超超临界汽轮机高中压转子X12CrMoWVNbN10-1-1钢热处理过程中微观组织演变与力学性能的研究

发布时间：2018-02-07 15:05

本文关键词： 超超临界高中压转子 X12CrMoWVNbN10-1-1铁素体耐热钢析出相微观组织表征力学性能　出处：《上海交通大学》2015年博士论文　论文类型：学位论文

【摘要】：超超临界汽轮机高中压转子是百万千瓦超超临界汽轮机组的关键部件,其服役蒸汽温度达到600℃、蒸汽压力达到30MPa。恶劣的工作环境要求转子拥有良好的材料综合性能,而材料的性能与微观组织有着直接的联系。因此研究转子材料在热处理过程中的微观组织演变规律及其与力学性能的关系,对于制定合理的热处理工艺以获得所要求的性能具有极其重要的学术价值和工程意义。本文采用多种材料表征手段,如物理化学相分析、X射线衍射、背散射电子衍射、扫描电镜、透射电镜和力学性能测试等,以X12Cr Mo WVNb N10-1-1钢为主要研究对象,探索其微观组织演变规律及其对力学性能的影响,从而为制定合理的热处理工艺,实现批量化生产中组织稳定性和均匀性的控制提供技术支撑。首先,研究了X12Cr Mo WVNb N10-1-1钢过冷奥氏体700℃(TTT图鼻尖温度)等温分解的相变机制、特征和动力学等问题。研究发现等温过程中所发生的扩散型相变产物主要为铁素体基体、Cr-rich M23C6、少量Cr-rich M2N和Nb-rich MN。析出相首先在原奥氏体晶界处形核长大,随着等温时间的延长,以胞状物的形式向奥氏体晶粒内部析出,并伴有γ→α的转变。以不同等温时间转变后所得到的微观组织作为初始组织,对其再次进行奥氏体化加热和晶粒尺寸的测定,结果发现随着等温时间的延长,奥氏体晶粒的细化效果越明显,当等温时间长达120h后,其细化效果不再显著增加。在此基础上提出了通过等温转变的方法实现奥氏体晶粒细化的机制,认为残余奥氏体含量是细化奥氏体晶粒的关键,并确定了该钢有效细化奥氏体晶粒的残余奥氏体临界体积分数约为3.2%。其次,研究了不同奥氏体化条件下,碳氮化物的溶解规律与奥氏体晶粒长大的关系。结果表明,1070℃保温60min后富Cr相全部溶解于基体中。Nb-rich MN相在1070℃奥氏体化过程中起初逐渐溶解,当保温时间达到180min后不再继续溶解。而在1010℃条件下,Nb-rich MN相没有发生溶解。1200℃保温90min之后,仅有少量的Nb-rich MN相存在。结合奥氏体晶粒长大规律,可发现Nb-rich MN相对奥氏体晶界有显著的钉扎作用,起有效钉扎作用的Nb-rich MN颗粒的临界直径和体积分数分别约为117nm和3.1×10-4。同时还发现,奥氏体晶粒的长大行为对后续冷却过程中的马氏体相变行为有显著影响,即奥氏体晶粒异常长大现象的存在可能是引起马氏体相变开始点分离的主要原因。再次,研究了不同淬火冷却条件对X12Cr Mo WVNb N10-1-1钢微观组织与冲击性能的影响。通过实验观察确定了铁素体形成的临界冷速在1~1.5℃/min之间,而抑制Cr-rich M2N和Fe-rich M3C析出的临界冷速分别大于1.5℃/min和600℃/min。将不同冷速冷却后的试样进行700℃/24h回火并测试其冲击性能。结果表明,水冷、空冷和炉冷后回火试样的冲击性能几乎相同,约为33J;冷速降为1.5℃/min时,其回火试样的冲击性能下降至23.3J左右。随着淬火冷速进一步下降,对应的回火冲击性能也随之降低。结合不同条件下的微观组织与冲击性能的分析,认为冷速过低造成硬化相Cr-rich M2N的析出是冲击性能降低的主要原因。最后,以1080℃/16h奥氏体化后炉冷得到的产物作为初始状态,系统地研究了回火温度与时间对微观组织演变规律和力学性能的影响。组织观察表明,550℃以下回火18h,有Cr-rich M2N、Cr-rich M7C3和Nb-rich MN的析出。570℃回火10h后有Cr-rich M23C6的析出。随着回火温度的升高,Cr-rich M7C3逐渐减少,Cr-rich M23C6逐渐增多。此外,基于物理化学相分析方法测定了提取的析出相中所含元素含量,着重分析了不同回火条件下析出相的析出行为。在此基础上确定了在回火过程中碳化物的析出序列为Fe-rich M3C→Cr-rich M7C3→Cr-rich M23C6。硬度测试显示,550℃以下回火时析出相较少,大量合金元素在基体中仍起着固溶强化作用,其硬度值基本稳定在430 HV10左右;550℃以上回火时,随着回火温度的升高,大量合金元素从饱和的基体中析出,削弱了固溶强化效果,硬度值逐渐降低。冲击性能显示,570℃以下回火时,由于大量Cr-rich M7C3的析出,在其周围存在微应力场,导致冲击值约为7.5J左右;570~700℃之间回火时,亚稳相Cr-rich M7C3逐渐向稳定相Cr-rich M23C6转变,微应力逐渐被释放,冲击性能略有提高;700℃以上回火时,马氏体基体发生回复,生成大量亚晶,导致冲击性能大幅度提高。
[Abstract]:Ultra supercritical steam turbine high pressure rotor is 1000MW ultra supercritical steam turbine is the key component of the service, the steam temperature reached 600 degrees, up to 30MPa. bad working environment with good steam pressure rotor material properties, material properties and microstructure have direct contact. So studying the relationship of microstructure of rotor material in the heat treatment process of the evolution and mechanical properties, to make reasonable heat treatment process to obtain the required performance has important academic value and engineering significance. This paper uses various materials characterization techniques, such as physical and chemical analysis, X ray diffraction, electron backscatter diffraction, scanning electron microscopy, transmission SEM and mechanical property testing, with X12Cr Mo WVNb N10-1-1 steel as the main research object, explore the evolution of the microstructure and its effect on mechanical properties, In order to develop a reasonable heat treatment process, to achieve control of mass production stability and uniformity of the organization to provide technical support. First of all, on the X12Cr Mo WVNb N10-1-1 700 degrees of undercooling austenite (TTT tip temperature) phase transition mechanism of isothermal decomposition, and the characteristics of the problem. The study found that the dynamic diffusion transformation products what happened in isothermal process is mainly ferrite matrix, Cr-rich M23C6, Cr-rich M2N and Nb-rich MN., a small amount of precipitates firstly in austenite grain boundary nucleation and growth, with the increase of isothermal time, the austenite grains precipitated by internal cell like form, accompanied by gamma, alpha change. To change the different isothermal time after the microstructure as the initial organization, again to its determination of austenitizing and grain size, the results found that with the increase of isothermal time, the austenite grain refinement effect The more obvious, when the isothermal time up to 120h, the refinement effect is no longer significant increase. On the basis of this proposed