DENSIFICATION OF PREALLOYED TOOL STEEL POWDERS - SINTERING MODEL

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 too l 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 de scribes microstructure development and densification, with considerati on of the particle size, carbon content, alloy composition, initial mi crostructure, green density, heating rate, peak temperature, hold time , and sintering atmosphere. A key factor is the formation of an intern al liquid that induces capillary bonding between particles while causi ng a loss of rigidity in the solid-liquid particles. Densification occ urs by viscous flow of the semisolid particles. Grain growth is a crit ical factor, since the required quantity of liquid for densification d ecreases as the grain size increases. The density and microstructure p redictions are compared favorably with experimental data, showing the expected sensitivity to sintering temperature and carbon content.