Comparison of microstructures and properties of X80 pipeline steel additively manufactured based on laser welding with filler wire and cold metal transfer

Additive manufacturing (AM) with low heat input is a crucial technology of on-line maintenance of long-distance oil & gas transmission pipeline. By utilizing two methods, i.e. laser and cold metal transfer (CMT) arc, with low heat input and a homemade filler wire, the AM test was separately carried out on the widely used X80 steel. Parameters and the overlap ratio of AM processes were optimized at first; afterwards, the additively manufactured parts (AMPs) were prepared on the surface of the X80 pipeline steel substrate. Subsequently, the microstructures of the AMPs were observed and also microhardness test, impact test and tensile test were carried out. The result showed that the heat affected zones (HAZs) of the substrate were both not softened by applying the two methods. The microhardne values of the laser- and CMT-based AMPs were separately 140% and 119% that of the substrate, respectively. The impact energies of the laser- and CMT-based AMPs at room temperature were 53% and 67% that of the substrate, respectively. The average tensile strengths of both laser- and CMT-based AMPs were higher than that of X80 substrate, while the average percentage elongations after fracture of laser- and CMT-based AMPs are similar, which both accounted for about 66% that of the substrate. It is noted that the mechanical properties of the laser- and CMT-based AMPs both showed anisotropy, in which the anisotropy of the former was more significant.