Sintering Behaviour And Mechanical Properties Of Powder Metallurgy M3:2 High Speed Steels

To obtain M3:2 high speed steel with better fabrication and mechanical properties, the solidification characteristics of rapid solidified M3:2 high speed steel powders produced by water atomization, the effects of carbon, silicon and TiC additions on sintering behavior, microstructure and mechanical properties were investigated systematically in this dissertation.The rapid solidification characteristics of M3:2 high speed steel powders were studied. Experimental results show that the shape of powders is irregular with the cooling rate of 10⁵ K/s-10⁷ K/s. The microstructure was found to exhibit an equiaxed morphology, with carbides of cubic MC and close-packed hexagonal M₂C existing in continuous network. at grain boundaries. 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 powders changes from martensite to austenite, andδferrite, 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 addition on sintering behavior and microstructure were investigated. M3:2 high speed steels with carbon addition in the range of 0.4-0.8 wt.% may decrease sintering temperature effectively, and enlarge temperature range which achieves full densification. Temperature ranges increase from 10℃of undoped high steel steels to 20℃of high steel steels with 0.4-0.6 wt.% carbon addition, and 30℃of steels with 0.8 wt.% carbon addition. The mechanism is that the crystal lattice distortion of austenite aggravates as the carbon addition increases. So the energy of the system increases and the temperature of solidus decreases, as a result, the sintering window is enlarged, favoring densification of the powders. The sintering and mechanical properties results show that the best sintering temperature for the sample with carbon addition 0.4 wt.% that has the best mechanical properties is 1240℃, and the sintering window is 1240¹260℃, which are 30℃lower and 10℃wider than undoped M3:2 high speed steels. Densification and microstructure of high speed steels with different silicon additions were studied. Experimental results show the addition of silicon exerts a crucial role on the densification of M3:2 high speed steels sintered at 1230℃. For the case of 0.7 wt.% silicon addition, the alloy exhibits the maximum sintering density. Samples with silicon addition up to 0.7 wt.% could obtain relative density more than that of alloys without silicon addition sintered at best sintering temperature 1240℃. The solution of carbides could inhibited by silicon addition in the range of 0.4⁰.7 wt.% and samples with 0.4 wt.% silicon addition could achieve minimum grain and carbide size. High speed steel with 0.7 wt.% silicon addition show maximum axial ratios of martensite matrix with less carbide exists. But excess silicon additions have no benefit to liquid formation and facilitate pearlite transformation.The influences of additives were studied on the mechanical properties of high speed steel in sintered and tempered state. Results show that temper could lead to increment of hardness, and same time a little sacrifice of bend strength of M3:2 high speed steel with carbon addition. Whereas for alloys with silicon addition temper process could lead to a great increment of both hardness and bend strength. M3:2 high speed steels with 0.7 wt.% silicon addition processes the best combination of high hardness and high bend strength after tempering at 550℃.The results of the friction and wear measurements of the M3:2-TiC shows that the weight loss of M3:2-TiC decreases initially with the increase of volume fraction of TiC particles. The weight loss of M3:2-TiC reaches minimum value of 4.21 mg when the volume fractions of TiC particles increase to 6 vol.% in the M3:2 high speed steel. The steels with addition 6% TiC show better wearability is for the reason of good wearability of TiC particles have approved cohesion with matrixes. Microstructure analysis of M3:2-TiC after wear shows that main mechanism of wear is abrasive and oxidative.