Continuous cooling transformation behavior in the weld coarse grained heat affected zone and mechanical properties of Nb-microalloyed and HY85 steels

Continuous cooling transformation (CCT) diagramswere determined for weld simulated coarse grained heat affected zone (CGHAZ) of Nb-microalloyed and HY 85 steels using a Gleeble (R) 3800 thermo-mechanical simulator. Samples were heated to a peak temperature of 1573 K (1300 degrees C) and cooled at different cooling rates from 0.5 K/s to 120 K/s. Microstructures, hardness, and impact toughness at 223 K were determined corresponding to simulated samples of weld CGHAZ thermal cycles. Transformation temperatures A(C3), A(C1), B-s, B-f, M-s, and M-f were also determined using dilatometric analysis. At slow cooling rates pro-eutectoid ferrite and pearlite were observed in Nb-microalloyed steel and a mixture of bainitic ferrite and granular bainite was observed in HY 85 steel but no pro-eutectoid ferrite was observed. At moderate cooling rates, only bainite was observed in both the steels. At high cooling rates, lath martensite was observed for both the steels. A strong dependence of prior austenite grain size on mechanical properties has been found. A prior austenite grain size of approximately 30 mu m (80 K/s) has shown impact toughness more than 100 J and 198 J for Nb-microalloyed steel and HY85 steel samples respectively. Based on microstructures, hardness, impact toughness, and transformation temperatures, CCT diagrams for the weld CGHAZ region were determined. (C) 2015 Elsevier Ltd. All rights reserved.