dimensional tolerances and mechanical
properties.

A rouglh guide to the temperature ranges can be obtained by considering the operational ternpera.ture/melting point ratio.
On this scale, hot working takes place when the ratio is >0.6, warm working when the ratio is 0.34.5 (the latter corresponds
to recrystallization conditions], and cold when the ratio is  <0.3.

 Shape-effect criterion

The effect of shape reflects the geometry of both the initial and final component and, consequently, the nature of the
change imposed on it by the forming operation. A process in which a component of a relatively small initial
surface area/thickness ratio is deformed in such a way that the ratio is increased, is often classed as a ‘bulk deformation
operation’. On the other hand, the component of an initially high surface aredthickness ratio, shaped in a process which
does not impose any change in the thickness but effects shape changes only, is said to be ‘sheet formed’. Any change in the thickness of such a component can easily lead to tensile plastic instability and incipient. localized yielding. Bulk processes are those of rolling, extrusion, forging and solid- and/or hollow-section drawing. Bending, pressing, deep drawing, spinning and shearing are the main sheet-forming operations.

Operational-stress system
Because of the inherent severity of many forming processes, particularly the rotary ones, a consideration of the type and
property of the induced stress field is of primary importance. The success of the operation may well depend on its compatibility
with the properties of the processed material. The presence of tensile and compressive stress fields results
in the appearance of shearing stresses which, in turn, lead to the sliding of molecular planes and, eventually, to the yielding
and plastic flow of the metal. Stress systems containing these components are most likely to give rise to plastic flow which, if
it is controlled, will produce the desired amount of deformation.Purely compressive or tensile systems create conditions of
hydrostatic pressure in a triaxial field (absence QE shear), or produce shearing stresses in uni- and bi-axial conditions.
Clearly, since it is the configuration of the individual stress system that is indicative of the type of deformation which can
be expected, its assessment prior to choosing a forming system is imperative. These various possibilities are illustrated,
diagrammatically in Figure 16.14.

As an indication of the incidence of any of the stress systems, the following, non-exhaustive, list can be considered:
Tensile-compressive systems

Biaxial tensioduniaxial compression:
1. under a roll of a two-roll piercer,
2. under a roll of a two- and three-roll piercer, and
3. under a roll in the helical rolling process.

Uniaxial tensionhniaxial compression:

1. between the rolls in roll forming, and

2. in the flange in deep drawing.

Uniaxial tensionlbiaxial compression:
1. in the drawing die.

Compressive stress systems
Triaxial stresses:

• In the oblique zone of a three-roll rotary plug piercing
  mill,
• in the closed forging die,
• near the die throat in extrusion of bar, and
• under the punch in tube extrusion.

Biaxial stress:

• between the rolls of a longitudinal rolling mill with no
  front and/or back tension, and
• in the upsetting, open dies.

Tensile stress system
Biaxial stress:

1. stretch forming, and
2. bulging.

Operational strain rate
A number of engineering alloys and even some practically 'pure' materials, e.g. commercially pure aluminium, are susceptible
to the changes in the rate of straining. Modern technological techniques have either 'speeded up' conventional
processes-for instance, wire can be drawn at some 120 m min-l ---or have introduced new ones that operate in
truly dynamic conditions. Impact extrusion, explosive forming, welding and compaction, and mechanically and electrically
induced discharges of energy producing high strain rates, have all combined to introduce an entirely new field of
high-energy rate fabrication, known commonly as HERF. The range of possibilities arising in this context are listed in
Table 16.2 which provides a detailed insight into the effect of different strain rates and the means of producing them in an
industrial environment.

Starting material
Since some modern processes do not require bulk solids as starting materials, but utilize particular matter and semi-solid
substances, a classification based on the initial physical state of the material offers an interesting alternative to the more
conventional approach. Typical examples of unconventional starting materials are: 'mashy' state processing, leading to conventional rolling of compositse sandwich components; the Conform-type extrusion, starting with a powder or gr,anulated material, or an exp!osive compaction of powders.

Characteristics of the basic groups of processes Of the major processes listed in Section 16.2.1, forging is the
most diverse and cannot therefore be described in more general terms. For this reason, the basic characteristics of only
four groups of processes are indicated here and those of forging, sheet forming, cropping, etc., are discussed later.
All rolling processes rely on the forces transmitted through the roils to the material to effect deformation and on the
rigidity of the roll system for the dimensional accuracy of the product. Sheet and plate are initially obtained from a slab by rolling
the slab in a relatively simple system (Figure 16.15). Driven rol!s introduce the material into the roll gap, or working zone
of the pass, and reduce the thickness. The success of any further processing to obtain strip rather than a sheet or large
area of pllate, depends on the ability of the system to maintain a constarit width of the processed metal and on reduction of
the thickness (this being equivalent to the reduction in the cross-sectional area). These requirements call for a plane
strain operation which is possible only if the lubrication of the pass is very efficient. Processing in this mode can proceed in
either cold or hot conditions.

