What You Need to Know about High-Strength Metals

July 1, 2009
Super-tough and lighter than ever, high-strength metals have been making their way into vehicles since 2006. Here’s what you need to know.

Imagine that a new Ford 500 with side collision damage comes in to your shop. Your tech makes the repairs, including some welding and straightening in the A and B pillars and the reinforcements for the rocker panel. Five months later, you hear that the same car had another collision—and this time, the parts your tech fixed failed in the crash. That sounds dramatic, but if you’re not wise to the new metals manufacturers are using, such a scenario could become all too real.

While all steel welds the same, not all steels are the same. If your techs are behind in learning about the new high-strength steels that vehicle manufacturers are using to make cars stronger and lighter, they could be compromising the very parts they’re trying to repair.

WHY these New Metals?

The auto industry has more than a financial problem. It also has a weight problem, and it’s getting bigger all the time.

The auto industry soon must comply with updated corporate average fuel economy, or CAFE, standards. By 2020, car manufacturers must produce vehicles that average 35 miles per gallon (mpg). That requirement was in the Energy Independence and Security Act of 2007. Now, the Obama administration is working toward all cars having to get at least 39 mpg and light trucks 30—and by 2016, rather than 2020.

Whatever happens with CAFE standards, manufacturers must lighten their load even as customers demand bells and whistles that add weight, says Jason Bartanen, technical director for I-CAR. “Customers want navigation systems, stereo systems, DVD players and other amenities,” he says. “They also want those cars to get 50 mpg, offer 400 horsepower, go from zero to 60 in the blink of an eye and have 5-star crash ratings. All of those things mean more and more weight, so to offset that and raise the fuel economy, manufacturers have to identify lightweight materials.”

Losing Weight With Metal

That lightweight material is here, and it’s showing up in cars the way aluminum and boron did a few years ago. A recent study by the Automotive Applications Council of AISI’s Steel Market Development Institute found that in light vehicles in the 2009 model year, use of advanced high-strength steels (AHSS) increased by 4 percent over the 2007 model year.

These are the most commonly used high-strength steels:

Dual phase (DP) steel. DP steels consist of two types of iron: ferrite and martensite. Ferrite is basically iron at room temperature. Iron in the ferrite phase is slightly softer, which gives the metal the ability to bend. But martensite is a harder phase of iron, and gives the steel the overall hardness that makes it stronger than conventional steels. Carbon, manganese, chromium, molybdenum and nickel, or a combination of these, are often added to increase the steel’s strength.
• Transformation-induced plasticity (TRIP) steel. TRIP steel is also a matrix of ferrite and martensite, and it includes bainite (a combination of ferrite, other metals and carbon) and retained austenite (a solution of carbon and iron that survives at room temperature if manganese, nickel or chromium are added). These contain a lot of silicon and carbon. Compared to DP steel, TRIP steel hardens slightly more as it is stretched and formed.
• Complex phase (CP) steel. CP steel has even higher tensile strength than DP steel. It’s made of a matrix of ferrite and bainite, and has smaller amounts of martensite, austenite and pearlite (a combination of ferrite and iron carbide), along with other elements like titanium and columbium.
• Martensitic (MS) steel. MS steel is made of a martensitic matrix with ferrite, bainite or both mixed in. MS has the highest tensile strength, up to 1,700 megapascals (MPa). By comparison, CP, TRIP and DP steels top out at about 1,000 MPa. MS steels can also be strengthened with carbon, silicon, chromium, boron, molybdenum, vanadium and nickel.

These metals are showing up in different models in a lot of different places. But their initial and ongoing use is definitely in the passenger compartment. “I’ve been seeing ultra-high tensile, advanced steel in the A and B pillars, in the reinforcements of the rocker panels, and the crosscar beams as protection from a side impact,” says Bob Keith, senior director of training and education for CARSTAR. “I haven’t been seeing them in the external or cosmetic panels.”

Repairing High-Strength Steels

The strength of these metals makes it a different undertaking to repair them than older, weaker metals. The tensile strength of metal determines not only how to repair it, but whether to repair it at all. For example, GM offers shops a reparability chart, but it differentiates by strength of metal rather than type.
With these metals, it’s important to know what not to do as well as what to do, because some can’t be repaired. “Anything over 800 MPa is not repairable and has to be replaced,” Keith says.

That’s because heat compromises the hardness of these metals. These advanced high-strength steels are stronger because the thermomechanical processing of the steel as it’s made creates both soft and hard phases in the metal. Reheat the metal, and the hard phases tend to revert to soft ones. “If you get these metals too hot, you’ll change the microstructure and adversely affect the metal’s properties, which in turn could affect its performance in the next crash if there is one,” says Dave Anderson, director of long products for the American Iron and Steel Institute.

