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Using heat to facilitate the straightening of a modern automobile or light truck is very tempting.
After all, if you can spend two or three hours heating and straightening
the upper-rail, lower-rail and strut-tower assembly of a typical
unibody vehicle, why spend 20 hours sectioning and replacing the
Before applying any heat-straightening methods
to a collision-damaged vehicle, however, there’s more to consider
than your time. Does the auto manufacturer recommend or even permit
the use of heat? Will the application of heat re-engineer the
vehicle so it’s no longer safe? What will happen to the corrosion
To answer these questions and to determine
whether heat straightening is the right repair procedure, it’s
important to understand the properties of metal and the effects
heat has on those properties.
Before discussing the use and application
of heat on modern steel automobiles, let’s talk a little metallurgy
– the study of metals from their extraction as an ore to their
use and application as a refined metal.
A very important part of metallurgy is the
study of any particular metal’s mechanical properties. Mechanical
properties determine the strength of each particular type of steel,
while strength is the ability to resist changing shape under load.
For our purpose, the load we’re concerned with is the external
force of a collision or impact. When the load or forces exceed
the yield strength of a particular steel, the steel changes shape
The type of steel ultimately determines the
strength of a particular panel. Modern automotive steel falls
into four main categories: mild steel, high-strength steel, high-strength
low-alloy steel and ultra high-strength steel. Mild steel has
the lowest yield strength (35,000 psi), while the yield strength
of ultra high-strength steel is extremely high (up to 110,000
psi). High-strength steel has a yield strength in excess of 35,000
psi but lower than 70,000 psi; high-strength low-alloy steel has
a yield strength in high-strength steel’s upper range.
It’s important to note that the high-strength
low-alloy steel has small additions of alloys to enhance its mechanical
properties. Ultra high-strength steel, with a higher yield strength,
is usually alloyed and heat treated or quenched and tempered.
Carbon content also affects steel’s strength.
What does carbon do? It’s the main hardening element in steel,
and as the carbon content increases, so does hardness and strength.
Plain carbon steels in the .05 to .30 percent range are often
called mild steels. The percentage of carbon and the process used
to thin the sheet metal will determine the strength of a given
steel delivered to the car maker for stamping and forming.
Turning Metal Into Sheet Metal
Automotive sheet metal starts out as a flat
sheet, usually in the form of a large coil. There are two common
processes for reducing thick steel sections into thin sheet metal:
cold rolling and hot rolling. Popular for making automotive sheet
metal, cold rolling results in a sheet with a superior surface
and uniform thickness. It also refines (makes smaller) the grain
The smaller the grain size, the greater the
strength; the larger the grain size, the lower the strength. When
strength is important, every effort is made to ensure a fine-grained
For example, if a 1018 low-carbon steel were
cold rolled, the yield strength would be 70,000 psi. If the same
steel were finished by hot rolling, it would have a yield strength
of only 40,000 psi. If the cold-rolled steel is annealed (the
process of heating the damaged area to a dull red temperature
and allowing it to cool slowly), the grains are restored to their
former state and the metal won’t be much stronger than the hot-rolled
One of the drawbacks of cold rolling is that,
eventually, the sheet metal becomes so hard that further cold
rolling will make the sheet brittle. If you were to take a section
of this more brittle sheet and try to form a complex shape, such
as a lower rail, the metal would crack.
To prevent cracking, the sheet steel is annealed,
which involves protecting the steel from the atmosphere, heating
it to a suitable temperature (usually around 1,000 degrees F)
and then allowing it to cool slowly. This process may take several
days. Often, the steel is cold rolled just a little to restore
its high-strength properties, zinc coated and then shipped to
the automaker for stamping and forming. Stamping and forming will
further enhance the strength of the panel.
As you think about this process, keep in mind
that the forces of a collision are another form of cold working
and subsequent work hardening. The problem this time is that the
work hardening is working against you. If you try to straighten
the damaged section, it will work harden again, causing cracks.
Using Heat Safely
Let’s suppose you must repair the left front
of a collision-damaged unibody vehicle. You know the crush zones
of both the upper and lower rail will require replacement or sectioning,
if permissible. However, you think the strut tower and apron can
be saved. The problem is that the damaged rails are going to prevent
you from restoring OEM dimensions to the strut-tower assembly.