A much more complex rolling system is that of longitudinal rolling, which is employed in the production of axisymmetrical
billets, bars and hollows (Figure 16.16). A train of suitably shaped rolls. mounted on stands (either in pairs or in three-roll
configura.tions) inclined at right angles (between the successive stands) is used, as shown diagrammatically in Figure
15.16(a). A gradual reduction in the cross-sectional area of the material takes place (Figure 16.16(b)) as the specimen moves
axially forward through the sets of driven rolls. While fully engaged in the train, the processed material experiences,
additionadly, axiai tensions resulting from a differential distribution of‘ successive stand velocities. The ovality of the early
passes is slowly reduced along the train until the last stand is reached. Here, the final, circular cross-section is expected to
be achieved.

An alternative to longitudinal rolling is offered by the oblique-rolling system in which a single set of two or three
driven rolls produces tractive, frictional forces which propel the specimen axially while, at the same time, causing it to

rotate. The motion of an element of the worked material is thus forward, but helical. Figure 16.17 illustrates, using an example of tube rolling, the basic principle involved. In this case, three profiled, driven rolls, disposed at 120” to each other, and inclined at an angle a (the feed angle) to the horizontal ail1 axis, and an angle p (the cone angle) in the vertical piane, introduce the bloom (supported internally in the bore by a mandrel) into the forming pass. The bloom is ‘sunk’ onto the mandrel in the zone AB
and has its wall thickness reduced on the roll ‘hump’ BC. Slight elastic recovery takes place along DE. The bloom is
thus elongated and its wall is thinned. The amount of deformation imposed depends on the size of the inter-roll opening or
the ‘gorge’.These basic characteristics of oblique rolling operations (the variants of which are discussed later) are common to all
operations, as indicated, for instance, in Figure 16.18. This shows, diagrammatically, the operation of the so-called
‘secondary piercing’, or ‘oblique plug rolling’ of a tube-a process in which a long cylindrical mandrel is replaced by a short profiled plug.
On the other hand, processes of profiling by rolling can take various forms, one of which is indicated in Figure 16.19 where
a stepped shaft, required to acquire a series of specific profiles, can be manufactured by oblique rolling in a single
three-roll stand. An operation in which the billet is rotated and fed through a system of driven rolls produces this effect.
In another variant of oblique rolling, a two-roll system of helically ribbed rolls (Figure 16.20) will produce metal balls
out of a solid cylindrical billet.

These few examples illustrate the versatility of rolling operations, a more detailed discussion of which is given in
Section 16.2.4.

When the initial shape of the work piece has been imposed on it by one of the processes described above, there often
arises the problem of how to achieve a degree of further deformation leading, possibly, to the final product. Drawing
processes answer this need by providing a means of producing either solid (bar, rod or wire) or holiow tubular sections,
either circular or non-circular in shape. The drawing operation is carried out in a die-or a set of consecutive dies forming a
tandem drawing system-into which the work piece, with a swaged leading end, is introduced (Figure 16.21). An axial
force is applied through a gripping device (as indicated by the arrow in Figure 16.21) and the work piece is pulled through
the die. In the case of a solid specimen, the outer dimension only is reduced, whereas with a hollow section there is also a
change in the wall thickness. Lubrication of the working zone of the pass (the part of the die surface along which the
deformation is effected) is of importance from the point of view of the magnitude of both the drawing load and the
induced drawing stress, and in view of the surface finish. Similar results can be obtained in extrusion, a process in
which the starting billet (sometimes referred to as the ‘slug’) is inserted into a cylindrical container and is then pushed mechanically
through a suitably profiled die (Figure 16.22). There is a number of variants of this process (see Section 16.2.6); but
the two basic operations are those of forward (or direct) and inverse (or backward) extrusion. In the forward extrusion a
solid moving ram is brought into direct contact with the billet and activates the latter by moving it axially forward through
the die. In inverse extrusion a hollow ram is in contact with a movable die which bears onto the billet, firmly held in the
container. When the pressure exerted by the tooling is sufficiently high to exceed the yield stress of the material, plastic
flow is initiated and backward extrusion into and through the hollow ram takes place Considerable control over the dimensional accuracy can be exercised in such systems but, again, solution of the lubrication problem is of importance. In this latter context, hydrostatic extrusion (to be described later) provides an important alternative to the conventional arrangements indicated here.
A large group of ‘unorthodox’, dynamic processes introduces a number of new elements and opens new operational
possibilities of using materials which are sometimes difficult to process and of reducing manufacturing costs by
dispensing with heavy plant and equipment. The high-energy-rate processes stem essentially from the
usually overlooked fact that the working of metal requires energy and not merely the application of force, and that, in
addition, the rate of dissipation of energy is of importance. A simple consideration of the basic equation for kinetic energy shows that a comparatively small change in the velocity of a body will have a more pronounced effect than will a change in
its mass. A typical conventional system approaching the conditions of high-energy forming, i.e. drop-hammer forming,
is limited in its usefulness by the necessity of using large masses and, therefore, unwieldy and costly equipment.
The sources of energy used in the high-velocity systems are chemical explosives, electrostatic and magnetic fields, and
pneumatic-mechanical devices. The basic processes are those of forming (shaping), welding and powderiparticulate-matter
compaction. A variety of forming systems exists, each displaying specific characteristics associated with either sheet or tube
forming, for which it is intended.

