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

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 elements composition close to the fusion line of both materials was then assessed using energydispersive 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. Presence of 1.3Ni, Rezime Moderne čelične structure i spojevi moraju da zadovolje različite zahteve kao što je visoki napon tečenja, visok odnos poprečnog preseka prema masi komada, i zadovoljavajući odnos osobina prelaznih osonina plasičnosti i krtosti. Na taj način, spajanje različitih tipova čelika visoke čvrstoće je postalo atraktivno sa stanovišta troškova i smanjenja težine i korozije. Pri zavarivanju različitih materijala, postoji potreba za bolje razumevanje odstupanja nastajanja mikrostruktura kao rezultat asimetrične raspodele toplote. U ovoj studiji, karakterizacija transformacije beinita, ferita i matrenzita u mikrostrukturi zone uticaja toplote (ZUT), nastale hlađenjem pri unosu toplote od 10 kJ/cm, je izvedena za spojeve različitih čelika visoke čvrstoće (S700MC/S960QC). Karakterizacija je izvršena skenirajućom elektronskom mikroskopijom (SEM), analiza slika je vršena sa softverom ImageJ Pro i određivan je udeo zapreminskog udela faza, kao što je zrnasti beinit (GB). Sadržaj legirajućih elemenata uz liniju spoja oba materijala je određen primenom energo – disperzivne spektroskopije X zracima (EDS). Rezultati pokazuju značajno prisustvo GB, koji je imao oko 70% zapreminskog udela u čeliku S700MC pri 615 °C i koji se sastoji od donjeg beinita i zaostalog austenite (RA) na 420°C. Prisustvo od 55% GB je konstatovano pri 470°C kod čelika S960QC, koje je prouzrokovano stvaranjem temperovanog martenzita (TMA) na 400 °C. Prisustvo 1.3Ni, 0.4Mo, i 1.6Mn u zoni porasta NAUKA∗ISTRAŽIVANJE∗RAZVOJ SCIENCE∗RESEARCH∗DEVELOPMENT 160 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE, 4/2020, str. 159-172 0.4Mo, and 1.6Mn 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. zrna u zoni uticaja toplote (CGHAZ) kod čelika S700MC potvrđuje rizik nastajanja krtog loma na strani čelika S700MC, zbog visokog sadržaja karbida i ferita u GB.