Supa-sander (miniature belt
Sander)
This machine (Figures 3.54, 3.62) has a 20 mm
wide belt and is designed to handle small and
accurate convex and concave contours. It has an
adjustable head feature for flexibility of operation.
118Repair of Vehicle Bodies
Figure 3.64Ratchet wrench (Desoutter
Automotive Ltd )
Figure 3.63Impact air screwdriver (Desoutter
Automotive Ltd )
Hand and power tools 119
Bonded windscreen electric
Cutter
This cutter (Figure 3.65) greatly simplifies what
was formerly a time-consuming task for two people.
With this tool, windows bonded in with
polyurethane adhesive can be taken out without
difficulty. There is no environmental hazard, and
no fumes which might constitute a health risk.
Residual adhesive can be removed without damaging
the body.
The special cutter uses an AC/DC motor. An
electric circuit permits the blade oscillating frequency
to be varied to suit the required cutting
speed. A wide selection of special-purpose steel
cutting blades is now available for all current
car models. To simplify operation at various
points on the body, the cutter blade has a twelvesided
mount to vary its position in relation to
the tool.
This precision tool is used to cut through seals,
joints and hard-bonded connections with exceptional
speed and ease. Bonded-in car windscreens,
for example, can be removed without damaging
either the glass or the frame.
The equipment consists of a high-performance
dust extractor unit and a range of plug-in air tools
which includes random orbital sanders, orbital
sanders, saws and grinders (Figures 3.66, 3.67).
Dust created during the sanding or sawing
process is immediately extracted into a handy,
easily emptied vacuum pump. In the case of the
sanders, a series of holes in the pad itself is used
Figure 3.65Bonded windscreen electric cutter
(Fein/George Marshall Ltd )
Figure 3.66Centralized dust extraction system for
use with FESTO tools (Minden Industrial Ltd )
Figure 3.67Dust extraction system in a workshop
(Minden Industrial Ltd )
3.25 Dust extraction for power tools
Built-in dust extraction should be a must in any
new equipment programme, for it is essential that
airborne dust levels in body shops are reduced to a
minimum for the safety of the body shop employees.
Sanding, grinding and cutting operations in
the body shop usually generate quantities of airborne
dust; this is a major hazard not only to health
but also for refinishers wanting a quality paint
finish. Systems therefore have been specifically
designed to overcome problems of dust, noise and
oil pollution.
120Repair of Vehicle Bodies
to remove the dust and debris which, with conventional
systems, form a cloud and obscure the
surface to be worked on as well as necessitating
breathing apparatus for safety’s sake. A single
quick-fix hose connects the hand tool to the
power source. The hose has three separate compartments
for compressed air supply, exhaust and
dust extraction (Figures 3.68, 3.69, 3.70). Most
sanding and flatting tools have adjustable speed
control. They have housings made of high-impact
GRP, which makes them light in weight and suitable
for prolonged usage without operator fatigue.
Figure 3.68Two extraction points with self-closing
flaps (Minden Industrial Ltd )
Figure 3.69Air tool with hose attachment for
centralized system (Minden Industrial Ltd )
Figure 3.70Dust-free hand sanding tools: Vacbloc
and Vacfile (Minden Industrial Ltd )
Hand and power tools 121
Questions
1 Sketch and describe each of the following hand
tools, giving an example of the type of work it
would be used for: (a) cross-pein and finishing
hammer (b) body spoon (c) planishing hammer.
2 List the hand tools which you would expect to find
in a body repairer’s toolkit.
3 Describe a suitable application for the use of
a flexible body file.
4 Describe a suitable application for (a) a toe dolly
(b) an anvil dolly.
5 Explain a repair situation where a body spoon
may be used instead of a panel dolly.
6 Sketch and describe four essential hand tools
used by the body repairer.
7 Name and explain the use of a hand tool which
would have the effect of spreading a blow, thus
reducing the stretching of the metal.
8 For what purpose would a sandbag and a
wooden hollowing block be used?
9 State a typical use for a pear-shaped mallet.
10 Describe a typical repair application in which a
pick hammer would be used.
11 Describe a repair situation in which each of the
following special hand tools would be used:
(a) impact driver (b) panel puller (c) edge setting
tool (d) door skinner.
12 Describe how the following four power tools could
be used by the body repair worker: (a) power saw
(b) spot-weld remover (c) bonded windscreen
cutter (d) miniature belt sander.
13 Describe the movement of a dual-action (DA)
sander.
14 In what circumstances would a grid dolly be
used?
15 Compare the advantages of a beating file with
those of a planishing hammer.
16 When referring to sheet metal snips, what do the
terms ‘right hand’ and ‘left hand’ mean?
17 Describe three types of sheet metal bench
stakes, and sketch one of them.
18 Explain two safety precautions which must be
observed when using a cold chisel.
19 What safety precautions must be observed
before using an electric drilling machine?
20 Describe the safety measures which would be
necessary when using hand and small power
tools in a workshop.
21 What action should be taken to render a panel
hammer safe for further use after its head has
become loose?
22 Why are the teeth on a body file milled in a
curved formation?
23 Some beating files and panel hammers have
serrated faces. Explain the reasons for this.
24 Name a pneumatic tool which can be used for the
removal of a damaged panel section.
25 Name three types of spanner that could be used
by a body repair worker.
26 Describe the advantages of socket sets in
removing and replacing damaged panels.
27 Sketch a Torx screwdriver bit that could be used
with an impact screwdriver.
28 State the function of an air transformer and its
importance when used with power tools.
29 Explain the two different types of air hose
couplings used with pneumatic tools.
30 Explain the reason why the face of a planishing
hammer should be maintained in perfect
condition.
31 State the dangers of carrying sharp or pointed
tools in pockets or protective clothing.
32 When a fault develops on an electrical power
tool, what action must be taken?
33 Explain the reasons why some tools have a
built-in dust extraction system while others are
connected to a centralized dust extraction
system.
34 Identify and sketch two types of mole grips used
in body repair applications.
35 With the aid of a sketch, identify the BS symbol
used on power tools to indicate double insulation.
