Preparation Of M2 Powder Metallurgy High Speed Steel And Research On Its Microstructure And Properties

High peed steel(HSS) usually exhibits high hardness, high wear resistance, high cutting toughness and good thermal hardness. In addition to the main applications in tool materials, HSS has been widely used in mold and structural materials as well. Particularly, HSS prepared by powder metallurgy(PM) technology has the advantage to fundamentally solve the problems such as uneven distribution and massive coarsening of carbides. However, due to the complex composition of HSS alloy, the density and microstructure of PM HSS are very sensitive to the variation of sintering parameters. Therefore, preparation processes of HSS are key steps for further developments of this material for researches on PM. In this paper, the M2 PM HSS were prepared by vacuum sintering technology. The microstructures and mechanical properties were observed and analyzed in details. The experimental results show that:(1) With the milling time increases, the refine rate of powder appears firstly quick back slow trend and then reached a steady state after 20 h. At the same time, the agglomeration degree of powder is continuously increasing and cold welding phenomenon began to appear. On the other hand, the vacuum sintered M2 PM HSS shows a uniform microstructure with small average grain size and without any carbides segregation after sintering at 1245℃ for 1h. Besides, at the same conditions, the relative density, Rockwell hardness, wear weight loss and transverse rupture strength(TRS) reach to 98%, 52.5, 0.032 g and 2100 MPa, respectively. It is worth noting that a higher HRC hardness value above 65 of HSS can be achieved after quenched at 1200℃ and tempered at 550℃ for three times.(2) The additive amount of carbon has significant effects on the microstructure of M2 PM HSS. The M2 PM HSS with carbon contents between 1.0wt.% and1.1wt.% have dense microstructures containing fine grains, and there are M6 C and MC type carbides evenly distribute on substrates. When the carbon content increased to 1.1wt.%, the carbides are obviously coarsened and there are apparently more carbides inside the grain interior. Further increasing the carbon content, both the hardness value and the densification degree of M2 PMHSS show an upward trend. Effects of carbon on the sintering temperature is mainly reflected in the fact that it can reduce the sintering temperature of HSS, as well as expanding the sintering temperature range.(3) Low cost PM HSS with alloy powder containing iron-tungsten and ironvanadium has sintering windows between 1245℃and 1255℃. When the temperature is 1255℃, a relatively high densification over 92% was achieved in M2 HSS. Moreover, hardness and wear resistance tests were carried out on HSS with alloy powder containing iron-tungsten and iron-vanadium. The results show that, after the addition of iron-tungsten and iron-vanadium, their Rockwell hardness declined compared with the general HSS, meanwhile the macro-hardness value is6 to 7 lower than that of the ordinary one. With the increase of sintering temperature, the wear resistance of M2 PM HSS presents a downward trend after the first increase. At last, the heat treatment with various temperatures were operated on low cost PM HSS, it is found that with the increase of quenching temperature, the hardness increased, and the rockwell hardness gradually reached to more than 55. But after quenched at a temperature over 1220℃, the hardness of the PM HSS fell instead. When the quenching temperatures are between 1200-1220℃, the PM HSS with fine grains and without carbide network structures were obtained.