that are mostly judged on their
8
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W HAT YOU S HOU LD K NOW B E FOR E YOU START
content than iron, the even dispersal
of what carbon it does have makes it
strong and somewhat plastic, or
deformable, unlike various irons.
Mild sheet steel, the stuff of auto-
bodies, is roughly .25-percent car-
bon. Above that concentration of
carbon, steels begin to fit into the
medium steel classification. Between
.6-percent and 1-percent carbon,
steels are considered hard or high-
carbon. Ultra hard steels, like tool
steels, may contain between 1-per-
cent and 2-percent carbon.
How panels are supported makes a tremendous difference in how you
The softness of panel steel allows
approach their repair. This 2008 Mitsubishi Galant’s upper fender attachments
it to undergo the highly organized
are very unusual. Short strut pieces attach the fender tops to the car’s inner
brutality of stamping it into complex
fenders. Anyone who repairs these fenders has to take this into account.
three-dimensional shapes like doors,
hoods, roofs, and fenders. Using heat
visual merits. At times, and in some
truly amazing material. It is a com-
and enormous pressure, automotive
situations, a good practitioner uses
plex alloy of iron, carbon, and other
body steel is stamped into final sheet
characteristics of panel configuration
elements. It has been heat treated in
format. While it is primarily an alloy
to slightly trick the eye. (There will
its manufacture to disperse the car-
of iron and carbon, several other ele-
be more on this topic, later.)
bon evenly into the steel’s granular
ments—which, in some cases, have
Along with crown, how a panel
structure. While steel has less carbon
names that are hard to remember and
is supported and attached to a vehi-
cle is critical in understanding how it
performs under impact, and how
best to remove impact damage from
it. Many panels have strengthening
structures welded or bolted under
them. Panels that are attached to
vehicles by welding them to sub-
structure perform differently from
those that are bolted to substructure.
Unless you deal with them, bent or
damaged
substructure
reinforce-
ments and fastening points that
impart strength to panels, cause pan-
els to resist restoration to their origi-
nal formats. Always consider this
factor when you plan panel repair or
restoration work.
Autobody Steel
Throughout most of automotive history, all panels were stamped out in
presses, like the ones shown here in a General Motors stamping room in the
The steel sheet stock that is
mid 1970s. More recently, some very large stampings are rolled into panels by
formed into automobile panels is a
dies that move in two dimensions. (Photo supplied by General Motors Corp.) AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
9
CHAPTE R 1
difficult to pronounce—are routinely
and repair: plasticity and elasticity.
added to it to give it the special char-
Plasticity is the ability of metal to
acteristics that are needed to form it
deform without fracturing. The point
into automotive panels.
of fracture is called the “yield” point.
New car panels are presently
Automotive panels are stamped at
in the range of 22-gauge to 23-gauge;
the factory from flat stock into com-
that is, .0299 and .0269 inch.
plex, three-dimensional shapes. The
Note that as the gauge number
fact that this can be done is proof of
increases, the thickness of steel sheet
their plasticity. When a body repair
stock gets thinner. The way that this
Thin panels are hard, presenting
technician works on them with ham-
works involves an arcane formula
several problems in repair. It is easy
mers, dollies, and other tools, they
that takes into account the weight of
to cut through, when welding them.
are again deformed, courtesy of their
a cubic foot of the material involved.
Their hardness and thinness make
plasticity.
To make things thoroughly confus-
them difficult to file because files
Plasticity under tension is called
ing, basing gauge on weight means skitter over them, rather than cut in.
ductility, and produces stretching
that the same gauge number applied
Worse, very little metal can be
when it occurs. Think of the bumper
to different metals gives different
removed before they become
over-rider on a truck smashing into
thicknesses. For example, while dangerously thin.
the door of your vehicle. It deforms
22-gauge sheet steel is .0299 inch
it—plasticity—and it probably will
thick, 22-gauge galvanized steel is .031
els. In most cases, the thinner that
put the metal under tension and
inch thick, 22-gauge aluminum sheet
body metal is the more problems it
stretch it—ductility. When plasticity
stock is .025 inch thick, and 22-gauge
tends to present in repair. That is
occurs
under
compression,
as
stainless steel is .031 inch thick.