Metals and
non-metals used
in vehicle bodies
4.1 Manufacture of steel coil and sheet
for the automobile industry
In the manufacture of steel coils, the raw material
iron ore is fed into a blast furnace, together with
limestone and coke; the coke is used as a source of
heat, while the limestone acts as a flux and separates
impurities from the ore. The ore is quickly
reduced to molten iron, known as pig iron, which
contains approximately 3–4 per cent carbon. In the
next stage of manufacture, the iron is changed into
steel by reheating it in a steel-making furnace and
blowing oxygen either into the surface of the iron
or through the liquid iron, which causes oxidation
of the molten metal. This process burns out impurities
and reduces the carbon content from 4 per cent
to between 0.08 and 0.20 per cent.
Casting
The steel is cast into ingots; these are either heated
in a furnace and rolled down to a slab, or more commonly
continuously cast into a slab. Slabs by either
casting process are typically 8–10 in (200–250 mm)
thick, ready for further rolling. These slabs are
reheated prior to rolling in a computer-controlled
continuous hot strip mill to a strip around twice the
thickness required for body panels. The strip is
closely wound into coil ready for further processing.
Pickling
Before cold rolling, the surfaces of the coils must
be cleaned of oxide or black scale formed during
the hot rolling process and which would otherwise
ruin the surface texture. This is done by pickling
the coils in either dilute hydrochloric acid or dilute
sulphuric acid and then washing them in hot water
to remove the acid. The acid removes both the
oxide scale and any dirt or grit which might also be
sticking to the surface of the coil.
Cold rolling
In the cold rolling process the coil is rolled either
in a single-stand reversing mill (narrow mills
using either narrow hot mill product or slit wide mill
product) or in a multiple-stand tandem mill to the
required thickness. Most mills are computer controlled
to ensure close thickness control, and employ
specially prepared work rolls to ensure that the right
surface standard is achieved on the rolled strip. The
cold rolling process hardens the metal, because mild
steel quickly work hardens. The cold rolled coils are
suitable for applications such as panelling where no
bending or very little deformation is needed. At this
stage the coil is still not suitable for the manufacture
of the all-steel body shell and it must undergo a further
process to soften it; this is known as annealing.
Annealing
Coils used for the manufacture of a car body must
not only have a bright smooth surface but must also
be soft enough for bending, rolling, shaping and
pressing operations, and so the hardness of cold
rolled coils to be used for car bodies must be
reduced by annealing. If annealing were carried out
in an open furnace this would destroy the bright
surface of the coil and therefore oxygen must be
excluded or prevented from attacking the metal during
the period of heating the coils.
The normal method of annealing the coils is box
annealing. The coils are stacked on a furnace base,
covered by an inner hood and sealed. The atmosphere
is purged with nitrogen and hydrogen to
eliminate oxygen. A furnace is then placed over the
stack and fired to heat the steel coils to a temperature
of about 650 °C for around 24 hours, depending
upon charge size and steel grade.
Temper rolling
During the process of annealing the heat causes a
certain amount of buckling and distortion, and a
further operation is necessary to produce flat coils.
The annealed coil is decoiled and passed through a
single-stand temper mill using a specially textured
work roll surface, where it is given a light skin
pass, typically of 0.75–1.25 per cent extension.
This is necessary to remove buckles formed during
annealing, to impart the appropriate surface texture
to the strip, and to control the metallurgical properties
of the strip. The strip is then rewound ready for
despatch or finishing as appropriate.
Finishing
The temper rolled coils can be slit to narrow coil,
cut to sheet, reinspected for surface critical applications,
or flattened for flatness critical applications
as appropriate. Material can be supplied with
a protective coating oil, and packed to prevent
damage or rusting during transit and storage.
4.2 Specifications of steels used
in the automobile industry
The motor body industry uses many different types
of steel. Low-carbon steel is used for general constructional
members. High-tensile steels are used for
bolts and nuts which will be subjected to a heavy
load. Specially produced deep-drawn steel including
micro-alloyed steel is used for large body panels
which require complex forming. Zinc-coated steel
sheets are increasingly being specified for automobile
production, both for body and chassis parts,
as improved corrosion protection is sought. Stainless
steel is used for its non-rusting, hard wearing and
decorative qualities. The many different types of
springs used in the various body fittings are produced
from spring steel, while specially hardened
steels make the tools of production. Drills, chisels,
saws, hacksaws and guillotine blades are all produced
from special alloy steels, which are made
from an appropriate mixture of metals and elements.
Steel varies from iron chiefly in carbon content;
iron contains 3–4 per cent carbon while carbon steels
may contain from 0.08 per cent to 1.00 per cent
carbon. The chemical composition and mechanical
properties of these carbon steels, especially when
alloyed with other elements such as nickel,
chromium and tungsten, have been gradually standardized
over the years, and now the different types
of steels used are produced to specifications laid
down by the British Standards Institution. A British
Standard specification defines the chemical composition
and mechanical properties of the steel, and also
the method and apparatus to be used when testing
samples to prove that the mechanical properties are
correct. The tensile strength, and in the case of sheet
and strip steel the bend test, are the properties of
most interest, but the British Standard specification
also defines the elongation, the yield point and the
hardness of the steel.
The steels used in the motor trade may be grouped
as follows:
1 Cold forming steels
2 Carbon steels
3 Alloy steels
4 Free cutting steels
5 Spring steels
6 Rust-resisting and stainless steels.
As each group may contain many different specifications,
some idea of the variety of steels may be
gained. However, in the motor body industry the
specifications which apply are those pertaining to
cold forming steels, namely BS 1449: Part 1: 1983.
The greatest percentage of steel used in motor
bodies is in the form of coil, strip, sheet or plate.
Sheet steel is a rolled product produced from a
wide rolling mill (600 mm or wider); to come
under the heading of sheet steel, the steel must be
less than 3 mm thick. Steel 3–16 mm thick comes
under the heading of plate.
Tables 4.1, 4.2 and 4.3 are the specifications for
steel sheet strip and coil for the manufacture of
motor body shells in the automobile industry.
