【作者】 杨云龙；

【导师】 曹占义；

【作者基本信息】 吉林大学 ， 材料学， 2013， 博士

【摘要】 在汽车的排气系统中，汽车排气歧管通过法兰直接与发动机连接，将发动机各气缸排出的高温废气汇集到一起排出车外。汽车在行驶过程中，排气歧管内部温度一直处于800℃以上高温工作状态，而外部直接与自然环境相接触，温度保持在100℃-200℃左右；汽车在行驶中的停车和启动致使排气歧管的工作状态会经历短期内的降温、升温的交替变换过程。考虑到排气歧管复杂的工作环境，故对其使用材料特性要求很高。从结构上看，排气歧管的构成依赖于发动机的结构，而随着汽车工业的快速发展越来越复杂的发动机结构会带动排气歧管结构的连锁变化。随着发动机排放标准和其效率的不断提高，汽车用排气歧管材质主要经历了普通灰铸铁、高强度灰铸铁、普通球铁、蠕墨铸铁、普通不锈钢、网状钢、高硅钼球铁、高镍球墨铸铁、高合金的不锈钢的发展过程。而我国目前的汽车工业尚处于稳定发展阶段。十二五期间，吉林省将以一汽为中心，构建整车研发制造、零部件配套和服务体系，提升整车制造能力。特别是长春市300万辆和吉林市100万辆汽车扩能工程更是推进了吉林省汽车产业的快速发展。到2015年，全省将实现整车产能达到400万辆、汽车工业增加值达到2000亿元的宏伟目标。这种情况下，铸造技术便成了汽车排气歧管量产的主要制造手段，所选择的材料主要是铸铁或合金。而近些年来，一汽集团的汽车排气管材料一直采用德国汽车标准进行制作，考虑到自主品牌的必要性和企业经营成本的降低，提出汽车排气歧管材料国产化的目标。为实现这一目标，吉林大学与一汽铸造公司合作开发新型的高硅钼球墨铸铁和高镍球墨铸铁，以满足近期一汽集团新型汽车的使用要求。本文以所开发出的高硅钼球墨铸铁和高镍球墨铸铁为基础，系统研究了排气歧管材料所需抗氧化性、抗热疲劳性、力学性能、抗生长性以及抗电化学腐蚀性。探索其在实际生产以及预测分析其工作温度下的服役行为和失效预期，从而能更好的进行生产和应用。希望通过具体研究为一汽集团开发符合性能要求的新型汽车排气歧管新材料提供可靠的理论依据和实验参考。本文的主要研究工作和取得研究成果概述如下：1、所研究的高硅钼球墨铸铁和高镍球墨铸铁参照德国标准GGG-SiMo51和GGG-NiSiCr3552进行材料设计和熔炼，熔炼过程中采用75SiFe进行孕育处理，利用砂型铸造成“Y”形试样。通过采用OLYMPUS-PMG3光学显微镜、OLYMPUS LEXTOLS3000激光显微镜、D/max2500pc型超大功率X射线衍射仪、JSM-5310型扫描电子显微镜进行观察分析。在高硅钼球墨铸铁中，其组织主要由铁素体和球状石墨构成，大量硅融入到铁素体中，在晶界处偶尔存在一些呈骨骼状的钼的碳化物；在高镍球墨铸铁中，其组织主要由奥氏体、金属间化合物FeNi3和富锰、铬的碳化物构成，同时，Ni容易在晶界处偏析，形成Fe、Ni、Si等元素的化合物。由于碳当量的差异，相同的熔炼工艺下，高硅钼球墨铸铁的球化率高于高镍球墨铸铁，而高镍球墨铸铁的球状石墨的直径小于高硅钼球墨铸铁的。高镍球墨铸铁因饱和度大于4.4，熔炼时存在球化率衰退现象，而高硅钼球墨铸铁的饱和度小于4.4而不存在球化率衰退现象。2、在热疲劳测试过程中，通过金相检测合格的试样通过箱式电阻炉对其进行加热，加热温度范围从800℃到950℃，每50℃为一梯度进行温度选取，也就是上限温度（Tmax）。为保证试样的温度均匀，加热试样在实验温度下保温10分钟，利用自来水进行淬火，水温作为测试的下限温度（Tmin）。待试样冷却到内外温度一致时方可继续试验。热循环温度区间（ΔT）对热疲劳的影响实质就是上限温度的影响。记录各实验温度下的裂纹形成与扩展同热循环次数的关系及此过程中试样形状变化情况。在本项研究中定义尺寸超过0.5毫米的裂纹为主裂纹。热疲劳裂纹的长度随冷热循环次数的增加而变长。冷热循环温度差值越高，产生热疲劳裂纹所需的次数越少。主裂纹的形成依赖于显微裂纹，裂纹一般形成于晶界处或碳化物附近；裂纹的扩展受碳化物的粗化作用和氧化作用的影响较大。热疲劳裂纹不是一开始就出现的，而是要经过一定的次数积累。相同测试情况下，高硅钼球墨铸铁热疲劳裂纹出现时的热循环次数远小于高镍球墨铸铁的；在测试中发现，显微裂纹的数目经历由多到少的变化，符合裂纹屏蔽效应的特征。高硅钼球墨铸铁和高镍球墨铸铁在测试过程中都产生热变形，但同种测试条件下，高硅钼球墨铸铁的变形远大于高镍球墨铸铁。热疲劳寿命受材料性能、冷却速度、工作的温度梯度等因素影响。根据研究结果表明高镍球墨铸铁的热疲劳性远优于高硅钼球墨铸铁。所生产的高硅钼球墨铸铁和高镍球墨铸铁理想的最大工作温度分别为850℃和950℃。3、通过对一定时期的高温加热后测量氧化膜厚度和重量变化来评价高硅钼球墨铸铁和高镍球墨铸铁的抗氧化性。抗氧化性过程中，试样分别在箱式电阻炉中加热到800℃至1000℃。无论是高硅钼球墨铸铁还是高镍球墨铸铁的氧化皮厚度都随加热时间的延长而增厚，一定程度上其变化特征基本呈现抛物线规律。通过利用扫描电镜对测试试样的观察认为，前期因表层石墨的流失而使重量减少，后期因氧元素的侵入而使试样的重量增加。氧化膜的产生在一定程度上减缓了氧化速度，所以氧化初期的氧化速度小。通过研究发现高镍球墨铸铁的抗氧化性明显好于高硅钼球墨铸铁。4、在电化学综合测试系统环境下对高硅钼球墨铸铁和高镍球墨铸铁的电化学腐蚀行为进行简单的测试分析，通过测试与计算，高硅钼球墨铸铁和高镍球墨铸铁的腐蚀电位为-1.002V和-0.523V，腐蚀电流分别为2.73×10^-4A/cm²和0.36×10^-4A/cm²，并且，高硅钼球墨铸铁在阳极极化过程中存在明显的钝化区域。5、为使汽车排气歧管在工作过程中尺寸保持稳定，从而使其与发动机保持良好的密封状况，对高硅钼球墨铸铁和高镍球墨铸铁进行抗生长测试。该项测试通过热膨胀仪完成。结果表明，加热后的尺寸恢复到初始状态的能力随工作温度的升高而逐渐变差，但高硅钼球墨铸铁的尺寸增幅明显大于高镍球墨铸铁的增幅。通过对抗热疲劳性、高温抗氧化性、耐腐蚀性和抗生长性测试，并对测试结果分析比较，在所开发的汽车排气歧管材料中，高镍球墨铸铁优于高硅钼球墨铸铁。更多还原

