Effect of line energy density in repairing 34CrNiMo6 steel by electron beam remelting

The role of the line energy density of electron beam remelting in repairing damaged 34CrNiMo6 parts was investigated. Remelting technology can be used to rapidly repair defects with superior performance, but the value of the heat input during operation was difficult to control, which has a substantial impact on the microstructure and mechanical properties of the parts. The line energy density Q was introduced to calculate the experimental parameters. The microstructure was observed at different regions of the sample, and its mechanical properties were analyzed. From the results, the following conclusion was drawn. An appropriate increase of line energy density could increase the depth and width of remelting to improve the remelting efficiency. Under a low line energy density (Q1), the mainly thermal cycle on 34CrNiMo6 steel was tempering rather than the preheating. With the increase of energy density, the preheating effect of ex-remelting came into sight, and the martensite of repaired zone with energy density Q2 and Q3 became coarse. As to the enhanced diffusion of carbon, the amount and size of carbides precipitated from ferrite increased. The microhardness decreased with the increase of energy density, the max value was about 660 HV (Q1) and the min was 330 HV (Q3). Tensile properties in different Q were similar, the max tensile strength σ_b was 845 MPa by remelting. The samples after quenching and tempering (QT) exceeded the forging standard even for the lowest strength value (1013 MPa). The fracture mode of remelted samples was brittle fracture in low energy density (Q1), and changed to quasi-cleavage fracture at energy density Q2 and Q3. All the modes of the QT samples were ductile fractures.