The alloy layer formed by high-temperature diffusion treatments in the air after adhering of aluminum foil thickness from 5 to 50 μm on metal mold steel SKD61 are examined. As a result, structural defect was not observed between FeAl and αFe in the alloy of high temperature aluminizing, and the most stable alloying layer was formed at the case of the initial aluminum thickness was 15 μm. The case of initial aluminum was thinner than 5 μm at the long time diffusion and that was thicker, and than 5 μm at the short time diffusion, structural defects were observed in the alloy layer. The former structural defect contained Cr oxide and aluminum oxide outside, and the latter structural defect contained void and high concentration of Cr. The stable alloy layer of high-temperature aluminized metal mold steel will be consist by Cr saluted FeAl and αFe which has toughness without structural defects.
