The microstructure of infiltrated high speed steel-tungsten carbide-copper composites

Marcin Madej, Jan Leżański

Quarterly No. 4, 2009 pages 305-311

DOI:

keywords: composites, high speed steel, tungsten carbide, copper, pressing, sintering, infiltration

abstract High hardness, mechanical strength, heat resistance and wear resistance of M3/2 grade high speed steel (HSS) make it an attractive material for manufacture of valve train components. In this application, the material must exhibit resistance to oxidation, high hot strength and hardness, and superior wear resistance. Metal matrix composites were produced by the infiltration technique. Since technological and economical considerations are equally important, infiltration of high-speed steel based skeleton with liquid copper has proved to be a suitable technique whereby fully dense material is produced at low cost. An ability to press and sinter to near net shape requires good compressibility of the powder. Even after annealing, tool steels powders can be pressed to only about 80% of the theoretical density by most commercial facilities. On sintering and infiltration, little or no shrinkage can be tolerated and so the necessary strength and toughness may be achieved without removal of the remaining porosity. A reasonable compromise between all of these requirements may be achieved by using mixtures of high speed steel powders with softer low alloy or pure iron powder. During sintering and infiltration of such mixtures, inter-diffusion of both carbon and metallic alloying elements occurs. The aim of the present study was to produce high speed steel-tungsten carbide-copper composites, which should have acceptable density, wear resistance and good sliding prosperities. Various amounts of WC powder were added to the HSS powder prior to compaction. The following compositions were investigated: 100% M3/2, M3/2 + 10% WC and M3/2 + 30%WC. Then the powders were cold pressed in a rigid cylindrical die at 800 MPa. Both green compacts and compacts pre-sintered for 60 minutes at 1150 C in vacuum were infiltrated with copper at 1150°C for 15 minutes. Microstructural analysis of the as sintered structures has been done by the following techniques: optical microscopy, X-ray diffraction and EDX analysis. The morphologies of capillaries and as-infiltrated microstructures are discussed in this work. It can be seen that the microstructure of the M3/2 grade HSS based composites consists of a steel matrix with finely dispersed carbides and islands of copper. The tungsten carbides located within the grains and on the grain boundaries as well. The qualitative EDX analysis revealed the presence of both MC type vanadium-rich carbides and M6C type tungsten and iron rich carbides. The SEM and EDX analysis performed on the specimens containing 10 and 30% tungsten carbide have re-vealed the carbide phase evenly distributed within the copper-rich regions. WC reacts with the surrounding HSS matrix and forms a tungsten and iron-rich M6C carbide grain boundary network.