【作者】 李新梅；

【导师】 邹增大；

【作者基本信息】 山东大学 ， 材料加工工程， 2010， 博士

【摘要】 节能和环保要求新建现代化发电厂必须提高生产效率,而作为最成熟的洁净煤发电技术之一的超超临界(USC)机组则成为目前火力发电的共同发展趋势,使其20年来在国内外得到了快速的发展,而这又首先得益于高温性能优良的新型耐热钢成功的开发和应用。中国USC机组中高温过热器和再热器的高温段大都采用的是Super304H钢,但目前对Super304H钢焊接接头的深入研究报道较少,因此研究焊接接头的组织与性能、尤其在高温时效条件下接头的组织结构变化及其与性能之间关系研究,不但对确保机组的安全运行具有重要的现实意义,而且对于钢材的应用和监督、实现钢材的国产化都有实际意义。本文综合应用金相显微镜、扫描电子显微镜、能谱分析仪、透射电子显微镜、电子探针、X-射线衍射仪,以及常温力学性能试验、高温持久试验和电化学动电位再活化法等手段,研究了Super304H钢焊接接头的微观组织结构及其对性能影响,高温时效条件下Super304H钢焊接接头的微观组织变化规律、析出相在焊接和时效过程中的变化、以及这些变化对接头常温力学性能、高温持久强度和晶间腐蚀性的影响,探讨了接头的弱化机理,解明了焊接工艺对接头性能影响的关键因素。在供货状态下Super304H钢的微观组织为晶粒细小的γ-基体+析出相,γ晶粒尺寸均匀细小。析出相主要由Nb(C,N)和富Cu相组成,Nb(C,N)有呈方向性分布的尺寸较大的条块状和呈弥散分布的细小颗粒状两种形态,富Cu相尺寸均匀细小呈弥散分布。在焊接条件下Super304H钢焊接接头的微观组织为γ+析出相,焊缝为典型的胞状枝晶形态,而靠近熔合区的热影响区晶粒有明显长大现象。接头中的析出相主要是Nb(C,N)和富Cu相,在焊缝枝晶界处分布有颗粒状或蠕虫状的Nb(C,N)。经最高1350℃高温处理后,Super304H钢基体组织仍为单一的γ相,温度低于1100℃时γ晶粒缓慢长大,高于1150℃时晶粒尺寸急剧长大。在高温处理过程中析出相的种类有Nb(C,N)、富Cu相、M23C6和NbCrN,随温度不同,析出相发生析出或溶解的变化,同时它们的形态、分布和数量也呈现出不同的变化规律,由此引起Super304H钢的冲击韧性和显微硬度产生相应的变化。因此,Super304H钢长期工作温度不宜超过700℃,为提高接头的抗高温蒸汽氧化性,需尽可能控制其在1150℃以上温度区域停留的时间,并应该通过改善冶金条件来细化焊缝组织。经过600℃-700℃高温时效后,Super304H钢焊接接头的微观组织仍为γ+析出相,基体组织特征基本没发生变化,析出相主要是Nb(C,N)、富Cu相和M23C6。时效条件不同,析出相的析出行为也不同,焊缝和母材中的Nb(C,N)相对析出量曲线形状与它们在原始组织中的含量有关,M23C6主要沿晶析出,其形态由细条状或颗粒状向链球状(长条状或块状)、孤立的颗粒状转变,并且温度对M23C6的影响远大于时间。试验条件下Super304H钢焊接接头的蠕变断裂强度高于母材,蠕变断口位于远离熔合线的母材上,蠕变空洞的主要形核位置在尺寸较大的条块状Nb(C,N)处,随着蠕变时间的延长,晶界上M23C6也是空洞形核的主要位置。在600℃-700℃时效条件下Super304H钢焊接接头发生显著的时效脆化现象,造成Super304H钢焊接接头时效脆化的根本原因是大量M23C6沿晶析出,而且析出量是决定因素。在使用温度范围内时效时,焊缝金属稳定的冲击功仅为13J,远低于母材和热影响区的冲击功,因此,尽管目前的Super304H钢选用的焊接材料与母材基本匹配,但这方面依然需要运行检验或进一步研究改进。Super304H钢焊接接头在供货状态和焊接条件下具有优良的抗晶间腐蚀性能。经600℃-700℃时效后则具有较大的晶间腐蚀敏感性,时效过程中Super304H钢焊接接头晶间腐蚀敏感性变化与M23C6的数量、形态和分布变化相一致,M23C6沿晶界析出是Super304H钢焊接接头在时效条件下发生晶间腐蚀的根本原因,而且析出量是决定因素。M23C6不仅沿晶析出引起贫Cr发生晶间腐蚀,而且在晶内析出也会引起其周围贫Cr发生腐蚀。因此对于炉内具有一定浓度高温腐蚀介质的锅炉中运行的Super304H钢应加强焊接接头晶间腐蚀性能的监督。更多还原

