Mechanical working of metals

A metal is worked mechanically either to generate a shape more economically than can otherwise be obtained or to
provide improved properties in one or all of strength, ductility or fatigue (not creep, for which a cast structure is superior).
The improvement in properties may be overall, directional to resist a directional stress system. or statistical to ensure that a
lower percentage of components fall below a specified level of 7roperties.

Metals can be mechanically worked because they differ from other crystalline solids in that the atoms in a crystal are
not linked by valency bonds tc adjacent atoms. The free electron metallic bond is non-specific. pulling equally hard in
all directions. Metal atoms are therefore bound tightly in certain regular crystalline structures. There are seven crystal
systems giving 13 possible space lattices and all engineering metals crystallize in one of three of these lattices (see Table
7.2 and Figure 7.11). Some metals are polymorphic: for example: in the case of iron. austenite is face-centred-cubic,
ierrite is body-centred-cubic.

These crystalline structures are resistant to tensile stress but can he sheared along certain crystallographic planes. The
planes of lowest resistance to shear have the closest packing of atoms in the plane. Each trio of atoms in a close-packed layer
surrounds 2 space (or hollow) in which an atom in the next layer can rest. Slip occurs when the atoms in a plane move into
the next hollow in the adjacent plane. The shear stress required to move a whoie plane of atoms is very high and a
perfect metal crystal (such as occurs in 'whiskers') would deform elastically between 306 and 10% before deforming
plastical.ly in shear.

The crystals in massive metals are not perfect. 'Dislocation' TOWS occur in the lattice where an atom is missing or an
additional atom is present. These dislocations allow the planes io shear one row of atoms at a time, thus greatly reducing the
critical shearing force. More dislocations come into play as the netal is deformed and in most cases, two or more dislocations

may interact and stop each other. The critical shear stress therefore increases and the metal work hardens, increasing its
tensile strength. Finally, no more shear is possible and the metal fractures. If temperature is increased, diffusion and
recrystallization can release stopped dislocations. In hot working, diffusion occurs while the metal is worked, but in cold
working and annealing the two processes occur separately. In either case the grain of the metal may. if the conditions are
chosen correctly. be refined.

Mechanical working has other beneficial effects. The structure of cast metal may contain micro segregates, lumps and
planes of constituents which separate out during casting. Mechanical working. correctly applied. may break up and
disperse these to a size at which they can he diffused during heat treatment.

Castings may contain cracks, voids and non-metallic inclusions.

Mechanical working, correctly applied. may close up and remove the cracks and voids (or aggravate them so that
the part must obviously he rejected) and break up nonmetallic inclusions. Alternatively. defects may be aligned and
elongated in the direction of the tensile component of stress so that they do not impair resistance to fracture caused by tension
or fatigue. Of the three space lattice systems shown in Figure 7.11 the face-centred-cubic has more planes on which slip can
occur than the other two (which are themselves greatly superior to other lattice systems). Face-centred-cubic austenite
is therefore more ductile than body-centred-cubic ferrite, and steel is usually hot worked in the austenite (7)
phase.
Hot working

Before working, the steel (unless already hot from continuous casting) is reheated into the y-phase. Care must be taken to
prevent excessive scaling and to avoid overheating (or burning) the steel. The limiting temperatures for carbon steels can
he estimated from Figure 7.12 and are given in Table 7.3. Further forging temperature ranges are given in Table 7.4.
Grain growth occurs when steel is held in the y-phase, but subsequent working reduces grain size. Figure 7.13 illustrates
these effects. The grain size of mild steel increases from ASTM 10 to ASTM 6 on heating to 900°C hut six rolling passes
reduce it to ASTM 10. Heating to 900°C has no effect on the grain size of two other steels hut in both cases heating to
higher temperatures increases grain size and rolling reduces it again. The finer austenite resulting from h3t working transforms
to a finer grain ferrite on cooling.

Rolling

Rolling is the most economic way of working steel if adequate quantities are produced. Steel for strip is cast into slabs,
scarfed to ensure a defect-free slab and hot rolled using automatic gauge control. Carbon steels may be ‘controlled
rolled’, that is, finish rolled at relatively low temperatures thereby inducing a fine ferritic gain structure with improved
tensile strength and notch toughness.