Metals and non-metals used in vehicle bodies 123
124Repair of Vehicle Bodies
4.3 Carbon steel
Carbon steels can be classified as follows (Table 4.3):
Low-carbon steel
Carbon-manganese steel
Micro-alloyed steel
Medium-carbon steel
High-carbon steel.
The properties of plain carbon steel are determined
principally by carbon content and microstructure,
but it may be modified by residual elements other
than carbon, silicon, manganese, sulphur and phosphorus,
which are already present. As the carbon
content increases so does the strength and hardness,
but at the expense of ductility and malleability.
Low-carbon steel
For many years low-carbon steel (sometimes
referred to as mild steel) has been the predominant
autobody material. Low-carbon coil, strip and
sheet steel have been used in the manufacture of
car bodies and chassis members. This material has
proved an excellent general-purpose steel offering
an acceptable combination of strength with good
forming and welding properties. It is ideally suited
for cold pressings of thin steel sheet and is used for
wire drawing and tube manufacture because of its
ductile properties.
Low-carbon steel is soft and ductile and cannot
be hardened by heating and quenching, but can be
case hardened and work hardened. It is used extensively
for body panels, where its high ductility and
malleability allows easy forming without the danger
of cracking. In general low-carbon steel is used
for all parts not requiring great strength or resistance
to wear and not subject to high temperature or
exposed to corrosion.
However, new factors such as a worldwide
requirement for fuel conservation for lighter-weight
Table 4.1Symbols for material conditions: BS 1449: Part 1: 1983
Condition Symbol Description
Rimmed steel R Low-carbon steel in which deoxidation has been controlled to
produce an ingot having a rim or skin almost free from carbon
and impurities, within which is a core where the impurities are
concentrated
Balanced steel B A steel in which processing has been controlled to produce an ingot
with a structure between that of a rimmed and a killed steel. It is
sometimes referred to as semi-killed steel
Killed steel K Steel that has been fully deoxidized
Hot rolled on wide mills HR Material produced by hot rolling. This will have an oxide scale
narrow mills HS coating, unless an alternative finish is specified (see Table 4.2)
Cold rolled on wide mills CR Material produced by cold rolling to the final thickness
narrow mills CS
Normalized N Material that has been normalized as a separate operation
Annealed A Material in the annealed last condition (i.e. which has not been
subjected to final light cold rolling)
Skin passed SP Material that has been subjected to a final light cold rolling
Temper rolled Material rolled to the specified temper and qualified as follows:
H1 Eighth hard
H2 Quarter hard
H3 Half hard
H4 Three-quarters hard
H5 Hard
H6 Extra hard
Hardened and tempered HT Material that has been continuously hardened and tempered in order
to give the specified mechanical properties
Metals and non-metals used in vehicle bodies 125
body structures, and safety legislation requiring
greater protection of occupants through improved
impact resistance, are bringing about a change in
materials and production technology. This has
resulted in the range of micro-alloyed steels known
as high-strength steels (HSSs) or high-strength lowalloy
steels (HSLAs).
Micro-alloyed steel
This steel is basically a carbon-manganese steel
having a low carbon content, but with the addition
of micro-alloying elements such as niobium and
titanium. Therefore it is classed as a low-alloy highstrength
steel within the carbon range. As a result of
its strength, toughness, formability and weldability,
the car body manufacturers are using this material
to produce stronger, lighter-weight body structures.
A typical composition utilized for a micro-alloyed
high-strength steel (HSS) is as follows:
Percentage
Carbon (C) 0.05–0.08
Manganese (Mn) 0.80–1.00
Niobium (Nb) 0.015–0.065
The percentage of niobium used depends on the
minimum strength required.
Formable HSSs were developed to allow the automotive
industry to design weight out of the car in
support of fuel economy targets. A range of highstrength
formable steels with good welding and
Table 4.2Symbols for surface finishes and surface inspection: BS 1449: Part 1: 1983
Finish Symbol Description
Pickled P A hot rolled surface from which the oxide has been removed by
chemical means
Mechanically descaled D A hot rolled surface from which the oxide has been removed by
mechanical means
Full finish FF A cold rolled skin passed material having one surface free from blemishes
liable to impair the appearance of a high-class paint finish
General-purpose finish GP A cold rolled material free from gross defects, but of a lower standard
than FF
Matt finish M A surface finish obtained when material is cold rolled on specially
prepared rolls as a last operation
Bright finish BR A surface finish obtained when material is cold rolled on rolls having a
moderately high finish. It is suitable for most requirements, but is not
recommended for decorative electroplating
Plating finish PL A surface finish obtained when material is cold rolled on specially
prepared rolls to give one surface which is superior to a BR finish and
is particularly suitable for decorative electroplating
Mirror finish MF A surface finish having a high lustre and reflectivity. Usually available
only in narrow widths in cold rolled material
Unpolished finish UP A blue/black oxide finish; applicable to hardened and tempered strip
Polished finish PF A bright finish having the appearance of a surface obtained by fine
grinding or abrasive brushing; applicable to hardened and
tempered strip
Polished and coloured blue PB A polished finish oxidized to a controlled blue colour by further heat
treatment; applicable to hardened and tempered strip
Polished and coloured yellow PY A polished finish oxidized to a controlled yellow colour by further heat
treatment; applicable to hardened and tempered strip
Vitreous enamel VE A surface finish for vitreous enamelling of material of specially selected
chemical composition
Special finish SF Other finishes by agreement between the manufacturer and the purchaser
126Repair of Vehicle Bodies
Table 4.3Summary of material grades, chemical compositions and types of steel available: BS 1449: Par t 1: 1983
Chemical composition
Material Rolled condition
CSi Mn
SP
grade (see Table 4.1) min. max. min. max. min. max. max. max.
