【作者】 罗杰；

【导师】 陈维平；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 大断面球墨铸铁件由于凝固冷却缓慢,使得铸件中心常出现碎块状石墨等石墨畸变现象,大大降低了铸件心部的力学性能。在过去的40多年里,国内外科研工作者对于大断面球墨铸铁件中的石墨畸变问题进行了大量的研究,也取得了很大的进展。但是,目前对于碎块状石墨的形成机理仍不是非常清楚,对于该问题的研究也在不断继续。本文首先在工业生产现场浇注了Φ590×800mm的大断面球墨铸铁件,分析了其不同断面处的凝固过程、组织形貌和力学性能,探讨了铸件中的碎块状石墨的形成原因。然后,本文研制了1500℃组合式多功能电阻炉,并利用该装置控制模拟铸件的凝固,实现了对大断面铸件凝固过程的物理模拟。在此基础上,本文进一步研究了微量元素Sb、Bi以及随流孕育剂量对模拟铸件组织性能的影响,研究了随流加入纳米SiC颗粒对球墨铸铁组织性能的影响。主要结论如下:(1)Φ590×800mm大断面球墨铸件凝固冷却缓慢,在铸件中心凝固时间最长,达到372分钟。在铸件边缘凝固时间最短,为232分钟;铸件不同壁厚处的显微组织和力学性能变化情况特别。在铸件距中心170mm处,石墨形态恶化严重,碎块状石墨最多,力学性能最差,铸件抗拉强度、硬度、延伸率分别为362MPa、195HBW、1.67%。在铸件距中心255mm处,石墨形态最好,力学性能最佳,铸件抗拉强度和硬度分别为494MPa和225HBW。在铸件距中心85mm的圆周范围内,存在一定量的球状石墨和碎块状石墨,铸件综合力学性能较好。(2)Φ590×800mm大断面铸件中,沿径向方向镁和稀土元素出现了一定的宏观偏析。在铸件距中心170mm处,镁和稀土元素富集,其含量分别比铸件距中心255mm处高26%和20%。(3)1500℃组合式多功能电阻炉结构简单、使用方便。利用该电阻炉能较好地控制模拟铸件的冷却,实现了模拟铸件对大断面铸件心部凝固过程的模拟。模拟铸件与大断面铸件实际凝固冷却曲线的误差小于1.7%。(4)微量元素Sb、Bi能够改善模拟铸件中畸变石墨的形态。当加入量分别为0.01wt.%时,Sb元素的改善效果强于Bi元素。当Sb合金的加入量为0.03wt.%时,模拟铸件的碎块状石墨形态消失,球墨形态最好。此时,铸件的抗拉强度、屈服强度和延伸率分别为最高值707MPa、610MPa和2.96%,比不加微量元素模拟铸件的各力学性能分别高74.4%、81.9%和22.9%。(5)通过随流孕育的方式在球化孕育处理后的铁液中继续加入0.1wt.%和0.2wt.%的孕育剂促进了铸件中石墨数量的增加,但也促进了石墨的畸变和基体中铁素体的增多,降低了铸件的抗拉强度和硬度。(6)通过随流孕育方式在普通铸件中加入0.2wt.%纳米SiC颗粒能够较好的增加铸件中的球墨个数、改善石墨形态,提高铸件力学性能。在模拟铸件中随流加入0.2wt.%的纳米SiC颗粒促进了石墨析出,降低了铸件的抗拉强度和硬度。更多还原

【Abstract】 Due to the slow solidification process of heavy section ductile iron, the graphite degeneration, especially the chunky graphite often appeared in the center part of the casting, which lowers the mechanical properties of the casting greatly. In the past 40 years, scientists and producers around the world has given this problem much attention, and made a great progress in realizing and resolving the problem. But until now, the reason and mechanism for the formation of chunky graphite are still unavailable. The related researches on this issue are still in process.This paper firstly produced a heavy section ductile iron castingΦ590×800mm in factory, then made a research on the solidification, microstructure and mechanical properties of the casting, as well as the reason for the formation of chunky graphite. Then A 1500℃Knockdown Multifunctional Resistance Furnace was developed and used to control the solidification process of a small casting in its hearth which was named simulation casting in the paper, in order to make the simulation casting solidify in the manner of the centeral part of the castingΦ590×800mm. The effects of minor elements Sb and Bi, addition of more inoculant through in-stream inoculation on the simulation casting was studied. The effects of nanoparticle SiC on simulation casting and common casting was studied. The results are as follows:(1) The solidification process of the castingΦ590×800mm is extremely slow. The solidification time in the center is 372 minute, which represents the longest time. The solidification time in position 255mm away from the center is 232 minute, which represents the shortest time.The change of microstructure and mechnical properties in different positions on the casting shows special trends. The worst graphite morphology and mechnical properties appeared in the position 170mm away from the center, where lots of chunky graphite appeared, the tensile strength, brinell hardness and elongation are 362MPa、195HBW、1.67% respectively. In position 255mm away from the center, the graphite morphology and the mechanical properties are the best, where the tensile strength, brinell hardness are 494MPa and 225HBW respectively. In the circle area with radius of 85mm around the center, there are many noduar graphites and much chunky graphite. The machenical properties in this area are comparatively good.(2) Along the radius direction of the casting, macrosegregation of Mg and RE were discovered. In the position 170mm away from the center, the contents of Mg and RE are extremely high, where the content of Mg and RE are 26% and 20% higher than in the position 255mm away from the center respectively.(3) It was easy and safe in using the 1500℃Knockdown Multifunctional Resistance Furnace. The cooling curve of simulation casting controled by the furnace is similar to the real cooling curve of the castingΦ590×800mm. The maximal comparative error between the two curves is 1.7%. The results show that it’s effective in simulating the solidification process of heavy section casting by this furnace.(4) The minor elements Sb and Bi was effective in improving the morphology of the graphite in the simulation castings, and the effect of 0.01wt.% Sb addtion was better than that of 0.01wt.% Bi addition. When 0.03wt.% Sb alloy is added into the iron, the morphology of graphite and mechnical properties of the simulation casting was the best; the tensile strength, yield stress and elongation are 707MPa, 610MPa and 2.96% respectively, which are 74.4%, 81.9% and 22.9% higher than that of simulation casting without minor elements.(5) The addition of 0.1wt.% and 0.2wt.% inoculant by in-stream inoculation has good effects in increasing the mounts of graphite, but bad effects on the morphology of graphite, which also promotes the ferrite in the matrix, lower the machenical properties at last.(6) The addition of 0.2wt.% nanoparticle SiC in common casting can increase the mounts of nodular graphite, improve the graphite morphology and enhance its mechnical properties. The addition of 0.2wt.% nanoparticle SiC in simulation casting promotes the growth of the graphite but lower its tensile strength and brinell hardness.更多还原