An aluminium alloy is heat treated by heating it for
a prescribed period at a prescribed temperature,
and then cooling it rapidly, usually by quenching.
The particular form of heat treatment which results
in the alloy attaining its full strength is known as
solution treatment. The alloy is raised to a temperature
of 490 °C by immersing it in a bath of molten
salt. The bath is usually composed of equal parts of
sodium nitrate and potassium nitrate contained in
an iron tank. This tank is heated by gas burners
and, except for its open top, is enclosed with the
burners in a firebrick structure which conserves the
Table 5.4 Tempering colours for plain carbon steel
Colour Temperature (°C) Type of article
Pale straw 220–230 Metal turning tools,
scrapers, scribers
Dark straw 240–245 Taps, dies, reamers,
drills
Yellow-brown 250–255 Large drills, wood
turning tools
Brown 260–265 Wood working tools,
chisels, axes
Purple 270–280 Cold chisels, press
tools, small springs,
punches, knives
Blue 290–300 Springs, screwdrivers,
hand saws
Table 5.3 Temperature colours for steel
Colour Temperature (°C)
Black 450–550
Very dark red 600–650
Dark red 700–750
Cherry red 800–850
Full red 850–900
Bright red 950–1000
Dark orange 1050–1100
Light orange 1150–1200
Yellow white 1270–1300
White (welding heat) 1400–1550
Metal forming processes and machines 167
heat. The temperature of the bath must be carefully
regulated, as any deviation either above or below
prescribed limits may result in the failure of the
metal to reach the required strength. The alloy is
soaked at 490 °C for fifteen minutes and then
quenched immediately in cold water.
At the moment of quenching the alloy is reasonably
soft, but hardening takes place fairly rapidly
over the first few hours. Some alloys, chiefly the
wrought materials, harden more rapidly and to a
greater extent than others. Their full strength is
attained gradually over four or five days (longer in
cold weather); this process is known as natural age
hardening. As age hardening reduces ductility, any
appreciable cold working must be done while the
metal is still soft. Working of the natural ageing
aluminium alloys must be completed within two
hours of quenching, or for severe forming within
thirty minutes. Age hardening may be delayed by
storing solution-treated material at low temperatures.
Refrigerated storage, usually at 6–10 °C, is
used for strip sheet and rivets, and work may be
kept for periods up to four days after heat treatment.
If refrigerated storage is not used to prevent
age hardening it may be necessary to repeat solution
treatment of the metal before further work is
possible.
Alloys of the hiduminium class may be artificially
age hardened when the work is finished.
Artificial ageing is often called precipitation treatment;
this refers to the precipitation of the two
inter-metallic compounds responsible for the hardening,
namely copper and manganese silicon. The
process consists of heating the work in an automatically
controlled over to a temperature in the region
of 170 °C for a period of ten to twenty hours.
Artificial ageing at this temperature does not distort
the work. The temperature of the oven must be
maintained to within a few degrees, and a careful
check on the temperature is kept by a recording
instrument. In order to ensure uniform distribution
of temperature a fan is fitted inside the oven to
keep the air in circulation. At the end of a period of
treatment the oven is opened to allow the work to
cool down. One of the chief advantages of this
process is that work of a complicated character
may be made and completed before ageing takes
place. Moreover, numerous parts may be assembled
or riveted together and will not suffer as a
result of the ageing treatment.
5.3 How metal is formed to provide
strength
It has been established that the strength of a material
is governed by its material composition and by
the method and direction of loading, i.e. tensile,
compressive, torsion, shear and bending. Generally
the majority of metals are capable of withstanding
greater loads in tension than any other type of
stress. One of the properties of steel is that, within
certain limits, it is elastic: that is, if it is distorted
by a load or force it will change shape, but it will
return to its original shape when the force is
removed. However, above a certain intensity of
load (the elastic limit) the metal will remain distorted
when the load is removed. Sheet steel, such
as is used in the manufacture of car bodies, has
reasonable strength in tension but has little resistance
to compressive and/or torsional loads. This
lack of resistance is due not so much to poor compressive
strength as to lack of rigidity. Low-carbon
steels are used extensively in the manufacture of
vehicle bodies, and the designer has to ensure that
the relatively thin sections of material are capable
of withstanding the various types of loading. In
addition to the permanent stresses present in the
material, the vehicle body as a whole is subject to
shock stresses due to road conditions, and these
must also be taken into consideration by the
designer.
With the development of deep drawing steels
and better press equipment, large streamlined panels
were designed and formed into contours that
were more attractive, gave longer life and greater
safety, and at the same time reduced the bulky
construction previously required to give similar
strength. It is known that the shape of any material
is held by the stresses set up in the material itself,
such as those given by angles, crowns, channels
and flanges. The original shape will be maintained
until the material is subject to a force sufficient to
overcome the initial stresses. Furthermore, it will
tend to return to its original shape providing it has
not been distorted beyond the point of elasticity.
Crowned surfaces
The building up of stresses at the bend or peak is
also an important consideration in the design and
manufacture of the modern car body. The most
168 Repair of Vehicle Bodies
common features of the body are the curved surfaces
(Figure 5.2); these are called crowns and
may be curved either in one direction or in all
directions. A crowned surface is stronger than a
flat panel, and whilst it will resist any force tending
to change its shape, it also has the ability to
return to its original shape unless distorted
beyond its point of elasticity. These are the features
of metal sheet which has been formed in a
press into a permanent shape, with die-formed
stresses throughout its entire area tending to hold
the shape. On the bend or crown, one side of the
sheet is longer than the other; and the metal at the
surface is more dense than at the centre of the
sheet. The final action of the press is to squeeze
the surface together, thus setting up stresses and
greater strength. The greater the crown or curve
of the panel, the greater its strength and rigidity
to resist change in its shape. This is illustrated by
the fact that a low crown, i.e. a surface with very
little curve, such as a door panel, is springy and is
not very resistant to change of shape. On the
other hand high crowns, that is surfaces with a lot
of curve, like wings, edges of roofs and sill panels,
are very resistant to change in shape by an
outside force.
Angles and flanges
A further method of giving strength to metal is to
form angles or flanges along the edges of sheets
(Figure 5.3). A right-angled bend greatly increases
the strength of a sheet, as can be demonstrated by
forming a right-angled bend in a thin sheet of
metal and then trying to bend the metal across the
point of the bend. This method is used on inner
door panels and at the edges of wings, edges of
bonnets and boot lids, and wherever stiffness is
required at unsupported edges.
