Correct structural repair is the big issue for I-CAR in 2008

Comments Comments

Special Report

The use of advanced high-strength steels (AHSS) and Ultra high-strength steels (UHSS) is increasing in popularity for almost every vehicle maker. The flow-on is challenging repairers.

THE NEW FORMS of steel are the result of a never-ending quest for a material that allows increased fuel efficiency while allowing for ease of manufacture, performance, and styling.


AHSS and UHSS panels are thinner, lighter, and stronger than mild and many high-strength steel panels and accomplish the same desired effect for collision energy management.

However, the addition of AHSS and UHSS used in vehicle construction has raised some concerns about the identity and characteristics of these steels, where they are commonly located, and to what extent they can be repaired. Repairers involved in structural repair are not involved in a simplistic repair process. They are required to remanufacture or re-engineer the vehicle, and their approach to the remanufacture process must be based on a true engineering philosophy. The structural integrity of the occupant capsule must be maintained.

There are a number of issues of which the professional collision repair facility must, as an absolute minimum, have a basic understanding in the changes in technology and what it may mean to the method of repair.

TENSILE AND YIELD STRENGTH

Tensile strength (or ultimate tensile strength) is defined as the measurement of the amount of force necessary to tear a piece of steel apart. Yield strength is the amount of stress a material can withstand without permanent deformation. Before the yield point, the steel will deform elastically and return to its original shape when the stress is removed. However, once the yield point is passed, the steel plastically deforms and the damage becomes permanent. The amount of strength is tymegapascals (MPa) or thousand pounds per square inch (ksi). A pascal is a unit of measurement equivalent to one Newton per square meter. A megapascal is one million Pascals.

BORON-ALLOYED STEEL

Of all the advanced high-strength steels, ultrahigh- strength steel (UHSS) alloyed with boron is the material that has received much notoriety over the past several years. Vehicle makers have made this boron-alloyed material an integral part of their vehicle design, including the inner bumper rails and door intrusion beams, B-pillar inner reinforcements, the centre roof bow, and inner rear body panels. However while manufacturers have increased the use of a number of other AHSS steels, only a limited number have specific repair recommendations.

DUAL-PHASE (DP) STEEL

Dual-phase (DP) steels are gaining in popularity with the vehicle makers. Characteristics of DP steels include a higher tensile strength than conventional steels of similar yield strength. DP steels also have a higher initial work-hardening rate, and lower yield strength/tensile strength than the similar strength High Strength Low Alloy (HSLA) steels. When working with DP steel, the repair issues may not be the same as when working on other types of AHSS. GM, for example, recommends to avoid heating any part made from DP steel, as the strength was substantially degraded by heating to 650°C (1,200°F).

OTHER TYPES OF AHSS

The best method to determine a steel type on a vehicle is to look at the vehicle-specific body repair information. When doing so, be prepared to encounter a variety of acronyms that test the limit of the alphabet. DP, IF, TRIP, MART, CS, DDS, HSLA, BOR - each of these represents a different type of steel with varying strengths. Whatís important to note is that when identifying different types of AHSS, not all have the exact same strength and characteristics for the designation listed. Each grade of steel is chosen by the vehicle maker based on specific characteristics, such as formability and strength.

STEEL STRENGTH FORMATION AND COMPARISON

Steel hardness is dependent on the alloying elements used during the manufacturing process. Carbon is the primary hardening element in steels, and is used in varying percentages depending on the desired strength. However, many AHSS steels derive their strength from a combination of ferrite (more commonly known as iron), bainite, martensite, and retained austenite. Bainite, martensite, and austenite are metallic materials that exist in steel after it has been heated to a specific point. The percentage of these left in steel is determined by the rate of cooling and determines the steel strength.

