新型奥氏体钢显微组织结构稳定性及力学性能的研究

发布时间：2018-06-04 15:41

本文选题：新型奥氏体钢 + 金属间化合物　；参考：《北京科技大学》2017年博士论文

【摘要】：为降低CO_2排放量并提高热效率,以解决目前日益突出的环境污染与能源需求之前的矛盾,先进超超临界电站预计将运行参数提高到7008℃/30 MPa,这对电站用结构材料提出了更高的要求。新型奥氏体钢(Alumina-Forming Austenitic Steel, AFA)由于具有优异的高温力学性能和抗氧化性能,被认为是很有应用前景的候选结构材料。目前,关于该新型奥氏体钢的研究主要集中在成分设计以及抗氧化机理方面,对其高温服役过程中组织结构的稳定性,特别是析出物的变化,还缺乏系统的研究。而材料在长期服役过程中组织结构的稳定性是筛选电站用结构材料的重要指标之一。为了了解该材料的组织结构稳定性,本文针对Fe-18Ni-12Cr基新型奥氏体钢的制备、700℃长期时效组织结构和力学性能的变化以及700℃蠕变性能进行了系统的研究。采用真空熔炼法制备了单相新型奥氏体钢。合金元素含量不同的新型奥氏体钢在1150℃左右热轧过程中均发生了动态再结晶,即原始热轧态样品为等轴晶粒组织。18-12-AlNbC新型奥氏体钢在700℃长期时效过程中,室温拉伸的加工硬化速率随时效时间的增加而增加,即具有加工硬化效应;而其700℃拉伸试验中,动态回复这种软化机制占主导地位。时效1000 h后,其抗拉强度为749 MPa,且均匀延伸率为26%,相对于时效前没有降低。其主要析出相为NbC,另外还有金属间化合物Fe_2(Nb,Mo) Laves和少量的B2-NiAl相。其中圆形NbC在时效过程中非常稳定,尺寸约为5nm;而片状的NbC时效1000h后其等效圆直径增加到了89 nm,这两种NbC颗粒为主要的强化析出相。Laves相具有较快的长大速率,时效1000 h后其长度为920 nm且长径比为7.4。尽管晶界上析出相的覆盖率达到74%,但是可以起到强化晶界,没有降低材料的拉伸塑形。18-12-A1新型奥氏体钢在700℃长期时效过程中析出动力学较为缓慢,因此时效10和100 h后材料的拉伸性能和组织结构没有发生太大的变化;时效1000 h后大量的B2-NiAl以及少量的Fe2Mo Laves开始析出,具有明显的强化效应,使材料的抗拉强度提高了约40%,不过其均匀延伸率降低到了22%左右。大量的B2-NiAl在室温具有很明显的强化作用,不过会在一定程度上降低材料的拉伸塑性；在700℃失去强化作用。对18-12-AlNbC和18-12-A1两种新型奥氏体钢在7000C、不同应力条件下的蠕变性能进行了测试。两种材料在7000C的蠕变应力指数均为7左右,即属于位错型蠕变；蠕变损伤容量系数λ均大于5,属于由组织结构退化造成的蠕变损伤。其次,18-12-A1NbC新型奥氏体钢具有更优异的蠕变性能,主要体现在具有更小的最小蠕变速率((?)m)、更长的蠕变断裂寿命以及较小的蠕变变形。相同蠕变应力条件下,18-12-AlNbC的最小蠕变速率比18-12-Al低一个数量级,也因此具有更长的蠕变断裂寿命。另外,18-12-AlNbC新型奥氏体钢在经历蠕变速率降低的第一阶段后即进入稳态蠕变速率,即其最小蠕变速率和稳态蠕变速率基本相同;而18-12-Al新型奥氏体钢在第一阶段蠕变速率先降低,到达最小值即最小蠕变速率后继续缓慢增加,因此其最小蠕变速率和稳态蠕变速率相差一个数量级。NbC可以使材料具有更优异的蠕变抗力,同时金属间化合物Laves相和B2-NiAl相不会损害材料的蠕变塑性。
[Abstract]:In order to reduce the CO_2 emission and improve the thermal efficiency in order to solve the contradiction between the increasingly prominent environmental pollution and the energy demand, the advanced ultra supercritical power station is expected to increase the operating parameters to 7008 /30 MPa, which puts forward higher requirements for the structural materials used in the power station. The new austenitic steel (Alumina-Forming Austenitic Steel, AFA) is due to the higher requirements. With excellent mechanical and antioxidant properties at high temperature, it is considered as a promising candidate structure material. At present, the research on this new austenitic steel is mainly focused on the composition design and the mechanism of oxidation resistance, and the stability of the structure in the process of high temperature service, especially the change of the precipitates, is still lack of system. In order to understand the structural stability of the material, the preparation of the Fe-18Ni-12Cr based austenitic steel, the changes in the structure and mechanical properties of the long-term aging and the creep properties at 700 degrees C, and the creep properties of the Fe-18Ni-12Cr based new austenitic steel have been studied in this paper. The single-phase new austenite steel was prepared by vacuum melting process. The new austenite steel with different alloy elements content occurred during the hot rolling process at about 1150 C, that is, the original hot rolled sample is the.18-12-AlNbC new austenite steel with equal axis grain structure during the long time