脉冲电流作用下过共晶高铬铸铁的组织演变研究

发布时间：2018-05-23 18:51

  本文选题：过共晶高铬铸铁 + 脉冲电流　； 参考：《昆明理工大学》2017年硕士论文

【摘要】：因为过共晶高铬铸铁中有大量的M7C3型初生碳化物和共晶碳化物存在,材料具有很高的硬度,在耐磨领悟具有很好的应用潜质。但由于常规铸态下M7C3型初生碳化物尺寸粗大,对基体的割裂作用明显,增加了材料的脆性,造成过共晶高铬铸铁的实际应用受到了极大的限制。因此,为了提高过共晶高铬铸铁的实际应用价值,只有控制初生碳化物尺寸及形态,降低其对基体的割裂作用,才能提高材料的韧性并保证其耐磨性。本文将在凝固过程及凝固后的热处理过程中过共晶高铬铸铁施加脉冲电流,然后通过LOM、SEM、EDS、TEM、XRD等手段研究其组织,分析脉冲电流的作用机理,并对比常规铸态和脉冲处理后过共晶高铬铸铁的性能。研究发现,在凝固过程中施加脉冲电流(Electric Current Pulse,ECP),可以显著细化初生碳化物尺寸,减少初生碳化物裂纹、孔洞等缺陷,提高初生碳化物致密度。初生碳化物等效直径从约150μm减小到约65μm,显微硬度由HV1398～1420提高到HV 1485～1501。脉冲电流处理后的凝固组织中初生碳化物依旧保持六棱柱形态,但对基体组织的割裂效果明显减弱减少,有利于提高其使用性能。脉冲电流细化初生碳化物的机理是:在凝固过程中,脉冲电流对熔体起到了磁致收缩效应,提高熔体压力,进而实现了“电致过冷”作用,能够显著提高熔体中初生碳化物的形核率,细化初生碳化物;同时,脉冲电流还会对熔体产生电迁移作用,使C原子的扩散能力得到明显的提高,增强了C原子向初生碳化物内部穿透扩散,增加了初生碳化物的含碳量,并使碳C原子固溶到了碳化物密排六方晶格中的四面体间隙中,增加了晶格畸变,提高了固溶强度和致密性,改善初生碳化物性能。但脉冲电流没有改变出生碳化物的整体生长过程,只是通过提高C原子的扩散能力,维持了碳化物生长后期的持续生长能力,从而减少了内部孔洞缺陷。另外,在700℃到1000℃奥氏体化过程中,在进行常规工艺及脉冲电流处理时,虽然过共晶高铬铸铁中初生碳化物及共晶碳化物尺寸及形态均不会发生变化,依然维持处理前的状态,但是脉冲电流能够降低过共晶高铬铸铁发生固态相变的温度,并提高相变发生的程度。在1100℃奥氏体化时,在常规工艺脉冲处理工艺下,过共晶高铬铸铁基体组织均转变为奥氏体,共晶碳化物大量溶解并粗化;脉冲处理处理工艺下,初生碳化物溶解程度很大,但常规工艺下,初生碳化物较好保持了处理前的形态。在700℃时脉冲电流热处理时,试样基体硬度和宏观硬度最高,分别为HV 958和HRC 82;随着脉冲电流热处理温度从700℃升高到800℃,基体组织显微硬度和试样宏观硬度均降低,在800℃到1000℃范围内出现一个平台;在1100℃时再次下降,分别为HV 390和HRC71。
[Abstract]:Because there are a large number of M7C3 primary carbides and eutectic carbides in hypereutectic high chromium cast iron, the materials have high hardness and have good application potential in wear-resisting comprehension. However, due to the coarse size of M7C3 primary carbides in the conventional cast state, the cleavage effect on the matrix is obvious, and the brittleness of the material is increased, which results in a great limitation on the practical application of hypereutectic high chromium cast iron. Therefore, in order to improve the practical application value of hypereutectic high chromium cast iron, only by controlling the size and morphology of primary carbides and reducing its splitting effect on matrix can the toughness and wear resistance of the materials be improved. In this paper, the hypereutectic high chromium cast iron is applied pulse current during solidification and heat treatment after solidification. Then the structure of hypereutectic high chromium cast iron is studied by means of LOMSEMSEM EDS Tem XRD and the mechanism of pulse current is analyzed. The properties of hypereutectic high chromium cast iron after conventional and pulse treatment were compared. It is found that the application of pulse current in the solidification process can significantly refine the size of primary carbides, reduce the defects of primary carbides, such as cracks and voids, and increase the density of primary carbides. The equivalent diameter of primary carbides decreased from about 150 渭 m to about 65 渭 m, and the microhardness increased from HV1398~1420 to HV 1485 / 1501. The primary carbides in the solidified microstructure after pulse current treatment remain in hexagonal shape, but the cleavage effect of the matrix structure is obviously weakened and reduced, which is beneficial to improve the performance of the solidified structure. The mechanism of refining primary carbides by pulse current is as follows: during solidification, pulse current plays a magnetically induced shrinkage effect on the melt, increases the melt pressure, and realizes the effect of "electrically induced undercooling". The nucleation rate of the primary carbides in the melt can be increased significantly and the primary carbides can be refined. At the same time, the pulse current will also produce the electromigration effect on the melt, and the diffusion ability of the C atoms will be improved obviously. The diffusion of C atoms into the primary carbides is enhanced, the carbon content of the primary carbides is increased, and the carbon C atoms are dissolved into the tetrahedron gaps in the dense hexagonal lattice of the carbides, thus increasing the lattice distortion. The solution strength and densification were improved, and the properties of primary carbides were improved. But the pulse current does not change the whole growth process of the born carbides, only by improving the diffusion ability of the C atoms, it can maintain the growth ability of the carbides in the later stage of growth, thus reducing the defects of the internal pores. In addition, during austenitization from 700 鈩，

本文编号：1925915