Effect of Heat Input on Microstructure and Mechanical Properties of Marine High Strength Steel Fabricated by Wire Arc Additive Manufacturing

Marine-grade high strength steel (yield strength = 590 MPa) is a low-carbon, low-alloy steel characterized by high strength and toughness, excellent weldability, and seawater corrosion resistance. Thus, it is suitable for structural applications and widely used in the shipbuilding industry. Recently, wire arc additive manufacturing (WAAM) has attracted significant attention worldwide because of its high deposition rates and material utilization ratios, low material and equipment costs, and good structural integrity. However, the research on WAAM of 590 MPa marine-grade high strength steel is limited. In this work, 590 MPa marine-grade high strength steel components were produced by cold metal transfer and pulse-arc additive manufacturing (CMT + P-WAAM). The effect of the heat input on the microstructures and mechanical properties of the developed steel were investigated using several techniques, including OM, SEM, EBSD, and TEM. The results indicate that at a heat input of 5.6 kJ/cm, the microstructures of the WAAM deposited metals are mainly upper bainite and granular bainite, the area fraction of the martensite-austenite (M-A) constituents accounts for about 14.82%, the length ratio of the effective highangle grain boundary (grain boundary angle alpha > 45 degrees) is 36.3%, the tensile strength of the deposited metals are 843 and 858 MPa in the horizontal and vertical directions, respectively, and the average microhardness is about 286 HV. However, its impact absorbed energy at -50 degrees C is only 15 and 16 J in the horizontal and vertical directions, respectively. At a heat input of 13.5 kJ/cm, the low cooling rate and the high inclusion (inclusion size d > 0.4 mu m) content promote the formation of a large quantity of acicular ferrites with lath bainites and a small amount of granular bainites. The area fraction of the M-A constituents is reduced to 4.21%, and the length ratio of the effective high-angle grain boundary is increased to 52.4%. The tensile strength of the deposited metals in the horizontal and vertical directions is reduced to 723 and 705 MPa, respectively. Similarly, the average microhardness is also reduced to 258 HV, but the low-temperature impact absorbed energy is greatly improved, reaching 109 and 127 J, respectively, which is 7-8 times that of the WAAM deposited metal at a low heat input. The impact fracture characteristics also changed from a quasi-cleavage fracture to a typical ductile fracture.