Abstract
To clarify the influence of destabilization heat treatment on martensitic transformation of high chromium cast iron, specimens with 6.5 to 26% chromium and with 1.5 to 4% carbon were annealed at 900, 1,000 or 1,100°C for 20 to 1.8×104 min and then rapidly cooled down to room temperature. When specimens containing chromium and carbon in the ratio of 3 : 1 are annealed, carbides precipitate from supersaturated austenite and distribute uniformly in a short time. With the progress of precipitation of carbides, Ms point rises to the temperature determined by equilibrium chromium and carbon concentration in austenite. In the specimens containing chromium and carbon in the ratio of 6 : 1 or 8.5 : 1, the precipitation of carbides initiates from the immediate vicinity of the eutectic carbides, and delays that of the central part of primary dendrite. With cooling, austenite transforms to martensite only in the region where chromium and carbon were depleted by the precipitation. Annealing more than 6×103 min is required to homogenize the austenite. Ms temperature of homogenized specimen rises both with the decrease in annealing temperature and with the increase in the ratio of chromium to carbon.
When the specimens are homogenized at 1,000°C, the hardness of matrix becomes larger with increase of the ratio of chromium to carbon. Conversely, the hardness of specimens homogenized at 900°C decreases with increase of the ratio, because the hardness depends on the carbon concentration in martensite as well as on the amount of martensite. The marix is mostly composed of austenite in the specimens homogenized at 1,100°C.
