【作者】 李秀兰；

【导师】 马幼平；

【作者基本信息】 西安建筑科技大学 ， 材料加工工程， 2013， 博士

【摘要】 铬系铸铁是工程中应用广泛的一种耐磨材料。高硬度碳化物与不同基体组成“复合材料体系”的性能和使用寿命主要受控于碳化物的结构、尺寸、形态、分布及基体组织的成分、结构等因素。所以凝固组织中碳化物结构、尺寸、形态、分布及基体组织的成分、结构控制一直为铬系铸铁多年研究的热点和难点问题。本文基于凝固理论和铬系铸铁的液态结构和性能特点探讨凝固组织中碳化物形核、长大机理和基体的凝固行为，在此基础上进而研究其性能特征，以期为今后开发高性能的铬系耐磨铸铁奠定基础。论文取得了以下创新成果：（1）以金属凝固形核理论为指导，以高铬铸铁中碳原子位于M₇C₃型碳化物四面体或八面体间隙为基本出发点，通过计算和推导建立了过共晶高铬铸铁初生碳化物的形核模型。高铬铸铁初生碳化物晶体结构为六方点阵，熔体中的四面体和八面体原子团簇存在向六方多面体晶核合并的热力学内在条件，由四面体合并成六方形多面体的驱动力远大于八面体合并成六方形多面体的驱动力，同时形核也受动力学条件影响，八面体间隙大于四面体间隙，为Fe，Cr所构成的原子团簇捕获碳原子提供了有利条件。体系中存在着四面体和八面体的竞争形核。（2）对铬系铸铁碳化物析出热力学条件及动力学机制进行了深入研究，碳化物固液界面微观结构对碳化物类型转变及生长方式有重要影响。Jackson因子随Cr含量的增加而增加，碳化物生长固液界面光滑程度增加，降低了网状M3C型碳化物的“疯长”程度。Cr量的增加改变了铸铁液态原子结构，提高了铁液共晶转变温度而降低了铁液导热系数，加剧了碳化物晶格畸变程度。解释了在不同凝固条件下铬系铸铁碳化物类型发生临界转变的内在原因，初步揭示了过共晶高铬铸铁初生碳化物的生长过程。运用周期键链理论、传热和传质原理解释了初生M₇C₃型碳化物的生长机制。认为初生碳化物在初始生长阶段以平界面形式向液体中生长，当沿碳化物某一面凝固潜热散热快而熔体中C原子来不及扩散至碳化物界面时，碳化物以向内包抄形式生长。过共晶高铬铸铁的最终形态不仅与碳化物晶体内部结合键、各个晶面间的相对生长速率和晶体缺陷等因素有关，在很大程度上也受生长环境的影响。冷却速度快时碳化物向内包抄的面积小，容易长成实心状，冷却慢时则呈现L型、U型或六方形状。（3）研究铬系铸铁在H₂SO₄水溶液和石英砂形成PH3.5的浆料中的冲刷腐蚀磨损行为，结果表明：磨屑中是否具有大量的Cr₂O₃与试样含Cr量有关，而且影响其腐蚀磨损抗力。在本课题实验条件下，含Cr量分别为10%、12%和15%试样，冲刷腐蚀磨损后磨屑几乎没有检测到Cr₂O₃，其冲刷腐蚀耐磨抗力小；含Cr量为23%和28%的试样，冲刷腐蚀磨损后磨屑检测到大量的Cr₂O₃，的，其冲刷腐蚀耐磨抗力大。（4）在静态热碱腐蚀条件下，凝固速度对腐蚀抗力的影响主要取决于其对碳化物与基体间的腐蚀电位差和晶界能双重作用机制的竞争结果。凝固速度的提高，基体成分更加均匀，降低了碳化物与基体的腐蚀电位差，提高了合金腐蚀抗力；同时凝固速度提高使凝固组织更加细小，导致晶界面积和晶界能增加，增加了晶间腐蚀敏感性，降低了合金腐蚀抗力。在本课题的试验条件下，含Cr量为15%的试样，随着腐蚀的进行，凝固速度的提高，晶界能增加占主导地位，降低了腐蚀抗力；而含Cr量分别为10%、12%、23%和28%的试样，随着凝固速度增加，成分的均匀性使其晶间的腐蚀电位差的减小占主导，从而提高腐蚀抗力。更多还原