【Abstract】 Exhaust manifold is connected directly to the engine in the exhaust system. The hightemperature flue gases which are given off in the engine are fed and accomplished throughexhaust manifold. In the course of driving cars, the exhaust manifold has been working athigh temperature. And external automobile exhaust manifold directly contact with the naturalenvironment. At last, exhaust manifold internal temperature is up to several hundred degrees,especially, beyond one thousand degrees, but external temperature of the exhaust manifold isbetween100℃and200℃. So that, the exhaust manifold worked in the large temperaturedifference between inside and outside. The exhaust manifold usually works on alternativethermal cycle of quick-cooling and quick-heating during start-up or stop. Considering thecomplex working conditions, the exhaust manifold material requirement is very high. Theexhaust manifold shape depends on the engine’s structure. With the rapid development ofautomobile industry, the structure of the engine becomes more and more complex. In result,the structure of exhaust manifold also changes. Manufacturing the exhaust manifoldbecomes more and more difficult. With attention focused on global environmental problems,reduced pollutant emissions and increased engine efficiency will increase the exhaust gastemperature. In developed countries of Europe and America, the exhaust manifold materialschanged from gray iron to high strength gray iron, to ductile cast iron, to compacted graphiteiron, to Stainless Steel, to high Si-Mo ductile cast iron, to ductile Ni-resist cast iron, to highalloy stainless steel. In contrast, the automobile industry development of our time is short.The exhaust manifold materials changed from gray iron to high strength gray iron, to ductilecast iron, to compacted graphite iron, to alloy cast iron, to Stainless Steel. China’s autoindustry is still in a stage of stable development. In the12th Five Year Plan Period, Jilinprovince will take FAW as the center, and construct the system of vehicle R&D andmanufacturing and supporting parts and service. Especially, the project of three million carsin Changchun and one million cars in Jilin will help the rapid development of the automobileindustry of Jilin Province. According to the Jilin provincial government planning, to2015,the ambitious goal of four milion cars and2000million Yuan of automotive industrial addedvalue will be achieved. Considering the complex structure of automobile exhaust manifold,exhaust manifold selected materials are mainly iron because their main manufacturing arecast. For a long time, the automobile exhaust manifold material has been using the Germanautomotive standards. Sothat, automobile manufacturing cost is high in First Automotive Works （FAW）. Considerating of China’s reality, the target of exhaust manifold materialslocalization is set up by FAW. In order to achieve this goal, the Jilin University and R&DCenter of FAW Foundry Co., Ltd research new high Si-Mo ductile cast iron （DCI） andductile Ni-resist cast iron （DNCI） to meet the need of new car. The results of the paper arebased on the newly developed high Si-Mo DCI and DNCI. Oxidation resistance and thermalfatigue resistance