Microstructure and Fracture Characteristics of HAZs of Ca- deoxidized Steel Plates with Different Ti/N Ratios After HHIW

The microstructure and fracture characteristic of heat affected zone (HAZ) of the Ca-deoxidized steel plates with the different Ti/N ratios of 1.61, 3.79, and 5.00 denoted as TN16, TN38, and TN50 steels were studied after the High Heat Input Welding (HHIW) of 400 kJ/cm. The ratio of the ductile zone to the total fracture surface area is 55% in TN38 steel, which is the largest among the three steels. The main cracks in TN16 and TN50 steels display a smooth shape propagating along grain boundaries, while in TN38 steel, main crack exhibits shape in a zigzag pattern. The EBSD results reveal that the brittle transgranular fracture occurs on the grain boundary ferrites (GBFs), ferrite side plates (FSPs) with θ < 15 ° in TN16 and TN50 steels, while the ductile fracture occurs on the intergranular acicular ferrites (IAFs) with θ > 45 ° in TN38 steel. The area fractions for IAFs in CGHAZ are 68.3%, 91.6%, and 50% for TN16, TN38, and TN50 steels, respectively. Thus, the low-temperature impact toughnesses of HAZ at -20℃ are 77, 165, and 33 J for TN16, TN38, and TN50 steels, respectively.