Characterization of bainite-ferrite structures formed on the heat-affected zone of a dissimilar welds of high-strength steel (S700MC/S960QC) and their dependency on cooling time

Abstract

Modern steel structures and joints must satisfy various increasingly demanding requirements, such as high yield strength, improved cross-section-to-mass ratio, and desirable ductile-to-brittle transition properties. Consequently, joining different types of high-strength steels has become an attractive option from the cost perspective and for weight and corrosion reduction. In dissimilar welding, however, there remains a need for better understanding of discrepancies in microstructure formation resulting from asymmetric heat distribution. In this study, a characterization of the transformation of bainite, ferrite, and martensite in the microstructure of the heat-affected zone (HAZ) formed by a cooling time of 10 kJ/cm of heat input was carried out for dissimilar high-strength joint steels (S700MC/S960QC). The characterization was performed by scan electron microscopy (SEM) sampling, the images of which were analyzed by ImageJ Pro and evaluated by volume fraction of block-like granular bainite (GB). The alloy element composition close to the fusion line of both materials was then assessed using energy dispersive X-ray spectroscopy (EDS). The results showed a strong presence of GB, which had about 70% volume fraction in S700MC at 615°C and which comprised formations of lower bainite and retained austenite (RA) at 420°C. The presence of 55% block GB was observed at 470°C in S960QC, which was caused by the formation of tempered martensite (TMA) at 400°C.The presence of 1.3 Ni, 0.4 Mo, and 1.6 Mn in the coarse grain heat-affected zone (CGHAZ) of S700MC confirmed the risk of brittle failure on the S700MC side due to the high presence of carbide and ferrite in the GB.