Rolling processes and products

Classification of processes

With the exception of special steels and/or profiled sections rolling processes follow one of two main routes:
1,
2.
Traditionally, both routes start with a cast ingot (Figure 16.23) which is then rolled down to slabs (route 1) by cogging. In route 2, cogging again leads to the production of a bloom (a product of over 10 cm2, or equivalent, in cross-section), and then to either a variety of small flats or large rounds or, through a billet mill, to a billet (a product of cross-sectional area less than 10 cm2). However, very satisfactory developments in the area of continuous casting have led to the introduction of casting machines into these cycles. In the new,
fully automated and computer-controlled, high-productivity works, continuous casting of slabs has to a great extent
eliminated the cast ingot.

In route 2, in a modern mill the stress is on the use of continuous billet casters (in preference to bloom casters), thus eliminating one stage of the production line. Where blooms are still required, normal practice is to employ two or three strands of material which are then rolled in two or three passes to produce blooms. With smaller sizes of billet, up to six strands can be cast. It is clear from Figure 16.23 that the manufacture of a wide range of either semifabricates or finished products calls for a variety of mills and plant settings. A very brief review of these
is provided here but, again, detailed information can only be obtained from the Further Reading at the end of this chapter. Basically, the process, whether hot or cold, begins with the preparation of stock such as an ingot (in older plant) or continuously cast bloom or billet. In hot operations this is followed by heating in a strictly controlled atmosphere and temperature, and then rolling proper. Finishing of the work piece includes a number of operations such as cutting, cooling and, very often, straightening. In cold operations, which are used to enhance the mechanical properties of the material and improve dimensional accuracy, the ancillary equipment consists
of furnaces for heat treatment and plant for surface finishing. Whereas modern plant comprise not only the rolling
mill(s) proper, but also a number of pieces of ancillary equipment concerned with the preparation of the material
prior to and post rolling, interest centres mainly on the actual mill since the dimensional quality of the product will depend mainly on its performance. According to their actual functions, rolling mills are subdivided into the following classes: 1. cogging mills (production of blooms, billets and slabs from
ingots, where these are still used);

Figure 16.23 Schematic, simplified representation of the sequences of the basic rolling processes. Route 1 leads to the production of plate and wide strip. Route 2 results in sheet, narrow strip, tube and sections

2.plate. strip and sheet mills;
3.tube mills (longitudinal rolling and cross-rolling);
4,section mills (production of profiled sections. rounds, flats and strip); and
5.special milis (production of machine parts).

An individual mill is characterized by the function for which it is intended and also by the type, position and number of
rolls. These are located in stands, which in some cases are incorporated in a stand train. Depending on the position of the
stands, mills can be further classified as linear (where the stands are in a line and there may be. one or two lines driven by
a single motor). or continuous (where a number of stands is placed in tandem and, at any given time, rolling takes place in
more than one stand).

In each stand the number of rolls involved defines the stiffness of the structure and, therefore, reflects on the dimensional
accuracy of the product, whereas the disposition of the rolls specifies the function of the system. Irrespective of the
complexity or otherwise of the roll arrangement, the actual work of deformation is normally performed by a pair (or a set)
of innermost rolls which are backed up (for the mentioned structural stiffness) by a number of other usually larger
diameter sets of rolls. The number of these gives rise to the name of the mill which is described as n-high.
A selection of basic arrangements for flat, horizontal rolling is given iin Figure 16.24. With reference to the figure, the
following types of mill can be distinguished.

1. Two-highdither (i) reversing or (ii) non-reversing. (i)Used for slabbing, large rail, profiled sections and
thick plate rolling.
(ii)Used for continuous rolling of billets. rods, rounds and plate, linear rolling of sheet, and for dimensional
calibration of the product.

