高频脉冲电流诱发过共晶高铬铸铁中碳化物细化和粒化机理研究

M7C3 carbides have high hardness and good wear resistance, but M7C3 carbides in hypereutectic high chromium cast iron (hypereutectic HCCI) were in a long thick rod shape and flake shape, which greatly reduced the impact toughness and wear resistance of the alloy. The application of the hypereutectic HCCIs in the working conditions of impact abrasive wear resistance could not be achieved. In this project, the high frequency electric pulse current (HFEPC) will be used to control the overall solidification course of the hypereutectic HCCI to refine and granulate the carbides both the primary and eutectic, and to improve the impact toughness and wear resistance of the alloys obviously. The dispersed fining primary carbide nuclei in the melting metal will be obtained firstly under the control of the appropriate HFEPC, and then the eutectic carbides will nucleate and grow up to granular shape under the co-role of the existing primary carbide nuclei and the appropriate HFEPC. Firstly, the solidification microstructure evolution of the alloy will be investigated systematically. Secondly, Carbide nucleation and grew thermodynamics, solute atom diffusion kinetics, the interface concentration gradient, carbides interface morphological changes, carbide defect structure and physical properties and melt characteristics will be focused on. Thirdly, the relationship between the inherent characteristic frequency of the molten alloy and the combined effects of the HFEPC discharge current, frequency and pulse width will be established. On the basis of the above mentionged research results, the mechanism of pulse current inducing carbide refining and granulating in the hypereutectic HCCI will be explored. The project focused on the improvement and control mechanism of the carbide morphology by the HFEPC. It has guiding significance for the compounds morphology control of other alloys, and also promotes the engineering application of the hypereutectic white cast iron.

M7C3型碳化物具有硬度高和耐磨性好等优点。过共晶高铬铸铁中M7C3型碳化物呈粗大长杆状和片状，大大降低其冲击韧性和耐磨性，限制其在冲击磨料磨损工况下的应用。项目借助高频脉冲电流对高铬铸铁凝固过程进行控制，首先获得弥散分布的细小初生碳化物晶核，在初生晶核和脉冲电流的共同作用下促进后续共晶碳化物的颗粒化，最终实现碳化物的细化和粒化，改善材料冲击韧性并提高耐磨性。项目在研究考察脉冲电流作用下高铬铸铁凝固过程中组织演变基础上，结合碳化物形核长大热力学、溶质扩散动力学、界面浓度梯度的变化、碳化物界面形态变化以及碳化物缺陷结构、物理性能和熔体特性等特点，建立脉冲电流放电电流、频率和脉宽的综合作用与熔融合金固有特征频率之间的关系，探索脉冲电流诱发高铬铸铁中碳化物细化和粒化机理。项目研究脉冲电流作用下碳化物形态的改善和控制机理，既对控制其它合金中化合物的形态具有指导意义，又促进过共晶白口铸铁的工程应用。

针对过共晶高铬铸铁中粗大长杆状和片状M7C3型碳化物的细化和粒化，以提高过共晶高铬铸铁的冲击韧性和耐磨性，项目借助高频脉冲电流对高铬铸铁凝固过程进行处理。在脉冲电流的作用下，实现了初生碳化物和共晶碳化物的细化和粒化，材料的冲击韧性有较大幅度的提高。项目对高铬铸铁不同凝固温度区间施加脉冲电流，研究了初生碳化物和共晶碳化物形态和尺寸的演变规律；采用SEM、TEM等手段考察了碳化物结构变化、碳化物界面形态变化、C和Cr等溶质元素在界面以及在碳化物内部和熔体中的分布变化情况；结合碳化物显微硬度、晶格常数和材料腐蚀性能的变化规律，以及脉冲电流的电迁移和磁致收缩效应，分析了脉冲电流对碳化物形核长大热力学和动力学的影响；采用正交试验方法研究脉冲电流放电电流、频率和脉宽分别对初生碳化物细化和粒化效果和作用程度；综合组织形貌、尺寸变化、元素分布等变化规律，揭示脉冲电流诱发高铬铸铁中碳化物细化和粒化机理。研究表明，由于脉冲电流的电迁移效应，促使熔体中溶质碳原子与铬、铁原子结合，同时磁致收缩效应加速浓度起伏微区形成，阻碍初生碳化物迅速长大，最终形成细粒化初生碳化物。颗粒状初生碳化物使得大部分共晶碳化物以初生颗粒状碳化物形核，共晶反应以离异共晶形式进行。正交试验表明，脉冲电流电压影响最明显，脉宽影响次之，频率影响最小。脉冲电流处理温度降低，熔体粘度增大，磁致收缩效应减小，电迁移效应作用下，电压增大会使得碳化物变粗。脉冲电流处理获得的试样抗拉强度提高约60%，冲击韧性提高约80%。脉冲电流处理后，铬元素在碳化物中含量降低，铁基体中铬元素含量升高，高铬铸铁的耐腐蚀性能有所提高。研究结果对于促进过共晶高铬铸铁的应用，以及在不产生晶间腐蚀前提下提高不锈钢中碳元素含量，获得既耐腐蚀又耐磨损不锈钢，都具有显著的工程应用前景。