aging process at 700. The working hardening rate of extensor is increased with the increase of aging time, that is, it has the effect of hardening, and the softening mechanism is dominated by dynamic recovery at 700 C. After 1000 h, the tensile strength is 749 MPa, and the uniform elongation rate is 26%, which is not lower than that before aging. The main precipitation phase is NbC, and there is also gold. The Intergenera compound Fe_2 (Nb, Mo) Laves and a small amount of B2-NiAl phase, of which round NbC is very stable in the aging process, the size is about 5nm, and the equivalent circle diameter of the tablet like NbC increases to 89 nm after aging 1000h, and the two NbC particles have a faster growth rate for the main intensification phase of the phase.Laves phase, and the length is 920 after aging 1000. And the ratio of length to diameter is 7.4., although the coverage rate of precipitation phase on grain boundary reaches 74%, but it can enhance grain boundary, without reducing the tensile shape of material, the precipitation kinetics of new austenite steel.18-12-A1 in the long time aging process is slow, so the tensile property and structure of the material have not changed too much after aging 10 and 100 h. After aging 1000 h, a large number of B2-NiAl and a small amount of Fe2Mo Laves begin to precipitate, which has obvious strengthening effect, which makes the tensile strength of the material increase by about 40%, but its uniform elongation is reduced to about 22%. A large number of B2-NiAl has obvious strengthening effect at room temperature, but it will reduce the tensile plasticity of the material to a certain extent. The creep behavior of two new austenitic steels of 18-12-AlNbC and 18-12-A1 under 7000C and different stress conditions were tested at 700 C. The creep stress index of two materials in 7000C was about 7, that is, dislocation type creep, and the creep damage capacity coefficient was more than 5, which was caused by the degeneration of tissue structure. Second, 18-12-A1NbC new austenite steel has more excellent creep properties, which is mainly embodied in a smaller minimum creep rate ((?) m), longer creep rupture life and smaller creep deformation. Under the same creep stress conditions, the minimum creep rate of 18-12-AlNbC is one order of magnitude lower than that of 18-12-Al, and thus has a longer period of time. In addition, the 18-12-AlNbC new austenitic steel enters the steady state creep rate after the first stage of the creep rate reduction, that is, the minimum creep rate and the steady creep rate are basically the same, while the 18-12-Al new austenite steel decreases first in the first stage, and the minimum creep rate is followed by the minimum. It continues to increase slowly, so the difference of the minimum creep rate and the steady creep rate of one order of.NbC can make the material have more excellent creep resistance, and the intermetallic compound Laves phase and B2-NiAl phase will not damage the creep plasticity of the material.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG142.1

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