So, for example, flame straightening is not an option with DP, MS or TRIP steel, except in the lower-strength metals like DP600. In the higher-strength metals, even if the repair is successful, the martensite softens and the metal won’t be as strong as it was before straightening. Awareness of the kind of metal in the car is key, Keith says, and so is knowing that if the manufacturer says not to repair it, you shouldn’t try. Most new vehicle models incorporate some of this material. “So if you see a new model, you need to investigate what’s in there, and you need to go to the OEM’s Web site and verify how or if the part can be repaired,” Keith emphasizes.

The tensile strength of metal determines not only how to repair it, but whether to repair it at all.

How Will You Know?

Most likely, your techs have already seen vehicles with AHSS, possibly without realizing it. There are tip-offs, one being that these metals are extremely difficult to drill. “If your drill bit turns red or you can’t even get a hole, chances are it’s one of the harder metals,” Keith says.

GM is beginning to add symbols to its parts so that technicians can more easily refer to its reparability chart and know whether MIG brazing, MIG welding or other procedures are acceptable for the type of metal in that part. Other manufacturers are adding visible cut locations, so that technicians know that they can safely cut and weld there without compromising the necessary strength. “These [locations] are in the less stressed areas,” Anderson says. “For example, the front rails are elaborate components with bends and thickness changes, but there are certain locations where you can safely cut or stitch weld.”

Generally, though, you need to assume that the newer a car is, the more likely it is that some of these metals will be present—and the more of them you’re likely to find. “Any car that’s 2006 or newer, there’s going to be a lot of high-strength steel in it,” Bartanen says.

Equipment and Training

To handle these metals, your shop will need the right equipment, and your technicians need some training. For example, spot welding is the preferred method (assuming there is one) for repairing these metals, so if you don’t have one, a spot welder may be necessary. “Most of the manufacturers are recommending an inverter squeeze-type of resistant spot welder,” Keith says. “Others have specs for MIG welding with a plug weld.” If your MIG welding equipment is capable of MIG brazing, you’re set; if not, you might need to get some that is.

Drilling these metals is also hard on drill bits, so you may need to invest in a harder grade of drill bit or even a higher-performance drill. “Some of these you have to have a special drill for, and others you can’t touch with a drill or a saw; they’re just too hard,” says Keith.

Again, the use, location and repair of these metals vary greatly by manufacturer. Fortunately, I-CAR has three courses that will help sort out the differences. I-CAR’s Bartanen says that “Steel Unitized Structures, Technology and Repair,” tells participants how to repair these steels, including how to identify them, how to set up a spot weld and maintain the equipment properly, and other important information. I-CAR also offers courses that concentrate on methods such as “Squeeze Type Resistant Spot Welding” and “GMA MIG Brazing.”

If your shop sees a lot of vehicles from a particular manufacturer, you could benefit from going straight to the source, either for information or training. “OEM1stop.com is a good place to start,” says Keith. “Some of [the OEM sites] are free and some have a one-time charge. Even if you have to buy a day’s worth of information, $30 is a cheap investment to make sure you’re repairing the vehicle correctly.”

Here to Stay

These metals will affect more than repairing individual parts. Their increased tensile strength may mean that stretching equipment will require more power, or that drills will have to be used for longer and with different bits. Not only that, their strength affects your estimating as well. “In vehicles that have these metals, the occupant structure is so strong that collision energy travels further than otherwise,” Bartanen says. “So if they’re present, you’ll have to do a more complete and thorough damage analysis. There may be damage in other areas that isn’t as easily noticed, so shops will have to make sure they’ve [adequately measured] to find the secondary damage.”

The need to replace parts made with high-strength metals rather than to repair them will affect estimates. Keith says that some information about these parts and metals is starting to show up in estimating systems, but it’s slow in coming. “For estimating accuracy, you need to identify these metals in the estimate,” he says.

Given all that these metals offer to manufacturers—lighter weight, more strength, better crash performance—these high-strength materials will be showing up in more and more cars, and in more and more places in those cars. Preparing for these new metals is an investment that can prevent you from having to turn away business now, and certainly one that is essential to your shop’s future.

“If you’re going to stay in this business, you’ll have to be able to deal with [these metals],” Keith says. “There are new alloys all the time, and they aren’t going away; there are too many factors driving this.”

High-Strength Resources  The following Web sites provide detailed information about repairing high-strength steels.
» oem1stop.com Link to all vehicle manufacturers and their technical repair information.
» goodwrench.com Search for “technical repair information” to access GM’s free technical repair site.
» autosteel.org The American Iron and Steel Institute’s Web page covers steel in automotive applications.
» i-car.com I-CAR offers several courses to help technicians identify and repair high-strength steels.

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