and mechanical properties and corrosion resistance which the exhaustmanifold materials required were systematically researched. The actual production situationwas simulated. And Service behavior and failure time were presented and analyzed in orderto produce and applicate better exhaust manifold materials. To hope to reaearch the project,the reliable experimental references will be provided to develop new performancerequirements material for exhaust manifold of FAW automobile. The produced resultsprovide the therretical basis for the naxt work.The main conclusions and the results obtained are listed as following:1. To refer with GGG-SiMo51and GGG-NiSiCr3552, high Si-Mo ductile cast iron andductile Ni-resist cast iron which are researched in the study are designed and melted. Duringsmelting operation,75SiFe was used in spheroidizing operation. Finally, the melts werepoured into Y-shaped sand molds. Analysis and measuring instruments of OLYMPUS-PMG3light optical microscopy （LOM） and OLYPUS LEXT OLS3000laser microscope andJSM-5310scanning electron microscopy （SEM） and D/max2500pc X-ray diffractometerand energy dispersive spectroscopy （EDS） were used to observe and analys test samples. Inhigh Si-Mo ductile cast iron, its organization is made of ferrite and spheroidal graphite. A lotof Si was integrated into the ferrite. There are some bone shaped carbides on grain boundary.In ductile Ni-resist cast iron, its organization is mainly made of austenite and intermetalliccompound FeNi3and carbide of Mn and Cr. At this time, Ni can produce segregation ongrain boundary. And compound of Fe and Ni and Si can be produced. Under the samemelting process, nodularity of high Si-Mo DCI is higher than that of DNCI. And sphericalgraphite diameter of DNCI is less than that of high Si-Mo DCI.2. During the thermal fatigue test, after metallographic analysis, the acceptable sampleswere put into a high temperature electric resistance furnaces and heated temperatures weredecided from700℃to1000℃on base of material, respectively. After holding time forabout10min, the heated samples were quenched with tap-water. The samples were put intothe furnace again after cool-down terminate. The cycle was done again. A low temperature（Tmin） is the tap-water temperature. The water temperature is always kept constant. It is about 20℃. So, the effect of test temperature gradient （ΔT） on fatigue is essentially superior limittemperature（Tmax）. ΔT is equal that Tmax subtract Tmin. In fact, changing test temperaturegradient is changing the maximum temperature. Number of the thermal cycles was recorded.In this study, it was defined that a crack whichever is longer than0.5mm as one crack, andthat the total length of the longest crack as the length of crack. During thermal cycles, moredistortions and more cracks and oxidations were created. With the maximum cycletemperature and the cycle number increased, the distortions