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

Abstract

Linear Friction Welding (LFW) is a solid-state joining process, in which a joint is obtained through the relative oscillation of two components under a high contact load. In this method, the welding temperature can be determined by the applied pressure, which was focused in the present study. Quenched and subsequently tempered SCM440 steel was welded by LFW under applied pressures of 150-1200 MPa. The influence of applied pressures on the Vickers hardness and microstructures was investigated. The welding temperature decreased with increasing applied pressure until 900 MPa applied pressure. However, the welding temperature rose again above A 3 temperature when the applied pressure was increased to 1200 MPa. It is presumed that the deformation during LFW was relatively limited to the interface region under the 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 structures with much smaller size than the usual quenched lath martensitic structure was formed; however, the misorientation distribution of grains are rather similar to the quenched one. On the other hand, in the case of high applied pressure, equiaxed extremely fine globular martensitic structure as small as 0.2 μm with large misorientation was formed. It is assumed that the martensitic transformation occurred in a single variant manner from the extremely fine dynamically recrystallized austenite grains. The hardness distributions exhibited a good accordance with microstructural variations with applied pressure as well as a distance from the weld center of the joints.