|
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
http://www.i-car.com.
|
The use of advanced high-strength steels (AHSS) is
increasing in popularity for almost every vehicle maker. They are the result of
a never-ending quest for a material that allows increased fuel efficiency while
allowing for ease of manufacturability, performance, and styling. AHSS panels
are thinner, lighter, and stronger than mild and many high-strength steel panels
and accomplish the same desired effect for collision energy management. However,
the addition of AHSS used in vehicle construction has raised some concerns about
the identity and characteristics of these steels, where they are commonly
located, and to what extent they can be repaired.
This week we’ll background AHSS and then next
week, look at some of the methods that can be used when working with these
materials.
Tensile and Yield Strength
Throughout this article, and wherever steel
strengths are defined or mentioned, yield strength and tensile strength are used
as the common measurements. Tensile strength (or ultimate tensile strength) is
defined as the measurement of the amount of force necessary to tear a piece of
steel apart. Yield strength is the amount of stress a material can withstand
without permanent deformation. Before the yield point, the steel will deform
elastically and return to its original shape when the stress is removed.
However, once the yield point is passed, the steel plastically deforms and the
damage becomes permanent.
The amount of strength is typically measured in
megapascals (MPa) or thousand pounds per square inch (ksi). A pascal is a unit
of measurement equivalent to one Newton per square metre. A megapascal is one
million pascals.
Boron-Alloyed Steel
Of all the advanced high-strength steels,
Ultra-High-Strength Steel (UHSS) alloyed with boron is the material that has
received much notoriety over the past several years. Vehicle makers, such as
Volvo, have made this boron-alloyed material an integral part of their vehicle
design. Volvo models such as the S40 and V50 have the inner bumper rails and
door intrusion beams made from steel alloyed with boron and the XC90 uses the
material for B-pillar inner reinforcements, the centre roof bow, and inner rear
body panels.
Dual-Phase (DP) Steel
Dual-phase (DP) steels are gaining in popularity
with the vehicle makers. Characteristics of DP steels include a higher tensile
strength than conventional steels of similar yield strength. DP steels also have
a higher initial work-hardening rate, and lower yield strength/tensile strength
than the similar strength
High Strength Low Alloy (HSLA) steels.
One application example of DP steel is the 2006
Jeep Grand Cherokee and Commander. According to DaimlerChrysler service
information, DP600 steel is used to increase crash performance (600 represents
the approximate MPa of the steel). The yield strength of this material on the
Jeep Grand Cherokee is 621–689 MPa (90–100 ksi). Examples of Jeep Grand Cherokee
and Commander parts with this material type include the frame rails and
A-pillars.
When working with DP steel, the repair issues may
not be the same as when working on other types of AHSS. For example, Land
Rover’s recommendation for replacing UHSS parts requires removing spot welds on
the part that is adjacent to the UHSS part, which is typically high-strength or
DP. This is because, according to Land Rover, DP and normal high-strength steel
pose no great issues.
GM recommends avoiding heating any part made from
DP steel, as the strength was substantially degraded by heating to 650°C
(1,200°F).
Other Types of AHSS
The best method of determining a steel type on a
vehicle is to look at the vehicle-specific body repair information. When doing
so, be prepared to encounter a variety of acronyms that test the limit of the
alphabet. DP, IF, TRIP, MART, CS, DDS, HSLA, BOR – each of these represents a
different type of steel with varying strengths. What’s important to note is that
when identifying different types of AHSS, not all have the exact same strength
and characteristics for the designation listed. For example, the characteristics
of DP steel may vary since DP steel can have a tensile strength between 450 and
1000 MPa (65 and 145 ksi). The strength chosen for a specific part will vary
from vehicle maker to vehicle maker or vehicle to vehicle.
Each grade of steel is chosen by the vehicle maker
based on specific characteristics, such as formability and strength. Below is a
list of common steel types. Notice that some of the steels overlap in
strength.
