DIRECT VACUUM SINTERING BEHAVIOR OF M2 HIGH-SPEED STEEL POWDER WITH COPPER AND GRAPHITE ADDITIONS

The sintering behaviour of systems based on a high speed steel powder with 2 to 10% Cu and 0.35% graphite additions during direct vacuum sin tering was analysed. The compacts pressed at 200-800 MPa were sintered using an anisothermal process (heating up to 1050, 1100, and 1200 deg rees C) or an isothermal one (heating at 1200 degrees C for 60 min). T he densification of compacts sintered by heating to 1050 degrees C doe s not exceed 3% and is controlled by the formation of Cu-Cu bonds by d iffusion in the solid state. Sintering by heating to 1100 degrees C, a ccompanied by the development of a liquid phase, results in a densific ation of only 5%. The positive influence of the liquid phase on densif ication through primary rearrangement is offset by the negative influe nce of gaseous reduction waste products enclosed in the pores. Sinteri ng by heating up to 1200 degrees C, or isothermal sintering at 1200 de grees C for 60 min, results in an increase of densification that, in t he case of compacts pressed at 800 MPa reaches values of above 99.5% o f the theoretical density. Densification does not increase continually with the increasing copper content but, indeed, it is found that high est densification is reached for the lowest copper content. The cause of the decrease of densification in systems containing 4-6% Cu is the negative effect of gaseous reduction waste products enclosed in the po res. When the copper addition exceeds 6% the processes of secondary re arrangement of rounded solid phase particles participate in densificat ion. The positive influence of graphite additions on the reduction of oxides and the decrease in solidus temperature is demonstrated by a pr onounced increase of densification and the systems with copper additio ns exceeding 6% and a graphite addition of 0.35% reach the theoretical density. PM/0630