Effect of Mn and Ni on Microstructure and Toughness of Submerged Arc Weld Metals

Effects of Mn and Ni content on the microstructure of weld metal and impact toughness of high-strength, low-alloy (HSLA) steel were studied. Deposited metals containing 0.9%, 1.2%, and 1.6% Mn and 1.0%, 1.5%, and 2.6% Ni are obtained using six types of welding consumables in multi-pass submerged arc welding. The microstructure and mechanical properties of the deposited metals were investigated by the Charpy impact test, hardness test, optical microscope, and scanning electron microscopy analysis. The result shows that the microstructure of columnar zone is dominantly composed of acicular ferrite and grain boundary by allotriomorphic ferrite. The microstructure of welded zone mainly consists of polygonal ferrite with the small amount of brittle alloys or metals, such as pearlite and martensite-austenite (MA) alloys, formed during the second phase. The hardness tends to decrease from the columnar zone to welded zone because of the presence of high-angle grain boundaries. Both Mn and Ni can change the hardenability, lower the A1 temperature (eutectoid transformation temperature), initiate the formation of acicular ferrite, but the acicular ferrite formed by Ni element is of higher quality than that of Mn. Excessive Mn results in the formation of a large amount of brittle alloys at the secondary phase such as pearlite and MA alloys and is responsible for crack formation and its propagation, and thereby deteriorates the impact toughness. The presence of Ni inhibits the precipitation of large-sized granular bainite as most of the polygonal ferrites are transformed into quasi-polygonal ferrite. Moreover, the presence of high-angle grain boundary strongly inhibits the crack propagation, and Ni dispersion in the ferrite alloy increases the toughness of the matrix, thereby decreasing the initiation of crack formation and its propagation. In the ductile to brittle temperature transition curve, the brittle transition temperature of the deposited metal of Si-Mn-Ni alloy will be lower than that of Si-Mn alloy, so the impact toughness of the top surface will be lower than that of Si-Mn alloy, and it shows that addition of Ni greatly improves low-temperature impact toughness of weld metal. © 2020, Editorial Board of Journal of Tianjin University(Science and Technology). All right reserved.