Materials having specific requirements based on formability
%% % % % % % %
1 HR, HS, –, –, – 0.08 – – – 0.45 0.030 0.025 Extra deep drawing aluminium-killed steel
1 –, –, CR, CS – 0.08 – – – 0.45 0.030 0.025 Extra deep drawing aluminium-killed
stabilized steel
2 HR, HS, CR, CS – 0.08 – – – 0.45 0.035 0.030 Extra deep drawing
3 HR, HS, CR, CS – 0.10 – – – 0.50 0.040 0.040 Deep drawing
4 HR, HS, CR, CS – 0.12 – – – 0.60 0.050 0.050 Drawing or forming
14 HR, HS, –, –, – 0.15 – – – 0.60 0.050 0.050 Flanging
15 HR, HS, –, –, – 0.20 – – – 0.90 0.050 0.060 Commercial
Materials having specific requirements based on minimum strengths
Carbon-manganese steels
34/20 HR, HS, CR, CS – 0.15 – – – 1.20 0.050 0.050 Available as rimmed (R), balanced (B)
37/23 HR, HS, CR, CS – 0.20 – – – 1.20 0.050 0.050 or killed (K) steels
43/25 HR, HS, –, –, – 0.25 – – – 1.20 0.050 0.050
50/35 HR, HS, –, –, – 0.20 – – – 1.50 0.050 0.050 Grain-refined balanced (B) or killed (K)
steel
Micro-alloyed steels
40/30 HR, HS, –, CS – 0.15 – – – 1.20 0.040 0.040 Grain-refined niobium- or titanium-treated
43/35 HR, HS, –, CS – 0.15 – – – 1.20 0.040 0.040 fully killed steels having high yield
46/40 HR, HS, –, CS – 0.15 – – – 1.20 0.040 0.040 strength and good formability
50/45 HR, HS, –, CS – 0.20 – – – 1.50 0.040 0.040
60/55 –, HS, –, CS – 0.20 – – – 1.50 0.040 0.040
_
Metals and non-metals used in vehicle bodies 127
40F30 HR, HS, –, CS – 0.12 – – – 1.20 0.035 0.030 The steels including F in their designations
43F35 HR, HS, –, CS – 0.12 – – – 1.20 0.035 0.030 in place of the oblique line offer superior
46F40 HR, HS, –, CS – 0.12 – – – 1.20 0.035 0.030 formability for the same strength levels
50F45 HR, HS, –, CS – 0.12 – – – 1.20 0.035 0.030
60F55 –, HS, –, CS – 0.12 – – – 1.20 0.035 0.030
68F62 –, HS, –, – – 0.12 – – – 1.50 0.035 0.030
75F70 –, HS, –, – – 0.12 – – – 1.50 0.035 0.030
Narrow strip supplied in a range of conditions for heat treatment and general engineering purposes
4 –, HS, –, CS – 0.12 – – – 0.60 0.050 0.050 Low-carbon steel available hot rolled,
annealed, skin passed or cold rolled to
controlled hardness ranges H1 to
H6 inclusive
10 –, HS, –, CS 0.08 0.15 0.10 0.35 0.60 0.90 0.045 0.045 For case hardening
12 –, HS, –, CS 0.10 0.15 – – 0.40 0.60 0.050 0.050 A range of carbon steels available in the hot
17 –, HS, –, CS 0.15 0.20 – – 0.40 0.60 0.050 0.050 rolled or annealed condition
20 –, HS, –, CS 0.15 0.25 0.05 0.35 1.30 1.70 0.045 0.045
22 –, HS, –, CS 0.20 0.25 – – 0.40 0.60 0.050 0.050
30 –, HS, –, CS 0.25 0.35 0.05 0.35 0.50 0.90 0.045 0.045
40 –, HS, –, CS 0.35 0.45 0.05 0.35 0.50 0.90 0.045 0.045 A range of carbon steels for use in the hot
50 –, HS, –, CS 0.45 0.55 0.05 0.35 0.50 0.90 0.045 0.045 rolled, normalized, annealed and (except
60 –, HS, –, CS 0.55 0.65 0.05 0.35 0.50 0.90 0.045 0.045 for grade 95) in the temper rolled (half
70 –, HS, –, CS 0.65 0.75 0.05 0.35 0.50 0.90 0.045 0.045 hard) conditions. Grades 40 and 50 may
80 –, HS, –, CS 0.75 0.85 0.05 0.35 0.50 0.90 0.045 0.045 be induction or flame hardened and grades
95 –, HS, –, CS 0.90 1.00 0.05 0.35 0.30 0.60 0.040 0.040 60, 70, 80 and 95 may be supplied in the
hardened and tempered condition
128Repair of Vehicle Bodies
painting characteristics have been developed. The
steels are hot rolled for chassis and structural components,
and cold reduced for body panels. Through
carefully controlled composition and processing
conditions, these steels achieve high strength in
combination with good ductility to allow thinner
gauges to be used: a reduction from 0.90 mm down
to 0.70 mm is a general requirement.
There was a danger that the new HSS, thinner
but just as strong, would lack the ductility
which allows it to be press formed into shape.
This problem was overcome by the use of microalloying.
In a metal crystal the atoms are in
layers; when the crystals are stretched (as in
forming), one layer of atoms slides over another.
The layers of atoms slide like playing cards in a
pack, and in doing so are changing shape in a
ductile way. The sliding can be controlled by
adding elements to the steel such as niobium
or titanium. The element reacts with the carbon
to produce fine particles which spread through
the steel. The element controls the ductility and
strengthens the steel, thereby improving the
properties of the material.
These steels are low carbon, employing solution
strengthening (cold reduced rephosphorized) or
precipitation hardening (hot rolled and cold
reduced micro-alloyed) elements to produce finegrained
steels which are suitable for welding by
spot and MIG processes only.
Medium-carbon steel
This can be hardened by quenching, and the amount
it can be hardened increases with its carbon content.
This type of steel can be used for moving parts such
as connecting rods, gear shafts and transmission
shafts, which require a combination of toughness
and strength, but it is being replaced in the car
industry by high-alloy steels.
High-carbon steel
This can be hardened to give a very fine cutting
edge, but with some loss of its ductile and malleable
properties. It is used for metal and wood
cutting tools, turning tools, taps and dies, and
forging and press dies because of its hardness
and toughness, but is seldom used now for motor
vehicle parts because of the introduction of highalloy
steels.
Zinc coated steels
The automotive industry, in seeking to provide
extended warranties, is turning increasingly to the
use of zinc coated steels. Modern automobiles
must be not only of high quality but also durable
and economical, as perceived by their purchasers.
