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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 http://www.i-car.com.
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Ultra-high-strength steels (AHSS) may be a dream
material for vehicle makers, but for anyone working on cars it can be the stuff
of nightmares, being apparently unworkable. But AHSS (shown here in blue) can be
worked with, short of straightening. It can be welded and it can be cut if the
right tools are used. We also found that it can be drilled, given the right bit
and procedure.
The type of AHSS used on vehicles today has
extremely high strength. The AHSS used on Volvo cars has a yield point of about
1,350–1,400 N/mm2 (196,000–203,000 psi). That’s about four times stronger than
average high-strength steel. But the process used to make it that strong takes
away some of the steel’s workability properties, such as being able to
straighten it.
Do Not Straighten AHSS
AHSS cannot be straightened because of the
extremely high heat used when it is being formed. When AHSS is damaged in a
collision, work hardening makes it too brittle to be restored to its original
state. Attempts to straighten an AHSS part will usually result in a cracked
part. The use of heat may allow the part to be straightened without cracking,
but heat destroys the strength of the part. The only solution is to replace the
part.
However, in some instances, it may be necessary to
pull on the AHSS part to remove indirect damage from a collision. But
straightening should usually be limited to parts that will be either partially
or completely replaced. When straightening parts made from AHSS, it is crucial
that the entire vehicle dimensions are monitored and that the vehicle is
visually inspected during the pulling operation. This prevents pulling damage
into an undamaged area of the vehicle.
AHSS cannot be galvanized. The galvanizing process
can’t take the high heat when the steel is being formed, and trying to apply a
galvanized coating after the part is shaped would require heating the part,
which would destroy the strength.
AHSS Applications
For now, AHSS is found primarily on European
vehicles, such as the dash panel on the Porsche Cayenne SUV, the door guard
beams on the Porsche 911 Carrera, and the inner B-pillars on the Mercedes-Benz E
Class.
Volvo probably uses AHSS the most. AHSS is used on
the bumper reinforcements and door guard beams on the Volvo S40 sedan and V50
station wagon. The Volvo XC90 SUV has several applications of AHSS, including
the inner B-pillar reinforcements, the roof bow between the B-pillars (if there
is no sunroof), and the inner rear body panels. The S80 and the V70 and S60 also
have AHSS inner and outer rear body panels. There are allowable sectioning cut
lines on these rear body panels. So how is this done?
Cutting AHSS
Volvo recommends cutting AHSS with a cut-off wheel
or a plasma-arc torch. A reciprocating saw should not be used. AHSS will remove
the teeth on a reciprocating saw blade. We had success using a 75 mm (3")
cut-off disk on a section of an XC90 rear body panel.
Using plasma arc cutting equipment has also shown
some effectiveness, although extensive practice on the technique is required.
The technique requires making a circular cut around the spot weld through the
outer layer, yet leaving the spot weld and the inner panel intact. Caution must
be used to prevent cutting into the inner panel. Before attempting to cut spot
welds on a vehicle, it is helpful to practice on a scrap part to obtain the
proper settings of the equipment and to practice the technique.
Drilling AHSS
It’s trying to drill spot welds out of AHSS where
many technicians have had the most frustration. One way to avoid drilling into
AHSS is to drill through the lower strength, softer steel that the AHSS part is
attached to. The restriction that this method has is that the backside of the
flange must be accessible to drill.
We tried different bits and methods to drill
directly into AHSS. Even very durable drill bits are dulled after a drilling a
series of holes in AHSS, and the bits are expensive. We tried a bi-metal
titanium carbide bit. Look at the close-up photo! A couple of attempts at
drilling into AHSS cracked the carbide insert. Attempts using a cobalt or
regular high speed steel drill bit just dulled the bit. We also tried the
two-drill bit method of drilling a pilot hole with a small bit and following
with a larger bit. The bits got dull just as fast. Applying oil did not help.
The oil seemed to not allow the metal shavings to come off the bit fast
enough.
What we found does work with some success is a
titanium drill bit combined with slow speed. We used an air drill with a maximum
freewheeling speed of 490 rpm. That’s much slower than the average 1,800 rpm of
spot-weld removal drills. With this setup, we drilled several holes in AHSS
without dulling or breaking the bit. This is not a promise that you would not
have to replace these bits far more than a regular spot welding drill bit on
regular steel.
Another approach is to use a spot weld drill bit
that has three flutes. This bit is made from tungsten carbide steel, a material
even stronger than AHSS alloyed with boron.
When used in combination with a pneumatic
C-clamp-style spot weld drill, this bit effectively removes spot welds.
Precision and control of the drill are keys to preserving the longevity of the
drill bit. A speed of 800-1000 rpm is recommended for removing the spot weld and
preventing heat build-up of the bit. Even when used cautiously, the drill bit
will become dull after drilling approximately 100 spot welds.
The C-clamp-style drill with the tungsten carbide
bit is also effective for making plug weld holes in a part made from AHSS
alloyed with boron.
Welding AHSS
One process where AHSS is worker-friendly is
welding. The low alloy and impurity content make the steel resistant to hot
cracking, so the welding process is not difficult.
[However, refer to Part 1 of
this series for some warnings on welding – see
Ultra High Strength Steels,
Part 1
Either the MIG welding process or squeeze-type resistance spot
welding can be used. Resistance spot welding is typically preferred because
there is minimal damage from heat effect.
To maintain the high strength characteristics of
the steel, it is important to limit the heat-affected area in the weld zone.
When MIG welding AHSS, a skip/stitch technique should be used, allowing the
metal to cool after approximately 13 mm (1/2") of welding. When using
squeeze-type resistance spot welding (STRSW), some equipment makers have
specific settings for welding AHSS. These settings pulse the welding output once
to preheat the weld zone, and then welds the metal together. This creates a weld
with less heat build-up in the weld zone, helping to maintain the strength of
the metal.
Corrosion Protection Important
One major concern with thin, strong steels is that
corrosion protection is even more critical to maintain vehicle strength. This is
especially important with AHSS, since AHSS is not galvanized during
manufacture.
Apply weld-through primer on bare steel mating
flanges, removing the primer from the direct weld area before welding. Take care
to not remove the primer from any area beyond the immediate weld area. Apply
epoxy primer to the entire parts after welding.
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