REPAIRABILITY

With all these new steels, questions are constantly being brought up regarding how it can be repaired (straightened, heated, welded). Recommendations for heating AHSS are very specific. According to the IISI and the vehicle makers, heating should not be used to straighten AHSS. The temperature required to straighten damaged steel causes degradation to the mechan-ical properties of the work-hardened part. Also, because the metals are thinner, they are more prone to fatigue and transfer more stress. The thin metal makes the shape of a weld a critical factor and any weld defects may result in increased fatigue and decreased strength when compared to HSS or mild steel. Welding could be an area where the industry will see possible differences in vehicle maker recommendations. One vehicle maker recommends welding on AHSS such as UHSS alloyed with boron using squeeze-type resistance spot welds and GMA (MIG) welds. Another vehicle maker is considering using MIG brazing and adhesives to join parts made from UHSS alloyed with boron. Until a uniform procedure is developed, it is critical to follow each vehicle maker’s recommendation when working on AHSS. The collision industry is currently in the infancy stage when it comes to repairing advanced high-strength steels. There is still a large amount of research that needs to be conducted to determine the limits of repairability. The steel and vehicle makers are working diligently to determine proper repair techniques such as welding, drilling, and cutting. Currently, the only universal recommendation for repairing AHSS is that heating for straightening purposes is not recommended due to the adverse affect on the strength of the steel.

IDENTIFYING AHSS LOCATIONS

Until recently there had been limited field tests available to determine the composition of a piece of steel. The only way to determine the strength of steel was to review the vehicle service information. However, even this information may be difficult to find depending on how in-depth the service information is. Common areas where AHSS may exist include rocker panels, B-pillars, A-pillars, and roof rails. Most of this is used to increase the level of side impact protection. Increasing the strength on the side stiffens the vehicle structure and reduces intrusion into the passenger compartment.
STEEL IDENTIFICATION

Steel identification is an essential step when determining the repairability of parts used on today's vehicles. When steel on a vehicle was either mild or high-strength, there was a general rule of thumb; if you didnít know what it was made of, treat it like high-strength steel. That is no longer the case. If you are working on a late model vehicle, it is more likely that the steel is either mild steel, conventional highstrength steel, or one of several strength variations of high-strength steel commonly referred to in the industry as advanced-high-strength steel (AHSS). Some vehicle makers identify the types or categories and repairability of the steels of a particular vehicle. The vehicle maker may also have specific recommendations for a part or area of a vehicle. It is important to check service information before determining how to repair a vehicle. But what do you do if the strengths of steel are not identified for a vehicle? Is there equipment available?
TESTING EQUIPMENT

There are numerous types of equipment available that can be used to help identify the strength of steel. Tensile-testing devices are available, but equipment that measures to the accuracies that are required is both sophisticated and expensive. Hardness testing, which can be converted to tensile strength, can also be used to identify steel strength. Most hardness- testing equipment determines hardness by making a dent in the steel and measuring the resistance of the steel to the deformation. Once the hardness of the steel is identified, there are conversion charts and formulas that can be used to help categorise the steel by tensile strength. Hardness testing can also be compared to that of a known steel. This allows the steel to be placed into a basic strength category based on this comparative data. It is important to note that the same equipment should be used when hardness testing and when making comparison readings. Do not compare results from one piece of equipment to results taken with a different piece of equipment.

CONCLUSION

What is important to note is that when working on late model vehicles, the old rules may no longer apply. A part such as a front rail may be made from several different types of steel, attached together by a variety of joining methods.

Due to the numerous types of steels used on today’s vehicles, technicians need to add steel identification to the list of tasks when preparing for a repair. This might be as simple as checking a chart in the service information, but these charts aren’t always available. The task of identifying different strengths of steel in vehicles will not be getting easier. Vehicle makers are taking advantage of incorporating the many different strengths of steel available today in new vehicle designs.

I-CAR has released programs that specifically address these varied types of steel, including the Steel Unitized Structure Technologies And Repair (SPS07) live training program. This course will form the basis of the 2008 Structural training focus and will be available nationwide as detailed in the I-CAR Australian Training calendar.

Any queries or requests for further information on the I-CAR Australia program should be emailed to admin@i-car.com.au

comments powered by Disqus