| Wire and arc additive manufacturing（WAAM）is a technology that uses arcs as a heat source and fabricates desired parts layer by layer along a specific path.This technology has the characteristics of high forming efficiency,low equipment cost,unrestricted molding size,especially suitable for the manufacturing of large-size metal parts.Martensitic heat-resistant steel is widely used in high-temperature components of thermal and nuclear power plants due to its high thermal conductivity,good resistance to stress corrosion and oxidation,and excellent high-temperature properties.This paper uses WAAM technology to systematically study the forming quality,microstructures and mechanical properties of martensitic steel.In this paper,the forming processes of wire arc additively manufactured martensitic heat-resistant steel are first studied.The results show that the appropriate parameters are 5.0 m/min wire feeding speed,6.5 mm/s welding speed,0.8 s arc extinguishing time,and 20-25 L/min shielding gas flow rate.Secondly,the microstructures and mechanical properties of the wire arc additively manufactured metal wall at different locations were studied by optical microscopy（OM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）and other testing techniques.The experimental results show that there is less splash on the surface of the deposited metal wall.An oxide layer was found on the surface of the deposited straight wall.The grains tend to exhibit typical epitaxial growth along the deposition direction.The grain size near the substrate is smaller than in the middle of the metal wall.In the top region of the deposited metal wall,equiaxed laths were obtained.Theα-Fe was found in the metal wall by XRD,and the diffraction peaks of other phases were not detected.The hardness of the deposited metal wall varies from 404 HV to 432 HV.The average hardness value near the substrate is higher than in the middle and top regions.The tensile strength of the straight metal wall at room temperature is about 1270 Mpa.The yield strength varies between990-1002 MPa,and the elongation of the tensile specimens all are less than 15%.The tensile properties of the samples at different locations showed some anisotropy.The impact toughness value of the straight metal wall in the horizontal direction is smaller than the vertical toughness value.The second phase particles are obtained in the impact fracture surface.Finally,the effects of heat treatments on the microstructures and mechanical properties of the wire arc additively manufactured martensitic steel were investigated.The results show that after normalizing and tempering heat treatment,the lath martensite decomposes and a large amount of pricipitates are precipitated.The EDS and XRD results of the metal wall indicate the presence of M23C6,MX and Cr7C3.After the normalizing and tempering heat treatment,the hardness of the sample was reduced by 36-48.5%.The strength of the sample was reduced by37.4-48.6%.However,the elongation of the sample increased by more than 38%.The impact toughness value of the sample in the as-deposited state is only 14.3-18.6%of the sample after heat treatment,and the impact toughness value tends to increase with the increasing of the tempering temperature.At the same time,heat treatment experiments also confirmed that the increase in impact toughness value under normalizing and tempering heat treatment is greater than the impact toughness value of only tempering heat treatment. |