These vehicles are expected to exceed an average
of seven years without structural or cosmetic deterioration
due to corrosion. Increasingly aggressive
environmental influences tend to shorten the life of
the car, whereas ever more specialized steel sheets
are being incorporated into vehicle construction in
the battle against corrosion.
The use of zinc coated steels has dramatically
increased to meet these challenges. Different
areas of a vehicle require different zinc coatings
and coating weights to meet appearance and performance
criteria. These are available in both
hot dipped (BS 2989: 1982) and electrolytically
deposited (BS 6687: 1986) versions in a range of
coating weights or thickness. Both types offer
barrier and sacrificial corrosion protection, and
the choice of product depends on the particular
application and requirements. The hot dip product
(available as plain zinc, or iron-zinc alloy) is generally
used for underbody parts. The electrolytic
product is used for exposed body panels, where a
full-finish surface quality is available to ensure
that a showroom paint finish is achieved. The
electrolytic product is available in single-sided
and double-sided coating. (See Figure 4.1.)
Single-sided zinc coated steel Free zinc is applied
to one side of a steel sheet by either the hot dip or
the electrolytic process for this material. Its
uncoated side provides a good surface for paint
appearance, so it is used mainly for outer body panels.
Since free zinc is towards the inside of the car,
it protects against perforation corrosion.
One-and-a-half-sided zinc coated steel In this
case, one side of the sheet is coated with free zinc,
and a thin layer of zinc-iron alloy is formed on the
other side. This product is produced mainly by the
hot dip process. It is primarily used for exposed
panels, where the zinc-iron layer is on the outside
for cosmetic protection and the free zinc side provides
perforation protection.
Double-sided zinc coated steel This product is
manufactured by applying free zinc to both
sides of the sheet with equal or differential
Figure 4.1Body shell panels showing galvanized protection (Motor Insurance Repair Research Centre)
A _ Galvanized one side only
B _ Galvanized both sides
C _ Galvanized layer applied individually
1 Front grille panel B
2 Reinforcement B
3 Front bumper mounting
reinforcement RH and LH B
4 Bonnet lock reinforcement RH
and LH B
5 Front wing RH and LH B
6 Bonnet lock panel B
7 Front suspension turret stiffener
RH and LH A
8a/8b Front wheel arch gussets RH
and LH B(a)/C(b)
9 Front bulkhead stiffener RH
and LH A
10 Front inner wing RH and LH B
11 Front suspension turret RH and LH A
12 A-post reinforcement RH and LH B
13 Body side gussets RH and LH B
14 Body side RH and LH A
15 Inner sill RH and LH A
16 Inner sill reinforcement RH and LH A
17 B-post gusset RH and LH A
18 Upper dash panel B
19 Bonnet skin B
20 Bonnet frame B
21 Dash stiffener RH and LH B
22 Front chassis leg closing panel
RH and LH A
23 Front chassis leg gusset RH and LH B
24 Front chassis leg RH and LH A
25 Front door frame stiffener RH and LH A
26 Front door skin RH and LH A
27 Rear door skin RH and LH A
28 Rear door frame RH and LH B
29 Rear door frame stiffener RH and LH A
30 Rear door frame gusset RH and LH B
31 Rear door skin stiffener RH and LH A
32 Rear door lock stiffener RH and LH A
33 Front window frame stiffener RH and LH A
34 Front door frame RH and LH B
35 Front door skin stiffener RH and LH A
36 Front door lock stiffener RH and LH A
37 Sill rear gusset RH and LH A
38 Rear chassis leg RH and LH A
39 Floor/heelboard gussets A
40 Rear chassis leg gusset RH and LH B
41 Boot floor cross members A
42 RH boot floor brace closing panel A
43 Boot floor brace RH and LH A
44 Boot floor bracket RH and LH A
45 Rear bumper mounting gusset RH and LH B
46 Boot lid frame B
47 Exhaust rear hanger stiffener B
48 Boot lid skin A
49 Boot lid lock mounting stiffener B
50 Rear wing RH and LH A
51 Rear suspension turret RH and LH A
52 Suspension turret capping RH and LH B
53 Boot floor side extension RH and LH A
54 Outer rear wheel arch RH and LH A
130Repair of Vehicle Bodies
coating weight. All types are readily paintable
and weldable. However, care should be taken to
ensure that welding conditions are comparable
with the material used: for example, higher weld
current ratings may be necessary on the heavier
coatings.
4.4 Alloy steels
Alloy steel is a general name for steels that owe
their distinctive properties to elements other than
carbon. They are generally classified into two
major categories:
Low-alloy steel possesses similar microstructures
to and requires similar heat treatments to plain
carbon steels (see Section 4.3.2 on micro-alloyed
steel).
High-alloy steel may be defined as a steel having
enhanced properties owing to the presence of one
or more special elements or a larger proportion of
element than is normally present in carbon steel.
This section is concerned primarily with high-alloy
steels.
Alloy steels usually take the name of the element
or elements, in varying percentages, having
the greatest influence on the characteristics of
the alloy.
Chromium Increased hardness and resistance to
corrosion.
Cobalt Increased hardness, especially at high
temperatures.
Manganese High tensile strength, toughness and
resistance to wear.
Molybdenum Increased hardness and strength at
high temperatures.
Nickel Increased tensile strength, toughness,
hardness and resistance to fatigue.
Niobium Strong carbide forming effect; increases
tensile strength and improves ductility.
Silicon Used as a deoxidizing agent, and has the
slight effect of improving hardness.
Titanium Strong carbide forming element.
Tungsten Greater hardness, especially at high
temperatures; improved tensile strength and resistance
to wear.
Vanadium Increased toughness and resistance to
fatigue.
Correct heat treatment is essential to develop the
properties provided by alloying elements.
There are many alloy steels containing different
combinations and percentages of alloying elements,
of which some of the most popular are as follows:
High-tensile steel Used whenever there is an essential
need for an exceptionally strong and tough
steel capable of withstanding high stresses. The
main alloying metals used in its manufacture are
nickel, chromium and molybdenum and such steels
are often referred to as nickel-chrome steels. The
exact percentage of these metals used varies according
to the hardening processes to be used and the
properties desired. Such steels are used for gear
shafts, engine parts and all other parts subject to high
stress.
