Safety regulations and stringent fuel consumption requirements have caused OEMs to use advanced high-strength steel, which has created a new era in collision repair.
When unibody vehicles first appeared, there was a buzz among collision repairers and insurers about how crash damage would be repaired. The same was true for aluminum-intensive vehicles.
Now, the industry is faced with a similar, likely bigger epidemic, but there's no buzz. Why? Because people didn't realize it was happening. If you hadn't guessed it, it's the use of advanced high-strength steel (AHSS).
Outside, it looks like a traditional vehicle. Try to straighten it, and it goes nowhere. Try to cut it, and your saw blades look like they need to visit a dentist. Try to drill it, and you have dull drill bits. Finally, you remove the outer panels and realize there are AHSS reinforcements inside. Welcome to the new era of collision repair.
If you have yet to encounter AHSS, you most definitely will; most vehicle makers use some type of AHSS. Safety regulations, coupled with more stringent fuel consumption requirements, have caused vehicle makers to investigate alternative materials to save weight, yet increase strength and rigidity. Using AHSS seems to be the material of choice for the majority of the global vehicle manufacturers (see Figure 1).To truly understand why vehicle makers are using more AHSS, we have to look no further than the U.S. Government and the Corporate Average Fuel Economy (CAFE) standard. First enacted in 1975, CAFE is aimed at reducing the overall fuel consumption to a specific vehicle maker's entire fleet of passenger cars and a separate class for light-duty trucks for each new model year, thus reducing the U.S. dependency on fossil fuels.
The way the CAFE system is designed, a vehicle maker can produce an extremely fuel-efficient model to offset a larger vehicle that consumes more fuel. Beginning in model year 2011, vehicle makers that produce light-duty trucks will be required to comply with a reformed CAFE standard that specifies the average fuel economy for the size, or footprint, of the vehicle, instead of averaging total fuel economy over a wide range of vehicles. The footprint is the stance, multiplied by the wheelbase of the vehicle, creating a square foot measurement that is used to determine the fuel economy standard, based on the projected number of units built per model year by each vehicle maker.
Safety characteristics
Let's face it, a "Five Star" rating on the National Highway Traffic Safety Administration (NHTSA) testing, or a "Best Pick" rating from the Insurance Institute for Highway Safety (IIHS) crash test is a strong marketing tool. There is no doubt that steel is king when talking about construction materials for the automotive industry. It had been the material of choice since the inception of the mass-produced vehicle. Then, in 1991, Honda Motor Company introduced the Acura NSX.
The complete vehicle was built from aluminum, greatly reducing the weight and demonstrating that aluminum could be used for a structural role during vehicle production. In 1994, the Ultra-Light Steel Auto Body (ULSAB) consortium was formed in an effort to demonstrate that steel could also be used effectively to reduce weight and increase vehicle occupant protection, creating both economical and positive environmental impacts for vehicle design engineers. While aluminum did create a stir to collision repairers, the majority of its use was aimed at low-production, specialty vehicles comprised mainly of the sports car segment. While aluminum is a bit different to repair, making the determination that the part was made from aluminum is as easy as taking a magnet to the part. This is not as simple when you are dealing with AHSS.
The trend is to use stronger, yet lighter, materials in areas surrounding the passenger compartment, and compliment stronger structural parts with lighter materials in areas that were traditionally made from steel. In essence, the vehicle structure has been placed on a major diet. Take a look at the radiator core support of some current platforms; what traditionally consisted of a welded steel tie bar is replaced with a rigid plastic radiator core support (see Figure 2).Another thing you'll notice when looking at the unitized structure of a late-model vehicle is the upper and lower frame rails will have collapse zones designed in the very front. While this is not an entirely new concept, the methods used to accomplish this have evolved. The collapse zone may be made of a thinner material, or with lower-strength steels.
These areas deform to absorb and dissipate collision energy. The lower rails in the strut tower area are increasingly stronger as collision energy approaches the passenger compartment. Instead of the rocker panels and pillars being filled with reinforcements to protect the passenger compartment during a side impact, AHSS is being used. The tailored-blanking process has enabled vehicle makers to design reinforced areas into the stamping itself, reducing the need for multiple additional reinforcements.
This does not mean we have seen the last of the inner panel reinforcements, but chances are the number of them will be reduced. With the tailored blanking process, a thicker, or higher strength piece of steel can be joined to a thinner, or lower strength steel using a laser weld (see Figure 3). Besides using tailor welded blanks with laser welds, there are also tailor rolled blanks where the steel sheet is rolled to different thicknesses before it's stamped into shape. The reduced number of reinforcements is directly proportional to the reduction of weight. These blanks eliminate steps in the manufacturing process.What is AHSS?
Steel strength can be classified in many different ways, and the terminology can be nothing short of mind-boggling. It seems that there is little or no consistency for the terminology. Some call it boron steel, while others call it martensitic steel.
