本文選題：高硅鉬球鐵 + 鉻元素�。� 參考：《西安理工大學》2017年碩士論文

【摘要】：高硅鉬球鐵具備良好的高溫強度,優(yōu)良的耐氧化性、抗熱疲勞性、抗生長性和良好的高溫抗蠕變性等,在汽車發(fā)動機耐高溫部件如排氣歧管、渦輪殼等鑄件上運用日趨廣泛。汽車發(fā)動機排氣歧管通過法蘭與發(fā)動機直接連接,并將發(fā)動機各個氣缸排出的廢氣匯集到一起排出。汽車在行進中啟動和停車導致汽車排氣歧管的工作狀態(tài)會經(jīng)歷短時間的升溫、降溫的交替變換過程,同時排氣歧管的表面與內(nèi)腔溫差較大,會造成熱疲勞裂紋的產(chǎn)生。因此改善高硅鉬球鐵在高溫下的熱疲勞性能和耐氧化性對材料的使用壽命具有重要意義。本文旨在通過添加鉻、釩等合金元素來改善高硅鉬球墨鑄鐵的組織和力學性能,借助抗拉強度測試、熱疲勞測試、抗氧化性測試等手段對比研究各元素對高硅鉬球墨鑄鐵性能的提高程度。研究結(jié)果表明:高硅鉬球墨鑄鐵的顯微組織由鐵素體基體、球狀石墨、富鉬碳化物和少量珠光體組成。石墨尺寸均勻,鑲嵌在鐵素體基體上,碳化物分布在晶界處。添加鉻元素的高硅鉬球墨鑄鐵中珠光體和晶界碳化物數(shù)量明顯增多,抗拉強度提高了8%,硬度提高了 41%;添加釩元素的高硅鉬球墨鑄鐵中微觀組織顯著細化,珠光體數(shù)量減少,抗拉強度提高了 15%,硬度提高了 7%;復合添加鉻、釩元素的高硅鉬球墨鑄鐵組織中珠光體數(shù)量趨于減少,晶界碳化物變得細小,抗拉強度提高了 9%,硬度值基本不變。在熱疲勞測試過程中,4種高硅鉬球鐵試樣首先發(fā)生不同程度的變形;當形變量達到一定程度時,于試樣表面萌生裂紋;裂紋均在大表面上形成;熱疲勞裂紋一般會在晶界的碳化物或消失的球狀石墨處產(chǎn)生;裂紋在擴展過程中球狀石墨能夠改變裂紋擴展的方向;裂紋的擴展速度隨著裂紋處氧化程度的加強而加快;在相同熱循環(huán)條件下,當出現(xiàn)相同長度的熱裂紋,含鉻元素的高硅鉬球墨鑄鐵所需的熱循環(huán)次數(shù)最多。在高溫環(huán)境下,試驗測定的4種高硅鉬球鐵的氧化增重皆隨時間的增加而增加。同時表層的球狀石墨因氧化作用而消失,試樣表層皆會形成Si02氧化膜,該膜會阻礙后續(xù)氧化過程的進行,對提高合金的抗氧化性有重要的作用。含鉻高硅鉬球鐵表層還會形成Cr203氧化膜,進一步提高該合金的抗氧化性。在相同測試條件下,含鉻高硅鉬球墨鑄鐵單位面積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.
【學位授予單位】：西安理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG250

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