【作者】 岳增武；

【导师】 田学雷；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 影响传统火力发电厂效率的主要因素是蒸气的温度和压力,自从二十世纪70年代的能源危机以来,全世界都在设法提高发电机组的温度和压力。近年来减少CO2排放的要求更加鼓励人们提高发电厂的效率。目前,大型发电机组的蒸气温度已经达到600℃左右。提高蒸气温度和压力的前提是研制出能承受更高温度和压力的耐热合金,而锅炉传热管在高温蒸气中的氧化性能则是耐热合金在高温使用的一个瓶颈。锅炉传热管内的介质为高温蒸气,管子的内壁不可避免地与蒸气发生氧化反应。当温度提高时,在一定的时间内生成的氧化膜更厚,增厚的氧化膜带来三个主要问题：(1)管子金属的厚度因氧化腐蚀迅速减薄,引起应力增大并导致蠕变破裂；(2)氧化膜的导热性比金属差,使蒸气对管子的冷却作用变差,引起管子金属的温度升高,反过来又会加速管子的氧化和蠕变破坏；(3)当氧化膜生长至一定厚度时,在机组停运时会发生剥落,剥落的部分氧化膜会聚集在锅炉管的下部,在机组重新启动后,因为剥落的氧化膜造成管子堵塞或使管子内介质流量减少,引起过热爆管事故。因氧化膜剥落、堵塞造成的过热爆管是引起锅炉管失效的第二大主要原因。本文研究的目的是通过沉积稀土氧化物涂层和喷丸处理工艺,降低Fe-Cr耐热钢在水蒸气中氧化膜的生长速率,防止或减少氧化膜的剥落,减少或避免由此引起的过热爆管事故。本文将电泳沉积稀土氧化物涂层(CeO2)应用于纯铁及T9、TP304H和HR3C等Fe-Cr系耐热钢,将喷丸处理工艺应用于TP304H和HR3C,在高温水蒸气中进行等温氧化试验,进行了氧化动力学测试,使用XRD, SEM和EDS等手段对氧化膜进行了物相结构分析和形貌、成分分析。试验结果表明,稀土氧化物(CeO2)涂层在一定的试验条件下可提高各种材料在水蒸气中的抗氧化性能,但对铬含量较高的HR3C效果最为显著；对TP304H和HR3C钢,沉积复合氧化物(CeO2+Cr2O3)涂层提高抗氧化性能的效果优于单纯Ce02涂层。喷丸处理可显著提高TP304H和HR3C的抗氧化性能,在650℃水蒸气中的氧化试验表明,喷丸处理的TP304H钢抗氧化性能优于HR3C,甚至优于经过喷丸处理的HR3C,180h氧化后,喷丸处理的TP304H试样的单位面积氧化增重是未处理试样的单位面积氧化增重的3.7%。在喷丸的基础上沉积稀土氧化物涂层能进一步提高TP304H和HR3C钢的抗氧化性能。使用自制的小径管内壁喷丸设备,对TP304H再热器管进行了内壁喷丸处理,在620-630℃服役7474h后,喷丸处理的管子内壁氧化膜的厚度只有未喷丸管氧化膜厚度的3%,且无剥落现象,喷丸处理提高抗氧化性能的效果显著。沉积稀土氧化物CeO2涂层对纯铁、T91钢和TP304H钢的初始氧化阶段产生明显影响,它在金属表面造成低的氧分压,促进形成富铬的氧化物、抑制铁氧化物的生成；而对稳态氧化阶段的阴阳离子扩散无明显影响,因而对氧化速度无显著影响。富铈带位于氧化膜的内部。对T91氧化行为的研究使用了金标记法,以确定扩散传质方向。对T91氧化膜的分析表明,沉积CeO2涂层并氧化后,富铈带位于外层柱状晶和内层等轴晶之间,在富铈带内侧产生新的氧化物形核点,生成等轴状的Fe3O4层,富铈带之外的柱状晶的氧化物颗粒被细化,；对TP304H氧化膜的分析表明,由于沉积CeO2涂层,氧化膜的粘附性提高,富铈带位于内、外氧化层之间,富铈带中含铈的物相为CeO2和FeCeO3。沉积CeO2涂层对提高HR3C的抗氧化性能效果显著。氧化膜内的扩散传质由铁离子向外扩散形成外层氧化物和氧离子向内扩散形成内层氧化物,转变为以氧离子向内扩散为主。喷丸造成的表面冷加工高能量区域及滑移带、位错和细小晶粒,提供了更多铬的快速扩散通道,提高了铬的扩散通量,促进形成均匀致密的富铬氧化膜,Fe3O4外层消失。由于TP304H属于亚稳态奥氏体钢,喷丸在TP304H表面诱发了马氏体相变。对喷丸的TP304H试样进行了高温X射线衍射,结果表明马氏体在氧化试验温度仍存在。喷丸产生的马氏体可能是TP304H钢能获得比HR3C更好的抗氧化性能的主要原因。更多还原

【Abstract】 The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Researches have been pursuing to increase both the temperature and pressure worldwide since the energy crisis in the 1970s. The need to reduce CO2 emission recently also becomes an incentive to improve the efficiency. Steam temperatures of the fossil power units with higher efficiency are now around 600℃. These lead to the consideration of materials capable of operating under higher stresses at ever increasing temperatures. One of the key factors to limite the materials using is their susceptibility to steam oxidation.It is inevitable that the metal is oxidized when it contacts with high temperature water vapor or steam. The higher the temperatue, the more quickly the scale grows. It results in three potential problems. Firstly, the thickness of the tube wall is decreased quickly and then the stress is increased and creep ruptures will happen. Secondly, the increased insulation of the tube material from the cooling fluid by the low thermal conductivity of the oxide scale leads to an increase in metal temperature, thus, oxidation and creep may be accelerated. The third concern is that the thicker oxide scale may spall more easily when the boiler is cooled down. On restart, the spalled oxide scale may lodge somewhere in the system with the potential for causing tube blockages. It is notable that long-term overheating failures due to flow restriction from exfoliated oxide scale is the second most important cause of boiler tube failures, thus increased attention must be paid to the oxide scale growth and exfoliation.Electrophoresis deposition of CeO2 coating and shot peening treatment were used for Fe-Cr alloy in this thesis. The purpose was to reduce the oxidation