铬、钒元素对高硅钼球铁组织和性能的影响

发布时间：2018-06-08 00:05

本文选题：高硅钼球铁 + 铬元素　；参考：《西安理工大学》2017年硕士论文

【摘要】：高硅钼球铁具备良好的高温强度,优良的耐氧化性、抗热疲劳性、抗生长性和良好的高温抗蠕变性等,在汽车发动机耐高温部件如排气歧管、涡轮壳等铸件上运用日趋广泛。汽车发动机排气歧管通过法兰与发动机直接连接,并将发动机各个气缸排出的废气汇集到一起排出。汽车在行进中启动和停车导致汽车排气歧管的工作状态会经历短时间的升温、降温的交替变换过程,同时排气歧管的表面与内腔温差较大,会造成热疲劳裂纹的产生。因此改善高硅钼球铁在高温下的热疲劳性能和耐氧化性对材料的使用寿命具有重要意义。本文旨在通过添加铬、钒等合金元素来改善高硅钼球墨铸铁的组织和力学性能,借助抗拉强度测试、热疲劳测试、抗氧化性测试等手段对比研究各元素对高硅钼球墨铸铁性能的提高程度。研究结果表明:高硅钼球墨铸铁的显微组织由铁素体基体、球状石墨、富钼碳化物和少量珠光体组成。石墨尺寸均匀,镶嵌在铁素体基体上,碳化物分布在晶界处。添加铬元素的高硅钼球墨铸铁中珠光体和晶界碳化物数量明显增多,抗拉强度提高了8%,硬度提高了 41%;添加钒元素的高硅钼球墨铸铁中微观组织显著细化,珠光体数量减少,抗拉强度提高了 15%,硬度提高了 7%;复合添加铬、钒元素的高硅钼球墨铸铁组织中珠光体数量趋于减少,晶界碳化物变得细小,抗拉强度提高了 9%,硬度值基本不变。在热疲劳测试过程中,4种高硅钼球铁试样首先发生不同程度的变形;当形变量达到一定程度时,于试样表面萌生裂纹;裂纹均在大表面上形成;热疲劳裂纹一般会在晶界的碳化物或消失的球状石墨处产生;裂纹在扩展过程中球状石墨能够改变裂纹扩展的方向;裂纹的扩展速度随着裂纹处氧化程度的加强而加快;在相同热循环条件下,当出现相同长度的热裂纹,含铬元素的高硅钼球墨铸铁所需的热循环次数最多。在高温环境下,试验测定的4种高硅钼球铁的氧化增重皆随时间的增加而增加。同时表层的球状石墨因氧化作用而消失,试样表层皆会形成Si02氧化膜,该膜会阻碍后续氧化过程的进行,对提高合金的抗氧化性有重要的作用。含铬高硅钼球铁表层还会形成Cr203氧化膜,进一步提高该合金的抗氧化性。在相同测试条件下,含铬高硅钼球墨铸铁单位面积的增重和氧化皮厚度最小,抗氧化性最好。
[Abstract]:High silicon molybdenum ductile iron has good high temperature strength, excellent oxidation resistance, thermal fatigue resistance, growth resistance and good high temperature creep resistance, etc. It is widely used in automotive engine parts such as exhaust manifold, turbine case and other castings. The exhaust manifold of automobile engine is directly connected to the engine by flange, and the exhaust gas from each cylinder of the engine is collected together. When the vehicle starts and stops, the working state of the exhaust manifold of the vehicle will undergo a short time of temperature rise, and the temperature of the exhaust manifold will change alternately. At the same time, the temperature difference between the surface and the inner cavity of the exhaust manifold will be large, which will lead to the thermal fatigue crack. Therefore, it is important to improve the thermal fatigue property and oxidation resistance of high silicon molybdenum ductile iron at high temperature. The aim of this paper is to improve the microstructure and mechanical properties of high silicon molybdenum ductile iron by adding chromium, vanadium and other alloying elements, and to improve the microstructure and mechanical properties of high silicon molybdenum ductile iron by means of tensile strength test and thermal fatigue test. The effect of various elements on the properties of high silicon molybdenum ductile iron was studied by means of oxidation resistance test. The results show that the microstructure of high silicon molybdenum ductile iron consists of ferrite matrix, nodular graphite, molybdenum rich carbide and a little pearlite. Graphite is uniform in size, embedded in ferrite matrix, and carbide distributes at grain boundary. The quantity of pearlite and grain boundary carbides in high silicon molybdenum ductile cast iron with chromium element increased obviously, the tensile strength increased 8%, the hardness increased 41%, and the microstructure of high silicon molybdenum ductile cast iron with vanadium element was significantly refined and the quantity of pearlite decreased. The tensile strength increased 15% and hardness increased 7%. The quantity of pearlite in high silicon molybdenum ductile cast iron with chromium and vanadium was decreased, the grain boundary carbides became fine, the tensile strength increased 9%, and the hardness value remained unchanged. In the process of thermal fatigue testing, four kinds of high silicon molybdenum ductile iron samples are first deformed to varying degrees, when the deformation amount reaches a certain degree, cracks occur on the surface of the specimens, and the cracks are formed on the large surfaces. Thermal fatigue cracks usually occur at the carbide or vanishing spheroidal graphite in grain boundaries, and the spheroidal graphite can change the direction of crack propagation in the process of crack propagation, and the crack propagation rate accelerates with the increase of oxidation degree at the crack. Under the same thermal cycling conditions, when there are hot cracks of the same length, the heat cycles of high silicon molybdenum ductile iron containing chromium elements are the most. Under high temperature, the oxidation weight gain of four kinds of high silicon molybdenum ductile iron was increased with the increase of time. At the same time, the nodular graphite in the surface layer disappeared because of oxidation, and SiO2 oxide film was formed in the surface layer of the sample, which will hinder the subsequent oxidation process and play an important role in improving the oxidation resistance of the alloy. Cr203 oxide film will be formed on the surface layer of Cr-containing high silicon molybdenum ductile iron, which can further improve the oxidation resistance of the alloy. Under the same test conditions, the weight gain and oxide thickness per unit area of high silicon and molybdenum ductile iron containing chromium are the least and the oxidation resistance is the best.
【学位授予单位】：西安理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG250

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