Details Matter – Using In-Depth Scientific Analysis to Interpret and Solve Production Issues with Examples from Production of TRIP Steel Tube Welds

Innovative materials and production methods are being developed within the steel industry for next generation vehicles. An example of this is transformation induced plasticity (TRIP) steels that due to their high strength and ductility allow automotive designers to make auto parts with thinner materials, which can still absorb more energy during crash; decreasing vehicle emissions while increasing safety. TRIP steels have also been adopted into hydroformed designs, requiring them to be tubed using high-frequency induction welding (HFIW). However, the high Al content of TRIP steels make them highly susceptible to bondline oxide formation during the HFIW process. Oxides found on bondline fractures in TRIP tubes appear similar to those found in conventional steel tubes containing oxides or penetrator defects. However, the oxides in the TRIP tubes fractures have higher Al content. This suggests that the Al-containing oxides in TRIP tube welds formed due to insufficient squeeze out, entrapped oxide, or excessive welding power, as these are the typical root causes for oxides formed in tube welds made in conventional steels. However, thermogravimetric analysis (TGA) showed that the Al-containing oxides were fundamentally different than the typically Fe-base oxides. Scanning and transmission electron microscopy (SEM and TEM) showed that the nature the Al-containing oxides found along the bondline of TRIP tubes was completely different than Al₂O₃ expected from the energy dispersive x‑ray (EDX). Finally, the formation location of the Al-containing oxides was different than seen in Fe-based oxides. The differences between the nature of the Al- and Fe- based oxides resulted in differences in the necessary corrective actions to mitigate the formation of these respective oxides. By understanding the metallurgical behaviour of TRIP steels and their response to the local environment during HFIW, welding process parameters may be tailored to weld these steels in a robust manner.