2.Double duo-used mainly in older mills for rolling small and medium-sized rods, strip and profiled sections.
3.Three high-used for rolling medium and large billets, rails, rods and profiled sections.
4.Three-high with the middle roll oscillating between the two outer rolls-all the rolls are driven; used for blooming or slabbing of ingots of around 3.5 ton in weight.
5.Three-high Lauth, with an idle middle roll-used for rolling thick plate and as a first stage in linear hot rolling of thin sheet.
6.Four-high-either (i) reversing or (ii) non-reversing.(i) (ii) Used for cold rolling thin strip and hot rolling thin and thick plate. Used for continuous hot rolling strip and plate and cold rolling strip.
7.HC rolling mill (Hitachi)-used mainly for cold rolling
plate and strip.
8.Taylor mill-used for precision rolling of thin sheet and
strip.
9.Six-high-used for cold rolling thin sheet and foil.
10.Sendzimir twelve-high mill-used for cold rolling thin
sheet and foil.
11.Twenty-high, planetary Sendzimir mili-used for cold
rolling thin sheet and foil.



The quality of product in terms of material properties depends considerably on whether processing takes place at an elevated or at ambient temperature. Rolling hot is not only
intended to minimize the forces and torques necessary to effect deformation, but also to improve the structure through recrystallization. Unless recrystallization takes place, strain hardening (produced by deformation) will adversely alter the structural characteristics of the metal, even at temperatures much higher than ambient. The processing temperature should, therefore, lie above that of recrystallization, in order

to produce new, small grains (Figure 16.25), but below the value at which oxidation takes place and affects the surface of
the product. The main function of cold rolling, as already mentioned, is to improve the mechanical properties of the alloy, to impart a surface finish suitable for the future use of the product, and to  enable the manufacture of dimensionally accurate very thin
sheet, strip and foil (0.001-0.2 mm thick) and ‘medium’ thick sheet (1.0-1.5 mm thick). Depending on the quality of the
surface, the rolled sheet can be used for stamping when the material displays fine-grain finish, for plating when the surface
is smooth, or for further artificial surface coating when the surface is dull.

 Rolling of plate, strip and sheet

The definition of each of these products is somewhat vague, but it is usually taken that, in hot processes, a rolled item is
referred to as ‘plate’ when its thickness is in the range 4-160 mm and its width is 600-3800 mm. Sheet that is
4-50 mm thick and 20G1500 mm wide is also sometimes called ‘universal’, whereas ‘thin sheet’ or ‘thin strip’ is material

of thickness 0.2 to <4 rnm with a width of 600-2400 mm. ‘Plate’, intended as a constructional material for bridge structures,
general building purposes, boilers. ship hulls, car and railway rolling stock bodies, etc., is rolled up to a length of some 36 m. depending on the final use of the product. Rolling is done either from slabs or flat billets. Thicker plates are
usually produced in linear, two-stand, two-high reversing systems ((1) in Figure 16.24) followed by a train of four-high
finishing stands. Typically, thinner plates may be rolled from slabs in a single stand. Lauth linear system ((5) in Figure
16.24) or, for greater accuracy, in a planetary mill. More recently, thick and thin products have been rolled in
semi-continuous systems employing two universal stands in tandem, ,followed by a continuous mill arrangement.
Strip and medium heavy sheet are produced in lengths of 4-20 rn on either linear or planetary mills; with the latter, ir,
particular, high thickness reductions of up to 95% are obtained. Thin sheet, intended as structural material, is often rolled,
as separate items, to a length of approximately 1800 mm, or alternatively in long lengths in continuous mills and is then
coiled. The car and electronic industries, in particular, make considerable use of steel and aluminium alloy sheets and strips
coated with thin layers of plastic material. The metallic product used in such applications varies in thickness from
some 0.3 to 2 mm.

Cold rolling operations have a two-fold objective: to improve the mechanical properties and dimensional accuracy of
the product and to enhance the economic feasibility of the operatiori (hot rolling of materials to less than about 2 mm
thickness is uneconomical). Due to economic considerations hot-rolled strip or sheet is further processed in a variety of
mills (very similar to those shown in Figure 16.24), but particulairly in planetary or cluster miils if very thin material is
required. A typical arrangement employed in the latter case is shown in Figure 16.26. The usual working sequence consists of
pickling ithe hot-rolled material, rolling to the required size, followed., where necessary, by heat treatment and, finally, by
general finishing operations.

The range of thicknesses that can be obtained depends, to a degree, on the properties of the starting material, since the roll
forces and torques required can be very high. It is often because of this that cluster mills, characterized by their high
rigidity, ,are used. Low carbon steel can be rolled down to a thickness of about 0.2-0.4 mm. Similar strip thicknesses are
obtained in Inconel, Monel, nimonic and bimetallic combinations, rolled in Sendsimir mills which use rolls of 450-13001 mm in length. Consecutive rolling operations are employed to produce very thin foils.

Rolling of structural sections

Semifinished or hot-finished structural sections are rolled in semicontinuous, continuous, Universal. linear, or H mills.
Although a variety of shapes and components can be made on such mills, of particular engineering interest is the rolling of
medium and heavy sections. Depending on the actual sizes and weight per length run, these comprise channels, I- and
T-beams, rails and angular sections, as well as semiproducts intended for further processing, such as rounds. squares and
flats. Without being specific about the type of mill, it is generally understood that ‘bar mills’ are used for rolling heavy
sections and ‘rod milis’ for light sections.

