treating an exemplary quenching is to heat the blank to 850 c quench in a polymer modified water bath followed by tempering at 210 250 c following the quenching operation the part is finish machined and balanced such a process sequence may result in a minimum yield strength of about 1200 mpa 170 000 psi ultimate tensile strength of about 1300 mpa 185 000 psi and ductility of at least 6 for an exemplary rotor of the dimensions discussed above it may be important to ensure adequate ductility so that the rotor when subjected to the pre conditioning process disclosed below will have the desired beneficial residual stress state that improves energy density and ensures adequate margins of safety referring to fig2 a system 10 shows a flywheel energy storage device that includes a rotor 12 that is located within a hermetically sealed housing including a top plate 14 a cylindrical vertical enclosure 16 and a bottom plate 18 two bearing packs 20 allow the rotor to rotate freely in rolling contact with the bearings held within each bearing pack dowel pins 22 accurately locate the upper and lower plates with respect to each other o ring seals 24 in the cylindrical enclosure 16 seal the top and bottom plates to form the vacuum enclosure ribs 26 in the top and bottom plates provide the.