Collision Repair Training | United States
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Current Events/News: Advantage Online: 2004 Archives
This article may be distributed and reproduced, but only by following the requirements stated in Permission to Reproduce. WORKING WITH BORON STEEL
May 10, 2004 - It could be called the “most ultra” of the ultra-high-strength steels. Boron steel may be a dream material for vehicle makers, but to the collision industry it can be challenging. To some collision repair technicians who have called the I-CAR Tech Centre, the title of this article doesn’t make much sense, since the words “working with” and “boron steel” don’t seem to go together. But boron steel can be worked with, short of straightening. It can be welded just like any other steel, 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 boron steel used on vehicles today has extremely high strength. The boron steel 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 Boron Steel Boron steel cannot be straightened because of the extremely high heat used when it is being formed. When boron steel is damaged in a collision, work hardening makes it too brittle to be restored to its original state. Attempts to straighten a boron steel 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. That same heating process doesn’t allow boron steel to 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. Boron Steel Applications For now, boron steel is found primarily on European vehicles, such as the dash panel on the 2002 Porsche Cayenne SUV, the safety bar around the rear seats on the 2003 Porsche Boxster, the door guard beams on the 2003 Porsche 911 Carrera, and the inner B-pillars on the 2003 Mercedes-Benz E Class. Volvo probably uses boron steel the most. Boron is used on the bumper reinforcements and door guard beams on the 2004 Volvo S40 sedan and 2005 V50 station wagon. The 2003 Volvo XC90 SUV has several applications of boron steel, 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 1999–2004 S80 and the 2001–2004 V70 and S60 also have boron steel inner and outer rear body panels. There are allowable sectioning cut lines on these rear body panels. So how is this done? Cutting Boron Steel Volvo recommends cutting boron steel with a cutoff wheel (see Figure 1) or a plasma-arc torch. A reciprocating saw should not be used. Boron steel will remove the teeth on a reciprocating saw blade. We had success using a 75 mm (3") cutoff disk on a section of an XC90 rear body panel (see video). A plasma-arc torch is only used for rough cutting. There are some plasma-arc torches available that can be easily set to only penetrate one panel thickness, leaving the inner panel intact. That feature is handy for cutting around spot welds on boron steel. Drilling Boron Steel It’s trying to drill spot welds out of boron steel where many technicians have had the most frustration. One way to avoid drilling into boron steel is to drill through the lower strength, softer steel that the boron 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 boron steel. Even very durable drill bits are dulled after a drilling a series of holes in boron steel, and the bits are expensive. We tried a bi-metal titanium carbide bit. Look at the close-up photo in Figure 2. A couple of attempts at drilling into boron steel 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 (see Figure 3) combined with slow speed. We used an air drill with a maximum freewheeling speed of 490 rpm (see Figure 4). That’s much slower than the average 1,800 rpm of spot-weld removal drills. With this setup, we drilled several holes in boron steel without dulling or breaking the bit. See the video for how the low-speed drill and special bit worked on one hole in boron steel. 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. Welding Boron Steel One process where boron steel 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. Either the GMA (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. 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 boron steel, since boron steel 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. Conclusion There promises to be more applications of boron steel, along with other types of high-strength steels, in future model years to reduce weight and strengthen the chassis. Perhaps along with the added applications will be more equipment and tool options for working with these strong and brittle materials. There is updated information on this topic in the article "Working with UHSS
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