mechanism of austenite grain refinement by isothermal transformation method, that is the key content of residual austenite austenite grain refinement, and to determine the critical volume fraction of retained austenite austenite grain size of the steel is about 3.2%. second refine the study, different austenitizing conditions, the relationship between dissolution of carbonitride and austenite grain growth. The results show that 1070 DEG C after 60min Cr rich phase dissolved in the matrix of.Nb-rich MN at 1070 DEG C austenitizing process initially gradually dissolved, when the holding time is up to 180min in 1010 and no longer continue to dissolve. C, Nb-rich MN had dissolved.1200 Deg. C for 90min, only a small amount of Nb-rich MN. Combined with the austenite grain growth law, Can be found in Nb-rich MN relative austenite grain boundary pinning effect, Nb-rich MN effective particle pinning effect of the critical diameter and volume fraction are about 117nm and 3.1 * 10-4. also found that the austenite grain growth behavior on the subsequent cooling process of martensite phase change behavior has a significant impact, there is austenite the abnormal grain growth phenomenon may be caused by the martensitic start point. The main reason for the separation again, the effects of different cooling conditions on the microstructure of N10-1-1 steel and X12Cr Mo impact WVNb performance. By experimental observations to determine the critical cooling rate of ferrite formation between 1~1.5 DEG /min, critical cooling and suppression Cr-rich M2N and Fe-rich M3C precipitation speed were greater than 1.5 DEG /min and 600 DEG /min. different cooling samples after cooling speed 700 DEG C /24h tempering and test its performance. The simulation results show that the impact of, The impact of water cooling, air cooling and furnace cooling performance after tempering is almost the same, about 33J; cold downhill 1.5 C /min, the impact toughness of tempering decreased to about 23.3J. With the quenching rate to decline further, tempering impact performance corresponding decreases. Combined with the analysis of microstructure and impact properties of different. Under the condition of the precipitation caused by the low speed that cold hardening phase Cr-rich M2N is the main reason for the impact of reduced performance. Finally, the product of 1080 DEG /16h after austenitizing furnace cooling are used as the initial condition, systematic study of the influence of tempering temperature and time on microstructure evolution and mechanical properties of the tissue were observed. Show that the 550 DEG Cr-rich tempering 18h, M2N, Cr-rich M7C3 and Nb-rich MN.570 C 10h after tempering precipitation precipitation of Cr-rich M23C6. With the increase of the tempering temperature of Cr-rich M7C3 decreased gradually, Cr-rich M23C6 Gradually increased. In addition, the phase analysis method based on physical and chemical extraction of precipitates containing elements were determined, focuses on the analysis of the precipitation phase under different tempering condition. The precipitation based on the precipitation sequence of carbides during tempering for Fe-rich M3C, Cr-rich M7C3, Cr-rich M23C6. hardness test showed that 550 DEG C during tempering with little, a lot of alloying elements in the matrix remains a solid solution strengthening effect, value basically stable at about 430 HV10 the hardness more than 550 DEG C; tempering, with the increase of the tempering temperature of alloy elements precipitated from the saturated matrix, weaken the effect of solid solution strengthening, hardness decreased. The impact of the performance shows that the 570 DEG C during tempering due to precipitate a large number of Cr-rich M7C3, the existence of micro stress field around it, resulting in impact value is about 7.5J between 570~700 degrees; tempering, metastable The phase Cr-rich M7C3 gradually changes to stable phase Cr-rich M23C6, and the micro stress is released gradually, and the impact properties increase slightly. When the temperature is over 700, the martensite matrix regenerates, resulting in a large number of sub crystals, resulting in a great improvement in impact properties.

【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1;TG161

【参考文献】