【Abstract】 Chromium cast iron is used widely as a wear-resistant materials. The service life ofcomposite system with high hardness carbides and different matrix was dominated bythe structure,size, morphology, distribution of carbides. The studies for many years havebeen foucus on the structure, size, morphology, distribution of carbides and thecomposition and microstructure control of the matrix. Based on solidification theoryand liquid structure and property of chromium cast iron, the nucleation and growthmechanism of carbides and solidification behavior of matrix were discussed. Theproperty characteristics was further studied on the basics of microstructure in order tolay the foundations for the future development of chromium cast iron with highproperty.The innovative achievements of the paper were as follows:（1） Guided by the metal solidification nucleation theory and started with carbonatoms in tetrahedral or octahedral interstice of carbides, the nucleation model of primarycarbides of hypereutectic high chromium cast iron was builded by calculation andderivation. The primary carbide crystal structure of high chromium cast iron ishexagonal lattice, the tetrahedral and octahedral clusters might merge to six polyhedracrystal nucleus from thermodynamic consideration. The driving force of tetrahedronatomic cluster combined into six polyhedra crystal nucleus was much larger than that ofeight polyhedron atomic cluster. However, the nucleation was influence by kineticsconditions. The fact of octahedral interstice greater than tetrahedral interstice providedadvantageous conditions to Fe and Cr clusters trapping carbon atom.The melt existed incompetitive nucleation between tetrahedral and octahedral. （2） The thermodynamic behavior of carbide precipitation and growthmorphology of carbides were thoroughly investigated. The microstructure ofcarbides-liquid interface had an important influence on carbide type changes and growthpatterns. Jackson factor increases with the increase of Cr content, the smoothnessdegree of the carbide in the carbides-liquid interface was increased accordingly, leadingto the decrease of net M3C carbides soaring level. Added Cr changed the liquid atomicstructure of cast iron, raised the eutectic transition temperature and decreased thethermal conductivity in iron liquid. Which aggravated the carbide lattice distortiondegree.The intrinsic reasons for carbide type critical transition in different solidificationconditions were explained. The growth process of primary carbide of hypereutectic highchromium cast iron was elementarily revealed. The growth mechanism of primarycarbides was explained by periodic bond chain theory, heat and mass transmissionprinciples. It was believed that the primary carbides grew in form of planar interface inthe initial growth stage in liquid. Carbides outflanked inward growth when latent heat ofsolidification quickly dissipated along certain sides of carbides and C atoms wasdifficult to diffuse to the carbides-liquid interface. The carbides morphology not onlyhad relationship with internal bond of carbides, the relative growth rate of variousplanes and crystal defects, but also largely depended on the growth environment. Rapidcooling rate led to small area of outflanking inward carbides, the carbides morphologywas easy to grow into a solid shape. Slow cooling rate led to L-shaped, U-shaped orhexagonal shape carbides.（3） It is useful to ascertain whether there existed a larger number of Cr₂O₃filmunder H2SO4（PH=3.5）and quartz sand erosion-corrosion wear conditions. The presenceof a larger number of Cr₂O₃film was associated with Cr contents. The number of Cr₂O₃film had a marked influence on the improvement of corrosion wear resistance. Underthe experiment conditions, Cr₂O₃was hardly detected as for the samples of white castirons containing10wt.%Cr,12wt.%Cr and15wt.%Cr after erosion-corrosion wearexperiments, which had a poor corrosion wear resistance. As for high chromium castirons containing23wt.%Cr and28wt.%Cr, a larger number of Cr₂O₃existed ingrinding chips, which improved corrosion wear resistance of the two kinds of alloys.（4） The influence of solidification rate on the corrosion resistance was mainly depended on the competition between potential difference between carbides and matrixand grain-boundary energy under the alkali static corrosion resistance conditions. Theincrease of solidification rate led to elements homogeneously distributed in matrix andfiner size. Correspondingly, potential difference between carbides and matrix wasdecreased and the corrosion resistance of alloys was improved. Meanwhile, the finermicrostructure increased the grain boundary area and grain boundary energies, soincreased the sensitivity of grain boundary, which resulted in the increase of corrosionresistance. Under the experiment conditions, the increase of grain boundary energiesowing to the increase of solidification rate was mainly recognized in15wt.%Cr whitecast iron with the experiment going on, which decreased the corrosion resistance. As for10wt.%Cr,12wt.%Cr,23wt.%Cr and28wt.%Cr white cast iron, the decrease ofpotential difference between carbides and matrix was dominant for the whole corrosionprocess with increase of solidification rate, so the corrosion resistance was improved.更多还原