【作者】 李慧；

【导师】 卫英慧；

【作者基本信息】 太原理工大学 ， 材料科学与工程， 2020， 硕士

【摘要】 奥氏体耐热钢由于其优异的耐高温性能广泛应用于超超临界火电锅炉的再热器、过热器等部位。随着火电锅炉机组参数的不断提高,锅炉机组对关键部位用钢要求也日益严苛。山西某钢铁企业研发出一种可以作为超超临界机组650℃-700℃锅炉再热器和过热器的奥氏体耐热钢候选材料C-HRA-5钢,该钢目前处于试制阶段,尚未实际应用。一般来说,服役态耐热钢的析出行为与性能变化是通过人工加速时效的方法来研究。本文通过对C-HRA-5耐热钢在1100℃、1150℃、1200℃、1250℃、1280℃不同温度进行固溶处理寻找最佳的固溶处理温度,然后在650℃、700℃和750℃下分别时效不同时间,通过金相显微镜（OM）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、X射线衍射仪（XRD）、显微维氏硬度计等对C-HRA-5钢在不同时效热处理方式下的析出规律进行分析,主要得到以下结论:本研究中,奥氏体耐热钢C-HRA-5的最佳固溶处理工艺为1200℃+1 h,获得奥氏体组织,TEM与EDS分析结果表明基体存在着没有完全溶解的Z相,可能是由于水冷速度较慢导致。C-HRA-5钢在短期时效后,析出相主要有M₂₃C₆相、MX相和Z相,时效时间延长至500 h后,还会析出σ相和Laves相。M₂₃C₆相主要分布在晶界,呈块状或不规则三角状;Z相在晶界和晶内均有分布,主要呈不规则长方块状或正方形块状;MX相在晶界和晶内都有分布,在晶界呈球形,在晶内呈方形块状;σ相以M₂₃C₆或MX相为成核位点呈长棒状分布在晶内;Laves相在晶内呈长的竹叶状分布。时效初期的亚稳析出相Cr₃C₂和Cr₇C₃随着时效过程的推移逐渐过渡为稳定的Cr₂₃C₆析出相,同时析出相数量明显增多,出现球化现象。C-HRA-5钢在650℃、700℃、750℃时效后的硬度值均为先上升后下降,且都在300 h后达到峰值。在时效初期,析出相尺寸小、易被位错切过,因此其强化机制主要是位错切过机制（Kelly机制）;随着时效的进行,析出相尺寸逐渐增大,位错难以顺利切过析出相,因此硬度值逐渐升高;随着时效过程继续,覆盖在晶界上析出相呈连续分布,晶内析出相颗粒也明显增多,此时,晶界、孪晶界以及亚晶界的强化效应以及晶内析出弥散强化效应相结合,导致钢的硬度达到峰值。随着时效时间的继续延长,析出相不断聚集粗化导致强化效应减弱,因此,钢的硬度逐渐下降。通过C-HRA-5耐热钢高温时效硬度值,建立了硬度值与Larson-Miller P函数的线性关系模型,即HV=869.48476-0.02757P,为超超临界机组的耐热钢运行状态评估具有重要意义。更多还原

【Abstract】 Austenitic heat-resistant steel is widely used in ultra-supercritical thermal power boiler reheaters,superheaters and other parts due to its excellent high temperature resistance.With the continuous improvement of the parameters of thermal power boiler units,the requirements of steel for key parts of boiler units are becoming increasingly strict.An iron and steel company in Shanxi has developed an austenitic heat-resistant steel candidate C-HRA-5 steel that can be used as a superheater 650℃-700℃boiler reheater and superheater.The steel is currently in the trial production stage and has not yet been practically applied.In general,the precipitation behavior research and performance change law of the service heat-resistant steel are studied by artificial accelerated aging method.In this paper,the best solution treatment methods are found by performing different solution heat treatments on C-HRA-5 heat resistant steel at 1100℃,1150℃,1200℃,1250℃,and 1280℃,and then the solution samples are aged at 650℃,700℃,and750℃for different times.The precipitation rule of C-HRA-5 steel under different aging heat treatment methods was analyzed through the analysis of met allographic microscope,scanning electron microscope,transmission electron microscope,X-ray diffractometer,micro Vickers hardness tester,etc.And the following conclusions were mainly obtained:In this study,the best solution heat treatment method for austenitic heat-resistant steel C-HRA-5 steel is 1200℃for 1 hour,and the solution structure is a single austenite structure.Transmission electron microscopy and EDS analysis showed that there was a small Z phase that was not completely dissolved in the matrix after solid solution,possibly due to the slower water cooling rate.The precipitation phases of C-HRA-5 steel after short-term aging mainly include M₂₃C₆ phase,MX phase and Z phase.After the aging time is extended to 500 h and 1000 h,there areσphase and Laves phase.The M₂₃C₆phase is mainly distributed in the grain boundary,which is a rectangular block or an irregular triangle;the Z phase is distributed in the grain boundary and the crystal,mainly in the shape of irregular long squares or small square blocks;the MX phase is distributed in the grain boundary and in the crystal,and it is spherical in the grain boundary and a small cube in the crystal;theσphase takes M₂₃C₆or MX phase as the nucleation site and it is distributed in a long rod shape within the crystal;the Laves phase appears as long bamboo leaves in the crystal.As the aging process progresses,the metastable precipitated phases Cr₃C₂and Cr₇C₃gradually transition to stable Cr₂₃C₆ precipitated phases.At the same time,the number of precipitated phases increased significantly,and nodularity occurred.The overall hardness of C-HRA-5 steel after aging at 650℃,700℃,and 750℃tends to increase first and then decrease.During aging at different temperatures,the hardness reaches its peak after 300 h.In the early stage of aging,the precipitate phase is small in size and easily cut by dislocations,so its strengthening mechanism is mainly the dislocation cutting mechanism（Kelly mechanism）;as the aging progresses,the size of the precipitate phase gradually increases,dislocation is difficult to cut through the precipitation phase,the hardness value gradually increases;after the aging time continues to increase,the precipitation phase on the austenite grain boundary is continuously distributed,and the intragranular precipitation phase particles also increase significantly.The combination of the strengthening effect of the boundary and the dispersion strengthening effect of intragranular precipitation results in the hardness of the steel reaching a peak.As the aging time continues to prolong,the precipitation phases continue to aggregate and grow,which weakens the strengthening effect.Therefore,the hardness of the steel gradually decreases.Based on the high temperature aging hardness value of C-HRA-5heat-resistant steel,a linear relationship model between hardness value and P function is established.That is,HV=869.48476-0.02757P,which is of great significance for the evaluation of the operating status of heat-resistant steel for ultra-supercritical units.更多还原