【作者】 汪存龙；

【导师】 张鹏；

【作者基本信息】 广东工业大学 ， 材料加工工程， 2012， 硕士

【摘要】 精密喷射成形是一种典型的短流程制备高致密度材料的方法,可实现工、模具的快速成形,属于快速凝固技术的一种。本论文在自行研制的20kg级精密喷射成形快速制模设备上,对制备高致密度HM1热作模具钢进行研究。采用金相、X射线衍射、扫描电镜及能谱分析等方法对铸态和喷射态HM1热作模具钢及其热处理后的微观组织、物相、元素分布等进行了分析,并对其力学性能和摩擦学性能进行了研究。主要研究内容和结果如下：首先,对喷射态HM1钢组织和性能进行了研究,并与铸态进行了对比。结果表明,铸态HM1钢组织为粗大的枝状晶,晶粒尺寸150～200μm,晶界处为粗大连续的网状碳化物组织；而喷射态HM1钢为细小的等轴晶组织,晶粒尺寸20～50μm,晶界处弥散分布少量细小的先共析碳化物相；这表明采用喷射成形,可有效消除元素偏析和网状碳化物组织。X射线衍射定量分析表明,喷射态和铸态材料的物相均以马氏体和残余奥氏体为主,残余奥氏体相对体积分数分别为14%和8%。铸态材料硬度为HRC46～48,而喷射沉积坯材料硬度稍低为HRC44～47,主要是因为铸态材料中存在粗大的高硬度碳化物所致。压缩实验表明,喷射态和铸态HM1钢的断裂压缩率均在30%～40%,而断裂强度分别为2770MPa和2546MPa。喷射沉积过喷粉末的分析研究表明,过喷粉末粒径质量分布呈高斯状,当雾化压力在0.6MPa左右时,粒径在200微米以下过喷粉占总量的累积质量的60%以上；过喷粉的微观结构为细小的等轴晶组织,通过测量不同粒径粉末的平均二次枝晶间距,并经回归计算得到不同粒径过喷粉末的凝固速率,结果表明雾化过程中大多数的雾化液滴凝固速率在103K/s以上。然后,对铸态和喷射态材料,经回火处理后的组织和性能进行了研究。经2小时,分别在440℃.480℃.520℃.560℃.600℃及640℃下进行回火处理,结果表明,材料硬度随着回火温度的提高而升高,至520℃时,达到峰值,其中喷射态硬度达HRC52～54,而铸态为HRC50～51；随后,随着回火温度的继续上升,硬度逐渐下降。但在600℃时喷射态材料仍能保持较高硬度,材料的回火稳定性提高。主要是由于喷射成形过程,较快的冷却速率,使得合金元素在沉积坯中均匀分布；后续的高温回火时合金元素与碳的扩散产生二次硬化效应,在沉积坯中形成比铸态组织更细小、弥散的合金碳化物相,使其硬度高于传统铸态。证明喷射态HM1钢回火后具有比铸态材料更优良的热处理性能,材料的高温耐热性提高。最后,对铸态和喷射态HM1钢滑动干摩擦性能进行了研究。表明,以GCrl5钢球为对磨材料时,喷射态和铸态HM1钢在轻载时磨损方式均为粘着磨损,随着载荷的增大材料开始部分地呈现磨粒磨损形貌特征。铸态和喷射沉积态材料摩擦系数均随外加载荷的增大而增大,进入稳态磨损后铸态材料的摩擦因数比喷射态的摩擦因数稍高。同样实验条件下,喷射态材料磨损量低于铸态材料而喷射态材料经回火后磨损量可大幅度降低,100N重载时,喷射态磨损量较之于铸态减少了34%,而经过回火后的沉积态HM1钢磨损量更比未回火时降低了21%。回火提高耐磨性的机理是依靠高温阶段析出的碳化物,尤其是高硬度的MC型碳化物,其在基体中均匀弥散分布形成硬质点阻碍材料被磨损。更多还原

【Abstract】 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.更多还原