Study on microstructure and low-temperature impact toughness of deposited metal form covered electrodes for 440 MPa grade high-strength steel

To meet the welding material requirements of 440 MPa grade high-strength steel, three types of covered electrodes have been developed, and the welding test of deposited metal was carried out. The microstructure and low-temperature impact toughness of deposited metal were analyzed by means of the optical microscope, scanning electron microscope, and transmission electron microscope. The results show that with the increase of Mn, Ni, Cr, and Cu contents, the average impact absorption energy of the deposited metal at −40 ℃ gradually increases from 35.7 J to 96.3 J, and the low-temperature impact toughness gradually increases; With the increase of Mn, Ni, Cr and Cu contents, although the content of M₂₃C₆ type carbide in the deposited metals gradually increases, the CCT curve of the deposited metals gradually moves to the right, and the transformation temperature gradually decreases, which makes the content of acicular ferrite gradually increase, the size of ferrite lath gradually decreases, and the interlaced distribution trend between ferrite lath gradually increases, and the content and size of M-A constituent gradually decrease, which is the main reason for the gradual improvement of low-temperature impact toughness; The outer layer of inclusions in the deposited metal containing Cu can form CuS, making it easier for acicular ferrite to form nuclei, which is conducive to improving low-temperature impact toughness.