Effects of Baking on the Structure and Properties of Resistance Spot Welds in 780 MPa Dual-Phase and TRIP Steels: The effects of baking treatments on weld microstructure and mechanical properties are explained through TEM analysis

摘要

Dual-phase and transformation-induced plasticity (TRIP) steels are finding increased application worldwide in car and truck bodies due to the many advantages they offer. These structures are typically joined by the resistance spot welding process. Subsequently, the welded car bodies and frames are painted and undergo an elevated-temperature paint baking process. Because the effect of baking treatments on weld microstructures and mechanical properties was not known, a systematic study was undertaken to evaluate the effect of paint baking on the tensile-shear strength and microstructure of resistance spot welds in 780 MPa dual-phase and TRIP steels. Peel tests, shear-tension tests, and microhardness traverses were conducted on the as-welded (nonbaked) and baked weld samples. Both as-welded and baked welds were examined using both a scanning (SEM) and a transmission (TEM) electron microscope. The results showed that postweld baking increased the load-bearing ability of the welds in shear-tension tests compared with that of the samples in the as-welded condition. However, for both steel grades, baking had no effect on the fracture appearance in shear-tension tests, as expected, and no noticeable changes were observed in weld hardness. TEM examination revealed that, in both the TRIP and the dual-phase steels, the dislocation density in the ferrite and on ferrite grain boundaries was low in the base or matrix material, but much higher in the heat-affected zone (HAZ) and the weld fusion zone, and large areas of lath and twin martensite were found, along with ferrite, in the weld. Epsilon (ε) carbide precipitates were found in twin martensite regions of the weld and HAZ regions in both the dual-phase and the TRIP steels after baking. It is believed that baking introduced Cottrell atmospheres around dislocations and grain boundaries, and thus changed the local yielding behavior, which was manifested in the small increase in shear-tension strength. Further, the low-temperature tempering from the baking treatment caused precipitation of transition (ε) carbides in the martensite within the weld and the HAZ, and is believed to have resulted in a certain amount of toughening within these microstructures.