Abstract
Hardened steel exhibits slow but remarkable flow during tempering under sustained stress, which causes no such plasticity in annealed state. To certify the plastic flow of tempered steel below tempering temperature in relation to the dimensional accuracy of ball bearings the twisting creep below 300°C was studied using variously heat-treated specimens of SUJ-2. On the heating of quenched specimens at 2°C/min under shear stress of 1.8∼15 kg/mm2, the total strain increment below the designated temperature (named cumulative flow) increases linearly with stress and is zero only at zero stress. The cumulative flow may relate to the quantity and initial carbon concentration of martensite. The cumulative flow of tempered specimens has a transition temperature, determined by the temperature and the holding time of tempering, below which it diminishes nearly independently of the carbon concentration of primary martensite. Above the transition temperature, the cumulative flow increases abruptly and reaches that of quenched specimens in the same temperature range. The creep rate, in the usual sense, of quenched and tempered specimens is inversely proportional to time. The removal of applied stress causes reverse flow and its rate obeys the same time law as the creep rate. The plastic flow of tempered ball-bearing steel at usual service temperature remains so small as compared with the elastic deformation, that it seems to have no influence upon the running accuracy of ball bearing.
