The dimensional stability of ball bearing steel was disscussed from the different points of view by M. Cohen. (Trans. A.S.M., 1945, vol. 34, p. 216). Apart from his studies, the authors studied on the influence of the ordinary industrial oil quenching and tempering, including no sub-zero treatment, on the dimensional stability of ball bearing steel. Specially, they investigated the effects of the difference of cooling method, the temperature of quenching oil, the time of tempering, the secondary tempering (after the grinding) and the repeated quench.
The effects, afforded by these treatments to dimensional changes which occurred in aging at room temperature, were mentioned on the basis of austenite-martensite reaction and dislocation theory.
(a) If the specimen is arrested at the higher temperature, the greater dimensional changes will occur in aging at room temperature.
(b) The most parts of dimensional changes which may occur in aging at room temperature, reveal in the period of one and a half days after the quench.
(c) Rapid cooling in the cold water to the room temperature directly after the arrested quench gives less dimensional changes.
(d) The tempering directly after the arrested quench, which is prevailed in the industrial heat treatments of steel in order to avoid the cracks and distortions, gives the greatest dimensional changes in aging at room temperature.
(e) High temperature of the quenching oil will afford no good effect to the dimensional stability, but in the case of arrested quench, it will not be influenced by the temperature of quenching oil.
(f) The increase of tempering time and the secondary tempering will affect favourably the dimensional stability.
(g) Repeated quench, involving the increase of the content of dissolved carbide in austenitic matrix during the repeated heating, increases the retained austenite. Nevertheless, the result that the dimensional changes have occurred unexpectedly less in aging at room temperature is noteworthy in respect of the behavior of retained austenite.