During the previous articles in this series, many grades of advanced high-strength steels (AHSS) were introduced. Additional collaborative work between the steel and automotive industries is leading to the development of a new third generation of AHSS (3rd Gen AHSS). This article will explore in greater detail the challenges faced by the repair community and the ongoing work to develop acceptable and practical AHSS repair procedures.
AHSS introduction
The introduction of AHSS to light vehicle body structure applications poses a significant challenge to organizations involved in vehicle repair. AHSS are typically produced by non-traditional thermal cycles and contain microstructural constituents whose mechanical properties can be altered by exposure to elevated temperatures. This temperature sensitivity can alter the mechanical behavior during repair welding or flame straightening, thus seriously affecting the structural performance of the AHSS components after the repair.
The Steel Market Development Institute (SMDI), with our automotive partners – FCA US LLC, Ford Motor Company, General Motors Company – and I-CAR, have completed studies examining the mechanical behavior of various AHSS products after exposure to typical repair arc welding and flame straightening temperature cycles. Recommended practices for repairing components made of these materials were also developed. The studies evaluated many of the AHSS grades being applied and built into vehicle structures today.
AHSS thermal evaluation
Several steel grades were evaluated for their sensitivity to thermal exposure taking place during heating to soften the material for straightening, typically by flame. The test results, conclusions and recommendations contained herein are the consensus views of the team members.
Vehicle manufacturing simulation
The sheet steel comprising vehicle body structure components does not exist in its as-produced state at the time of repair, but rather has been subjected to several mechanical deformations and thermal treatments during stamping, assembly, painting, and subsequent collision damage. These treatments could alter the response of a component to repair processes. To simulate the actual state of material at the time of repair, interstitial free (IF), high-strength low alloy (HSLA), dual phase (DP) and transformation-induced plasticity (TRIP) steels were first subjected to eight percent strain in uniaxial tension (to simulate part forming) and heated to 170 degrees Celsius for 20 minutes to simulate paint baking. The martensitic steel was subjected only to the paint bake treatment, as martensitic steels rarely undergo substantial deformation during fabrication.
A time-temperature test matrix was developed to represent the various thermal conditions encountered during repair welding and flame straightening as shown in Table 1. Individual steel performance result discussions are based on this test matrix and discussed by grade category and specific type.
Representative samples of each time-temperature test were subjected to tensile tests (see Figure 1) to provide information on the post-repair properties and anticipated subsequent performance. Force is applied to the sample to stretch the sample and various properties are measured based on the amount of force the sample can carry without permanent deformation and subsequent failure (fracture).
A few key measurements of mechanical properties are determined in a tensile test. Yield strength is a measure of the strength level steel starts to permanently deform. This is an important metric in sheet metal forming and also for the on-set of crash. By changing this value during repair, you may affect the performance of safety critical components like an air bag sensor. Elongation is a measure of the amount of stretch in the steel before fracture occurs. Decreasing elongation by a repair procedure can affect the amount of total energy the repaired component can absorb. The total strength of a material before the failure is the ultimate tensile strength (UTS). If this value is lowered during repair, the part may fail at a lower force than initially designed for the vehicle. Strength values are measured in force (newton) over area (meters squared) and reported in units of megapascal (MPa).
Although all of these factors are important to the performance of a part, UTS will be used to illustrate the impact of heat during repairability processes. Note, all samples begin at strengths greater than their as-received material (or specification level) because of the effect of strain from forming and heat from paint bake treatment.
Steel performance
Conventional Steels: IF and HSLA steels are conventional steel products which are essentially a single phase ferrite microstructure and obtain their strength by the addition of chemical elements. These steels have been used in body structures and closures for many years and are well-known to be repairable without substantial performance degradation by arc welding and flame straightening.
A Grade 4 IF steel (300 MPa as-received UTS) and HSLA 340 steel (450 MPa as-received UTS) grades were evaluated. The result of the temperature impact on UTS for each is shown in Figure 2. The IF grade shows a slight increase in strength up to 650 degrees Celsius then a decrease at higher temperatures. The UTS drops below the grade specification when heated to 1,000 degrees Celsius. The HSLA grade loses strength when heat is applied at and above 600 degrees Celsius and drops below the material specification around 800 degrees Celsius.
For repair processes, this means conventional steels can be subjected to heat during repair with the finished repaired component having mechanical properties greater than the properties of the as-received steel. However, it is recommended heating is kept below 750 degrees Celsius to ensure no degradation in properties.
