Browsing by Author "Elliot Biro"

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A Comprehensive Evaluation of Tempering Kinetics on 3rd Generation Advanced High Strength Steels
(Elsevier, 2022-12-02) Dileep Chandran Ramachandran; Elliot Biro
Emerging third-generation advanced high strength steels (3G AHSS) grades are being increasingly considered for autobody applications owing to their complex microstructures providing an excellent combination of strength and ductility. When welded with RSW, 3G AHSS such as 3G 1180 and 3G 980, exhibits a solid-state transformation in the sub-critical-heat-affected-zone (SCHAZ) due to the heat generated during the RSW process. The resulting changes in microstructure can affect the mechanical properties of the material within and around the spot welds and thereby influence the performance of welded components in case of a vehicle crash event. To study metallurgical transformations in the SCHAZ during the RSW process, 3G 1180 and 3G 980, and a reference martensitic press hardened steel, PHS 1500, were selected. These materials were subjected to isothermal tempering experiments at temperatures ranging between 350 °C and 650 °C and different tempering times from 0.2 s to 24 h. It was shown that the Hollomon-Jaffe model accurately describes the tempering kinetics of 3G 1180 and PHS 1500 but does not predict the transformation processes in the SCHAZ of 3G 980. The PHS 1500 and 3G 1180 showed softening (tempering) for all combinations of investigated tempering times and temperatures due to the decomposition of martensite. In contrast, the 3G 980 showed a combination of softening by the formation of martensite and secondary ferrite combined with secondary hardening due to the formation of fine M2C typed plate-like carbides. Furthermore, during long tempering, the M2C particles dissolve into the matrix and were replaced by cementite particles in 3G 980.
Effect of Material Chemical Composition on the Formation of Halo Ring in Gen 3 Q&P 980 Steel
(Springer, 2024-12-17) Dileep Chandran Ramachandran; Elliot Biro
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.