Effect of Applied Pressure on Microstructure and Hardness of Linear Friction Welded Martensitic Steel

<p>Linear friction welding (LFW) is a solid-state joining process in which a joint is formed through the relative oscillation of two components under a high contact load. In this method, the joining temperature can be determined from the applied pressure, which is the focus of this study. Quenched and subsequently tempered SCM440 steel was joined using LFW at applied pressures of 150–1200 MPa. The effect of applied pressure on the Vickers hardness and microstructures was investigated. The joining temperature decreased with increasing applied pressure until a pressure of 900 MPa was reached. However, the joining temperature rose again above the A₃ point when the applied pressure increased to 1200 MPa. The deformation during LFW was presumed to be relatively limited to the interface region under extremely high applied pressure, which caused an overshoot in the temperature of the joint interface. In the case of low applied pressure, slightly elongated lath martensitic microstructures with a much smaller size than the usually quenched lath martensitic microstructure were formed; however, the misorientation distribution of the grains was rather similar to that of the as-quenched one. On the other hand, in the case of high applied pressure, an equiaxed extremely fine globular martensitic microstructure as small as 0.2 µm with large misorientation was formed. Martensitic transformation was assumed to have occurred in a single-variant manner from extremely fine, dynamically recrystallized austenite grains. The hardness distributions exhibited good agreement with the microstructural variations with the applied pressure and distance from the joint center.</p>