| High speed steel (HSS) is one of the most widely used materials for cutting tools. Compared with conventional HSS, HSS produced by powder metallurgy technology has some advantages such as perfect mechanical properties, economical usage of material and saving energy consumption. In this dissertation, the solidification characteristics of rapidly solidified M3/2 high speed steel powders produced by water atomization, the effects of carbon and silicon contents on the sintering behavior of powders, for the purpose of reducing the sintering temperature and enlarging the sintering window, were investigated systematically. The mechanism and sintering kinetics were analyzed.The solidification characteristics of high speed steel powders were studied. Experimental results show that the shape of powders is irregular with the average particle size of 50μm and the cooling rate of 105~107 K/s. The microstructure of the powder exhibits an equiaxed crystal morphology, with carbides of cubic MC and close-packed hexagonal, M2C, existing in continuous network between the crystal grains. Because the nucleation of martensite is difficult for the reason of melt-segmentation and the atom status of high-temperature stable phase is retained to low temperature for the reason of high cooling rate, the crystallization phase of HSS powders changes from austenite to austenite/ ferrite, as the particle size decreases. The rapid solidification process of powders is a semi-critical supercooling status, namely a interim stage from total diffusion to non-diffusion.The effects of carbon content on the sintering behavior were studied. It is shown that the matrix of the sintered samples was martensite/austenite, with a distribution of carbides of M6C and MC. The best sintering temperature for sample with carbon content of 1.4 wt% that has the best mechanical properties (HRC 46.8 and bending strength 2124.5 MPa) is 1240℃, and the sintering window is 1240℃～1260℃, which are 30 K lower and 10 K wider than the conventional M3/2 HSS, respectively. The mechanism is that the crystal lattice distortion of austenite aggravates as the carbon content increases. So the energy of the system increases and the temperature of solidus (liquid+austenite+ carbides area) decreases, as a result, the temperature for initial liquid formation is reduced and the volume fraction of the liquid phase increases at the same sintering temperature. Accordingly, the sintering window is enlarged, favouring densification of the powders.Similar to carbon addition, the samples with silicon content up to 1.0 wt%, sintered at 1230℃, can obtain nearly 100% relative density, which means that increasing of silicon content can also contribute to densification process. And it has the best mechanical properties (HRC 51.8, bending strength 1639 MPa and fracture toughness 26.3 MPa·m1/2). But the properties are worse than those of sample with 0.3 wt% silicon content, sintered at 1240℃except the hardness, which means silicon content can also influence the grain size and solution strengthening of martensite as well as carbides distribution and size, which control the mechanical properties of the sintered products.The sintering kinetics of HSS powders were analyzed. The sintering process can be divided into three stages: initial stage (relative density <3%), at which sintering neck forms and grows, intermediate stage (relative density <90%), at which interconnected pores shrink to isolated pores, and final stage (relative density>90%), at which isolated pores shrink and the contact between two particles flats. The three stages are sequential and governed by a diffusion mechanism. In the final supersolidous liquid phase sintering stage, more liquid phase formation at a lower temperature can help to achieve full density, which is the essential reason for that increasing carbon and silicon contents can reduce the sintering temperature of high speed steel powders.
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