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.
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.
|Table 1: Time-Temperature Test Matrix|
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).
|Figure 1 - Representative mechanical property performance curve|
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).