【作者】 李萍；

【导师】 刘宁；

【作者基本信息】 合肥工业大学 ， 材料物理与化学， 2017， 博士

【摘要】 经济和社会的发展对于材料提出了越来越高的要求,WC/钢基复合材料将增强相WC的高强度高硬度和钢基体的韧性结合起来,具有可导电加工、热处理强化、耐磨等一系列优点,成本大大低于WC—Co硬质合金,在工模具、耐磨件、轧辊等领域获得广泛应用。本文设计了一种超高强度钢作为基体的WC/钢基复合材料,通过粉末冶金法,成功制备出WC/钢基复合材料,优化了配方和制备工艺(压制工艺、烧结工艺、热处理工艺等),制备出的材料具有高硬度、韧性和耐磨性。此外,采用电冶熔铸法和离心铸造法制备了 WC/钢基复合材料,并进行了热处理和化学热处理研究,优化了材料的综合性能。主要研究工作与成果如下:1)设计了 一种超高强度钢45CrNi2Mo2作为粘结剂的新型WC/钢基复合材料,通过优化工艺确定了粉末冶金法最佳压制力为300Mpa,烧结温度在1220℃左右。结果发现:300MPa下试样硬度最高,孔隙最少,分布均匀;1220℃下烧结体的性能最佳,40wt%WC在该温度下体积收缩率为33.68%,显微硬度404.7(HV0.3)。2)探索了粉末冶金法下WC含量的改变对钢基复合材料力学性能与微观组织的影响。结果表明:WC的加入提高了复合材料的显微硬度,40wt%WC、45wt%WC性能相差不大,组织致密,孔隙少,但性能远超其他配方。1220℃下40wt%WC试样的硬度最高达到404.7HV0.3,洛氏硬度达到67HRC,且综合性能达到最佳。40wt%WC的组织均匀,WC与基体相互溶解产生Fe3W3C、Fe3C、FeW3C等物相,从经济角度综合考虑,最终确定最佳配方为40wt%WC型。3)研究了不同的热处理制度对离心铸造法制备的WC/钢基复合材料组织与力学性能的影响。结果发现:离心铸造试样经退火后缺陷减少,硬度降低,抗弯性能变强。淬回火后样品组织更加均匀,马氏体增多,网状结构更规则,980℃淬火后,洛氏硬度为81.7HRA,维氏硬度为595HV10,抗弯强度为1263MPa。离心铸造试样965℃渗硼为最佳制度,维氏硬度(876HV10)、洛氏硬度(71.6HRA)达到最大,抗弯强度为1078MPa,试样既有一定的韧性,又有较高的硬度。4)分析了不同的热处理制度对电冶熔铸法制备的钢基复合材料组织与性能的影响。结果可见:DGJW20退火后硬度略有降低,而淬火后洛氏硬度大幅提高。随着淬火温度的升高,试样的硬度不断上升,到980℃硬度达到最高(84.4HRA,873HV10),抗弯强度为811MPa。980℃淬火、200℃回火为最佳热处理制度。电冶熔铸试样在约965℃时渗硼保温7h效果最好,维氏硬度(1120HV10)比原始样维氏硬度提高约100%,洛氏硬度达到77HRA,抗弯强度为695MPa。渗硼后的表层为FeB、Fe2B、CoB和Ni3B等硼化物。5)研究了不同的整体热处理与化学渗硼制度对粉末冶金法制备的40wt%WC复合材料的影响。结果表明:退火后试样的硬度降低,淬回火后硬度又提升,970℃淬火200℃低温回火性能最佳,硬度达到最高(1107.7HV0.2),淬火试样中主要物相为Fe3W3C、Fe3W3C-Fe4W2C。对粉末冶金法制备的试样,使用渗硼剂配方(5%KBF4、5%B4C、5%木炭粉、85%SiC)在975℃渗硼时硬度最高,耐腐蚀性能也好。对于配方(B4C、KBF4、硝酸铈Ce(N03)3、SiC、活性炭),5%硝酸铈掺入渗硼剂,980℃下渗硼后的试样具有优异的性能。更多还原

