The microstructure and toughness of simulated coarse grained heat affected zone (CGHAZ) in low carbon steel have been investigated in this study. In order to simulate microstructure evolution in CGHAZ, specimens were subject to weld thermal cycle with heat input of 100 kJ/cm, 400 kJ/cm and 800 kJ/cm at 1 400°C peak temperature using thermal simulator. As increase in heat input energy of weld thermal cycle, prior austenite grain size increases due to longer holding time at peak temperature. Excellent impact toughness of CGHAZ with heat input of 800 kJ/cm was obtained because of high volume fraction of fine acicular ferrite (AF) inside gain acting as an obstacle to cleavage propagation owe to its high angle grain boundary, forcing cleavage crack to change the route of propagation and effectively impedes the propagation of crack. The primary AF mainly originated from Zr-oxide particle with MnS and subsequently secondary AF nucleated sympathetically in form of side arms and grew from the primary AF in CGHAZ.

Microstructure evolution and impact toughness of simulated coarse grained heat affected zone (CGHAZ) in Zr bearing low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution of CGHAZ with high heat input welding thermal cycle at 1 400°C peak temperature. Microstructure of CGHAZ consisted of high volume fraction of AF inside grain and GBF at prior austenite grain boundaries. Prior austenite grain size of CGHAZ increases with heat input increasing. Excellent impact toughness (more than 100 J) of CGHAZ with heat input of 1 000 kJ/cm was obtained in this experiment. Impact toughness of CGHAZ with 400 kJ/cm (230 J) is the highest, because austenite grain size of CGHAZ with 400 kJ/cm favors the development of AF inside grain. Impact toughness is not only related with high angle boundaries but also with effective grain size. High supercooling of CGHAZ provided driving force for the AF transformation during welding thermal cycle, increasing the number of AF.KEY WORDS: coarsen grained heat affected zone (CGHAZ); acicular ferrite (AF); impact toughness; microstructure; electron backscattered diffraction (EBSD).