because the thinner body metal is,
opposed to tension, it is called mal-
The important things to remem-
the more difficult it is to form and to
leability, and produces the opposite
ber are that as gauge numbers
weld. The alloys used in thinner
of stretching by compacting or
increase, thickness decreases, and
panel sections tend to be harder than
“upsetting.” In upsetting, metal is
that the same gauge numbers for dif-
the older, thicker panels, because
piled into itself.
ferent metals may translate into
they contain more carbon. That
Let’s go back to that unfortunate
slightly different thicknesses.
makes them more difficult to deform
damage to your vehicle’s door that
Finally, there is a misconception
with body tools, without taking
occurred when a truck hit it. After
that gauge designations involve the
them beyond their yield points (frac-
the accident, a technician removed
number of sheets of a particular
turing them). Their hardness also
the inner panel from the door. Then,
gauge that can be fit into 1 inch.
makes them very difficult to surface
the technician began to fix the dam-
This, simply, is not true. Common
file for the purpose of leveling them.
age by hammering the ridge near the
gauge numbers for automotive
Welding thinner metal is always
center of the dent down and out
outer-body steels are:
more challenging, due to the ten-
against a dolly, centered under it on
dency of thinner sections to melt
the outside of the door. If the techni-
• 18-gauge .0478 inch
and “drop out” at welding tempera-
cian had read this book, he or she
• 19-gauge .0418 inch
tures. That outcome also can be very
would probably have had a better
• 20-gauge .0359 inch
hard on a metal worker’s shoes.
first move. The accident probably
• 21-gauge .0329 inch
stretched the metal in the door’s skin
• 22-gauge .0299 inch
Plasticity and Elasticity
because it was deformed while being
• 23-gauge .0269 inch
held rigidly at both ends by the
• 24-gauge .0239 inch
When I speak of the hardness of
door’s substructure. The attempt to
metal, I am generally describing sev-
hammer it out put the area near the
Thickness is important because,
eral significant characteristics, two of
hammering
under
compression
in part, it determines how difficult it
which are particularly important to
because the dolly was supporting the
will be to repair damaged body pan-
anyone working in panel fabrication
undeformed metal on either side of
10
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W HAT YOU S HOU LD K NOW B E FOR E YOU START
SUV, and had the queasy sensation of
feeling your hand deform the hatch
metal where you were pushing
against it. But then, as you released
the panel, you felt the metal under
your hand return to its rightful
shape. You can thank elasticity for
that good outcome. If the metal did-
n’t spring back, it was because you
Upsetting can be useful. Here, it is
exceeded its elastic limit.
Good news! This dent looks worse
used to shrink a stretched area. The
Elasticity is critical because dam-
than it is. Most of the displaced metal
metal is heated until it bulges, and
aged panels usually contain a small
is being held out of place by the ridge
then hammered down. The hot metal
minority of surface area that has
in its middle. Once that ridge is
piles into itself because it is bounded
been pushed, or deformed, beyond
unlocked, most of the damaged area
by unyielding cold metal. The
its elastic limit. Most of what may
will spring back into its proper place,
resulting upset makes the heated
look like damaged metal—because it
on its own.
area thicker and laterally smaller.
is out of position—has not been
deformed beyond this limit, and will
under the cover of a pile of books or a
the ridge. The result of hammering
return to its pre-accident shape when
knapsack, so that your teacher would
down on the obvious ridge, with a
you release the small areas of badly
not see you performing this metallur-
dolly under it, was to compress the
deformed metal that are holding it
gical experiment—was that before
metal there latterly, or to upset it.
out of place in the damage. I don’t
any of the three bends in the paper
This is a critically important dis-
want to sound excessively rosy about
clip could be straightened, the metal
tinction in autobody work. When
these matters but, to the untrained
stopped moving in the bends and
you stretch metal you are effectively
eye, panel damage almost always
bent on either side of them, leaving
exchanging some of its thickness for
looks worse than it is.
