USC电站锅炉用T92钢高温下微观组织演化及寿命预测

发布时间：2018-08-14 09:05

【摘要】：T92钢(NF616)是上世纪90年代日本新日铁公司在T/P91钢的基础上,将材料进行了进一步的合金化,增加W含量到1.8%,减少Mo含量到0.5%,并增加了适量的B元素。与T/P91钢相比,T92钢的持久强度有较大幅度的提高,可用于625℃以下的高温蒸汽管道。目前,T92钢凭借其优异的综合性能已成为新一代超(超)临界火力机组的理想用钢。然而,国产T92钢投入使用的时间较短导致T92钢长期运行后的的数据资料匮乏,同时国内外超(超)临界机组的运行调峰方式的差异,造成材料在长期高温服役过程中微观组织结构与性能退化不一样,对机理的研究也不够深入,尤其是目前还没有有效的T92钢蠕变寿命预测方法,更谈不上剩余寿命的预测及寿命预测的验证。基于此,本文以某国产T92钢的模拟加速老化样品为研究对象,研究材料在蠕变过程中的组织结构演变与力学性能变化的规律,并基于Larson-Miller参数法的蠕变寿命预测方程对其寿命进行了预测。本文运用扫描电镜、透射电镜和纳米压痕仪等现代材料分析方法对模拟不同工况参数T92钢的显微组织结构、断裂机理及力学性能进行了分析。研究表明,供货态T92钢的组织为典型的板条状回火马氏体+碳化物,在长期的高温及应力的作用下,材料的微观组织无法保持其原始组织的稳定性,逐渐开始退化。板条马氏体发生回复,材料中呈弥散分布的原始第二相向原奥氏体晶界、亚晶界及板条马氏体边界偏聚,并发生聚集和粗化,并伴随着新相Laves相的析出。T92钢中的M23C6碳化物颗粒呈棒状和球状两种形态,在蠕变过程中,具有球状形态的颗粒更为稳定,Laves相则一般依附在大尺寸的M23C6颗粒上析出,析出位置主要在原奥氏体晶界、亚晶界等界面上。与M23C6相相比,Laves相的粗化速度较快,当Laves相长大到一定的尺寸时,将会诱发材料中蠕变孔洞的产生,孔洞的聚集和联结造成材料的断裂及过早失效。对比蠕变样品中受应力区域与不受应力区域的析出相平均尺寸发现,应力明显加速Laves相的形核和粗化,而M23C6的平均尺寸则略有增加。此外,在整个蠕变过程中,基体的主要合金元素(Cr、W、Mo等)的百分含量均逐渐减少,合金元素逐渐由固溶态向化合态进行转移,这些微观组织结构的软化共同造成材料宏观显微硬度的下降。其次,本文结合微纳米压痕硬度测试技术获取了700℃下不同时间断裂后的T92钢蠕变样品中板条马氏体基体的硬度,排除了高角边界对结果的影响,再结合微观组织分析方法,探讨了不同模拟工况条件下马氏体基体的退化机制。结果表明T92钢在长时高温及应力的作用下,板条马氏体中基体的主要强化因素都将发生不同程度的削弱,蠕变过程中Laves相和M23C6相等第二相的析出和粗化造成第二相间距增大;板条马氏体发生回复,晶宽增加;基体中重要的固溶强化元素W、Mo等不断流失,这些因素的综合作用,导致了T92钢在长时蠕变过程中的基体性能的下降,可以通过检查马氏体基体的硬度来判断,结果表明基体硬度随蠕变时间的增加而不断下降。最后,本文对600℃,649℃和700℃下的T92钢的加速老化数据进行拟合,建立了T92钢高温过程中的对数曲线,提出了一种基于Larson-Miller参数法的寿命预测方法,通过该方法尝试了T92钢外推10万小时的持久强度,与ECCC的外推结果相当。
[Abstract]:T92 steel (NF616) was further alloyed on the basis of T/P91 steel by Nippon Steel Co. in the 1990s. The content of W was increased to 1.8%, the content of Mo was reduced to 0.5%, and the content of B was increased. Compared with T/P91 steel, the durable strength of T92 steel was greatly improved, which can be used for high temperature steam pipes under 625 C. At present, T92 steel has become an ideal steel for the new generation of ultra-supercritical (ultra-supercritical) thermal power units because of its excellent comprehensive properties. However, the short time of using domestic T92 steel leads to the lack of data after long-term operation of T92 steel. Meanwhile, the differences of peak-shaving modes in operation of ultra-supercritical (ultra-supercritical) thermal power units at home and abroad result in the long-term high temperature wear of materials. The microstructure and properties of T92 steel are different during service, and the research on the mechanism is not enough. Especially, there is no effective creep life prediction method for T92 steel, let alone the validation of residual life prediction and life prediction. The evolution of microstructure and the change of mechanical properties during creep were studied. The creep life prediction equation based on Larson-Miller parameter method was used to predict the creep life of T92 steel. The cracking mechanism and mechanical properties of T92 steel are analyzed. The results show that the microstructure of T92 steel is a typical lath tempered martensite + carbide. Under the action of long-term high temperature and stress, the microstructure of the material can not maintain the stability of its original structure and gradually degenerates. The primary secondary phase segregates to the original austenite grain boundaries, subgrain boundaries and lath martensite boundaries, and aggregates and coarsens with the precipitation of the new phase Laves. The M23C6 carbide particles in T92 steel are rod-like and spherical in shape. During creep, the spherical particles are more stable, while the Laves phase generally adheres to the large-sized M. Compared with M23C6 phase, the coarsening rate of Laves phase is faster. When the Laves phase grows to a certain size, it will induce the formation of creep voids in the material. The voids aggregation and bonding will cause the fracture and premature failure of the material. Mean sizes of precipitates in stress and non-stress regions show that stress accelerates the nucleation and coarsening of Laves phase, while the mean sizes of M23C6 increase slightly. In addition, the percentage of main alloying elements (Cr, W, Mo, etc.) in the matrix decreases gradually during the whole creep process, and alloying elements gradually change from solid solution state to chemical conformity state. Secondly, the hardness of lath martensite matrix in creep specimen of T92 steel was obtained by micro-nano indentation hardness testing technique, which excluded the effect of high angle boundary on the results. Then the microstructure analysis was combined with micro-nano indentation hardness testing technique. The results show that the main strengthening factors of lath martensite matrix will be weakened in varying degrees under the action of long-term high temperature and stress, and the precipitation and coarsening of Laves phase and M23C6 phase will result in the increase of the second phase spacing during creep. The results show that the matrix hardness of T92 steel decreases with the increase of creep time. Finally, the accelerated aging data of T92 steel at 600, 649 and 700 degrees Celsius are fitted, and the logarithmic curves of T92 steel at high temperature are established. A life prediction method based on Larson-Miller parameter method is proposed. By this method, the rupture strength of T92 steel is extrapolated for 100,000 hours, which is equivalent to that of ECCC.
【学位授予单位】：华南理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG142.1

【参考文献】