Effect of Material Chemical Composition on the Formation of Halo Ring in Gen 3 Q&P 980 Steel

Abstract

This study investigates how fusion zone (FZ) chemical composition influences the formation of the halo ring, a transient softening region that can form along the fusion boundary (FB) of resistance spot welds in Q&P980. For this purpose, spot welds were made in both similar Q&P steel joints as well as joints sandwiching low carbon (LC) or high carbon (HC) steels with Q&P steels, which tailored the chemical composition of the FZ. Electron probe microanalysis (EPMA) characterization was performed on a spot-welded sample to understand the effect of material chemical composition on the halo formation. Alloying elements such as C, Mn, and Si were significantly less in the FZ composition of the welds made with LC steel, as compared to 3-sheet Q&P weld, which resulted in associated 33% reduction in hardness of the LC steel FZ. On the contrary, the C and Mn were higher in the FZ of the weld made with HC steel compared to that of the 3-sheet Q&P weld. However, like the welds made with the LC steel the FZ of the welds with the HC steel had a lower Si content than the FZ of the 3-sheet Q&P steel. The increase in C and Mn contents in the welds made with the HC steel resulted in an 14% increase in FZ hardness compared to the FZ of the 3-sheet Q&P steel. The halo formation is more prominent in welds made with LC steel. It was wider in welds where the FZ had a lower alloy content than the Q&P steel as compared to the higher alloyed FZ of the weld made with the HC steel. In LC steel weld a high Mn and Si segregated region inside the FZ was also observed. It was shown that the transient softened zone can be affected by the difference in chemical composition between the FZ and heat-affected zone (HAZ). Therefore, any changes in mechanical properties associated with the halo are more likely to be more prominent in dissimilar welds where high and low alloy steels are combined.