T23钢粗晶热影响区再热裂纹敏感性研究

发布时间：2018-04-08 13:21

本文选题：T23钢　切入点：再热裂纹　出处：《上海交通大学》2015年博士论文

【摘要】：T23钢是一种新型铁素体耐热钢,由于其具有良好的焊接性和较高的蠕变强度,被广泛应用于超(超)临界锅炉的水冷壁、过热器等组件。然而,在制造厂和电厂的T23钢膜式水冷壁焊接接头中均出现了裂纹,导致了水冷壁管的爆管及泄漏,严重影响了机组投运后的安全。因此,对T23钢焊接接头再热裂纹(RC/SRC)的敏感性进行更深入的研究具有重要意义。采用实际焊接接头的斜Y型坡口试验及基于Gleeble-3500热力耦合试验机的热模拟CGHAZ(coarse-grained heat-affected zone)的STF(strain-to-fracture)试验,对国产T23钢CGHAZ再热裂纹敏感性进行评估。采用透射电镜(TEM),电子背散射衍射(EBSD)及Jmat Pro软件对晶界析出相,裂纹所在晶界的取向差分布,以及晶界附近应变的协调过程进行表征分析。结果表明,再热裂纹在不同温度下的敏感性受晶界及晶内的相对强度影响。晶界强度由晶界取向差,晶界析出相共同作用,晶内强度由晶内二次析出相及晶内位错密度共同决定。此外,建立了再热裂纹位置与晶界取向差的关系,为从晶界工程角度降低裂纹敏感性提供了理论依据。为阐明影响晶内和晶界强度因素的作用,进行了试验设计。通过750℃焊后热处理来调控晶内强度,采用STF试验对再热裂纹敏感性进行评估,结合SEM,EBSD,TEM及小角度衍射(SAXS)对析出相种类,位错密度,空洞及微裂纹位置的变化进行了研究。结果表明,一定时间范围内(1min-1.5h),位错密度的快速下降有效软化晶内强度,塑性变形主要在晶内而非在晶界上发生,再热裂纹敏感性下降。超过该时间范围后,晶内位错密度下降速度放缓,同时晶内析出了大量的MC相,使得晶内强度有所增加,塑性变形可能会通过晶界的开裂进行协调;但是,由于在晶界上新析出了尺寸较小且与基体有较好共格/半共格关系的M23C6,它与其他碳化物相比增加了晶界的相对强度,因而抵消了晶内强度增加带来的不利影响。由于晶内及晶界强度的同时增加,塑性变形只能通过强度较低的block的开裂或者剪切的方式进行协调,因此,再热裂纹敏感性依然较小。上述结果为焊后热处理工艺的设计提供了指导,以确保焊接接头投入运行后能够安全服役。本文还设计了一种临界热循环工艺,该工艺改变了晶界特征。采用TEM和EBSD对临界热循环工艺得到的再结晶晶粒的晶体学特征,晶界取向差分布,应变集中程度进行了表征和分析,研究表明,再结晶晶粒降低了原奥氏体晶界上大角度晶界的比例,并增加晶界的曲折度,阻碍裂纹沿着原奥氏体晶界进行直线扩展。另外,再结晶中的M/A(martensite-austenite)组元消耗了一定的碳元素,造成析出相在原奥氏体晶界上难以粗化长大。同时M/A组元形成时带来的应变集中避开了原奥氏体晶界。该工艺实现了晶界的有效强化,降低了再热裂纹敏感性。最后基于再热裂纹的开裂机理及影响晶内和晶界强度的因素,构建了T23钢CGHAZ再热裂纹的开裂模型。此模型的建立为T23钢焊接接头的焊后热处理提供了依据,并为其他材料再热裂纹的研究提供了新的方法和思路。
[Abstract]:T23 steel is a new type of heat-resistant ferritic steel, because of its high creep strength good weldability and high, is widely used in ultra (ultra) supercritical boiler water wall, superheater and other components. However, welding on T23 steel factory and power plant membrane type water wall joints are the crack, LED tube explosion and leakage of the water wall tube, seriously affecting the unit after operation safety. Therefore, the T23 steel welding joint of reheat cracking (RC/SRC) susceptibility to further research has important significance. The actual welding cable Y groove joint test and thermal simulation of CGHAZ Gleeble-3500 thermal coupling based on the testing machine (coarse-grained heat-affected zone) STF (strain-to-fracture) test, evaluation of domestic T23 steel CGHAZ reheat cracking. By transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and Jmat Pro software on grain boundary precipitation The grain boundary phase, crack orientation difference distribution, and grain boundary strain coordination process were analyzed. The results show that the reheat cracking susceptibility at different temperatures by the relative strength of effect of crystal boundary and in the grain boundary strength by grain boundary misorientation, grain boundary precipitates interaction, intragranular strength by intergranular phase and two a crystallization of dislocation density in the joint decision. In addition, to establish the relationship between the reheat crack position and boundary misorientation, provides a theoretical basis for reducing the crack sensitivity from grain boundary engineering perspective. To elucidate the influence factors in the grain and grain boundary strength, the experimental design. Intragranular strength controlled by 750 DEG C after welding heat treatment with STF test of reheat cracking susceptibility was assessed with SEM, EBSD, TEM and small angle diffraction (SAXS) on the precipitate, dislocation density, void variation and micro crack location are studied . the results show that within a certain time (1min-1.5h), the rapid decline of the dislocation density effectively soften intragranular strength, plastic deformation is mainly in the grain rather than in grain boundaries, decrease reheat cracking. Over the time range, intragranular dislocation density and the rate of decline slowed, intragranular precipitation of a large number of MC the crystal phase, strength increase, plastic deformation may be coordinated through the grain boundary cracking; however, due to the new grain boundary precipitation is smaller in size and with the matrix has good coherent / semi coherent M23C6, compared with other carbide increases the relative strength of the grain boundary, and thus offset the increase intragranular strength adverse effects. Due to the increase of grain and grain boundary strength and plastic deformation only through coordination, lower strength block cracking or shear mode. Therefore, reheat crack sensitivity is still small. Provides guidance for the design of the above results for heat treatment after welding, to ensure welding joint operation safe service. The paper also designs a critical heat recycling process, the process changes the grain boundary characteristics. The recrystallization crystal characteristics of TEM and EBSD on the critical thermal cycling process, grain boundary misorientation the distribution, degree of strain concentration were characterized and analyzed, research shows that the recrystallization grain reduces the original austenite grain boundaries the proportion of high angle grain boundaries, and increased grain boundary tortuosity, block the crack along the original austenite grain boundary line expansion. In addition, the recrystallization of M/A (MARTENSITE-AUSTENITE) component of carbon consumption of certain that caused the precipitates at the grain boundaries to coarsening. At the same time bring strain M/A component is formed when the concentration from the original austenite grain boundary. The process of implementation of the grain boundary Effect of strengthening, reduce the reheat cracking. Finally, based on the cracking mechanism of reheat crack and the influence factors in the grain and grain boundary strength, construct the model of CGHAZ cracking reheat crack of T23 steel. Provide the basis for the establishment of this model for T23 steel welded joints after welding heat treatment, and provides the new research methods and ideas for other materials reheat crack.

【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】：TG142.73

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