Abstract
Spheroidal graphite cast iron specimens with various ratios of decarburized layers were made by heat treatment for 86.4-345.6 ks at a temperature range of 973 K to 1373 K. The impact property of the specimens was examined at a temperature range of 233 K to 293 K. The impact value decreased with the increase in the thickness of the decarburized layer for each test temperature. This was suggested to be due to the formation of a pearlite-ring in the decarburized layer. Increasing the thickness of the decarburized layer increased the ratio of the pearlite-ring layer to the decarburized layer. However, the pearlite-ring layer disappeared when the specimen was overdecarburized. Hence, the impact value recovered in the overdecarburized state.
As a result, the ductile-brittle transition temperature (DBTT) increased with the increase in the thickness of the decarburized layer. The DBTT rose to about 300 K when the specimen was decarburized at 1373 K. On the other hand, the fracture surface after impact test showed brittle fracture even at a room temperature. This tendency was pronounced with the increase in the thickness of the decaruburized layer.
The impact values obtained were discussed in terms of crack initiation energy and crack propagation energy. These values were sufficiently higher than the standard values for auto parts.
