Study on the Surface Microstructure of a Modified STD61 Steel Mold Used for the Die Casting Process

The surface microstructure and crack formation of an aluminum die-casting mold were investigated.The mold was made of a modified STD61 steel, and was used for more than 165,000 cycles.The mold surface consisted of four layers; an oxidized layer, a decarburized layer, a network carbide layer and a tempered martensite matrix.The depth down to the hardest network carbide layer was ~200 μm.Inside the matrix, M3C transition carbides were dissolved so that solute C joined pre-existing M23C6 and MC carbides to make them coarse.About 60% of thermal fatigue cracks had a depth less than 200 μm due to the hard network carbide layer, which obstructed the propagation of cracks.Cracks of over 200 μm were filled with oxide wedges of Al and Si which was which flown from the molten alloy as well as the Fe oxide.