Micromagnetic microstructure analysis during high cycle fatigue loading of single-lip deep hole drilled quenched and tempered AISI 4140 steel

Deep hole drilling techniques, such as single-lip deep hole drilling, have been at the centre of research in machining technology in the last decades. One major upside of these techniques is the surface integrity of deep drilled components. The drilling process leads to beneficial work hardening inside the boundary layer of the borehole and further generates compressive residual stresses. Yet a cyclic load can cause further evolution of microstructure inside this boundary layer and has an influence on the surface integrity of the drilled parts by e.g. causing relaxation of compressive residual stresses. In this study, notched and single-lip deep drilled AISI 4140 specimens are tested under a cyclic load with a stress amplitude of approx. 25% of the yield strength. The notch causes a stress concentration and enables a precise application of measurement devices focusing on the microstructure in this area. Two different lubrication strategies and their effect on the microstructure are compared. Different fatigue states are observed with the help of an eddy current testing device. These states are examined in depth with magnetic Barkhausen noise analysis, magneto-optical Kerr effect and X-ray diffraction. In combination with Electron Backscatter Diffraction orientation mappings as well as Electron Channelling Contrast Imaging and Scanning Transmission Electron Microscopy images a softening behaviour of the boundary layer of emulsion-processed specimens, especially between 4∙10 5 and 8∙10 5 cycles, can be observed. This softening leads to a reduction of compressive residual stresses in the axial direction of approx. 200 MPa inside the boundary layer of the borehole.