Channels (50-450 mm high, designated as heavy sections when the height exceeds 200 mm) are usually processed in
continuous mills, as are T-sections (30-220 mm high). The method of manufacture of I-beams is less uniform. Smaller
sizes (below, say, 200 mm in height) are produced in continuous mills. whereas heavy sections (up to 600 mm and,
occasionally, up to 1 m in height, with flanges of 200-400 mm in width) are rolled in universal-type mills. The method Qf
rolling steel rails depends on the weight per metre run. In general, sizes range from 8 to 75 kg m-’, the 4@75 kg m-l
sizes representing heavy sections.

In most cases, two-line, two- or three-high, two- to sevenstand linear systems are used but, more recently, special
universal and H-type mills have been introduced. The semifabricates (rounds 6200 mm in diameter; square
sections 8-200 mm side) are produced on semicontinuous and continuous mills, as are rods (5-8 mm in diameter) intended
for wire manufacture. These products are rolled in up to 39 stands.

Rolling of seamless tubing

Seamless tubing is produced by either longitudinal or cross (oblique) rolling of hot finished rounds. Both manufacturing
routes start with pickling, dressing, heating and piercing the billet to provide a roughly made rube which can then be
further hot and, if necessary, cold processed. The latter stages are intended to give dimensional tolerances to the product
and, in the case of cold processing, to impart both better mechanical properties and, often. better surface finish.

The two routes comprise the following intermediate processes:

1. Longitudinal rolling consists of rotary or punch piercing, plug or mandrel rolling. It occasionally may incorporate
the Erhardt or push-bench process, followed by the finishing sequences of sinking and stretch reducing.

2. Cross-rolling involves rotary piercing, Assel elongating (possibly a combined piercing-elongating process) or Diescher elongating.

Irrespective of the route followed, the piercing stage precedes further operations and consists either of rotary piercing
of a solid round or hydraulic punch piercing of a square billet. Of these two methods, cross-rolling (Figure 16.27) is carried
out in a two- or three-roll system in which the rolls are inclined at an angle (feed angle a) to the central axis of the mill, and a
plug (situated on a long bar) is inserted between the rolls. The plug bar can either be free to rotate or driven.
With the rolls set obliquely to each other, the billet is drawn into the pass by frictional tractive forces, the axial component
of the forces being responsible for the forward movement and the tangential component for the rotation. The physical conditions
that develop in the pass are equivalent first to oblique rolling and then to cross-rolling. The cross-rolling zone extends
from the plug nose to the exit from the pass, and the billet is worked between the roll and plug surfaces.
The process is not continuous, in the sense that the successive arcs of the billet circumference come, in turn, into contact
with the rolls. The set-up clearly tends to develop ovality in the billet, unless horseshoe-type adjustable guides are introduced
between the rolls. Up to a point, these guides rectify this undesirable condition and help to preserve circularity of
the tube.

Longitudinal rolling The main elongation of the bloom. to produce a hot finished hollow, is carried out in either a plug or
a mandrel (continuous) mill and is then completed in one or two finishing stages. In the plug-mill system, the plug, a short cylindrical. detachable tool, is maintained in position by a plug bar, an arrangement similar to that in the rotary piercing plant, and controls the dimensions of the bore of the hollow. The work piece passes through grooved, oval rolls that are usually positioned
in two stands and maintained at 90" to each other. The mill is generally used as a breaking-down stage and operates on
tubing of 3-40 mm wall thickness and 2MOO mm diameter. In the mandrel-mill system the hollow bloom is threaded on
a long mandrel and is fed into a continuous train of 7-10 pairs of rolls arranged in tandem. The tube is rolled out on the
mandrel without interruption to approximately the finished thickness. The axes of the rolls are alternately at right angles
to one another. The earlier breaking-down passes have considerable ovality. The finishing passes are designed to leave the
tube loose on the mandrel and more or less circular so that no reeler is required to enable the mandrel to be withdrawn.
The method is no more continuous than any other seamless process which starts with a billet of finite length and rolls it
into tube. The continuous mill is outstanding for producing long lengths of all thickness at a very high rate of output.
The final rolling stage uses a sizing or reducing mill to give the tube its final outside diameter with, preferably. little
change in wall thickness. Reducing mills fall into three broad categories:

1. sizing,
2. stretch reducing, and
3. sinking.

Siting mills are used to produce an accurately sized finished tube without appreciably reducing the outer diameter. They
are usually equipped with three to seven stands, each containing two rolls, and are normally designed for an overall
diameter reduction of 3-8%. In general, the diameter reduction is accompanied by a thickening of the tube wall.
Sizing mills are usually installed in plants designed to produce the larger tube-size range, but can be used with
advantage in the manufacture of smaller tubes when following main seamless processes that are reasonably flexible in the
sizes that they can produce.