The fact is, it could be the same, or it could be different. To make steel strengths easier to understand, the U.S. collision industry has grouped steels into the general categories of low-strength (mild) steel, high-strength and ultra-high-strength steels (UHSS). The classifications are determined by steel strength.
Although straightforward in concept, this is where the confusion starts. Steel strengths are typically rated in yield strength and/or tensile strength. When attempting to identify the approximate strength of a steel to determine correct repair methods, it's helpful to know which rating system is used by the manufacturer.
To clarify the terminology, yield strength is the stress limit where plastic deformation starts, which means it is the level of force that must be achieved or exceeded to straighten the metal. Yield strength is the minimum force required to achieve permanent deformation of the steel. Tensile strength is the maximum tension load that is reached before the metal fractures and is measured in force per unit of cross section area. Tensile strength can be described as the maximum force the metal can be subjected to before it tears or breaks (see Figure 4). The typical units of measurement for steel strength are either megapascals (MPa), or pounds per square inch (psi). One MPa equals 145 psi. We will discuss more on tensile strength and yield strength later in the article.Molecular structure
Steel making is a complex process. The mechanical characteristics can vary depending on the amount of alloying elements added, the degree of mechanical manipulation and work hardening, and by the thermal treatment process.
The most significant element that is added to steel during the alloying process is carbon. The more carbon that is added and remains linked to the iron atom, the harder the steel will be. The formability and the welding capabilities decrease with increased carbon content. Other alloying elements are added to the steel to better tailor the steel for the specific purpose.
The thermal treatment process may be more influential than the addition of alloys when determining the characteristics of automotive grade carbon steel. The temperatures used, as well as the rate of cooling, affect the amount of carbon atoms that attach to the iron atoms and remain suspended in the steel molecule. This directly affects strength and hardness.
A very non-scientific description is to think of a steel molecule as a cardboard box. If you stand on the empty box, it crushes fairly easily. As you begin to fill the box with solid items, it becomes stronger, allowing you to stand on it without it crushing. To a point, the more items you pack in the box, the stronger the box will be. While you are cramming additional items into the box, the shape of the box becomes distorted. Eventually, the items overflow out of the box, since it is not large enough to hold all of them. This is similar to what is going on with the steel molecule as it is being thermally treated.
The molecular structure of carbon steel can be broken down into phases: the ferrite phase, the austenite phase and the martensite phase. Each of these phases occurs when temperatures change and result in two different cubic structures to the basic steel molecule. The ferrite phase is when the steel is in its softest and most formable stage. In this phase, the molecule has a body centered cubic structure.
In the ferrite phase, very few carbon atoms attach to iron atoms; the space between iron atoms isn't large enough to contain the larger carbon atoms, creating a molecule with a very small amount of dissolved carbon. The box is empty.
Next is the austenite phase. In this phase, the steel is heated above 727° C (1,340° F) where more carbon atoms can dissolve in the iron atoms of the steel molecule. This stretches or changes the shape of the box to create space for the larger carbon atoms and creates a face centered cubic structure for the steel molecule. The amount of carbon that remains dissolved in the iron is affected by the rate of heating and cooling, since the carbon falls out of the solution as it cools. The faster the steel cools, the more carbon atoms that remain suspended within the iron molecule.
The box is full when it is heated, and as it cools, the items start falling out of the box. The slower it cools down, the more items fall out of the box. When the steel has cooled completely, the box ends up being half full.
The next phase is the hardest, strongest phase of carbon steel – the martensite phase. Martensite phase steel is made by quick-quenching steel in the austenite phase. The rapid cooling traps much of the carbon atoms inside the iron atoms in the steel molecule.
Here, the box is full as the metal cools quickly. The carbon atoms have compressed into the steel molecule, causing the box to be distorted. The distorted shape of each molecule locks it in place with the other molecules, making it difficult to move around. In essence, when the metal is bent, the molecules move around. When they are locked in place very tightly, as with the martensite phase, the steel does not bend very easily. Heating and cooling of AHSS is a critical process that is directly related to the strength and durability of the steel.
How does AHSS affect collision repair?
Now we have a very basic understanding of AHSS. The next question is: How do we repair vehicles that are constructed from AHSS? The first step is knowing if the vehicle that you are working on has it. The most obvious sign is not being able to drill it or cut it, but waiting until you try this may be too late in the process.
Now more than ever before, it's necessary for us to do our homework. Log on to the vehicle maker's technical information and determine what the vehicle structure is made from, and know and understand any particular precautions that the vehicle maker may have when working with specific types of AHSS. A source for all of the vehicle makers Web sites can be found on the Technical Information tab at www.i-car.com.
After you determine that the vehicle you're working on does have AHSS and you understand the precautions that the vehicle maker has, there are a few more things to consider. The traditional means of straightening a vehicle structure by anchoring on the four corners of the torque box area may not get the job done when it comes to pulling on a structure with AHSS parts. The amount of force required to straighten AHSS far exceeds what it takes to straighten mild steel or high-strength, low alloy (HSLA) steel. So can it be straightened?