High-speed steels These are mostly used for cutting
tools because they will withstand intense heat
generated by friction and still retain their hardness
at high temperatures. It has been found that by
adding tungsten to carbon steel, an alloy steel is
formed which will retain a hard cutting edge at
high temperatures. High-speed steels are based on
tungsten or molybdenum or both as the primary
heat-resisting alloying element; chromium gives
deep hardening and strength, and vanadium adds
hardness and improves the cutting edge.
Manganese steel An addition of manganese to
steel produces an alloy steel which is extremely
tough and resistant to wear. It is used extensively
in the manufacture of chains, couplings and hooks.
Chrome-vanadium steel This contains a small
amount of vanadium which has the effect of intensifying
the action of the chromium and the manganese
in the steel. It also aids in the formation of carbides,
hardening the alloy and increasing its ductility.
These steels are valuable where a combination of
strength and ductility are desired. They are often
used for axle half-shafts, connecting rods, springs,
torsion bars, and in some cases hand tools.
Silicon-manganese steel This is a spring steel using
the two elements of manganese and silicon. These
steels have a high strength and impact resistance and
are used for road springs and valve springs.
High-strength steel
High-strength steel have been introduced into automotive
production slowly only because of the need
for specialised press tools to form body panels
from this stronger material. The die tools need to
be harder than for normal low carbon (LC) steel,
Metals and non-metals used in vehicle bodies 131
and the presses need to be stronger and more
accurate. HSS came about because of the need to
make vehicles lighter following the 1970 fuel crisis.
Lighter car means better fuel economy. This
lead to American car makers forming the Ultra
Light Steel Auto Body (ULSB) group and the Ultra
Light Steel Body – Advanced Vehicle Concepts
(ULSB – ACV) group. This further research has
led to the concept of advanced high-strength steel
(AHSS) as the materials have been developed and
understood.
Cost
Steel costs about one-fifth of the price of aluminium
when bought in the quantities needed by a
car maker. Also the iron and steel industry has hundreds
of years of practical experience in shaping
and forming steel compared to the other materials
which could be used to make vehicle bodies.
Properties of HSS
High-strength steel has a yield strength ranging
from 300 to 1200 MPa compared to LC steel
which has a range of 140–180 MPa. However,
although the metal is stronger, it is not necessarily
stiffer. That is, the body parts can not necessarily
be made of thinner metal as they are likely to sag.
If you look at the swage lines on the latest vehicles,
you will see that many panels are stiffened
by the use of swaging. The current modern shapes
are to allow the usage of thinner sheet steel which
is lighter and of course cheaper. Oddly however,
the new vehicles are not lighter in weight; this is
because of the addition of electrical body controls
such as electric windows and seats. HSS is not as
easy to form as LC steel; also some types of HSS
can be drawn better than other. Generally the
extra strength of HSS is brought about by changes
in the steel microstructure during the steel processing.
The following paragraphs discuss the different
types of HSS and AHSS steels.
HSS are also known as re-phosphorized – added
phosphorous; isotropic – added silicone and bake
hardened – strain age hardened. The two most
common types used in vehicle body construction
are MSLA and HSLA.
Medium-Strength Low Alloy steel has a yield
strength of between 180 and 300 MPa. This steel is
made by dissolving more phosphorous or manganese
alloy into the molten steel during manufacture.
High-Strength Low Alloy steel has a yield
strength of between 250 and 500 MPa. This is made
by adding small amounts of titanium or niobium to
the molten steel which produces a fine dispersion of
carbide particles.
Advanced high-strength steel types are aimed at
producing steel with suitable mechanical properties
for the forming of vehicle body parts, usually
through the hydro forming process – using water
pressure to mould the metal over the die.
Dual phase (DP) steel has a yield strength of
between 500 and 1000 MPa. It is made by adding
carbon to enable the formation of (hard) martensite
in a more ductile ferrite matrix. Manganese,
chromium, vanadium or nickel may also be added.
The DP steel may have its strength triggered by
either bake hardening or work hardening when it is
stressed under the stamping or other forming process.
Transformation induced plasticity (TRIP) steel
has a yield strength of between 500 and 800 MPa
with greater figures attainable in some cases. TRIP
steels may be alloyed with higher quantities of carbon
and silicone and aluminium. The strength is
triggered by work hardening by the stress induced
during the stamping or forming process. That is,
the retained austenite is transformed into martensite
by the increasing strain during the stamping or
other forming process.
Complex phase (CP) steel has a yield strength of
800–1200 MPa. CP steel has a very fine microstructure
using the same alloying elements as in DP or
TRIP steel with the possible additions of niobium,
titanium and/or vanadium. Again the high strength is
triggered by applied strain.
Applications of AHSS
TRIP and CP steel is ideal for use in crash zones. It
is excellent for absorbing energy during impact.
CP steel is often used for ‘A’ and ‘B’ posts and
bumper attachments. Increasingly AHSS steel is
used for strengthening members to which other
steel panels are welded, in other words a steel
composite structure.
Repair of HSS and AHSS panels
One of the problems is that it is not possible to recognize
HSS and AHSS panels by sight. Therefore it is
essential to follow the guidelines offered by the vehicle
manufacturer on recommended repair methods.
As a general guide, look out for parts such as ‘A’, ‘B’
132Repair of Vehicle Bodies
and ‘C’ posts, screen pillars, cant rails and strengthening
cross members on cars made after about 1990.
On new cars look out for panels with large swage
lines, remember that the pressing process triggers the
hardening of the metal and so makes the panel stiff
both in shape and in microstructure. Damaged AHSS
and HSS panels are not readily repairable as the
impact changes the microstructure, making the metal
harder. So, panel beating is not an option, replacement
panels must be fitted in most cases.
The normal method of joining HSS and AHSS
panels is by spot welding. The heat and pressure
involved in the welding process changes the
microstructure of the metal, so great care is needed
in this process. Again, follow manufacturer’s
instruction on these repairs. It is sensible to do
tests before spot welding the new panels. You can
use the undamaged sections of the panels which
you have removed for test welds, changing weld
time and current, then cutting through the welded
area to check for penetration and adhesion.
