核电异种金属激光填丝焊接头组织与性能及热裂纹形成机理

发布时间：2018-02-04 22:51

本文关键词： 激光填丝焊核电异质接头微观组织热裂纹力学性能耐腐蚀性　出处：《上海交通大学》2015年博士论文　论文类型：学位论文

【摘要】：在压水堆核电站设备中,异种材料之间的焊接是一项复杂、技术难度大、质量要求高的工程,迄今为止仍是核电加工制造领域的难点。激光焊接技术是近几十年来发展起来的一种优质、高效的新型焊接方法,具有深宽比大、冷却速度快、热影响区窄等特点,成为激光加工领域的研究热点之一。针对核电一回路Alloy 52M/316L异种金属关键焊接构件,本文提出了采用窄间隙激光填丝焊的方法,探索核电工程中异种金属激光填丝焊的特点及组织变化规律,为推广激光焊接技术在核电工程的应用提供技术支撑。本文首先开展Alloy 52M/316L异种金属激光填丝焊工艺和焊缝成形的研究,研究表明激光填丝焊Alloy 52M/316L异质焊缝具有匙孔形貌特征,两层焊缝之间有较深的搭接量,揭示了焊缝高度和熔宽随激光功率、焊接速度和送丝速度等焊接参数变化的规律以及焊接侧壁未熔合和气孔的特征与形成原因。利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)等分析测试方法,研究Alloy 52M/316L焊缝区的微观组织、成分分布、第二相的析出行为以及异质界面结构特征、形成特点和界面区元素的偏聚行为,对比研究带极堆焊(ESW)阻隔层和GTAW阻隔层两种状态下异质界面的组织结构,结果表明Alloy 52M/316L焊缝区靠近熔合线边界的组织是细小的胞状晶,随着凝固的推进胞状晶转变为柱状树枝晶,在焊缝的中心处会合形成沿焊缝中心对称生长的组织形态；在激光多层焊缝的层间出现了重熔细晶区,形成了等轴细晶组织；在焊缝枝晶间主要是由富Nb的析出相和A1203与Ti02聚集一起生长形成球形复合氧化物,揭示了Alloy 52M/316L焊缝区微观组织和第二相析出的规律；在ESW52M/WM的界面区域富Nb的析出相呈链状分布或颗粒状聚集长大特征,由于Nb、Si、Cr和Fe等元素在晶界上富集在冷却凝固阶段引起富Nb的第二相析出并聚集、长大；同时因GTAW52M与ESW52M微观组织的差异,在GTAW52M/WM界面上没有出现富Nb的碳化物或Laves相；WM/316L界面是由宽度约30-40μm细小的胞状晶组成,激光填丝焊缝与316L不锈钢表现出良好的相容性,体现良好的界面结合特性；在WM/316L界面附近区域存在Cr和Mo元素的富集,并诠释了沿晶界析出呈链状σ相的原因。基于Alloy 52M/316L异质接头热裂纹产生的位置、特征、化学成分和断口分析以及高温拉伸试验和STF试验的研究,结果表明富Nb的低熔点共晶相和界面氧化物引起Alloy 52M/316L异质焊缝凝固裂纹的重要因素,Alloy 52M/316L异质焊缝凝固裂纹的断口呈凹凸显著的树枝晶状,表面光滑、无撕裂棱；基于Alloy 52M/316L焊缝区凝固裂纹液相回流的分析,推导出裂纹萌生和扩展条件下液相回流速度vlx；发现在液化裂纹处伴随着富Nb的白色链状析出相,具有晶界低熔点液膜的特征,液化裂纹断口面的晶粒呈卵石状,晶粒边界出现液膜,检测表明存在富S和Si的低熔点共晶相,或呈液膜特征的富Nb的析出相附在晶粒表面尖端上；发现富S的低熔点共晶相和富Nb的析出相是引起Alloy 52M/316L异质接头液化裂纹的冶金因素,并阐明了液膜形成的原因、富Nb的碳化物的组分液化和液膜迁移的裂纹形成机理；Alloy 52M/316L异质接头的高温失塑裂纹的走向沿柱状枝晶发生开裂,其扩展方向可能偏离枝晶生长的方向,高温失塑裂纹大多起裂于三叉晶界处,而裂纹在晶界上的形成过程也是孔洞在晶界上形核、长大和聚合连接的过程,晶界滑移直接影响晶界上孔洞的形核、长大与聚合连接,诠释了Alloy 52M/316L异质接头高温失塑裂纹形成条件、萌生与扩展机制。通过力学性能研究表明Alloy 52M/316L异质接头从Alloy 52M阻隔层经过焊缝到316L不锈钢的硬度呈下降的趋势,焊缝区的平均硬度为180HV；ESW52M/WM界面是冲击韧性薄弱环节,在ESW52M/WM界面上不规则的富Nb的碳化物或脆性的Laves相、氧化夹杂物是导致冲击韧性降低的主要原因；ESW52M/316L和GTAW52M/316L两种异质接头的拉伸断口均呈现出韧性断裂的特征,但在ESW52M/316L拉伸断口韧窝处存在大量的析出相,ESW52M/WM界面是整个焊接接头的断裂的薄弱位置；ESW52M/316L异质接头在350℃温度拉伸的断裂位置位于316L侧,其断口上存在较多空洞,断裂机制是空洞聚集型韧性断裂。电化学试验表明,在含Cl-和由Cl-与S2O32-组成的混合介质两种腐蚀溶液体系中,焊缝金属的极化曲线表现出多次钝化的特征,腐蚀的形貌呈现差异性,S2O32-促进了焊缝金属的阳极溶解,延缓或阻碍其钝化,促使表面钝化膜稳定性降低作用,揭示了Alloy 52M/316L异质焊缝在含C1-和含Cl-与S2O32-的混合介质两种溶液体系中的腐蚀机制,诠释了微观组织、析出相和腐蚀介质对Alloy 52M/316L焊缝金属腐蚀的影响。
[Abstract]:In the nuclear power plant equipment in PWR, welding between dissimilar materials is a complex, difficult, high quality requirements of the project, so far is still difficult to manufacture nuclear power. Laser welding technology is developed in recent years, a new type of high quality, high efficient welding method, has deep and wide ratio, cooling speed, narrow heat affected zone etc, become a hot research topic in the field of laser processing. Aiming at the nuclear power plant Alloy 52M/316L dissimilar metal welding key components, this paper proposes a method using narrow gap laser welding with filler wire, to explore the characteristics and microstructure of dissimilar metal laser welding in nuclear power engineering technology, provide technical support for the promotion of nuclear power engineering application in laser