Advanced High-Strength Steels
Dual Phase (DP) Steel
DP steels range in strength from 500 MPa to 1200 MPa and obtain their properties from the introduction of a martensitic phase into the ferrite microstructure. The ferrite phase provides formability, while the martensitic phase provides the improved strength. This category of steel grades obtains its microstructure, and thus its mechanical properties through a combination of alloying elements and thermomechanical processing. The processing involves some holding time at elevated temperatures and cooling at specific rates.
Two specific grades of DP steel were tested, DP 600 and DP 780, and higher strength levels are currently under evaluation. The number indicates the ultimate strength level of the material in MPa and is the common way to name these grades. UTS for both grades decreases with elevated temperature at a much faster rate than for conventional steels and is illustrated by DP 600 in Figure 3. At temperatures above 650 degrees Celsius the strength suddenly increases then upon additional heating decreases. This behavior is a result of changing the microstructure created during the original thermomechanical processing of the material. Once the microstructure is changed, it is very difficult to return it to its original state in a repair shop environment. Therefore, it is not recommended to subject DP steels to any kind of elevated temperature process for straightening or removing dents. The recommended repair procedure is to remove and replace the DP component. OEM repair guidelines and procedures should be referenced for approved cut and weld lines for replacement.
Transformation Induced Plasticity (TRIP) Steels: TRIP steels have a similar range of strength as DP steels, 500 MPa to 1200 MPa, while providing improved formability. The improved formability is obtained with the introduction of additional phases of austenite and bainite into the microstructure. These phases improve the work hardening properties of steel and provide additional energy absorption characteristics. TRIP steel microstructures are obtained in a similar manner as DP steels, and therefore have similar behaviors when heating.
TRIP 600 and 780 were evaluated in the studies and confirmed the expected results as demonstrated by TRIP 600 in Figure 3. Heating during repair of TRIP steel will also adversely affect their mechanical properties and thus the performance of the as repaired component may be compromised. OEM recommended repair procedures are similar to DP steels.
Martensitic Steel (MS)
MS steels typically have a microstructure of 100 percent martensite and have tensile properties greater than 980 MPa. Martensite is the strongest microstructural phase in steel and is obtained by alloying and rapid controlled cooling. This grade is used in areas where exceptional strength and anti-intrusion are needed, including such applications as the A-pillars, B-pillars, rockers and rails.
The effect of heat on MS 1300 is shown in Figure 3. Like other AHSS, it is adversely affected by heat and the performance of the as repaired component may be compromised. Thus, heat should be used only as outline in OEM repair procedures.
Response to MIG welding
To begin assessing the response of AHSS to repair metal inert gas (MIG) welding, selected samples of the AHSS were MIG welded at GM's Service Parts Operations repair lab in Warren, Michigan. Samples subjected to MIG welding included HSLA 340, DP 600, DP 780 and TRIP 600 steels. Samples received a single MIG weld bead at a welding speed sufficient to assure complete weld penetration through the thickness. After welding, standard two inch gauge length ASTM weld coupons were machined from the weldments, with the MIG weld beads at the center of the reduced gauge section and the weld bead transverse to the gauge length as shown in Figure 4. Test coupons were pulled under standard ASTM testing conditions. Tensile test results from the welded coupons (or as-fabricated coupons) were compared to those for the base (un-welded) condition and for the as-received condition.
The results of the preliminary MIG welding studies are encouraging and shown in Table 2. Whether the AHSS of this study were MIG welded in the as-fabricated or flame straightened state, yield and tensile strengths remained above the minimum levels specified for the steel grade (as-received). Elongation remained reasonable, indicating these steels are not significantly embrittled by the welding process. Therefore, MIG welding is an acceptable repair process for AHSS with no concerns over reducing the performance of the as-repair components. OEM-specific repair procedures should be referenced for verification.
Summary
The steel industry, working closely with automotive OEMs and the repair community, have developed and validated repair procedures applicable to the new AHSS used in today’s vehicles. Each OEM has taken the results from the AHSS repairability studies and developed their own repair guidelines.
The steel industry continues to develop AHSS grades with strength levels at and above 1000 MPa. These microstructures contain martensite and will be affected by heat exposure during repair as shown in previous studies. Collaborative studies will continue to update repair procedures for higher strength AHSS, including DP, MS and Press Hardened grades, and new 3rd Gen AHSS as they are introduced.