Here’s a situation that might allow you to
use heat to stress relieve the damaged rails and to restore the
OEM dimensions of the rails, saving the strut tower and apron.
You cut off the rails, weld in the replacement parts or section,
restore corrosion protection and you’re done.
There are other instances in which you can
also use heat – as long as you stay within the temperature and
time recommendations of the car maker. Keep in mind that some
vehicle manufacturers recommendations change almost yearly. Make
sure you know what the heating recommendations are for the particular
make, model and year you’re working on, and don’t forget that
some car makers don’t recommend the use of heat at all. The new
1997 Ford F-150 pickup truck is a good example – no heat should
be used to straighten the frame.
If you decide to use a torch as your heat
source, make sure that you follow the torch manufacturer’s operating
guidelines. Multiflame heating heads typically require a large
volume of fuel gas. Don’t starve that tip. Single-orifice welding
tips may actually be safer to use. Adjust the flame to neutral,
hold the inner cone of the flame about 2 to 4 inches away from
the metal and keep the torch moving. Allow the heated portion
to cool slowly and never quench it.
Keep in mind that heating and straightening
may require additional help. It’s difficult to operate the torch
and operate the straightening equipment at the same time. Some
light hammering will also help in the straightening process.
How Hot is Hot?
How do you determine the temperature of the
steel to be certain you’re working within the manufacturer’s recommendations?
Because the lighting in the typical collision-repair shop varies,
determining the steel’s temperature by color is risky. The correct
method for determining the temperature of steel is to use some
type of temperature-indicating marker. Whether it be a crayon,
some paint, an adhesive strip or something else, you have to use
the material properly to get accurate results.
Temperature indicators will work best on clean,
bare steel, so make sure you remove paint, primer and other contaminants,
such as grease or oil. Failure to follow these recommendations
may cause improper temperature readings. Also, try not to direct
the heat source directly at the heat indicator. Some technicians
who use crayons prefer to start heating the metal first and then
periodically apply a strip of the temperature crayon until it
If you’re looking for some high-tech help
in determining surface temperature, you may want to check out
a noncontact temperature-measuring instrument. These units are
becoming quite popular with auto technicians. The unit I tried
was hand held, powered by a single 9-volt battery and had a temperature
range of 0 to 750 degrees F. At a distance of 6 inches, it would
read a spot measuring .8 inches; at 20 inches, it would read a
spot measuring 2.5 inches; and at 10 feet, it would read a spot
measuring 16 inches. This particular tool read painted or dull
surfaces quite accurately – plus or minus 3 degrees F. However,
the manufacturer warns against reading shiny surfaces, such as
stainless steel or aluminum.
Another high-tech temperature-measuring tool
is a temperature adapter that works with a digital multimeter.
This unit uses a thermocouple surface probe, or you can purchase
a temperature meter with a surface-tip probe. The advantage of
this unit is that it will typically read a higher temperature
than the noncontact unit.
The use of heat to straighten collision-damaged
structural members is not highly endorsed by the car companies.
First, it’s extremely difficult and time consuming to identify
cold rolled versus hot rolled, carbon content and alloy content
and to determine how much the damaged panel has work hardened
from the collision.
Also, using a torch to stress relieve a piece
of heat-sensitive steel is not an exact science. Dimensions might
be restored to preaccident condition, but the performance of key
structural components may be questionable.
If you do decide to use heat, first make sure
the car maker allows it. If the car maker does, then be sure you
know the temperature and time limitations. Also:
- Correctly identify all high-strength steel panels. If you’re
not sure about a particular panel, treat it as if it were high-strength
- Don’t try to heat and straighten ultra high-strength steel
unless the vehicle manufacturer recommends it;
- Make sure you know where the internal reinforcements are in
key structural members. Never try to straighten key structural
members that have collapsed crush zones;
each time a new vehicle platform is introduced;
Following these key points will ensure that you’re using heat-straightening
methods at the appropriate times and in the appropriate manner.
Remember, using heat to facilitate the repair may seem like the
best option for you, but it also needs to be the best one for