Standard Steels
Commercial steel (CS)
Drawing steel
(DS)
Deep drawing steel (DDS)
Interstitial-free (IF)
Mild
High-Strength Steels
Bake hardenable (BH)
Isotropic
(IS)
Carbon-Manganese (CMn)
High-strength low alloy (HSLA)
Dual Phase
(DP) - Complex phase (CP)
Transformation induced plasticity
(TRIP)
Martensitic (MART)
Ultra High-Strength Steels
Dual Phase (DP) - Complex phase
(CP)
Transformation induced plasticity (TRIP)
Martensitic (MART)
UHSS
alloyed with boron (BOR)
Because of the wide range of strength for one
particular type of steel, such as HSLA, which 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. If a part is listed as
being made from TRIP steel, don’t assume that it is a high-strength steel. Each
steel type has a range of strength and many steel types overlap with regards to
hardness. For example, TRIP, DP, and HSLA could all have the same tensile
strength depending on how they were manufactured.
High-strength steels are generally defined as
having tensile strengths between 270–700 MPa (39–102 ksi). Ultra-high-strength
steels (UHSS) are defined as steels with tensile strengths greater than 700 MPa
(102 ksi). Advanced high-strength steels may start at 400 MPa (58 ksi).
|
Steel Grade |
Yield Strength |
Ultimate Tensile Strength |
|
BH 180/300 |
180 MPa (26 ksi) |
300 MPa (44 ksi) |
|
HSLA 350/450 |
350 MPa (51 ksi) |
450 MPa (65 ksi) |
|
DP 300/500 |
300 MPa (44 ksi) |
500 MPa (73 ksi) |
|
TRIP 350/600 |
350 MPa (51 ksi) |
600 MPa (87 ksi) |
|
CP |
700 MPa (102 ksi) |
800 MPa (116 ksi) |
|
MART |
1250 MPa (181 ksi) |
1700 MPa (247 ksi) |
|
BOR |
1350 MPa (196 ksi) |
|
Steel Strength Formation and Comparison
Steel hardness is dependent on the alloying
elements used during the manufacturing process. Carbon is the primary hardening
element in steels, and is used in varying percentages depending on the desired
strength. However, many AHSS steels derive their strength from a combination of
ferrite (more commonly known as iron), bainite, martensite, and retained
austenite. Bainite, martensite, and austenite are metallic materials that exist
in steel after it has been heated to a specific point. The percentage of these
left in steel is determined by the rate of cooling and determines the steel
strength.
This chart graphs strengths of different steels
(tensile strength) versus ease of being formed (malleability or elongation). It
can be seen that lower-strength steels are much more malleable than martensitic
steels. Note that the steel alloyed with boron is in the far right section of
the blue martensitic category.
The chart also shows that some steel types
overlap, depending on added properties. For example, some AHSS, such as some
TRIP and DP steels, are stronger and less malleable than some martensitic
steels.
Identifying AHSS Locations
Currently, no field tests exist to determine the
composition of a piece of steel. The only way to determine the strength of steel
is to review the vehicle service information. However, even this information may
be difficult to find depending on how in-depth the service information is.
Common areas where AHSS may exist include rocker
panels, B-pillars, A-pillars, and roof rails. Most of this is used to increase
the level of side-impact protection. Increasing the strength on the side
stiffens the vehicle structure and reduces intrusion into the passenger
compartment.
Repairability
With all these new steels, questions are
constantly being brought up regarding how it can be repaired (straightened,
heated, welded). According to the International Iron and Steel Institute (IISI),
test results indicate that MIG welding is acceptable as a repair method for AHSS
such as DP, MART, and TRIP. Mechanical properties are within the expected range
for each material in close proximity to the repair weld. Of course, any
recommendations from the vehicle maker that are contrary to this recommendation
should be followed.
Recommendations for heating AHSS are very
specific. According to the IISI and the vehicle makers, heating should not be
used to straighten AHSS. The temperature required to straighten damaged steel
causes degradation to the mechanical properties of the work-hardened part.
Also, because the metals are thinner, they are
more prone to fatigue and transfer more stress. The thin metal makes the shape
of a weld a critical factor and any weld defects may result in increased fatigue
and decreased strength when compared to HSS or mild steel. Welding could be an
area where there are differences in vehicle maker recommendations.
One vehicle maker recommends welding on AHSS such
as UHSS alloyed with boron using squeeze-type resistance spot welds and MIG
welds. Another vehicle maker is considering using MIG brazing and adhesives to
join parts made from UHSS alloyed with boron. Until a uniform procedure is
developed, it is critical to follow each vehicle maker’s recommendation when
working on AHSS.
Next week we’ll look at some methods for
working with these steels
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...
|
More of our most popular articles.
|
|
|
Email a friend
Print article
|