【作者】 高秋志；

【导师】 刘永长；

【作者基本信息】 天津大学 ， 材料学， 2012， 博士

【摘要】 高Cr铁素体耐热钢广泛用于超（超）临界火力发电。为解决日益突出的能源和环境问题，开展旨在提高高Cr铁素体耐热钢耐热温度的研究势在必行。在此背景下，本文就自行开发的新型高Cr铁素体耐热钢在奥氏体化和淬火后的相变行为及其焊接性进行了系统的研究，结合显微组织分析研究了该钢种在650℃高温时效过程中的组织演化规律。在大量试验研究和理论分析的基础上得到了如下结论：（1）相变行为研究：第一，临界区奥氏体化后室温组织为奥氏体+铁素体的双相组织；完全奥氏体化时，M23C6沉淀相约在950～1000℃析出溶解，-铁素体在1050℃后开始逐渐形成。模型拟合确定的奥氏体晶粒长大激活能为58.7kJ/mol，马氏体相变的生长激活能和界面迁移速率均随奥氏体化温度和程度的提高而减小。第二，新型高Cr铁素体耐热钢淬火时只发生马氏体相变，室温组织中含有-铁素体和少量的残余奥氏体。随淬火温度降低，马氏体相变起止温度均升高，沉淀相的体积分数和颗粒尺寸均增大，残余奥氏体在700℃淬火时完全消失。实验结果与扩展相变动力学模型拟合结果吻合良好，马氏体板条横纵比随淬火温度降低而升高，形核率降低。（2）焊接性研究：第一，在达到焊接热循环峰值温度时，组织中只存在-铁素体相，且因不能完全转化为奥氏体而部分残留下来，并发现-铁素体内析出富硼M23C6沉淀相。基于晶界位置饱和形核理论和氮扩散控制相变进行了动力学建模，模型模拟结果与实验结果吻合较好，表明在焊接过程中氮可能是控制高温相变的主要合金元素。第二，对焊接接头的研究表明，焊缝组织发生回复再结晶，且受M23C6相的析出位置影响，超过70%的M23C6相在晶界析出，晶内和晶界的M23C6相颗粒尺寸都较大，通过理论计算证实MX沉淀相析出量很少。焊后热处理过程中，保温时间对马氏体板条尺寸的影响更显著。（3）等温时效研究：随着时效时间的延长，新型高Cr铁素体耐热组织中马氏体板条特征逐渐消失，形成等轴晶粒，晶界析出相形状和组分逐渐变化，最终形成Laves相。成分偏聚和粗大沉淀相颗粒周围位错网胞的形成促进焊缝组织发生回复，并形成富Mo的Laves相；此外，母材组织中的Laves相优先在M23C6相周边区域形核并长大。更多还原

【Abstract】 High Cr ferritic heat-resistant steels are widely used in the keycomponents of the super/ultra-supercritical advanced power plants. Withthe increasingly prominent energy and environmental issues, the study onelevating the heat-resistant temperature of high Cr ferriticheat-resistant steel becomes imperative. In this paper, for the modifiedhigh Cr ferritic heat-resistant steel developed by our group, its phasetransformation behaviors during quenching and austenization process, theweldability were systematically analyzed, respectively. Furthermore, themicrostructure evolutions and precipitation processes during isothermalaging were also studied. The following conclusions could be drawn basedon the experimental observation and theoretical analysis.(1) Phase transformation: firstly, the microstructure ofaustenitization at the intercritical temperature was composed ofaustenite and ferrite dual-phase structures. At full austenizationtemperature, it was found that the dissolution of M23C6precipitates waswithin the range of950～1000℃. The delta ferrite gradually starts toform after1050℃. The experimental data were fitted by the phasetransformation model. The results indicate that: The activation energyfor growth of austenite grain is58.7kJ/mol; both the activation energyof growth and the interface velocity of martensite plates decreases withthe increase of austenization temperature. Secondly, only the martensitictransformation was found during quenching at various temperatures; thedelta ferrite and a small amount of residual austenite is also found inthe final microstructure. With decreasing quenching temperature, it isfound that both the starting and finishing temperatures of martensitictransformation increase, as the same as the size and amount of theprecipitates particles, whereas the delta ferrite content decreased. Whenquenching at700℃, retained austenite between martensite laths disappearcompletely. The results of calculation based on spread model indicatethat the aspect ratio of martensitic lath increases with the decrease ofquenching temperature, and the nucleation rate decreases.(2) Welding: first, during welding heat cycles, only the delta ferrite phase exists at the peak temperature,1320℃, which can not completelytransform to austenite. Amount of the residual delta ferrite is left atroom temperature. It is also found that the Boron-rich M23C6phaseprecipitates in the delta ferrite. Based on the grain boundaries sitesaturated nucleation theory, assuming nitrogen diffusion-controlledaustenite transformation, the results of model demonstrates that nitrogenis the main alloy elements to control the phase transformation duringwelding. Second, based on analyzed the welded joints, it is found thatthe location of M23C6precipitated affectes the recovery andrecrystallization of microstructures. More than70%M23C6phase isprecipitated at the grain boundaries of austenite and the lath boundaries.Annealing time is the significant factor for the size of martensite lathduring post weld heat treatment.(3) During isothermal aging, with time prolonging, recovery occursin the martensite lath, which leads to the decrease of dislocationdensities, the formation of equiaxed grains, and the change ofprecipitates shape at grain boundaries. The alloying elements around M23C6due to diffusion and the formation of dislocation network cells promotethe formation of Laves precipitates. The Mo-rich Laves phase forms in thewelded joints. For the high alloying elements contents and kinds of themodified high Cr ferritic heat resistant steel, it is easier to form Lavesphase which is prior located around M23C6precipitates and rapidly growsup.更多还原