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1、 工学硕士学位论文工学硕士学位论文 厚大断面球铁的显微组织 及力学性能 MICROSTRUCTURE AND MECHANICAL PROPERTIES IN HEAVY SECTION DUCTILE IRON 马晓锋 2009 年 6 月 国内图书分类号：TG143.5 学校代码：10213 国际图书分类号：620 密级：公开 工学硕士学位论文工学硕士学位论文 厚大断面球铁的显微组织 及力学性能 MICROSTRUCTURE AND MECHANICAL PROPERTIES IN HEAVY SECTION DUCTILE IRON 硕 士 研 究 生：马晓锋 导 师：宁志良 副教授 申
2、请学位：工学硕士 学科：材料加工工程 所 在 单 位：材料科学与工程学院 答 辩 日 期：2009 年 6 月 授予学位单位：哈尔滨工业大学 Classified Index：TG143.5 U.D.C.: 620 Dissertation for the Degree of Master in Engineering MICROSTRUCTURE AND MECHANICAL PROPERTIES IN HEAVY SECTION DUCTILE IRON Candidate： Ma Xiaofeng Supervisor： Asso. Prof. Ning Zhiliang Academi
3、c Degree Applied for： Master of Engineering Speciality： Materials Processing Engineering Affiliation： School of Materials Science and Engineering Date of Defence： June, 2009 Degree-Conferring-Institution：Harbin Institute of Technology 哈尔滨工业大学工学硕士学位论文 I摘 要 随着重型机械工业特别是核电工业的发展，厚壁、大型球铁件得到越来越广泛的应用。由于断面过厚
4、，凝固时间长，由此导致球化衰退与孕育衰退而发生石墨的畸变，形成各种非球状石墨，造成球铁性能的下降。 本文利用外加冷铁强制冷却的扇形试块模拟百吨级厚大断面球铁容器的凝固过程，采用光学显微镜，扫描电镜对厚大断面球铁的微观组织进行了分析，并研究了凝固时间对拉伸性能的影响规律及其断裂机制，分析了常温和低温（-40）条件下的冲击韧性以及低温（-40）条件下的断裂韧性。 从扇形试块的凝固曲线可以发现，由于采用了内外强制冷却，缩短了共晶转变时间，扇形试块中心的凝固时间为 190 分钟；内外冷铁中都存在着比较明显的温度梯度，增强了铸型的激冷能力，使铸件内部温度梯度增大，糊状区域变窄，缩短了扇形试块的凝固时间，
5、石墨球趋于圆整，保证了球化级别。 凝固速度对石墨球数量的影响明显，随着凝固时间延长，石墨球数量逐渐减少，尺寸随之增大，而石墨球所占面积基本保持不变；扇形试块边缘石墨球的形状因子（SF）峰值出现在 0.80.9 处，而心部 SF 峰值出现在 0.60.7 处，说明随着冷却速度降低，石墨圆整度也随之下降；同时，基体组织几乎全部为铁素体组织，但在最后凝固的心部反而出现一定数量的珠光体组织。 扇形试块不同位置的拉伸实验表明，凝固时间对厚大断面球铁抗拉强度的影响较小。断裂机理为铁素体基体发生长位移的塑性变形，石墨和铁素体基体形成的空穴不断扩大，导致脱离。常温冲击断裂的机理为压缩侧的试样边缘为韧-脆性混合
6、型断裂，压缩侧和断口中间区域以韧性断裂为主；低温冲击断裂机理为以脆性断裂为主，辅以局部性的韧性断裂。低温（40）三点弯曲断裂韧性试验表明，在-40的环境温度下，球化良好并且以铁素体基体的球铁铸件，在缓慢加载外力的情况下，以韧性断裂为主；在环境温度低于-40，以铁素体为主要基体，还有少量珠光体的球铁，当缓慢加载外力的情况下，其石墨球周围将发生韧性断裂，而在石墨球相对不集中的区域会产生脆性断裂，并且为穿晶断裂。 关键词 球墨铸铁；厚大断面；微观组织；球状石墨；力学性能 哈尔滨工业大学工学硕士学位论文 IIAbstract With the heavy machinery industry, par
7、ticularly the nuclear power industry development, heavy section ductile iron has been more and more widely used. The solidification time is increased for the casting with heavy section, resulting in spheroidization degeneration and inoculation degeneration occurred in the recession of non-spherical
8、graphite and the formation of various non-spherical graphite as well as a decline in the properties of ductile iron. A sectorial block with thickenss of 500mm was used to simulate the solidification process of a 100 ton heavy section ductile iron container by forcing cooling. The microstructures of
9、the simulated block were analyzed by optical microscope, scanning electron. The effect of solidification time on the properties and fracture mechanism of ducticle iron with heavy section were dicussed. The dynamic ductility and fracture toughness at room temperature and low temperature of -40 were m
10、easured. From the the cooling curves of the simulated fan-shaped block, it was found that the solidification time of eutectic solidification in the center time was shortened to 190mm due to the use of both inside and outside chills. There were obvious temperature gradients existing in the chills, le
11、ading to an increased cooling capacity in chills. The temperature gradient in the fan-shaped black was also enlarged and the solidification time was shortened. The mushy solidified region was narrowed, which was benefit for the obtaining of round graphite and higher spheroidal graphite level. The ef
12、fect of cooling rate on number of graphite nodules was significant. The long solification time resulted in a gradual decrease in number of graphite nodules and an increase in graphite size, while the total area of graphite remained basically constant. The shape factor peak of graphite in the edge of
13、 the simulated block was 0.80.9 and it decreased to 0.60.7 for the sample taken in the center, indicating that the graphite roundness was decreased with increased solidification time. The matix consisted of total ferrite and only a small amount of pearlite existing in the center of the block. The te
14、nsile property measurements at room temperature for the various positions in the simulated block show that the solidification time has little effect on the tensile strength and yield strength. The fracture mechanism is that the long displacement takes place on the matrix and the cavity in both graph
15、ite and ferrite enlarges gradually, leading to detachment. The impact fracture mechanism at room temperature is tough-brittle mixed fracture mode at the edge and ductile fracture in the compression side and the middle area. For the impact fracture at the low temperatue of -40, the fracture is mainly
16、 brittle fracture accompanied with ductile fracture at local area. For the fracture toughness measurement at the low temperature of -40 with slow loading speed, the 哈尔滨工业大学工学硕士学位论文 IIIfracture is ductile for the casting with sound grahite and ferrite matrix and the fracture took place around the graphite for the casting with main ferrite matrix and a small amount o
