Toughness and Microstructure of Coarse Grain Heat Affected Zone with High Heat Input Welding in Zr-bearing Low Carbon Steel

Abstract

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.