shapes like saddles between two
increased lateral dimension. When
opposite-facing humps, in kind of a
you upset metal, you are exchanging
Work Hardening:
camelback configuration. The saddles
some of its lateral dimension for
The Metal Remembers
were what was left of the original
increased thickness. At various
bends. The humps were new bends,
points in working with body metal,
The great elephant hiding dis-
in the opposite direction, that
you need to create upsets, and even
creetly in this sheetmetal living
occurred when the metal in the origi-
stretches, on purpose. At other times,
room is called work hardening. This
nal bends stiffened as you bent it, and
you will need to avoid these dimen-
is the tendency of metals, like mild
approached its elastic limit. Then, the
sional transformations, or have to
sheet steel, to become progressively
opposite-facing humps were made as
correct them. It is critical that you
harder as they are deformed beyond
understand exactly what stretches
their elastic limits.
That poor paper clip began its
and upsets are, and why and how
Doubtless you have already per-
life as a straight piece of wire. Form-
they occur. Later, I will discuss how
formed experiments involving this
ing it into a paper clip work hard-
to purposely create them, and what
factor, although you may not realize
ened the metal in its bends. When
situations call for creating them.
it. If you, like most people, ever tried
you tried to straighten it, you made
Elasticity in metal is its ability to
to straighten out a paper clip with
some progress, but work hardening
flex to a limit—its elastic limit—and
your fingers, you encountered work
made complete straightening impos-
still return to its original shape, on
hardening. What you discovered was
sible, so the metal bent on either side
its own. Some call this characteristic
that it is all but impossible to get the
of the work-hardened area. This is
memory, or spring back. You might
three bends out of a paper clip with
not trivial. Work hardening is terrifi-
have encountered this when you
your bare hands. What happened
cally important in body work. You
slammed the hatch on a minivan or
when you tried to do this—probably
must learn to identify it, predict it,
AUTOMOTIVE BODY WOR K AN D R
UST R E PAI R
11
CHAPTE R 1
An Example of Work Hardening
Here is a simple but dramatic example with his fingers. But the bend has work first work-hardened bend stubbornly of the work-hardening effect.
hardened and the metal wants to bend
refuses to budge.
Herb clasps a strip of 22-gauge mild
everywhere else, in the non-work-
Finally, Herb is able to hammer the
steel in a pair of sheetmetal pliers and bends
hardened metal, and not in the first bend
original bend and the side bends flat on
its middle to as close to a right angle as the
that he made. Frustrated, Herb tries to
an anvil. However, evidence of all three
jaws of the pliers al ow. Then, he closes the
straighten the bend by holding the metal
bends remains visible on the flattened
bend as far as he can in the pliers’ jaws.
in the pliers and forcing it, but that doesn’t
piece.
After removing the strip from the pli-
work. Then, he tries to straighten it with his
This sequence is a testimonial to the
ers, Herb attempts to straighten the bend
hands against a wood table top, but the
persistence of work-hardened metal.
12
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
W HAT YOU S HOU LD K NOW B E FOR E YOU START
Annealing Effects
One way to mitigate work-hardening
effects is to anneal metal. In this
process, metal is heated to its critical
temperature, roughly 1,600 degrees F in
the case of mild sheet steel, and allowed
to cool slowly in air. The effect is to relieve
the metal’s stiffness and reverse the work-
hardening effect.
In this demonstration, a strip of sheet-
metal is bent as close to a right angle as it
is possible to do with bare hands. Then,
unlike the demonstration of work harden-
ing, it is heated with an oxy-acetylene torch
to roughly 1,600 degrees F and allowed
to cool.
Now, it is easy to straighten the bend
with bare hands. The two strips were bent
almost identical y. Both were straightened by
hand, one with annealing and the other with-
out it. It’s pretty easy to tel which is which.
and deal with it, because it tends to
in your frustration, when you
Heat also has the ability to
be a factor in almost all of your colli-
couldn’t straighten that paper clip,
rearrange those grains for important
sion damage and fabrication efforts.
you bent it back and forth until it