Sintering behaviour and microstructure development of T42 powder metallurgy high speed steel under different processing conditions High speed steel powders (T42 grade) have been uniaxially cold-pressed and subsequently densified through different sintering routes including: supersolidus liquid phase sintering (SLPS) under vacuum and different nitrogen pressures (0.2, 0.9, and 8bar) and through solid state sintering (SSS) by hot isostatic pressing (HIP). HIP temperatures as low as 850C led to near full densification of the material (98% theoretical density) with average size of M6C and MC carbides lower than 1m and grain size 3m. Pressureless sintering under different nitrogen pressures (up to 0.39wt.%N absorption) led to a significant reduction of the optimum sintering temperature (OST) and a pronounced increase in the sintering window (SW) as compared to vacuum sintering. Pressureless sintering under 8bar N2 led to a further reduction in OST together with the precipitation of massive eutectic structures. Therefore, the SW was judged to be negligible. The response of the as-sintered materials to the heat treatment is basically determined by the amount of C available in the matrix prior to quenching and the grain size. The highest hardness achievable for the sintering conditions evaluated ranges 7001100 HV2 after austenitizing at 1100C, oil quenching and multitempering at 500550C. Tool steels serve a large range of applications including hot and cold working of metals and injection moulding of plastics or light alloys. High speed steels (HSS) are more specifically used as cutting tools and wear parts. More recently, these materials have also been used for structural applications. The high performance exhaust valve seat inserts for passenger vehicles constitute the most notable example 1. In general terms, for these structural applications, a combination of high strength, wear resistance and hardness together with an appreciable toughness (compared with other materials used as tools) and fatigue resistance is required. From a microstructural point of view, HSS can be described as metallic matrix composites formed by a ferrous matrix with a dispersion of hard, wear resistant carbides. The type, size, morphology, distribution and volume fraction of carbides as well as the characteristics of the ferrous matrix depend on both the composition of the material and the manufacturing process 2 and 3. The basic alloying elements of high speed steels are approximately 1530wt.% of carbide formers (Cr, Mo, W, V), sometimes Co and sufficient carbon to promote the formation of carbides. Tungsten and molybdenum mainly contribute to the formation of the primary M6C and M2C carbides and vanadium is the main constituent of the MC type. Conventional manufacturing processes for the production of components with these materials include wrought metallurgy and powder metallurgy (direct sintering and hot isostatic pressing; HIP). The main manufacturing steps for wrought processing are melting, casting, hot working, machining and heat treating. Normally, extensive hot working (area reductions 90%) is necessary to disperse the carbide networks formed during the solidification of the as-cast ingots. This hot working process leads to the alignment of carbide in strings, which is responsible for anisotropic properties 2. Powder metallurgy (PM) techniques were initially developed to overcome these problems. The starting raw materials are pre-alloyed gas or water atomised powders. Gas atomised powders are cleaner than water atomised powders and both of them are free of segregations due to the high cooling rates involved. Gas atomised powders are used for HIP 4 and powder injection moulding (PIM) 5. HIP is devoted for a prime quality product due to the cleanness of the raw material and to the fact that densification takes place by a solid state sintering (SSS) process. Consequently, a fine and homogeneous distribution of carbides embedded in a pore-free ferrous matrix is obtained leading to exceptional properties. PIM is best suited for small components with complex geometries and densification takes place by direct sintering (i.e., pressureless sintering) through a supersolidus liquid phase sintering (SLPS) mechanism 5 and 6. Water atomised powders are normally processed by the direct sintering route. Partial densification is achieved by cold-pressing the powders with a suitable compaction lubricant. Subsequently, sintering to full density takes place by a SLPS mechanism. The direct sintering route has inherent advantages in terms of achievable properties versus processing costs and environmental considerations related to the highly efficient material use. During the last 20 years, a high research effort has been mainly addressed at the understanding of the physical and chemical mechanisms involved in the densification via SLPS 7 and SSS 4. Additionally, research has also been focused on