Effect of magnesium addition in low carbon steel part 2: toughness and microstructure of the simulated coarse-grained heat-affected zone

ABSTRACT

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°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.

KEYWORDS:

• Al-deoxidised low carbon steel
• Mg-Al-O inclusion
• precipitate
• coarse-grained heat-affected zone (CGHAZ)
• acicular ferrite

Acknowledgements

The authors appreciate Prof. Lijun Wang in Northeastern University, Prof. Deyong Wang in Schoow University, and Dr W. Mu in McMaster University of Canada for discussions in this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors wish to thank The National Natural Science Foundation of China for providing financial support during this study (Project Nos. 51374059, 51374060).