Effect of aluminium on microstructure and mechanical properties of self-shielded flux cored welds in low carbon steel plate

Abstract

In the present paper, the results are reported of an investigation dealing with the inclusions, microstructure and mechanical properties of self-shielded flux cored welds with different aluminium contents. Results show that the total number and number density of inclusions in the low aluminium weld are higher than those of the high aluminium weld. However, the average size and the contamination rate of inclusions in the low aluminium weld are 0·77016 μm and 0·022%, which are lower than those of the high aluminium weld. The weld with a low aluminium content contains finer inclusions and the weld with a high aluminium content contains mainly coarser inclusions. There are remarkable differences in microstructure between the welds with different aluminium contents. The reheated zones in the weld made using flux no. 1 are larger than those in the weld made using flux no. 2, which are occupied by skeletal δ ferrite columnar crystals. As a result, a drastic reduction in impact toughness is observed in the high aluminium weld. Fracture examination shows that the low aluminium fracture surface consists mainly of small ductile dimples and a quasi-cleavage fracture area. The fracture surface of the high aluminium weld comprises mainly larger dimples and a cleavage fracture area. Inclusions in the low aluminium weld are mainly small Al₂O₃–MgO spinel with an approximate round shape, and some AlN inclusions. In the case of the high aluminium weld, inclusions are almost all large AlN. Finally, the results of thermodynamic analysis agree with the trends observed in the experimental data.