【Abstract】 Demands on energy saving and environmental protection have been driven the improvement of production efficiency of modern electric power plant, and the last 20 years has seen the rapid development of ultra supercritical (USC) power station unit which is recognized as one of the most matured clean-coal electric power generating technologies. The rapid development of USC power station unit benefits firstly from the research and application of new heat resistance steels with superior properties. It is well documented that Super304H steel has gained wide applications as superheater/reheater in USC unit in China. However, there is still a lack of comprehensive investigation on Super304H steel and its welded joint in our country. As a result, it is of importance to study the welded joint, especially the influence of high temperature aging on its microstructure and properties. It is also of practical significance to conduct these investigations to support the safety running, commercial application, supervising and customizing of this steel in our country.In the present paper, comprehensive characterization of the welded joint was performed by using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, transmission microscopy, electron probe microscopy, X-ray diffractometer, room-temperature test, high temperature creep test and electrochemical potentiokinetic reactivation test. Objections of this investigation include microstructure characterization and its effect on properties of Super304H steel and its welded joint, microstructure evolution of Super304H steel and its welded joint during high temperature aging, as well as precipitates evolution during welding and aging treatment. Besides, it is also studied systematically in the present paper that the influence of the above-mentioned factors on room temperature mechanical properties, high temperature creep strength, intergranular corrosion behaviors and the degrading mechanism of the welded joint. It is expected that this investigation can be used as a technical support for supervising the safety running of the welded joint, as well as to provide technical data for the customization of this steel in our country.The as-received Super304H steel shows microstructure of y matrix made up of fine and evenly distributed y grains with precipitates. Precipitates consist of Nb(C, N) and Cu-rich phase. Nb(C, N) shows morphology of finely dispersed small particles and relatively large block and stripe which distribute along certain direction. Microstructure of the as-welded joint of Super304H steel consists of y phase and precipitates. Weld metal shows typical cellular dendrite morphology and the HAZ adjacent to the fusing zone demonstrates obvious grain growth. Precipitates in the welded joint mainly comprises Cu-rich phase and Nb(C, N) which distributes in the dendritic grain boundaries with particulate and worm-like morphology.After being high temperature treatment with high temperature of 1350℃, matrix of Super304H steel still consists of y phase. When the treatment temperature is below 1100℃,γgrain grows slowly in contrast to its rapid growth at temperature higher than1150℃. During high temperature treatment, precipitates presented include Nb(C, N), Cu-rich phase, M23C6 and NbCrN, which show different precipitating and dissolution behaviors, morphology, distribution and amount with the changing of treatment temperature. And these also subsequently cause the corresponding changes of impact toughness and hardness of Super304H steel. As a result, it is suggested that long-term serving temperature should be below 700℃for Super304H steel and in addition to refining the weld metal microstructure through improving metallurgical conditions the time experienced above 1150℃should be controlled in order to improve its oxidation resistance to high temperature steam.After being aged from 600℃to 700℃, microstructure of Super304H steel and the welded joint consists of y phase and precipitates. The matrix is basically absent of any noticeable microstructure changing. Precipitates are mainly made up of Nb(C, N), copper-rich phase and M23C6. Precipitating behavior varies with the changing of aging condition. It is found that the shape of curve showing relative amount of Nb(C, N) and M23C6 is related to their content in the initial microstructure. M23C6 mainly distributed along grain boundaries and its morphology changes from thin stripe/particle to chain-like (block or stripe) and isolated particles. Compared with time, temperature has a more profound influence on the precipitating behavior of M23C6.Under the experimental condition creep strength of the welded joint is higher than that of the base metal. Creep fracture occurs in the area of the base metal far from the fusing line. Furthermore, it is found that creep cavity nucleates near the relatively large Nb(C, N) block and stripe, and with the increasing of aging time, cavity also forms near M23C6 particles on grain boundaries.After being aged at high temperature from 600℃to 700℃, obvious aging brittleness occurs for the welded joint, which is attributed to the precipitation of M23C6 along grain boundaries. Besides, it is found that the amount of precipitates is a determinate factor. When aged at the serving temperature, impact energy of the weld metal is only 13J, which is much less than these of the base metal and HAZ. Therefore, welding consumable used for the Super304H steel needs further evaluation and improvement.Both the Super304H steel in as-received condition and the as-welded welded joint possess superior intergranular corrosion resistance but sensitization occurs after being aged at high temperature from 600℃to 700℃. Susceptibility of intergranular corrosion of Super304H steel and its welded joint during aging treatment is in accordance with the amount, morphology and distribution of M23C6. Intergranular corrosion of Super304H steel and its welded joint is attributed to the precipitation of M23C6 along grain boundaries and is determined by the amount of precipitates. Intergranular corrosion induced by Cr depletion was caused by precipitating of M23C6 along grain boundaries, and it is also found that the precipitating of M23C6 inside grains also leads to corrosion near the Cr-depleted area. Therefore, supervising of Super304H steel and the welded joint should be enhanced which was normally serviced in the boiler in highly corrosive medium.更多还原