【作者】 范会超；

【导师】 曹志强；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2011， 硕士

【摘要】 铸造不锈钢叶轮具有优良的机械性能和耐蚀性能,但其铸造性能比较差。一方面由于不锈钢材料熔点较高,易氧化、凝固温度范围较宽,钢水的流动性差、收缩大；另一方面叶轮的结构复杂,壁厚差异大。采用普通铸造工艺生产,凝固组织粗大,在恶劣工作条件下,很容易在高应力区域,如叶片根部,产生裂纹。为了研究提高叶轮质量的工艺方案,本论文以中小型叶轮为研究对象,将电磁搅拌细晶技术与真空精密铸造技术相结合,探索研究了一种奥氏体不锈钢叶轮的真空电磁搅拌精密铸造新工艺,即在熔模精密铸造的基础上,采用真空冶炼浇注,严格控制叶轮的合金成分,并在金属液凝固过程中施加电磁搅拌,改善其凝固组织。结果表明：影响叶轮成型性能的主要因素是浇注温度、模壳预热温度和电磁搅拌启动时间,获得完整叶轮的最佳条件为：模壳的预热处理为1000℃下保温30min,浇注温度选择1590℃,电磁搅拌在浇注完金属液,静置3s后启动。电磁搅拌增加了冒口的补缩压力,有效地改善了叶轮铸件缩孔的大小和位置,提高了金属液的利用率。双向搅拌进一步改善了金属液内的温度分布,实现了铸锭的顺序凝固,消除了叶轮铸件的疏松缺陷。采用新工艺制备的叶轮轮廓清晰完整,表面光洁,无裂纹、气孔、缩松等铸造缺陷,为整体叶轮的企业生产提供一定的技术参考。利用电化学工作站、金相显微镜、扫描电镜、电子探针等现代材料分析检测设备,分析讨论了新工艺参数对叶轮组织性能的影响,结果表明：在双向电磁搅拌作用下,单向搅拌时间越长,等轴晶比例越大,其平均尺寸也越小。在单向搅拌8s时,整个心部断面等轴晶率达到100%,等轴晶的平均尺寸达到25μm。施加电磁搅拌后,铁素体相在铸态下变得短小分散,端部圆钝,含量在5%左右。电磁搅拌通过细化叶轮组织和抑制元素偏析及碳化物的析出,改善了不锈钢的耐蚀性能。其中,在0.5mol/LH2SO4溶液中耐晶间腐蚀的ERP再活化率由45.8%降到13.0%,在3.5%NaCl溶液中的耐点蚀电位由0.18105V提高到0.31622V。更多还原

【Abstract】 Cast stainless steel impeller has excellent mechanical properties and corrosion resistance, but has relatively poor casting properties. On the one hand, the stainless steel has high melting point, easily oxidized, a wide range of solidification temperature, poor fluidity, a large contraction, on the other hand, the impeller has complex structure and large differences in wall thickness. therefore, produced by common casting process, it has the coarse structure. If it work in poor working conditions, cracks easily occur in high-stress areas, such as blade roots.In order to study the process to improve the quality of the impeller, using the small impeller as study objects, the experiment combines the fine grain of electromagnetic stirring technology and vacuum casting technology, so as to explore and research a new process of stainless steel impeller by vacuum electromagnetic stirring precision casting technology., Based on the precision casting melting and casting are carried our in vacuum, to strictly control the alloy composition of impeller, and exert the electromagnetic stirring during the solidification process, to controlling the solidification structure. The result show that the main factors effecting the impeller forming are pouring temperature, mold shell preheat temperature and the time to start the electromagnetic stirring, for the complete impeller, the optimal conditions are preheating the shell molded at 1000℃incubated 30min, pouring temperature at 1590℃, opening the EMS 3s after pouring liquid metal. EMS can increase the feeding pressure on the impeller, so as to effectively modify the size and position of casting shrinkage and improve the utilization of liquid metal. Two-way EMS further homogenize the temperature distribution in liquid metal, so as to achieve the ingot solidification, eliminating the shrinkage porosity defects. The impeller by new process, with a new clear and complete outline and smooth surface, has no cracks, porosity, shrinkage and other casting defects, providing a technical reference for the impeller manufacturer.Modern material testing and analysis equipment, such as electrochemical workstation, optical microscope, scanning electron microscope, electron microprobe, are used to study and discussion the affection of the new process parameters on the microstructure and properties of the impeller, the results show that, appropriately increasing the stirring time, is benefit to improve the grain refinement. When the one-way stirring time was 8s, the equiaxed grains were evenly refined to the size of 25μm. Ferrite in the casting structure is about 5% and becomes short and scattered, with blunt end. EMS can improve the corrosion resistance of stainless steel, by refining the organizations and inhibiting the element segregation and carbide precipitation. Among them, the EPR reactivation rate of intergranular corrosion in 0.5mol/LH2SO4 solution decrease by 32.8%, from45.8% to 13.0%, the pitting potential in 3.5% NaCl solution increased by0.13517V, from 0.18105V to 0.31622V.更多还原