Boris Petkov

This article presents the effects of novel electron beam hardening (EBH) process parameters in terms of residual stresses (RSs) and microstructure modification in as-received C45 cylindrical specimens. The EBH was performed using continuous irradiation with power in the range of $720–2070\mathrm{W}$ on an Evobeam µEBW Cube 400 machine. A distinctive feature of the novel surface hardening process is the linear scanning mode in the axial direction of the treated cylindrical surface, which makes it suitable for machining shafts and axles. Using a one-factor-at-a-time technique, the individual effects of the electron beam current ${\mathrm{I}}_{\mathrm{b}}$, workpiece peripheral velocity ${\mathrm{v}}_{\mathrm{p}}$, scanning frequency (SF), and focal length (FL) on the RSs and microstructure in surface layers were evaluated. The X-ray diffraction results, scanning electron microscopy (SEM) images, and phase analyses confirmed the significant potential of the EBH process for forming compressive RSs due to martensitic transformation in the surface zone and gradient microstructure in terms of structure and phase composition. The measured maximum compressive axial and hoop RSs of $-289.5$ and $-345\mathrm{M}\mathrm{P}\mathrm{a}$, respectively, and compressive zone at a depth of approximately 0.3 mm correlate with the phase transformation region at a depth of approximately 0.2 mm. Based on the results for RSs and microstructure modification, the limitations with respect to the suitable operating parameter values were established. After excluding these operating parameter values, the following suitable ranges of the operating parameters were determined: ${\mathrm{I}}_{\mathrm{b}}\in \left(16,36\right)\mathrm{m}\mathrm{A},{\mathrm{v}}_{\mathrm{p}}\in \left(18,45\right)\mathrm{m}\mathrm{m}/\mathrm{s}$, $\mathrm{S}\mathrm{F}\in (5000,\mathrm{20,000})\mathrm{H}\mathrm{z}$, and $\mathrm{F}\mathrm{L}\in (+5,-5)\mathrm{m}\mathrm{m}$. The specified ranges are the basis for conducting a planned experiment on the novel EBH process.