Supersolidus liquid phase sintering of high speed steels: Part 3: computer aided design of sinterable alloys

Abstract

Calculated multicomponent phase diagrams were used to identify high speed steel (HSS) type alloys having the potential to exhibit enhanced sinter ability. The requirement was for an extensive austenite + carbide + liquid phase field. Of the six tungsten and molybdenum based systems studied, Fe–14Mo–C + 4Cr–8Co systems were potentially the most promising. Appropriate compositions were water atomised and additional alloys prepared by blending annealed powders with graphite powders. Powders were compacted to green densities of about 70% theoretical and then vacuum sintered. Sinterability was assessed in terms of sintered densities and microstructures. Alloys containing Fe–13Mo–1·3C, Fe–14Mo–4Cr–1·3C, and Fe–14Mo– 8Co–4Cr–1·4C were sintered to full density at temperatures as low as 1170°C, 70–150 K lower than for existing HSSs. Sintering windows were 20– 30 K, a significant improvement on existing HSSs. As sintered microstructures consisted of angular M₆ C carbides dispersed in martensitic matrixes, which is typical for correctly sintered HSS. Heat treatment response and cutting performance for the sinterable grades were assessed and found to be comparable to existing HSS. The cutting performance of Fe–14Mo– 8Co–4Cr–1·4C tools at 45 and 52·5 m min^-1 was superior to both cast wrought M2 and T1 tools of identical geometry. Lower carbon contents resulted in an increase in sintering temperature and a reduction in the width of the sintering window. Higher carbon contents destroyed sinterability, since they led to the formation of M₂ C eutectic structures in the undersintered condition. Alloy sinterability was correlated to differential thermal analysis data obtained during heating of powders. The variations in sinterability with alloy composition are discussed with reference to phase diagrams; the degradation in sinterability observed at carbon contents above 1·4% is attributed to the presence of ternary eutectic phase fields. The commercial implications of the relationship between sinterability and alloy composition are discussed.