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1、 华华 中中 科科 技技 大大 学学 硕硕 士士 学学 位位 论论 文文 I摘摘 要要 熔模精密铸造具有尺寸精度高，表面光洁度好，可生产形状复杂、超薄壁铸件的特点，因而成为球铁件生产的一种重要手段，自上世纪上世纪六十年代开发应用以来，精铸球铁工艺在国外得到了迅速发展，最近几年在我国也有迅速发展，其产量和产品种类、复杂程度都有大幅度增加。但是，熔模铸造球铁因其工艺特性，在成分、浇注条件等方面与砂铸不同，相应的球化处理、孕育处理也不同。首先，精铸球铁化学成分选择的碳当量偏低；其次，热壳浇注使球铁凝固的冷却速度减小，糊状凝固特性更突出，与砂铸球铁比更易产生缩孔缩松缺陷。因此，开展熔模铸造球墨铸铁的凝
2、固特性和组织性能的研究对球铁的熔模铸造工艺制定及组织和性能控制具有重要的理论意义和现实意义。 本文以快速火车发动机上盖铸件为研究对象，通过计算机模拟结合实际温度场测定以及球铁的组织和性能测试，研究了熔模铸造球铁的凝固特性和组织性能。结果表明，浇注温度在 13201420范围内，缩松倾向随着温度的升高而减小；随着模壳温度的下降，缩松倾向有增大的趋势。铸件在浇注温度为 1420，模壳温度为 700时，铸件的缩孔缩松倾向最小。铸件的凝固冷却曲线显示，热壳铸件冷却曲线平稳，共晶平台持续时间长于冷壳铸件，存在明显的共晶回升温度；冷壳铸件冷却曲线冷却速度快，没有明显的共晶回升温度。 生铁中的 V、Ti 严
3、重影响石墨球化，促进碳化物形成。因此，在熔模铸造球铁时，应选用不含 V、Ti 的低硫、低磷生铁。在熔模铸造球铁的生产工艺方面，采用低稀土镁球化剂和二次孕育，可获得圆整度好、尺寸均匀的石墨球。提高模壳的强度、焙烧温度，降低浇注温度有利于降低铸件的废品率，提高球铁铸件的质量。冷壳铸件球墨数量多尺寸小，石墨大小为 7 级，而热壳铸件石墨大小为 5 级。冷壳铸件基体组织中渗碳体含量大于 2%，热壳铸件的渗碳体含量小于 1%。热壳铸件本体的抗拉强度大于 600MPa， 延伸率大于 3%， 满足发动机上盖对球铁组织和性能要求。 关键词：关键词：熔模铸造；球墨铸铁；华铸CAE；凝固特性；组织；性能 华华 中
4、中 科科 技技 大大 学学 硕硕 士士 学学 位位 论论 文文 IIAbstract Investment casting has the characteristics of high dimensional accuracy, good surface finish and being able to produces complex shape and ultra-thin section castings, investment casting has already become an important preparation means of ductile iron parts
5、. Since 1960s, the investment casting technology of ductile iron has rapidly developed abroad, and it also has developed rapidly in China in the recent years. The production and complexity of ductile iron castings produced by investment casting are all significantly increased. However, the compositi
6、on, pouring conditions, nodulizing treatment and inoculation treatment of investment casting are different from sand casting. First of all, in the investment casting, the carbon equivalent is lower. Secondly, because of the hot shell in the investment casting, the cooling rate decreases, the tendenc
7、y of shrinkage enlarges. So it will be of great theoretical and practical significance for establishing process and the control of the performance of ductile iron to research on microstructure, properties and solidification characteristic of ductile iron by investment casting. By taking engine casti
8、ngs of fast train as the research object, through computer simulation and actual cooling curve of ductile iron and the testing of organization and performance, this paper researched the microstructure, properties and solidification characteristic of ductile iron by investment casting. The results sh
9、owed that the shrinkage tendency of the ductile iron declined when the pouring temperature increased from 1320 to 1420, and the shrinkage tendency increased when the temperature of shell dropped. The shrinkage tendency was minimal when the pouring temperature was 1420 and the temperature of shell wa
10、s 700. It is seen from the cooling curve that compared with the casting made from cold shell, the casting made by the hot shell has 华华 中中 科科 技技 大大 学学 硕硕 士士 学学 位位 论论 文文 IIIhigher eutectic temperature, longer eutectic reaction time and the eutectic platform is more stable. It was found that when the c
11、ontent of V, Ti elements is high, the spheroidization was poor. There were a large amount of distorted graphite nodules in the microstructure. The graphite form was grade 3 to 4 in Chinese graphite nodule standard; also there were a lot of carbides in the metal matrix. Therefore, in the investment c
12、asting, pig iron of non-v, Ti and low sulphur and phosphorous should be chosen. In the production of ductile iron in investment casting, using low-RE Mg-nodulizer and secondary inoculation, prepared ductile iron with whole round graphite of uniform size. Increasing the intensity of mold and the baki
13、ng temperature could reduce casting rejects and improve the quality of ductile iron castings. Cold shell castings had a large number of nodular graphite with small size, the size degree of the nodular graphite of cold shell castings was grade 7 while the size of hot shell was grade 5. In the matrix
14、of cold shell castings, the content of cementite was more than 2%, meanwhile, the content of cementite of hot shell castings was less than 1%. The tensile strength and elongation were more than 600MPa and 3%, respectively. It can meet the requirements of microstructure and properties for engine cast
15、ings. Keywords： Investment Casting；Ductile Iron；Intecast；Solidification Characteristic； Microstructure；Properties 独创性声明独创性声明 本人声明所呈交的学位论文是我个人在导师指导下进行的研究工作及取得的研究成果。尽我所知，除文中已经标明引用的内容外，本论文不包含任何其他个人或集体已经发表或撰写过的研究成果。对本文的研究做出贡献的个人和集体，均已在文中以明确方式标明。本人完全意识到本声明的法律结果由本人承担。 学位论文作者签名： 日期： 年 月 日 学位论文版权使用授权书学位论文版权
16、使用授权书 本学位论文作者完全了解学校有关保留、使用学位论文的规定，即：学校有权保留并向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅。本人授权华中科技大学可以将本学位论文的全部或部分内容编入有关数据库进行检索，可以采用影印、缩印或扫描等复制手段保存和汇编本学位论文。 保密，在_年解密后适用本授权书。 本论文属于 不保密。 (请在以上方框内打“”) 学位论文作者签名： 指导教师签名： 日期： 年 月 日 日期： 年 月 华华 中中 科科 技技 大大 学学 硕硕 士士 学学 位位 论论 文文 11 绪绪 论论 由于球墨铸铁(下称球铁)具有与钢相似的韧性和铸铁本身特有的优良铸造性、耐磨性和耐压性，所以自从 1947 年问世，便以一种新型的工业金属材料投入工业生产，并得到了迅猛的发展1。 1.1 球铁的发展球铁的发展 球铁件的产量在铸铁件总产量中所占的比例逐年上升,国外从 1
