| High-speed steel(HSS) is a ledeburite steel, with the composition of high carbon and high alloy, with the properties of high hardness(HRC 63~67), high hardness, high wear resistance and heat resistance. Therefore HSS is widely used in machining of cutting tool, processing of mold making, and some require high wear resistance and high temperature working parts. But the HSS produced by the traditional process has defects like composition segregation and coarse microstructure. Powder metallurgy high-speed steel(PM/HSS) is a new material between cemented carbides and HSS, which has the uniform fine microstructure without carbide segregation and excellent properties. At present, the drawbacks of the production of PM/HSS include the high demand of equipments and complex process, which lead to the vast production cost and make the application of PM/HSS limited.PM/HSS with the composition of Co7W6Mo5Cr4V2 was successfully prepared from the raw materials of Fe, Co, WC, Mo2 C, Cr3C2, VC powders via solid state sintering in vacuum. The solid state sintering and heat treatment process were studied to explore the effects of temperature parameters on the microstructure and properties of PM/HSS. Microstructural observation was realized by means of metallographic microscope and scanning electron microscope. Composition and phase analysis was carried out by X-ray diffraction and energy-dispersive X-ray spectrometry. The sintering density, hardness, transverse rupture strength were determined, and the fracture morphology was also observed. In addition, the effects of trace element Si and hard particle TaCand NbC on the microstructure and properties of PM/HSS were also analyzed. The main results are as follows:Nearly full density is obtained without microstructural coarsening in the sintering temperature range of 1190–1210 °C. The optimum sintering temperature for the steel is 1200 °C. The suitable quenching temperatures range from 1200 to 1240 °C, and the triple tempering process is 560°C×1h. After heat treatments, the microstructure consists of a martensitic matrix with a homogeneous distribution of MC and M6 C carbides(average size lower than 1 μm). The highest hardness of 68.5 HRC and the maximum transverse rupture strength of 3.58 GPa is obtained for the PM/HSS tempered at 560 °C, and with NbC addition, the hardness and the strength can reach up to 71 HRC and 4.40 GPa. Si can lower sintering temperature and promote the sintering process. The addition of TaC and NbC can improve microstructure and increase the hardness and transverse rupture strength. |
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