Abstract
In order to improve the ductility and toughness of high strength sintered alloy steels, an austemper heat treatment was applied to Fe-2%Si-3%%Ni-0.3-0.5%C compacts sintered at 1250°C for various time in vacuum. In addition to tensile strength and elongation the dependence of impact strength on testing temperature for austempered Si-Ni steel compacts are investigated, and the correlation between the retained austenite and the properties is discussed. The results are summarized as follows:
Mixed structure of upper bainite and retained austenite is produced by austempering (400-500°C) the Si-Ni steel compact, which shows the more excellent properties than those of conventional sintered alloy steels, especially the value of elongation increases with increasing the amount of retained austenite.
The ductile-brittle transition temperature is considerably lower and the impact strength at room temperature is higher for the austempered compact than for the compact sintered alone. The reason for this may be attributed not only to the existence of retained austenite but to the transformation plasticity induced by the retained austenite.
Also, it is found that the mechanical properties of austempered compacts are remarkably improved by the sintering conditions. It is therefore expected that the properties of the same level as the conventional sinter-forged steels are attainable, when the uniform distribution of alloying elements and the reduction of finer pores are achieved for Si-Ni steel compacts by such a high-temperature and a long-time sintering.
