Abstract
The relation between the fine spherical cementite precipitated in the proeutectoid ferrite of a plain carbon steel by the quick thermal cycling and the holding time of the thermal cycle was investigated mainly by the point counting method and the hardness test. The specimen, solid bars 6 mm in diameter, were subjected to a repetition of rapid heating and cooling in air under no load by direct electric resistance heating and jeted water cooling. The maximum temperature of the thermal cycle was 690°C and the keeping time was 0.5, 25 and 200 sec.
The result of obtainded are summarized as follows:
(1) When the keeping time of the thermal cycle was 0 sec, the fine spherical cementite (cf) was most finest and the number of cf was great. With increasing keeping time, the diameter of cf increased and the number of cf decreased.
(2) The ageing hardness of the specimens after thermal cycling was higher than the as-annealed specimens. The ageing hardness increased as the keeping time of the thermal cycle decreased.
