STEEL IDENTIFICATION USING HARDNESS TESTING
Steel identification is an essential step when determining the repairability of parts used on today’s vehicles. The type of steel can alter or limit the ability to straighten, remove, and replace parts.
When steel on a vehicle was either mild or high-strength, there was a general rule of thumb; if you didn’t 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 high-strength 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. This may be illustrated in a color-coded chart of the vehicle (see Figure 1). General repair methods may also be provided according to the steel type (see Figure 2). 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? There is equipment available to help with that task.
Tensile strength is the single characteristic that is most often used to determine the repairability of a steel. Tensile strength (or ultimate tensile strength) is defined as the measurement of the amount of force necessary to tear a piece of steel apart. The amount of strength is typically measured in megapascals (MPa) or thousands of pounds per square inch (ksi). One megapascal is also equal to 145 psi or 0.145 ksi.
The American Iron and Steel Institute (AISI) has developed a chart that can be used to help illustrate and compare the different steel names, tensile strength ranges, and percentage of elongation. The percentage of elongation is how much the metal will stretch before it fractures or breaks. Notice in the chart, in Figure 3, how some steel types overlap into multiple steel categories. Because of the wide range of strength for one particular type of steel, such as HSLA that can range from 300–700 MPa (44–102 ksi), the steel strength cannot be determined by name alone. This is important to keep in mind when making a determination about repair. It is more important to pay attention to actual strength rather than a vehicle maker’s or steel manufacturer’s trade name.
The many different names and classifications for steels used on today’s vehicle can make strength identification confusing. You may see one name used to describe two different strengths or categories of steel. For example, high-tension steel, extra-high-strength steel, higher-strength steel, etc. are all names for steels that may be similar in strength. Or, individual vehicle makers may use different names to describe the same strength category of a particular type of steel.
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. This equipment is typically only used in laboratories and testing centres (see Figure 4).
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. Hardness testing can be done by using either a bench-mounted piece of equipment or a hand-held portable device. A bench-mounted hardness tester requires a sample piece of the steel for taking to the bench (see Figure 5), while a portable hardness tester allows testing without having to cut a sample from the vehicle part (see Video).
Once the hardness of the steel is identified, there are conversion charts and formulas that can be used to help categorize the steel by tensile strength. Some machines are able to convert hardness to tensile strength automatically and other machines may use their own chart to divide the test sample into a steel category (see Figure 6). 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.
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. For this reason, facilities may want to consider acquiring a steel identification device.
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 two programs available that specifically address these varied types of steel, including the I-CAR Steel Unitized Structure Technologies And Repair (SPS07) Live training program, which will be available nationwide after its debut at NACE. Advanced high-strength steels are also discussed in the Advanced High-Strength Steel Overview (AHS01) online training program. To review this program, please go to www.i-caronlinetraining.com.