| Precision Spray Forming is a typical single-step process for producing low porous material, and it is a rapid solidification process. This thesis researched the high relative dense HM1hot work tool steel, which was synthesized on a self-developed20kg precision spray forming equipment for rapid tooling process. Use optical metallographic, X-ray diffraction, scanning electron microscopy and energy spectrum methods analysis micro structure, phase evolution and element segregation of both as cast and as spray-formed HM1hot work tool steel. And have discussed the mechanical properties and the friction and wear properties of this material. The main research contents were as follows.Firstly of all, the microstructure and properties of the as spray-formed and as cast HM1steel were discussed. The results show that, the as cast HM1steel has a thick dendrites with large continues carbide network; the grain size is between150and200microns. While the as spray-formed material has a refined equiaxed grains with small amount of dispersed proeutectoid carbides at the grain boundaries, the grain size is of20to50microns. And this indicates that spray forming can effectively eliminate elements segregation and reduce the carbide network. The X-ray diffraction quantitative analysis of the material shows that the as spray-formed and the as cast HM1steel have a similar phase of martensite and residual austenite, the volume fraction of residual austenite of them is14%and8%respectively. The Rockwell C hardness of as cast material is46～48, while the as spray-formed is44～47, the main reason is because the thick carbide network has a high hardness. The compact experiment shows that, both as spray-formed and as cast HM1steel have a fracture deformation volume of30%, and the breaking strength is2848MPa and2661MPa respectively.Researches on over-spray powders indicate that, the over-spray powder particle size distribution is Gaussian-like distribution, when the deposition process was carried out at a atomization pressure of0.6MPa, particles size below200microns account for60%of the total cumulative mass. The microstructure of over-spray powders is ultra-fined equiaxed grains. By measuring the second dendrite arm space (SDAS) of over-spray powders in different sizes, and by regression calculating the relate functions between SDAS and solidification rate, we can find that during the spray deposition process, most of the atomized metal liquid particles has a solidification rate of no less than103K/s.And the thesis studied the effects of tempering on microstructure and mechanical properties of both as cast and as spray-formed materials. Tempering experiment was carried out in440℃,480℃,520℃,560℃,600℃and640℃for2hours, respectively. The results show that, the Rockwell C hardness increased as the tempering temperature increases, and it reached the peak point at520℃, the as spray-formed hardness of HRC52～54, the as cast is50～51. Subsequently, with the increase of temperature, the hardness decreases. It is surely that after tempering at600℃, the as spray-formed material has a high hardness values, therefore, the tempering resistance was improved. The main reason is because the spray forming process has a large solidification rate, make the alloy elements equally distributed in the deposition, and the subsequent high temperature tempering make it possible for the alloy elements and carbon to diffusion with each other, and consequently they have a secondary hardening effect, and precipitate a refined and dispersed alloy carbides in the deposition, therefore the hardness is higher than it is in the conventional cast material. And these indicate that the as spray-formed HM1steel has a better heat treatment property than the cast alloys, the heat resistance increased in the process.Finally, the thesis studied the friction and wear properties of both ass-cast and as spray-formed material. The results show that, during light loads, both as cast and as spray-formed HM1steel were adhesive wear when whey were worked with GCr15sphere ball. The wear mechanism changed to abrasive wear partly when the load increases. The friction coefficient of both as cast and as spray-formed material increased when the load is increasing, and the friction coefficient of as cast material is larger than it is in the as spray-formed when the wear comes to a stable stage. The wear loss of as spray-formed HM1steel is lower compared with as cast, and it is lowest when it is with the tempered deposited material. Under a heavy load of100N, the wear loss of the as spray-formed decreases34%as that of the as cast material, and the tempered deposited material reduced21%more as that of the not been tempered. The wear loss of the as spray-formed steel decreases after tempering. The main reason for this is the carbides precipitate during high temperature tempering, especially for the MC type carbides. These carbides distributed uniformly in the matrix and formed hard spots stop further abrasion. |
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