Microstructural Changes of SKD11 Steel during Carbide Dispersion Carburizing and Subzero Treatment

Abstract:

Microstructural changes and hardness variations in SKD11 steel have been investigated during the processes of carbide dispersion (CD) carburizing, austenitization, subzero treatment and tempering. The carbon content of the surface region increased up to 3.0% after CD carburizing, and the surface phases consisted of two predominant types of M7C3 carbides (large primary eutectic M7C3 carbide and secondary M7C3 carbide), retained austenite and martensite. After austenitization, the carbon content of the surface region decreased to 2.4%. At the same time, surface hardness was reduced from 900Hv for the CD carburizing condition to 830Hv after austenitization. On the other hand, the hardness at the interior region of the austenitized steel displayed a 100Hv higher value than that of the CD carburizing steel. In spite of removal of the retained austenite, subzero treatment (at -100
) of the austenitized steel resulted in a decrease in hardness, probably due to the softening of the martensite matrix. However, tempering (at 200
for 4 hours) of the subzero treated steel raised its hardness up about 70Hv compared to steel tempered without the subzero condition, due to precipitation of fine nano size carbides below 50nm.

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