would be bigger and bigger, andthe main crack length became longer and longer, and oxidation degree became more andmore massive. At the same messuring conditions, the distortions of ductile Ni-resist cast ironare less than that of high Si-Mo DCI. And with cycle number increased, and manymicro-cracks expanded into the main crack. The cracks were always found at the biggersurface, and appeared cross shaped budding in the end. Micro-cracks could not occurrenceimmediately when the thermal cycle initiated. Initiations of micro-cracks need some energyaccumulations. At the same conditions, the cycle number of ductile Ni-resist cast iron ishigher than that of high Si-Mo DCI. And the thermal cracks were zig-zag-shaped across thesample’s surface. During thermal cycles, the number of micro-crack varied from multi tolittle. The thermal fatigue life is governed by many material properties and cooling velocityand the working temperature gradient. Based on these results, it is thought that thermalfatigue resistance of DNCI is better than that of high Si-Mo DCI. The maximum reasonableworking temperature of high Si-Mo DCI and DNCI is840℃and950℃, respectively.3. To evaluate oxidation resistance properties during high temperature service, somesheet specimens of high Si-Mo DCI and DNCI were aged at between800°C and1000°C forthe maximum of80h in a high temperature electric resistance furnace. The oxide thicknessof both high Si-Mo DCI and DNCI vary larger and larger with the development of time. Thethickness of oxide layer of high Si-Mo DCI is much thicker than that of DNCI during thesame heating time. The oxide consists of Fe2O3at the gas/oxide interface, then Fe3O4andfinally FeO. The porous iron oxide allows fast diffusion of oxygen and thus internaloxidation of the alloy. Protective silica may form or transform into Fe2SiO4in presence ofFeO or Fe3O4. In the cases, protective nickelous oxides can protect DNCI from the oxidedeeper into the material. The relations of between the oxide added weight and heat time atdifferent temperature are obstained. The oxidation process of the two materials was up toparabolic curve law. It has been found that DNCI has an excellent property of the oxidationresistance. 4. To evaluate the corrosion resistance of high Si-Mo DCI and DNCI, the electro-chemical test is operated. By testing and analyzing, the corrosion potential of high Si-MoDCI and DNCI is-1.002V and-0.523V. And their corrosion current is2.73×10^-4A/cm²and0.36×10^-4A/cm². There is a transpassive region during anodic polarization.5. In order to keep exhaust manifold size from varying in operation, because it is relateto the seal condition of engine, it is important to measure growth resistance. To use thermalexpansion instrument, it is found that the size of both high Si-Mo DCI and DNCI can changeafter working at high temperature. The size growth of DNCI is less than that of high Si-MoDCI.更多还原