welding. This research was first carried out Alloy 52M/316L dissimilar metal wire filling laser welding process and weld formation, research shows that laser wire filling Alloy 52M/316L has heterogeneous welding weld keyhole morphology, there is overlap between the two deep layer weld, reveals the weld height and weld width with laser power, welding speed and wire feeding speed of welding parameters and welding characteristics and forming reasons of the side wall without fusion and porosity. By using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) analysis method, the distribution of Alloy 52M/316L on microstructure, weld composition, precipitation behavior of second phase and the interface structure characteristics, formation characteristics and segregation of elements of the interface of strip cladding (ESW) structure barrier the interface layer and GTAW barrier layer under two conditions, the results show that the Alloy 52M/316L weld zone near the fusion line boundary organization is cellularcrystal small, with the advance of the solidification of cellular crystal into the dendrite in the weld. At the junction of center formation along the weld center symmetrical structure; in the laser multilayer welding layer between the fine grain zone remelting, formation of equiaxed grains in the weld microstructure; dendrite is mainly composed of A1203 and Ti02 Nb rich precipitates together to form spherical composite oxide growth, revealing the microstructure Alloy 52M/316L weld zone and the precipitation of the second phase of the law; in the interface region of the precipitation of ESW52M/WM rich Nb phase of chainlike distribution or granular aggregation and growth characteristics, due to Nb, Si, Cr and Fe enrichment at the grain boundary in the cooling stage caused by the precipitation of the second phase Nb rich and aggregation, also because of differences in GTAW52M growth; ESW52M and the microstructure of the interface in the GTAW52M/WM no Nb rich carbide or Laves; WM/316L interface is composed of width of columnar crystal is about 30-40 m small, wire filling laser weld and 316L stainless steel Exhibit good biocompatibility, represents good interfacial characteristics; in the area around the WM/316L interface enrichment of Cr and Mo elements, and the interpretation of the cause is precipitated along grain boundary phase characteristics of Alloy 52M/316L chain. A heterogeneous joint hot crack position, based on the analysis and research of fracture and high temperature tensile test and STF the test results show that the chemical, Nb rich and low melting point eutectic phase and interfacial oxide an important factor in causing Alloy 52M/316L heterogeneity of weld solidification crack, fracture of Alloy 52M/316L heterogeneity of weld solidification crack is of concave convex significant dendritic shape, smooth surface, no tear edge; Alloy 52M/316L analysis of solidification cracks in weld metal based liquid phase reflux. Derived from the crack initiation and propagation under the condition of liquid reflux rate