Fe 合金粉末の低等方圧加圧焼結

Abstract

An end-sealed thin stainless-steel pipe in which a high-speed steel compact was inserted was evacuated with a vacuum pump during heating at an elevated temperature using an atmosphere furnace. The sintering behavior of this low isostatic press sintering (LIPS) was examined in detail. The compact is subjected to isostatic compression stress through the deformation of stainless steel owing to atmospheric pressure. As the gas pressure in the stainless-steel pipe is low (about 2×10-2 Pa), as the sintering temperature is high (1573 K), and as the holding time is long (4 h), densification and hardness of the LIPS compact are large (about Hv640). This is because the decomposition of FeO film formed on the surface of the as-received powder is vigorous at low gas pressure in the stainless-steel pipe at high LIPS temperature. Hence, the degree of powder contact increases aided by large plastic flow in the powder particles owing to both the degradation of the strength of high-speed steel powder at a high temperature and the compression stress caused by atmospheric pressure. Higher rates of Fe diffusion is assisted by heating at high temperature. At an early stage of LIPS, pores of the center of the LIPS compacts migrate to the surface, resulting in the reduction of densification near the surface. However, by holding for a long time, the LIPS compact that has high densification from the surface to the center is obtained because of enhanced Fe diffusion. Under optimum LIPS conditions, it is possible to obtain a homogeneous LIPS compact which has porosity and hardness levels equivalent to those of the HIP compact.