rate and to avoid or soothe the overheating failures due to oxide exfoliation.Electrophoresis deposited CeO2 coating was used for pure iron and Fe-Cr alloys such as T91, TP304H and HR3C. Shot peening treatment was used for TP304H and HR3C. The oxidation kinetics was studied under isothermal condition in water vapor. The oxide scales were then studied with XRD, SEM and EDS.The results show that the oxidation resistance of pure iron, T91, TP304H and HR3C was improved by CeO2 coating. The effect of CeO2 coating on the oxidation resistance of HR3C is the most remarkable. The oxidation resistance of TP304H with composite coating (CeO2+Cr2O3) is better than that with CeO2 coating only.Shot peening can significantly improve the oxidation resistance of TP304H and HR3C. The oxidation resistance of TP304H with shot peening is better than HR3C, and it is even better than HR3C with shot peening. The mass gain of shot peened TP304H is only 3.7% that of un-peened TP304H. The combined treatment of shot peening and CeO2 coating is superior to shot peening only.The inner surface of TP304H reheater tubes were shot peened with self-designed shot peening apparatus for small diameter tubes. The oxide scale thickness at the inner surface of shot peened reheater tube was only 3% of the untreated one after service for 7474h at 620-630℃, and no exfoliation occurred on tubes with shot peening. The effect of shot peening on the oxidation resistance was remarkable.CeO2 coating obviousely affects the oxidation behavior of pure iron, T91 and TP304H at the initial transient oxidation stage. The coating can introduce lower oxygen partial pressure, promote the formation of Cr rich oxides, and inhibit iron oxides; The Ce rich oxide band is located inside the scale after oxidation, it has no obvious effect on the diffusion process of anion and cations, so its effect on oxidation rate is unsignificant at the steady oxidation stage. Gold marker was used to define the mass transport direction in the study of T91. Analysis of the scale on T91 steel shows that Ce rich oxide band is located at the interface of the inner equiaxed layer and the outer columnar layer after oxidation, and this Ce rich band is not consistent with the original surface. A new oxide nucleating and growing site (reaction front) was induced at the inner surface of the Ce rich band. Analysis of the scale on TP304H steel shows that the adherence of the scale is improved by CeO2 coating. Ce rich oxide band is located between the inner and outer layers. CeFeO3 and CeO2 are characterized in the Ce rich band.The electrodeposited CeO2 coating can improve the oxidation resistance of HR3C greatly in water vapor. The diffusion process was changed from inward oxygen diffusion and outward iron diffusion to predominantly inward oxygen diffusion. Shot peening introduces cold worked, high energy region at the surface, Cr diffusion is accelerated along the slip bands, dislocations and copious amounts of grain boundaries at elevated temperatures, and Cr rich oxides form at the early stage of oxidation, which acts as a strong barrier against further oxidation. The formation of Fe3O4 layer is inhibited. TP304H is a kind of metastable austenitic stainless steel. Shot peening can induce martensite at the surface. The shot peening induced martensite still exists at the testing temperature. The existence of shot peening induced martensite at the surface may play an important role in Cr diffusion and affect the oxidation behavior of TP304H steel significantly.更多还原