Hot-rolled strip is cold rolled to reduce thickness and give the required surface finish and forming qualities. Finishing
capabilities include rewinding to coil or cutting to length, side trimming and oiling. Roll trains for producing hot- and
cold-rolled strip are shown in Figure 7.14. By the use of a cluster mill, such as the Sendzimir (see
Figure 7.15) strip down to 0.1 mm thickness can be produced. Grooved rolls are used to manufacture blooms, billets, bar
and sections. Hollow sections can be produced by suitably designed mill trains and strip can be passed through forming
rolls which turn the edges towards each other to form a seamed tube which is then welded continuously by inert metal
arc or electric resistance. Seamless tube may be formed by cross rolling a billet in a Mannesman piercer in which the
rolling action produces a tensile stress at the centre of the workpiece.

Forging

Forging is the process of working hot metal between dies either under successive blows or by continuous squeezing. It
may be used to break down an ingot into a bloom or bar, to work down an ingot or billet to a rough finished shape before
finishing or to make a forging. There are two essential differences compared with rolling. It is almost always possible
(1) to design the dies and to arrange the sequence of forging to impose a higher ratio of compression to tension forces than is
possible by rolling and (2) to ensure that the grain of the metal is in a preferred direction and not purely longitudinal as in
rolling. For breaking down an ingot in, for example, tool, high-speed and some stainless and heat-resisting steels that
have a two-phase structure, hot rolling would (at least before the development of ESR or vacuum arc casting) lead to
ruptures due to the strong tension forces induced. Such ingots are usually broken down by hammer cogging. Forging is also
used for making very large components such as turbine rotors which are usually ‘open-die’ forged in a press. Large forging
ingots have a cross section whose circumference comprises a number of arcs meeting at cusps, because this shape minimizes
surface cracking during casting. The first forging operation removes the cusps to form an approximately circular cross
section and the forging is then drawn out through successive shape changes from octagon to square and back to octagon
using dies of shapes shown in Figure 7.16. If the geometry of forging and press permits, the forging is upended and upset to
produce some radial grain flow and it is then drawn out again. Hollow forgings are made by punching a hole in the centre
of a cylindrical workpiece and ‘becking’, working the die against a stiff bar passing through the forging and supported
on ‘v’ blocks.

Small closed-die forgings are made in two dies attached, respectively, to the hammer ram and bed which have successive
cavities to mould the stock progressively into a final shape in the last or finish cavity. For larger forgings a number of dies
are made to perform one operation each. A wide variety of shapes can be made, depending only on ability to make and
extract the component from two meeting dies with a parting line which may or may not be planar.

Steel from a basic oxygen furnace IS either continuously cast into slabs or cast into ingots which are then rolled to slabs. Prior to
re-heating machine scarfing is used to ensure a defect free slab. Automatic gauge control ensures strict control of thickness -@ throuQhout hot rolling. Hot rolledcoil may be supplied mill finish or pickled and oiled before despatch either in coil or sheet form.

The cold rolling processes not only reduce the thickness of the input hot rolled coil, but also give it the required surface finish and
forming qualities Finishing capabilities include re winding or cutting to length, side trimming, tension levelling oiling and branding to the customers' requirements

Most of the steels listed in BS 970 can be forged and will give properties appropriate to their section. Carbon steel
forgings for engineering purposes are listed in BS 24 and BS 29 and forgings for fired and unfired pressure vessels in
BS 1503.

The most demanding requirements for aircraft and similar requirements are met by ESR or consumable vacuum arc cast
ingot usually made to manufacturers' own specifications (agreed, where appropriate, by official inspecting bodies).
There are some 30 hot and cold metal-working processes which can be considered to be some version of forging.

Drawing and extriisiori

Extrusion Hot extrusion consists of placing a hot cylindrical billet into a container and either forcing a die with a centrally
placed orifice onto one end (indirect extrusion) or applying pressure to a ram at the other end, the die being held
stationary (direct extrusion). In either case metal is extruded though the die in the form of an elongated bar having the same
cross section as the orifice.

Extrusion is applied widely to non-ferrous metals which soften at lower temperatures than the steel or hard metal dies.
Bar and very complex sections can be economically produced and cut into a wide variety of shapes. Tubes and hollow
sections can be made either by extruding metal through bridge-type dies in which the metal stream separates and
rewelds at a later point or by extruding a hollow billet over a central mandril. Non-ferrous metals are extruded hot or cold.
Only the softer steels are extruded cold.

Hot extrusion of steel requires a glass lubricant. Very rigid and powerful presses are required and their high cost plus the
need for machined billets limits the process to high-cost steel, unusual shapes and tubes (which are mandril extruded). There
are besides extrusion other methods of tube production Drawing Usually after a 'semi' has been produced hot it is
cold drawn to reduce diameter, wall thickness oi both. Cold drawing through die.s is used to produce wire, tube and light
bar. This process requires considerable skill and attention to detail in die design, lubricants, wire rod cleansing and baking
to remove hydrogen introduced during cleansing.