New steels, new challenges
By Mark MacEachern
Automotive Collision Repair & Refinishing, NSCC Akerley Campus, Dartmouth, NS
Another downside of added weight is that in the event of a collision, the forces generated must be dissipated through the structure of the vehicle. In this case, lighter is better.
How can the manufacturers build a vehicle that gives us all the comforts we demand as well as being light weight, safe in the event of a collision and return good fuel economy? The latest solution has been to make use of UHSS (ultra high strength) steels. One of the members of this new family of steels is boron steel. Boron is added to steel in very low concentrations, usually less than 0.003 percent.
The Lincoln MKS features a robustly strong body structure that incorporates ultra-high-strength steels.
When boron is alloyed with steel, the resulting product becomes very strong, allowing the manufacturers to use a reduced thickness to achieve the same strength. So it seems to be a simple solution, make more use of boron steel and end up with a stronger, lighter vehicle structure.
While the solution may be simple for the OEM, the same cannot be said for repair shops. Boron steel, while being very strong, is very hard, and when the steel is bent it becomes subject to work hardening. What this means for the collision repairer is that boron steel must not be straightened; replacement is required. Attempting to realign bent boron steel parts will result in cracking.
Applying heat will destroy the strength of the part, resulting in reduced crashworthiness if the vehicle is involved in another accident. When realigning damaged structures, care must be taken not to cause further damage to associated lower-strength steel assemblies which may distort before the boron steel part has been pulled into proper dimension before removal.
Typically, when replacement is required, the entire part must be installed. Removing the spot welds will require a tungsten carbide bit or a cutting wheel can be used. If a cut off wheel is used be careful to leave the underlying layer intact. In most cases, sectioning is not recommended due to the heat associated when welding.
Where sectioning is allowed, make every attempt to keep the heat affect zone (HAZ) as small as possible. Using a stitch welding technique and keeping continuous welds to 13 mm or less while allowing for cool down time is the best course of action. GMA or MIG welding may be used however squeeze type resistance spot welding is preferred. Boron steel, because of the high heat used in its manufacture, cannot be galvanized, therefore restoring proper corrosion protection is of utmost importance.
Information on where boron steel is being used and the recommended repair procedures can be obtained by visiting manufacturers web sites. Some sites are free while others will require a fee or subscription. Most of the OEM’s have specific procedures for repair and replacement. Another source of information is I-CARs advantage online. Repairing vehicles that make use of boron and other UHSS should not be viewed as an impossible challenge, but simply a new type of repair which can be made simpler by acquiring the proper training and information.
The days of saying “I didn’t know” simply will not do. When it comes to repairing the structure of a modern automobile, one should follow the mantra of “do it once and do it right.”.
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