Any form of applied heat to HSS and AHSS
panels should be avoided, this includes trying to
anneal or soften the panel for the purposes of
straightening, heat shrinking or oxyacetylene cutting.
Neither MIG plugging nor cold working are
recommended. Remember that the nature of these
processes will affect a change to the properties of
the steel, and that the energy of the impact will
have had the same effect on the panel.
Guide to spotting AHSS and HSS panels
• Look for reinforced areas such as door pillars
and cross bracing areas, and where two or more
panel parts over lap each other.
• Look for body panels with pronounced sharp
swage lines.
• Feel for very thin panels.
• Listen for panels which when tapped gently
give a crisp metallic ring.
Boron steel
A particular type of HSS is referred to as Boron
steel, containing a very small amount of boron,
typically 0.001–0.004 percent. This steel is used by
many of the major manufacturers such as Vauxhall
and requires special repair techniques. To join
boron steel panels the method used is often MIG
brazing. See 12.13.3.
4.5 Stainless steel
The discovery of stainless steel was made in 1913
by Harry Brearley of Sheffield, while he was experimenting
with alloy steels. Among the samples
which he threw aside as unsuitable was one containing
about 14 per cent chromium. Some months later
he saw the pile of scrap test pieces and noticed that
most of the steels had rusted but the chromium steel
was still bright. This led to the development of stainless
steels. The classic Rolls-Royce radiator was one
of the first examples of the use of stainless steel.
The designer, engineer or fabricator of a particular
component may think that stainless steel is
going to be both difficult to work and expensive.
This is quite wrong, and perhaps stems from the
fact that many people tend to fall into the trap of
the generic term ‘stainless steels’. In fact, this is
the title for a wide range of alloys. Therefore if
such materials are to be used effectively and maximum
advantage is to be taken of the many benefits
they have to offer, there should be very close collaboration
and consultation over which grade of
stainless steel is best for the particular job in hand.
There are over 25 standard grades of stainless
steel specified by BS 1449: Part 2. Each provides a
particular combination of properties, some being
designed for corrosion resistance, some for heat
resistance and others for high-temperature creep
resistance. Many, of course, are multipurpose alloys
and can be considered for more than one of these
functions. In terms of composition, there is one element
common to all the different grades of stainless
steel. This is chromium, which is present to at least
10 per cent. It is this element which provides the
basis of the resistance to corrosion by forming what
is known as a ‘passive film’ on the surface of the
metal. This film is thin, tenacious and invisible and
is essentially a layer of chromium oxide formed by
the chromium in the steel combining with the oxygen
in the atmosphere. The strength of the passive
film, in terms of resistance to corrosion, increases
within limits with the chromium content and with
the addition of other elements such as nitrogen and
molybdenum. The formation of the passive film,
therefore, is a natural characteristic of this family of
steels and requires no artificial aid. Consequently, if
stainless steels are scratched or cut or drilled, the
passive film is automatically and instantaneously
repaired by the oxygen in the atmosphere.
Metals and non-metals used in vehicle bodies 133
Stainless steels can be conveniently divided into
the following three main groups:
Austenitic Generally containing 16.5–26 per cent
chromium and 4–22 per cent nickel.
Ferritic Usually containing 12–18 per cent
chromium.
Austenitic/ferritic duplex Usually containing 22 per
cent chromium, 5.5 per cent nickel, 3 per cent
molybdenum, and 0.15 per cent nitrogen.
Martensitic Based on a chromium content of
11–14 per cent, although some grades may have a
small amount of nickel.
Of the above groups, the austenitic steels are by far
the most widely used because of the excellent
combination of forming, welding and corrosionresisting
properties that they offer. Providing that
the correct grade is selected as appropriate to the
service environment, and that the design and production
engineering aspects are understood and
intelligently applied, long lives with low maintenance
costs can be achieved with these steels.
HyResist 22/5 duplex is a highly alloyed
austenitic/ferritic stainless steel. It has more than
twice the proof strength of normal austenitic stainless
steels whilst providing improved resistance to
stress corrosion cracking and to pitting attacks. It
possesses good weldability and can be welded by
conventional methods for stainless steel. The high
joint integrity achievable combined with good
strength and toughness permit fabrications to be
made to a high standard. It is being increasingly
used in offshore and energy applications.
Table 4.4 shows typical stainless steels used in
motor vehicles.
4.6 Aluminium
In the present-day search for greater economy in the
running of motor vehicles, whether private, public
or commercial, the tendency is for manufacturers to
produce bodies which, whilst still maintaining their
size and strength, are lighter in weight. Aluminium
is approximately one-third of the weight of steel,
and aluminium alloys can be produced which have
an ultimate tensile strength of 340–620 MN/m2. In
the early 1920s the pioneers of aluminium construction
were developing its use for both private
and commercial bodies; indeed, the 1922 40 hp
Lanchester limousine body had an aluminium alloy
construction for the bulkhead and bottom frame, and
aluminium was used for all the body panels. Before
the Second World War aluminium was used mainly
for body panels, but since the war aluminium alloys
have been and are now being used for body
Table 4.4Typical stainless steels used in vehicles: BS 1449: Part 2
Steel grade Typical alloying elements (%) Characteristics Typical applications
Austenitic
301 S21 17Cr 7Ni Good corrosion resistance Riveted body panels, wheel covers,
hubcaps, rocker panel mouldings
304 S16 18Cr 10Ni 0.06C Good corrosion resistance Mild corrosive tankers
305 S19 18Cr 11Ni 0.10C Low work hardening rate Rivets
316 S31 17Cr 11Ni 2.25Mo 0.07C Highest corrosion resistance Road tankers for widest cargo
of the commercial grades flexibility
Ferritic
409 S19 11Cr 0.08C _ Ti Reasonable combination Exhaust systems and catalytic
of weldability, formability converter components and freight
and corrosion resistance container cladding
430 S17 17Cr Good corrosion resistance. Tanker jackets, interior trim, body
Can be drawn and formed mouldings, windshield wiper arms
Martensitic
410 S21 13Cr 0.12C Corrosion resistant. Titanium modified for silencer
Heat-treatable composition components
capable of high hardness
134Repair of Vehicle Bodies
structures. Although aluminium is more expensive
than steel, it is easy to work and manipulate and
cleaner to handle. It also has the advantage of not
rusting, and, provided that the right treatment is
adopted for welding, corrosion is almost non-existent.
