Direct Vacuum Sintering Behaviour of M2 High Speed Steel Powder with Copper and Graphite Additions

Abstract

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 sintering was analysed. The compacts pressed at 200–800 MPa were sintered using an anisothermal process (heating up to 1050, 1100, and 1200°C) or an isothermal one (heating at 1200°C for 60 min). The densification of compacts sintered by heating to 1050°C does not exceed 3% and is controlled by the formation of Cu–Cu bonds by diffusion in the solid state. Sintering by heating to 1100°C, accompanied by the development of a liquid phase, results in a densification of only 5%.The positive influence of the liquid phase on densification through primary rearrangement is offset by the negative influence of gaseous reduction waste products enclosed in the pores. Sintering by heating up to 1200°C, or isothermal sintering at 1200°C for 60 min, results in an increase of densification that, in the case of compacts pressed at 800 MPa reaches values of above 99·5% of the theoretical density. Densification does not increase continually with the increasing copper content but, indeed, it is found that highest 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 pores. When the copper addition exceeds 6%the processes of secondary rearrangement of rounded solid phase particles participate in densification. The positive influence of graphite additions on the reduction of oxides and the decrease in solidus temperature is demonstrated by a pronounced increase of densification and the systems with copper additions exceeding 6% and a graphite addition of 0·35% reach the theoretical density. PM/0630