【Abstract】 Economic and social development puts forward higher and higher requirements for the material,WC/steel matrix composites which combine high strength and high hardness of reinforcing phase with toughness of steel matrix have a series of advantages such as be conductive processing,heat treatment strengthening,wear-resisting.The cost of composites is much lower than the WC-Co carbide,and it has a wide range of applications in mould,wear parts,roll and so on.In this paper,WC/steel matrix composites are designed which matrix is a kind of super high strength steel.With powder metallurgic method,WC/steel matrix composites are made by seeking the optimum formulation and processing conditions(pressing pressure,sintering temperature and heat treatment process,etc.).The materials have good hardness,toughness and abrasive resistance.With electroslag melting and casting method and centrifugal casting method,two kinds of WC/steel matrix composites are made,and whole heat treatment and chemical heat treatment are applied on two kinds of WC/steel matrix composites.The comprehensive performance of materials is optimized.Main achievements are shown as follow:1)A new WC/steel matrix composites are designed which binder is a kind of super high strength steel 45CrNi2Mo2.The best pressing force(300Mpa)and sintering temperature(1220℃)about powder metallurgy method are determined by the optimization of process.The results showed that the samples under 300MPa have the highest hardness,the least pore,and the uniform distribution.The performance of the sintered samples is best at 1220 ℃,and meanwhile the samples with 40wt%WC have 33.68%volume shrinkage rate and micro hardness 404.7(HV0.3).2)The paper explores the effect of the WC content on the mechanical properties and microstructure of the steel matrix composites which prepared by powder metallurgy.The results showed that the addition of WC improves micro hardness of composite material.40wt%WC and 45wt%WC have little difference on performance.Both of them have dense tissue and minimum porosity,meanwhile they get higher performance than the others formula.The maximum micro hardness of 40wt%WC reaches 404.7HV and Rockwell hardness reaches 67HRC at 1200℃.At this moment,comprehensive performance reaches the maximum.The organization of 40wt%WC distribute well.Fe3W3C、Fe3C、FeW3C are produced when WC and matrix phase solute each other.From the point of economy,40wt%WC is the best formula in this paper.3)The effects of different heat treatment systems on the structure and mechanical properties of WC/steel matrix composite materials by centrifugal casting are studied.The results showed that the defects reduce after annealing,the hardness decrease,while bending performance increase.Samples have more uniform tissue and more regular network after quenching-tempering,meanwhile martensite inside increase.The structure is the most uniform and regular at the quenching temperature of 980℃.At the moment,Rockwell hardness is 81.7HRA,Vickers hardness is 595HV10 and bending performance is 1263MPa.When centrifugal casting sample bornding at 965℃which is the best process,both Vickers hardness(876HV10)and Rockwell hardness(71.6HRA)reach the maximum,and bending strength is 1078MPa.Boriding at 965℃ ensure the sample has certain toughness and high hardness.4)The influence of different heat treatment systems on the structure and performance of steel matrix composite materials by the electroslag melting and casting method is analyzed.The results showed that the hardness of DGJW20 is slightly lower after the annealing and the hardness of Rockwell is greatly improved.With the increase of quenching temperature,hardness of the sample rises.It reaches the maximum of 84.4HRA,873HV10 at 980℃.At this moment,bending strength is 811MPa.When the quenching temperature rises over 980 ℃,the hardness decreases.Optimum heat treatment system is quenching at 980 ℃ and tempering at 200 ℃.When electric melting and casting sample boriding at 965 ℃,and then keep it warm for 7h works best,Vickers hardness(1120HV10)doubled than original sample.At this moment,Rockwell hardness reaches 77HRA,and bending strength reaches 695MPa.After boriding,FeB、Fe2B、CoB 和 Ni3B are appeared on surface.5)The effect of different overall heat treatment and chemical boriding system on the 40wt%WC composite materials prepared by powder metallurgy is studied.Results show that the hardness of samples decrease after the annealing,however it increases obviously after quenching-tempering.For sample of powder metallurgy method,the performance reaches the best when quenching at 970 ℃ and low tempering at 200 ℃,meanwhile the Vickers hardness reaches the maximum of 1107.7HV0.2.Main phase in quenching sample are Fe3W3C、Fe3W3C-Fe4W2C.When using boriding medium formula(5%B4C、5%KBF4、85%SiC、5%charcoal powder)for powder metallurgy method samples,hardness reaches at the maximum when boriding at 975 ℃,meanwhile decay resistance are the best as well.For new formula(B4C,KBF4,SiC,Ce(NO3)3,activated carbon),with the addition of 5%Ce(N03)3 in boriding medium,sample after borinding have excellent performance when boriding at 980℃.更多还原