In recent years the use of aluminium and aluminium
alloys for motor bodies, especially in the
commercial field, has developed enormously.
In the modern motor body the saving of weight
is its most important advantage, and although on
average the panel thickness used is approximately
double that of steel, a considerable weight saving
can be achieved. One square metre of 1.6 mm thick
aluminium weights 4.35 kg while one square metre
of 1.00 mm thick steel weighs 7.35 kg; the use of
aluminium results in a saving in weight of just
under 40 per cent.
The non-rusting qualities of the aluminium group
are well known and are another reason for their use
in bodywork. An extremely thin film of oxide forms
on all surfaces exposed to the atmosphere, and even
if this film is broken by a scratch or chip it will
reform, providing complete protection for the metal.
The oxide film, which is only 0.0002 cm thick, is
transparent, but certain impurities in the atmosphere
will turn it to various shades of grey.
Production
The metal itself has only been known about
130 years, and the industrial history of aluminium
did not begin until 1886 when Paul Heroult in
France discovered the basis of the present-day
method of producing aluminium. Aluminium is
now produced in such quantities that in terms of
volume it ranks second to steel among the industrial
metals. Aluminium of commercial purity contains
at least 99 per cent aluminium, while higher
grades contain 99.5–99.8 per cent of pure metal.
In the production of aluminium, the ore bauxite
is crushed and screened, then washed and pumped
under pressure into tanks and filtered into rotating
drums, which are then heated. This separates the
aluminium oxide from the ore. In the next stage the
aluminium oxide is reduced to the metal aluminium
by means of an electrolytic reduction cell.
This cell uses powdered cryolite and a very heavy
current of electricity to reduce the aluminium
oxide to liquid metal, which passes to the bottom
of the cell and is tapped off into pigs of aluminium
of about 225 kg each.
Types of sheet
Sheet, strip and circle blanks are sold in hard and
soft tempers possessing different degrees of ductility
and tensile strength. Sheet is supplied in gauges
down to 0.3 mm, but it is generally more economical
to order strip for gauges less than 1.6 mm.
Manufacturing process
Sheet products are first cast by the semicontinuous
casting process, then scalped to remove surface
roughness and preheated in readiness for hot rolling.
They are first reduced to the thickness of plate, and
then to sheet if this is required. Hot rolling is followed
by cold rolling, which imparts finish and temper
in bringing the metal to the gauge required.
Material is supplied in the annealed (soft condition)
and in at least three degrees of hardness, H1, H2 and
H3 (in ascending order of hardness).
4.7 Aluminium alloys
From the reduction centre the pigs of aluminium are
remelted and cast into ingots of commercial purity.
Aluminium alloys are made by adding specified
amounts of alloying elements to molten aluminium.
Some alloys, such as magnesium and zinc, can be
added directly to the melt, but higher-melting-point
elements such as copper and manganese have to be
introduced in stages. Aluminium and aluminium
alloys are produced for industry in two broad groups:
1 Materials suitable for casting
2 Materials for the further mechanical production
of plate sheet and strip, bars, tubes and extruded
sections.
In addition both cast and wrought materials can be
subdivided according to the method by which their
mechanical properties are improved:
Non-heat-treatable alloys Wrought alloys, including
pure aluminium, gain in strength by cold working
such as rolling, pressing, beating and any
similar type of process.
Heat-treatable alloys These are strengthened by
controlled heating and cooling followed by ageing
at either room temperature or at 100–200 °C.
The most commonly used elements in aluminium
alloys are copper, manganese, silicon, magnesium
and zinc. The manufacturers can supply
these materials in a variety of conditions. The
Metals and non-metals used in vehicle bodies 135
non-heat-treatable alloys can be supplied either
as fabricated (F), annealed (O) or strain hardened
(H1, H2, H3). The heat-treatable alloys can be
supplied as fabricated (F) or annealed (O), or,
depending on the alloy, in variations of the heat
treatment processes (T3, T4, T5, T6, T8).
4.7.1 Wrought light aluminium alloys:
BS specifications 1470–75
Material designations
Unalloyed aluminium plate, sheet and strip:
1080A commercial pure aluminium
99.8 per cent
1050A commercial pure aluminium
99.5 per cent
1200 commercial pure aluminium
99.0 per cent
Non-heat-treatable aluminium alloy plate, sheet and
strip:
3103 AlMN
3105 AlMnMg
5005 AlMg
5083 AlMgMn
5154A AlMg
5251 AlMg
5454 AlMgMn
Heat-treatable aluminium alloy plate, sheet and strip:
2014A AlCuSiMg
Clad 2014A AlCuSiMg clad with pure aluminium
2024 AlCuMg
Clad 2024 AlCuMg
Clad 2024 AlCuMg clad with pure aluminium
6082 AlSiMgMn
Abbreviations for basic temper
As fabricated: F The temper designation F applies
to the products of shaping processes in which no special
control over thermal conditions or strain hardening
is employed. For wrought products there are no
specified requirements for mechanical properties.
Annealed: O The temper designation O applies
to wrought products which are annealed to obtain
the lowest strength condition.
Abbreviations for strain hardened materials
The temper designation H for strain hardened
products (wrought products only) applies to
products subjected to the application of cold
work after annealing (hot forming) and partial
annealing or stabilizing, in order to achieve the
specified mechanical properties. The H is always
followed by two or more digits, indicating the
final degree of strain hardening. The first digit
(1, 2 or 3) indicates the following:
H1 strain hardened only
H2 strain hardened and partially annealed
H3 strain hardened and stabilized
The second digit (2, 4, 6 or 8) indicates the degree
of strain hardening, as follows:
HX2 tensile strength approximately midway
between O temper and HX4 temper
HX4 tensile strength approximately midway
between O temper and HX8 temper