To answer this question, understand that strength ranges of AHSS are very wide and some types are considered to be in the high-strength category. Others would be considered to be ultra-high strength when using charts provided by the American Iron and Steel Institute.
The collision industry had adopted the stance that UHSS, which was generally used for door intrusion beams and bumper reinforcements, should not be straightened or repaired. For safety these items were typically replaced when damaged. Based on that, UHSS parts now being used to limit intrusion into the passenger cage during a side impact are likely to be treated with the same consideration to ensure that collision repaired vehicles offer the same level of occupant protection in a crash for which the vehicle was originally designed.
As mentioned earlier, there are no definite answers here, as there are several different types that could possibly be used. Remember when we discussed tensile strength and yield strength earlier? The obstacle when straightening AHSS is the tensile strength can be very close to the yield strength, meaning there is less room between a part being returned to the original shape and state before the part tears, cracks or fractures. You also need to remember that the AHSS, in most cases, is attached to lower strength steels. It is quite possible that the part you intend to straighten will actually pull additional damage into the surrounding structure instead of returning to its original shape. This does not mean that the structure cannot be straightened. When attempting to straighten the structure, we need to be certain that the anchor attachment points are continually monitored and understand that additional anchoring points other than just the four torque box locations, may be required. It is also important to understand that we may need to brace the structure before straightening operations begin (see Figure 5). This will reduce the likelihood of pulling more damage into the structure.Can heat assist in straightening?
There's no doubt that heat makes straightening the structure easier. Heat relaxes the carbon atoms in the steel molecules, making it softer and more malleable. But applying heat has a drastic affect on the strength of the steel. Unless we can be absolutely certain that the steel has been heated to an exact temperature, and allowed to cool down at precisely the same amount of time that it did originally, heat should not be used, unless the vehicle maker has made that determination.
What about welding?
The vehicle makers weld AHSS mainly with the squeeze-type resistance spot welding (STRSW) process during vehicle assembly. Most modern collision repair facilities have the equipment required to use this process, as well. Some equipment makers have developed welding machines programmed specifically for spot welding on AHSS (see Figure 6).In some cases, parts made from AHSS may be included as a pre-welded assembly. This eliminates the need to weld directly to the AHSS. Can the material be (GMA) MIG welded? The jury is not completely back on this question. Differences in opinion among vehicle engineers exist when using the MIG welding process on AHSS. Some have procedures for GMA (MIG) welding it; others may recommend STRSW, weld-bonding, MIG brazing or riveting. Because there are differences in joining recommendations between the vehicle manufacturers it should not be assumed that the method used at the OEM is the joining method recommended for collision repair.
Remember the vehicle maker's repair information Web sites? It's time to visit them to see if the manufacturer allows the material to be welded. As we all know, MIG welding takes heat, enough of it to cause steel to get to the melting point. The metal around the actual weld zone has been subjected to temperatures that could alter the molecular structure, either by letting carbon atoms into or out of the box. This is known as the heat-affected zone (HAZ) (see Figure 7).When GMA (MIG) welding AHSS, it's important that the HAZ remain very small. The slower the metal cools, the more carbon atoms leave the box. As mentioned earlier, the more carbon atoms that leave the box, the less strength the metal will have. This does not mean grab a squirt bottle of water and quench the weld zone as soon as it has been welded. The cooling process at the steel plant is very controlled, something that we cannot duplicate in a collision repair setting.
Best advice, if the vehicle maker allows GMA (MIG) welding of AHSS, follow that recommendation. If they allow welding but do not provide specific instructions, weld in very small stitches, allowing plenty of time for the heat to dissipate between each stitch. The smaller the HAZ, the more likely it is to preserve the original characteristics of the steel.
Drilling and cutting
As mentioned earlier, welding and cutting some of the AHSS may create problems. Some steels classified as UHSS are in the martensetic phase, meaning they're very hard and strong. This makes them very difficult to drill or cut. This doesn't mean blow the dust off of the oxyacetylene torch and use it to cut or make plug weld holes. There's equipment that, when used properly, will drill or cut UHSS.
The Quiet Revolution, the use of AHSS in vehicle structures, didn't happen overnight. Yet many people in the collision industry did not readily recognize it was happening. As each new model year is released, we will be seeing more and more of it. Now is the time to prepare, as it will be affecting everyone who is involved in repairing collision damage. Repairing AHSS correctly is a MUST to maintaining the safety and integrity of the vehicle structure. The relationships that I-CAR has with many of the vehicle makers worldwide has led to a training program clarifying the repair considerations that have arisen with the increased use of AHSS.
The I-CAR Steel Unitized Structure Technologies and Repair (SPS07) training program is currently available in the United States and Canada. Visit the I-CAR Web site and search the class schedules to see when it has been scheduled in your area. I-CAR also has developed an online training program, Advanced High-Strength Steels Overview, to assist the collision industry with identifying and understanding AHSS. Visit: www.i-car.com/online.