| he main objective of this project is to design a new tool steel powder, which is less expensive (raw materials and process) than h igh speed steel (HSS) powders. The reference material which will be used throughout this project contains M2 HSS powder. Therefore, a new low cost tool steel powder has been designed to replace expensive HSS powders like the M2 powder. The new low cost tool steel powder (TS4 powder) contains a high concentration of Cr, V and C (TS4: 3.8 %wC, 12.5 %wCr, 1.5 %wMo, 2.5 %wW and 6 %wV). This project presents the different steps involved in the design of the new TS4 powder and the manufacturing process of VSI containing TS4 powder.;Due to its unique composition, the TS4 powder can be water atomized without excessive oxidation of the alloying elements such as Cr or V. Because of its high carbon content, the TS4 powder contains few internal oxides; the oxygen is mainly located in a relatively thin surface oxide skin on the powder particles. However, the presence of dissolved oxygen in powders containing a high carbon content, can lead to porous powders, which adversely affect the mechanical properties of the sintered parts. Thus, the mechanisms responsible for the protective effect of carbon and of pore formation were studied to allow a better control of the microstructure of atomized powders.;The continuous oxide surface skin, although relatively thin, is detrimental to sintering and a grinding process was designed to improve the sinterability of the TS4 powder. Moreover, additives that promote liquid phase sintering were studied to further improve the kinetics of sintering. The study of the effect of these additives has shown that the bonding between TS4 particles and alloyed steel particles is good with copper addition. In addition, the results obtained with MoS2 and Fe3P additions showed interesting results; the formation of Cr rich sulphides with MoS2 addition (dissociation of the MoS2 during sintering) and an increase of the size of the carbides in TS4 particles with Fe3P addition.;After the design of the new TS4 powder and of an optimized mix with the TS4 powder, samples were sintered to evaluate the properties after sintering and to study the microstructural evolution of the TS4 particles. The nanoscaled microstructure of the as-atomized TS4 powder becomes mainly micrometric in TS4 particles after sintering. The microstructure of TS4 particles after sintering contains about 50 %v alloyed austenite and about 50 %v of carbides and carbonitrides (M7C3 and M8(C, N)7). The evaluation of mechanical properties (transverse rupture strength, radial crushing strength and apparent hardness) showed that the new optimized mix containing the TS4 powder is comparable to that of a reference mix containing M2 HSS powder used industrially. Moreover, measurements of the wear resistance, by using a specialized bench (rig test) which simulates the operating conditions of VSI, and of the hot hardness showed that the new optimized mix is comparable to the industrial reference mix.;Finally, due to the success of this project, the composition of the new powder, the powder production process and the manufacturing process of VSI were patented. In addition, the new TS4 powder is currently produced industrially and the new optimized mix is commercially available to customers of our industrial partner Federal-Mogul. The design of the new TS4 powder and of the new process led to a cheaper tool steel powder which performs just as well as HSS powders such as M2 for VSI. Despite the fluctuating cost of raw materials in the last decade, the new TS4 powder (6.94... |