Reducing sinking mills comprise a continuous train of passes designed to reduce a larger tube to a smaller one without a
mandrel or plug to support the bore. The amount of reduction per pass is usually 3-5% according to the type of mill.

A reducing mill is usually employed in conjunction with a main unit to improve the performance of the latter. This may
be done by:

1. reducing the number of size changes to be made in the mill;
2. increasing the weight of the billet worked by the mill and, therefore, the rate of output; and
3. increasing the maximum length available, and decreasing the available minimum diameter. In obtaining these advantages the following disadvantages must be taken into account:

1.a reheating furnace is necessary;
2.an additional process is thus introduced increasing costs of fuel, power. labour and tenance;
3.tubes become thicker during reduction and, therefore, the mill must roll thinner gauge
4.; and the quality of the bore of the tube deteriorates for various reasons and this limits the amount of reduction possible.


These mills are usually designed with up to about 24 two-roll stands, and are capable of overall diameter reductions of up to
55%. The stands are driven through bevel gear boxes from common line shafts, and the behaviour of the tube-wall
thickness depends on the relationship between the gearbox reductions. It is normal for sinking mills to operate with no
axial tension between the stands so that the wall thickens freely. If, however, some tension is applied, wall thickening
can be restricted to about half of that which would occur without tension. Stretch reducing mills are primarily used for the economic production of small diameter thin-walled tubes. As the name implies, the wall thickness is reduced in the process and, to
achieve this condition, considerable tension must be applied between the stands (Figure 16.28).

The mills are designed with up to 22 stands, each with two or three rolls, and are capable of overall diameter reductions
of more than 7570, and wall reductions of up to 40%. To obtain the control required over the interstand tensions, the
mills are designed with either d.c. motor drives to each stand or hydraulically regulated drives. As the ends of the tube cannot be subjected to full tension in the mill, t‘hey are inevitably heavy walled and of a length that is proportional to the elongation, stand spacing and percentage wall reduction. The end crop can have a significant effect on the yield and, therefore, it is necessary to put the longest possible length into the mill to minimize the percentage loss. In addition to the essential change in the outer diameter of
the tube in the reducing operation, there is always some change in wall thickness. The thickness change plays a vital role in the design and operation of reducing mills and depends on the rolling condition. No-stretch conditions prevail when a tube passes through a single stand, so tha? its diameter is reduced without any external axial forces being applied to it; the thickness also
increases. In this case, where the change in wall thickness occurs puirely as a result of the compressive action of rolls on
the tube, the reduction is said to take place under the ‘no-stretch condition’.In a mill consisting of a number of stands that are all
reducing the tube simultaneously, this condition can arise providing that the roll speeds in successive stands are set such that there is no tension OF compression in the length of tube between each pass. Since there is always an increase in the linear speed of a tube as it is reduced, the roll speeds in successive stands must be increased in relation to the elongation that occurs. It follows that if a tube is being continuously processed through a number of reducing mill stands, the volume of flow must be constant. The cross-sectional areas multiplied by the linear speed must, therefore, be the same at all points in the miE, and as the tube area is reduced its speed must corrlespondingly increase.

Under no-stretch conditions, the whole tube thickens uniformly throughout its length, the amount of thickening being
dependent on the diameter to thickness ratio of the in-going tube and the total diameter reduction given. It is unaffected by
the reduction given in each stand or by the number of stands working on the tube at any time.
Conversely, in no-stretch conditions, if tension occurs between the stands, wail thickening can be partly or fully
compensated for, or the wall thickness can be reduced. Tension is applied by arranging for each pass to run at a higher speed, relative to the preceding pass, than is required for no-stretch conditions. With stretch conditions, not only is the thickness reduced
between the stands but the diameter is also reduced so that the actual reduction in diameter brought about by the compressive
action of the rolls is slightly less than that which takes place with no stretch, resulting in slightly less thickening of the tube
wall between the rolls. Stretching can occur only on a section of tubing which is between two passes. Consequently, the
extreme ends of the tube can never be subjected to stretch nor can an appreciable length at the ends suffer the full stretching
action. Therefore, a mill that is designed to stretch or in which stretching occurs produces a tube that is thinner in the centre
than at the ends, the thickness at the extreme ends being that which would arise under no-stretch conditions and the centre
thickness being that arising from the stretching action. The thickness change is not abrupt but gradual and there is
normally an appreciable length of uniform thickness between the thickened ends The push-bench process (Figure 16.29) forms an alternative to the above routes. The standard push-bench system incorporates punch-piercing of the hot billet, while retaining a solid base on the leading end, elongating on the bench proper and hot-reducing on an appropriate mill. To retain a reasonable
degree of concentricity, the depth of penetration of the piercing punch must not exceed seven times the punch diameter.
The ‘bottle’ which is fed into the bench is relatively short and the process may not be fully economical.
Modern developments have led to the introduction of roller-type dies (the Manfred-Weiss process) and, even more
recently, to the incorporation of an elongator in the cycle. The elongator increases the depth of penetration to nine times the
punch diameter while retaining good concentricity through the medium of equalization of the wall thickness. Since the
elongator will also reduce the diameter of the bottle, the amount of deformation to be obtained on the bench is lower
and the quality of the manufactured tube is higher. Some of the advantages of the elongator are, however,
offset by the fact that the gauge correction cannot be carried out at the ends of the bottle and that some reduction in output
must therefore be expected.

