A flywheel rotor comprising a rotationally symmetric mass made of a single piece of through-hardened and tempered 300M steel formed using a vacuum-arc-remelting process having: a yield strength of at least 900 MPa, a fracture toughness of at least 40 and 200 MPa·m 0.5, and a maximal intrinsic defect size that is 2 mm or smaller, the mass, which comprises most of the mass of the rotor and stores the majority of the kinetic energy stored by the rotor, and having a diameter of its outer shape greater along a first axis than its widest thickness along a second axis, the widest thickness being between 203,2 mm and 355,6 mm, the mass configured to rotate about the second axis, the mass further comprising: two journals, each journal on an opposite side of the mass, wherein each journal assists in coupling the rotor to a separate shaft, each journal extending perpendicularly outward along the second axis from a mound of increased thickness, each mound extending outwardly from the origin of the central section of the mass and having a tapered shape that has a thickness greatest adjacent to the corresponding journal and which tapers gradually in thickness as the radius increases, wherein a fillet is present where the journal and the mound are joined, the journal's outer/top surface being substantially planar, and perpendicular to the polar axis, wherein the outer surface of the journal is narrower than a connecting end of the shaft configured to attach to the journal, wherein the shaft then narrows to a narrower outer diameter for the majority of its length, wherein the diameter where the interference fit is located at the boundary between the outer diameter of the journal and the inner diameter of the connecting end of the shaft is greater than the diameter of the majority of the length of the shaft. a yield strength of at least 900 MPa, a fracture toughness of at least 40 and 200 MPa·m 0.5, and a maximal intrinsic defect size that is 2 mm or smaller, the mass, which comprises most of the mass of the rotor and stores the majority of the kinetic energy stored by the rotor, and having a diameter of its outer shape greater along a first axis than its widest thickness along a second axis, the widest thickness being between 203,2 mm and 355,6 mm, the mass configured to rotate about the second axis, the mass further comprising: two journals, each journal on an opposite side of the mass, wherein each journal assists in coupling the rotor to a separate shaft, each journal extending perpendicularly outward along the second axis from a mound of increased thickness, each mound extending outwardly from the origin of the central section of the mass and having a tapered shape that has a thickness greatest adjacent to the corresponding journal and which tapers gradually in thickness as the radius increases, wherein a fillet is present where the journal and the mound are joined, the journal's outer/top surface being substantially planar, and perpendicular to the polar axis, wherein the outer surface of the journal is narrower than a connecting end of the shaft configured to attach to the journal, wherein the shaft then narrows to a narrower outer diameter for the majority of its length, wherein the diameter where the interference fit is located at the boundary between the outer diameter of the journal and the inner diameter of the connecting end of the shaft is greater than the diameter of the majority of the length of the shaft. two journals, each journal on an opposite side of the mass, wherein each journal assists in coupling the rotor to a separate shaft, each journal extending perpendicularly outward along the second axis from a mound of increased thickness, each mound extending outwardly from the origin of the central section of the mass and having a tapered shape that has a thickness greatest adjacent to the corresponding journal and which tapers gradually in thickness as the radius increases, wherein a fillet is present where the journal and the mound are joined, the journal's outer/top surface being substantially planar, and perpendicular to the polar axis, wherein the outer surface of the journal is narrower than a connecting end of the shaft configured to attach to the journal, wherein the shaft then narrows to a narrower outer diameter for the majority of its length, wherein the diameter where the interference fit is located at the boundary between the outer diameter of the journal and the inner diameter of the connecting end of the shaft is greater than the diameter of the majority of the length of the shaft.