DENSIFICATION OF PREALLOYED TOOL STEEL POWDERS: SINTERING MODEL

摘要

The sintering densification of prealloyed tool steel powders is a long-term problem in powder metallurgy. Attention to this problem returns anew as powder injection molding turns its attention to processing tool steels. Small tool steel powders can be molded into complex shapes, but densification to a pore-free condition requires close temperature control. Internal liquids form when the particles are heated over the solidus temperature, giving densification due to liquid film spreading on the grain boundaries. A model is presented for the process that describes micros tructure development and densifieation, with consideration of the particle size, carbon content, alloy composition, initial microstructure, green density, heating rate, peak temperature, hold time, and sintering atmosphere. A key factor is the formation of an internal liquid that induces capillary bonding between particles while causing a loss of rigidity in the solid-liquid particles. Densifieation occurs by viscous flow of fhe semisolid particles, Grain growth is a critical factor, since the required quantity of liquid for densifieation decreases as the grain size increases. The density and microstructure predictions are compared favorably with experimental data, showing the expected sensitivity to sintering temperature and carbon content.