When the elongator forms an integral part of the cycle, reheating of the bottle, after punch piercing. will be necessary
and will, of course, add to the cost of the operation. The modern push-bench is normally used for the manufacture
of tubing of outer diameter of 6-15 cm. Thin-walled tubing can be obtained in lengths of up to 10 m when an
elongator is used, and in lengths of up to 6 m without the elongator. The advantage of the modern roller-die beds of the bench
proper over the old ring-die system lies in the reduction of thrust required by about 30%, and a reduction in the size of
the die bed by about 25%. Although the provision of three or four rolls in a cluster is more expensive than the installation

and maintenance of ring dies, the considerable increase in speed of the working cycle, resulting from the possibility of
using more passes on a given bed, and from the actual increase in the axial speed of the operation, offsets the apparent
economic disadvantage. High speeds of operation are, in fact, required in the case of alloy steels which harden rapidly with a
drop in temperature.

Cross rolling The two major rotary production processes employed are those of Mannesmann and Assel. In the Mannesmann
cycle a cold centred round billet is heated in a roller-type furnace, pierced in an oblique rotary mill, possibly
repierced on another rotary piercer, hot forged, reheated in a continuous furnace and stretch reduced. The piercing and
repiercing operations are of primary importance from the point of view of the incidence of redundancy.
In the Assel cycle (Figure 16.30), the billet is heated in a roller-type or rotary-hearth furnace, pierced in an oblique
rotary mill, elongated in the Assel mill, reheated in a continuous furnace, and then either sized or sunk on a longitudinal
rolling mill. The first two stages, i.e. piercing and elongating, account for most of the redundant shears induced in the
worked material.
To increase the range of possible deformations, it is occasionally necessary to follow the piercing stage by a repiercing
or secondary piercing operation in which the bore of the pierced bloom is increased while its wall thickness is reduced.
A purely elongating operation in which the bore remains unchanged and the wall is further deformed will follow.
Secondary piercing operations have been developed from the basic Assel elongating technique and are considered in that
context.
An Assel mill is shown diagra-mmatically in Figure 16.17. In this process, the previously pierced tube, with a cylindrical
floating mandrel inserted in the bore, is drawn by frictional forces into the pass defined by the setting of the rolls. In the
entry zone of the pass, the diameter of the tube is slightly reduced. The hump of the roll then reduces the wall thickness,
and a smoothing operation takes place in the reeling zone of the pass to remove the triangulation of the vertical section of
the tube. Final rounding of the tube proceeds in the exit zone of the roll. Elongation occurs primarily as a result of wall
thinning, but it is also slightly influenced by the reduction in tube diameter. Of special interest in tube making is the production of
stainless-steel tubing. This, as will be seen later, is generally manufactured in extrusion, but rotary processes are also used,
since the rate of yield is likely to be higher. The production routes used in this case are somewhat complex and reflect both the cost of the material and basic processing difficulties. The main process lines are:

1. punch piercing, Calmes elongator, rotary hot forging;
2. punch piercer, push-bench, polishing mill;
3. disk rotary piercer, plug mill; and
4. Mannesmann rotary piercer, hydraulic push elongator, rotary hot forging.
A possible alternative to asseling is offered by the Diescher elongator. This mill incorporates two oblique barrel rolls and
two disks (set at right-angles to the rolls) which replace the horseshoe guides necessary (see piercing) to correct the ovality
of the tube. Elongation is carried out between the rolls and a bore-supporting mandrel. The mill improves concentricity
of the tube and, by effecting elongation in both forward and backward directions relative to the gorge, imparts a
burnishing finish to the tube.

 Flow forming
Practical difficulties of producing large diameter, thin-walled long tubing in a variety of materials, including stainless steel,
has introduced a new approach because the ordinary manufacturing techniques could not always cope with the sizes and
materials involved and, even where this was possible, the cost of production was very high.

