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This
material first appeared in the I-CAR Advantage Online, which is published and
distributed free of charge. I-CAR, the Inter-Industry Conference on Auto
Collision Repair, is a not-for-profit international training organization that
researches and develops quality technical education programs related to
collision repair. To learn more about I-CAR, and to subscribe to the free
publication, visit www.i-car.com.
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Steel identification is an essential step when
determining the repairability of parts used on today’s vehicles. The type of
steel can alter or limit the ability to straighten, remove, and replace
parts.
When steel on a vehicle was either mild or
high-strength, there was a general rule of thumb; if you didn’t know what it was
made of, treat it like high-strength steel. That is no longer the case. If you
are working on a late model vehicle, it is more likely that the steel is either
mild steel, conventional high-strength steel, or one of several strength
variations of high-strength steel commonly referred to in the industry as
advanced-high-strength steel (AHSS).
Some vehicle makers identify the types or
categories and repairability of the steels of a particular vehicle. This may be
illustrated in a colour-coded chart of the vehicle.
General repair methods may also be provided
according to the steel type. The vehicle maker may also have specific
recommendations for a part or area of a vehicle. It is important to check
service information before determining how to repair a vehicle.
But what do you do if the strengths of steel are
not identified for a vehicle? There is equipment available to help with that
task.
Tensile Strength
Tensile strength is the single characteristic that
is most often used to determine the repairability of a steel. Tensile strength
(or ultimate tensile strength) is defined as the measurement of the amount of
force necessary to tear a piece of steel apart. The amount of strength is
typically measured in megapascals (MPa) or thousands of pounds per square inch
(ksi). One megapascal is also equal to 145 psi or 0.145 ksi.
The American Iron and Steel Institute (AISI) has
developed a chart that can be used to help illustrate and compare the different
steel names, tensile strength ranges, and percentage of elongation. The
percentage of elongation is how much the metal will stretch before it fractures
or breaks. Notice in this chart how some steel types overlap into multiple steel
categories. Because of the wide range of strength for one particular type of
steel - such as HSLA - that can range from 300–700 MPa (44–102 ksi), the steel
strength cannot be determined by name alone. This is important to keep in mind
when making a determination about repair. It is more important to pay attention
to actual strength rather than a vehicle maker’s or steel manufacturer’s trade
name.
The many different names and classifications for
steels used on today’s vehicle can make strength identification confusing. You
may see one name used to describe two different strengths or categories of
steel. For example, high-tension steel, extra-high-strength steel,
higher-strength steel, etc. are all names for steels that may be similar in
strength. Or, individual vehicle makers may use different names to describe the
same strength category of a particular type of steel.
Testing Equipment
There are numerous types of equipment available
that can be used to help identify the strength of steel. Tensile-testing devices
are available, but equipment that measures to the accuracies that are required
is both sophisticated and expensive. This equipment is typically only used in
laboratories and testing centres.
Hardness testing, which can be converted to
tensile strength, can also be used to identify steel strength. Most
hardness-testing equipment determines hardness by making a dent in the steel and
measuring the resistance of the steel to the deformation. Hardness testing can
be done by using either a bench-mounted piece of equipment or a hand-held
portable device.
A bench-mounted hardness tester requires a sample
piece of the steel for taking to the bench...
...while a portable hardness tester allows testing
without having to cut a sample from the vehicle part (see video).
Once the hardness of the steel is identified,
there are conversion charts and formulas that can be used to help categorize the
steel by tensile strength. Some machines are able to convert hardness to tensile
strength automatically and other machines may use their own chart to divide the
test sample into a steel category. Hardness testing can also be compared to that
of a known steel. This allows the steel to be placed into a basic strength
category based on this comparative data.
It is important to note that the same equipment
should be used when hardness testing and when making comparison readings. Do not
compare results from one piece of equipment to results taken with a different
piece of equipment.
Conclusion
Due to the numerous types of steels used on
today’s vehicles, technicians need to add steel identification to the list of
tasks when preparing for a repair. This might be as simple as checking a chart
in the service information, but these charts aren’t always available. For this
reason, accident repair facilities may want to consider acquiring a steel
identification device.
The task of identifying different strengths of
steel in vehicles will not get easier. Vehicle makers are taking advantage of
incorporating the many different strengths of steel available today in new
vehicle designs.
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