MICROSTRUCTURAL CHARACTERS AND TOUGHNESS OF DIFFERENT SUB-REGIONS IN THE WELDING HEAT AFFECTED ZONE OF LOW CARBON BAINITIC STEEL

It is generally recognized that welding heat affected zone (WHAZ) is the poorest toughness region in the welded joint of low carbon bainitic steels. The thermomechanical simulator was employed to simulate the welding thermal cycle processes of different sub-regions in WHAZ of low carbon bainitic steel in this work. The toughness of simulated specimens were tested on the instrumented drop weight impact tester with oscilloscope, and miscrostructure features were observed by means of OM, SEM; TEM and EBSD. The results showed that when cooling time (t(8/5)) was 30 s, the crack initiation energy of various sub-regions was similar, and the range of their values was between 40 and 70 J. However; fine grained heat affected zone (FGHAZ) exhibited excellent crack arrest properties because the impact load-time curve included wide crack ductile propagation and crack brittle propagation stages. By contrast; the crack propagation energy of intercritical heat affected zone (ICHAZ) and coarse grained heat affected zone (CGHAZ) obviously deteriorated. With the increase in cooling time, both crack initiation energy and crack propagation energy of various sub-regions decreased, in which the crack initiation energy of CGHAZ and the crack propagation energy of FGHAZ decreased notably. Under different cooling rates, the variation of morphology and size of M-A constituents was mainly responsible for the deterioration of crack initiation energy. As for crack propagation energy, the FGHAZ had a good resistance to crack propagation due to high density of high angle grain boundary. Therefore, its crack propagation energy was far superior to other sub-regions. There was uneven effective grain size in the ICHAZ and ferrite grain grew with the decease in cooling rate, which decreased the crack propagation energy. In the CGHAZ, prior austenite grains coarsened and the density of high angle grain boundaries decreased greatly, which resulted in the decrease in crack propagation energy.