vlx; found in liquation crack is accompanied by a white chain Nb rich precipitates, the grain boundary has a low melting point liquid The characteristics of crack fracture surface liquefaction grains are oval shaped, grain boundary appeared film, testing showed the presence of S rich and Si low melting point eutectic phase, precipitation or a feature rich Nb film attached on the grain surface on the tip; found that the precipitation of S rich and low melting point eutectic phase and Nb rich phase is caused by Alloy 52M/316L heterogeneous joint metallurgical factors liquation crack, and illustrates the reason of liquid film formation, crack formation mechanism of Nb rich carbide components of liquefied and liquid film migration; high temperature Alloy 52M/316L heterogeneous joint loss to plastic crack along the columnar dendrite fracture, the propagation direction may deviate from the direction of dendrite growth. High temperature plastic crack initiation in the loss of most of the triple junctions and crack formation at the grain boundary is void in grain boundary nucleation, growth and coalescence of the connection process, grain boundary sliding directly affect the grain boundary void nucleation, growth The polymerization and interpretation of the Alloy connection, 52M/316L joints ductility dip crack formation conditions, initiation and propagation mechanism. Through the study of mechanical properties showed that the Alloy 52M/316L connector from the Alloy 52M heterogeneous barrier layer of 316L stainless steel after weld hardness decreased, the average hardness of the weld zone is 180HV; the ESW52M/WM interface is the weak link of impact toughness. Irregular interface in the ESW52M/WM rich Nb carbides or brittle Laves phase, oxide inclusion is the main cause of the fracture toughness decrease; ESW52M/316L and GTAW52M/316L two kinds of joints showed the characteristics of ductile fracture, but ESW52M/316L tensile dimples were found a large number of precipitates, the ESW52M/WM interface is weak the position of welded joint fracture; ESW52M/316L joints at the fracture location to the temperature of 350 DEG C stretching in the 316L side, the fracture There are many holes, fracture mechanism is void coalescence fracture. The electrochemical test showed that in the mixed medium containing Cl- and composed of Cl- and S2O32- two kinds of etching solution, the polarization curve of weld metal characterized by multiple passivation, corrosion morphology difference, S2O32- promoted the anodic dissolution of weld metal that delay or prevent the passivation, prompted the passivating film on the surface of reduced stability, revealing the corrosion mechanism of Alloy 52M/316L in heterogeneous weld mixed medium containing C1- and Cl- and S2O32- two in the solution system, the interpretation of the microstructure, effects of precipitated phases and corrosion medium on Alloy 52M/316L weld metal corrosion.

【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】：TM623.91;TG456.7

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