Effect Of Magnesium Addition In Low Carbon Steel Part 2: Toughness And Microstructure Of The Simulated Coarse-Grained Heat-Affected Zone

A critical investigation into the role of Mg on the toughness and microstructure of coarse grain heat-affected zone (CGHAZ) in low carbon steel has been investigated. In this research, the specimens (Mg-free and Mg-added) underwent weld thermal cycle with heat input of 54, 80, and 100 kJ cm(-1) at 1350 degrees C peak temperature using a thermal simulator. The typical inclusions characteristics were characterised by means of scanning electron microscopy and equilibrium calculations. The precipitates were characterised by transmission electron microscopy and energy-dispersive spectroscopy. It is revealed that the occurrence of Mg in steel mostly exists in the form of Mg-Al-O oxide inclusions, but a few in the form of solid solution state and (Nb,Ti)(C,N)+MgO precipitates when the concentration of Mg is 0.0026%. The improvement of CGHAZ toughness is obtained when the heat input is 80 and 100 kJ cm(-1). The possible reasons about the effects of Mg on the toughness of CGHAZ, including Mg-Al-O inclusions, precipitates, and soluble Mg, are discussed in detail.