A solution to this problem was first proposed, on a modest scale, in the early 1950s when the horizontal flow-turning lathe
was introduced. This was followed by a vertical cold ‘floturning’ machine in which three stands, carrying five rollers each,
were introduced. In this, short thick-walled bloom is supported in the bore by a solid cylinder of hardened tool steel
machined to the exact tolerances and to the degree of surface

finish which is required of the bore. Each station, capable of angular adjustment to obtain optimum forming conditions, is
powered hydraulically. Rollers are advanced into the forming positions and retracted at the end of the cycle. The wall
thickness iof the bloom is easily reduced by one-sixth of the total reduction by each roller. As an example of the possibilities,
in orie pass a steel cylinder of 1.25 m outer diameter, 1.25 mm thickness and 3.5 m in length can be formed from a
hollow of 8 mm thickness and about 0.75 m in length. In effect, therefore, the machine reduces the wall thickness of the
hollow, elongates the work piece between the rollers, but retains the original diameter of the bore. The control of the
mechanical properties of the material can be maintained throughout annealing, and that of the surface finish can be
maintained throughout the finish of the roller surfaces. Some limitations on the length of the tubing obtained on a
vertical machine can be expected and this appears to be the reason for a parallel development of horizontal 'floturning'
equipment. The floturn process does, in fact, utilize, in general, the horizontal-type machine. The development of horizontal machines was initially stimulated by the demand for large diameter, long tubing required for the bclosters on space vehicles. The equipment designed fox this purpose is capable of turning out precise tubing of up to 635 mm in oute: diameter and of 30 m length in alloy steels, titanium. Monel, aluminium and stainless steel. Essentially, the process consists of reducing the diameter of
an initially thick-walled, short hollow by threading and clamping the latter on a rotating mandrel, and driving a carriage
with the forming rolls along the work piece. The carriage contains three rollers located at 120" to each other which,
when driven Slowly over the surface of the hollow, will reduce its thickness. This particular process. known as 'par-forming',
can retain wall-thickness tolerances to within f0.51 mm on, say. a 655 mm outer diameter, 2.95 mm thick and 13 m long
aluminium tube, and on a 508 mm outer diameter and 2.15 mm thick stainless steel component. To achieve these high tolerances, a certain amount of preparatory dressing of the hollow is necessary. Initially, the hollow is turned, faced and machined in the bore. The bore is usually h'oned to remove too1 marks and to impose the required tolerances. The tube should fit the mandrel to within
200 Fm and, for the final tube sizes stipulated above, it should be about 1.5 m long. Mandrels are made of high quality
chromiumi-molybdenum-tungsten steels and are ground to give toler,ances of the order of 51 pm. The aciual forming operation takes place in two stages.

Again, for the sizes discussed, something of the order of five passes of the forming rolls over the hollow will be needed,
these will be followed by annealing and, finally, by two further finishing passes. Machines of this type are fully capable of forming tubular components with shoulders, tapers and partial or full closures. The range of tube sizes varies from about 75 to 700 mm in
outer diameter, the range of wall thickness being 0.38-3.8 mm. Surface finish can be controlled to within 380 ,urn on the bore and 760 pm on the outer surface.

Floturning machines are often incorporated in mechanized plant and this facilitates cleaning of tubing prior to adhesive
bonding, where required. of two dissimilar metallic components. Cleaning sequences depend on the material processed
but; in general, will include emulsion. water and alkaline cleaning for non-feerrous materials (e.g. aluminium) and pickling
in nitric hydrofluoric acid for, say, stainless steels. The capital cost of the equipment involved in these operations
is oloviously high, as is the cost of hollow dressing, and the cost increases with the increase in the outer diameter of
the tube. Clearly, the existing industrial units serve very specific purposes and are not necessarily intended for ordinary
commercial purposes. In their own field of application, i.e. large diameters, thin walls, and substantial lengths, they are
well advanced compared with other methods.The possibility, however, of using the same principle for processing small diameter tubing has not been neglected. One of the main problems arising here is that of buckling. In the nose forming of steel tubes, where the Components are compressed on a mandrel by conical rollers, the effect of wall thickness on the stability and formability of the tube is noticeable.Rolling of machine parts Although developed primarily for tube manufacture, crossrolling processes are now often employed, for reasons of economy, in the production of a variety of machine parts. These range from a three-roll, oblique rotary grooved shaping of a shaft (Figure 16.19) to the helically two-roll manufactured special hubs (Figure 16.31). Three-roll (inclined at 120" to each other) finned-roll systems are used in the production of finned tubing of high dimensional accuracy,
whereas grooved two-roll mills are used to produce ball bearings (Figure 16.20). Bevel and spur gears, as well as
bearing races and similar components are manufactured by cross-rolling with a very substantial saving in material as
compared with many machining production routes in a variety of rolling systems.Sheet roil bending An interesting development in the area of cold-rolling is that of using especially profiled rolls to bend metal sheet (usually aluminium, but also hot or cold rolled steel) into simple or even complex shapes that serve as structural components in the car and aircraft industries Bending is carried out in machines incorporating roll trains of driven and idle tools, which can produce long length (up to 30 m) suitably profiled components, without changing the initial thickness of the sheet.