Some characteristics of decarburized layer in spheroidal graphite cast iron

Spheroidal graphite cast irons are widely used for auto parts because they have large degrees of freedom in shape and are inexpensive. However, they have serious drawback of crack generation due to excess carbon at the hardened region of heat-affected zone when they are welded. We have studied on decarburized spheroidal graphite cast iron, which has a possibility of welding because of graphite free in the surface region. In the present study, some characteristics of the decarburized layer in spheroidal graphite cast iron were investigated. The results obtained are as follows. Growth of decarburized layer is controlled by diffusion of carbon atoms toward the surface region during the heat-treating and there is a critical temperature of 930K for decarburization, below which decarburization does not occur. When the area ratios of the decarburized layer to all sectional area in the tensile test specimen was assumed to be a ratio of decarburized layer, the tensile strength was not influenced for the ratio of decarburized layer. However, when the overdecarburizing was processed, the tensile strength showed a tendency to decrease. Therefore, it should be noted in practical use of the decarburized spheroidal graphite cast iron that the excessive decarburization makes the strength of the thin parts of iron to decrease.