Acta Metallurgica Sinica, 2017, 53(1): 57-69
doi: 10.11900/0412.1961.2016.00135
国产核电安全端异种金属焊接件的微观结构及局部性能研究
Microstructure and Local Properties of a Domestic Safe-End Dissimilar Metal Weld Joint by Using Hot-Wire GTAW
明洪亮1,2, 张志明1, 王俭秋1,, 韩恩厚1, 苏明星3

摘要:

利用OM、SEM、显微硬度仪、微小试样拉伸实验及慢应变速率拉伸实验对国产带热丝隔离层核电安全端焊接件不同部位的微观结构及局部的力学性能和应力腐蚀敏感性进行了研究。发现,在SA508/52Mb界面处的52Mb中具有大量的对应力腐蚀敏感的I型晶界及II型晶界,导致此界面具有最高的应力腐蚀敏感性;SA508热影响区存在明显的组织过渡;316LN热影响区中随着距熔合线距离的增加,重位点阵(CSL)晶界的数量分数逐渐增大,Σ3晶界与理想的Σ3晶界的偏差角减小,残余应变逐渐减小,残余应变的最高值出现在对接焊底焊位置处的316LN热影响区中,导致316LN的热影响区也具有较高的应力腐蚀敏感性。焊接件不同部位的力学性能存在较大的差异。对于硬度分布而言,显微硬度变化最剧烈的位置在SA508/52Mb界面附近,且此界面附近的52Mb具有最高的硬度,此界面附近的SA508脱C区具有最低的硬度。强度的变化趋势与硬度的变化趋势类似。一般强度高的地方断裂应变低。焊接件不同位置的性能差异主要取决于不同部位的微观结构(包含组织、成分等)差异。

关键词: 异种金属焊接 ; 微观结构 ; 局部力学性能 ; 应力腐蚀敏感性 ; 残余应变

Abstract:

Dissimilar metal weld joints (DMWJ) widely exist in the nuclear power plants to join the different parts which are made of different structural materials. Among these DMWJs, safe-end DMWJ has attracted much attention of researchers and operating enterprises, as premature failures, mainly stress corrosion cracking failures, have occurred in these kinds of joints. However, DMWJ with 52M as filler metal in the nuclear power plants has no in-service experience. To ensure the structural integrity of the weld joint and the safe operation of the future plants, the microstructure and local properties of a domestic safe-end DMWJ by using hot-wire gas tungsten arc welding (GTAW) technology was studied in detail by OM, SEM, micro-hardness testing, local mechanical tensile testing and slow strain rate tests. The tensile tests were performed at room temperature with the tensile speed of 5 μm/s while the slow strain rate tests were conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃. A large amount of type I boundaries and type II boundaries which are susceptible to stress corrosion cracking (SCC) exist in 52Mb near the SA508/52Mb interface and result in the highest SCC susceptibility of this interface. Microstructure transition was found in the SA508 heat affected zone (HAZ). In 316LN HAZ, increasing the distance from the fusion boundary, the number fraction of CSL boundaries increase while the residual strain decreases, resulting in the second-highest SCC susceptibility of 316LN HAZ. In 52M, residual strain distributes randomly but not uniformly, the residual strain is prone to accumulate at the grain boundaries. Dramatic changes of mechanical properties are observed across the joint, especially at the SA508/52M interface. The differences of the local microstructure and chemical composition lead to the differences of the local properties of the weld joint.

Key words: dissimilar metal ; weld joint, ; microstructure, ; local mechanical property, ; stress corrosion cracking susceptibility, ; residual strain

核能作为一种清洁、经济、高效的能源,受到各国的广泛重视,我国已经成为世界上在建核电站最多的国家。但是核电站的建设极其复杂,用到的材料种类也多种多样,不同材料间的相互焊接会导致许多问题,如C迁移、焊接残余应力产生、焊缝金属的稀释等,这些均成为造成众多核电关键设备失效的重要原因。大量的研究及运行经验表明,压水堆核电站一回路系统中连接低合金钢压力容器壳体管嘴与奥氏体不锈钢安全端的异种金属焊接接头(dissimilar metal welded joint,DMWJ),是一回路循环系统的薄弱部件,服役时间远低于设计寿命,应力腐蚀开裂(SCC)是其主要的失效形式之一[1]

应力腐蚀开裂的三大影响因素为材料、应力/应变及腐蚀性环境。在运行过程中,一回路水环境可以得到较好的控制,材料因素及其所处的应力/应变状态就成为决定焊接接头是否会发生应力腐蚀开裂的关键因素。因而就焊接件而言,其不同部位的微观结构及力学性能的研究对于焊接件的安全性评价至关重要。所以,了解核电安全端异种金属焊接件的微观结构、力学性能及不同部位的应力腐蚀敏感性对于确保其在核电高温高压水环境下的安全运行具有重要意义。

对于安全端异种金属焊接接头部位的微观表征及其在核电高温高压水环境中的应力腐蚀行为已经有大量的研究[2~12]。Li和Congleton[13]研究发现,在292 ℃的一回路水中,奥氏体不锈钢-低合金钢异种金属焊接接头的熔合线附近比焊缝金属及母材具有更高的应力腐蚀敏感性。Lu等[3]研究了600合金热影响区的晶界类型分布、残余应变等对其在高温高压水中的SCC行为,发现热影响区的随机大角度晶界数量分数高、残余应变大,应力腐蚀裂纹扩展速率高。但是仍然存在许多问题,比如,一些研究所用的异种金属焊接接头是模拟件,材料尺寸及焊接工艺与核电站中实际运行的异种金属焊接接头相差甚远[4,14];采用有限元模拟来预测异种金属焊接接头中残余应力的分布时,由于焊接过程及焊接热循环的复杂性(尤其是厚件的多层多道次焊接)等导致模拟结果不可靠,必须进行实验验证[15];大多数的研究主要集中在以不锈钢(308L/309L)[6,12]以及镍基合金(Alloy52/152、Alloy 82/182)作为填充金属的安全端焊接接头上[4,9,16,17],缺乏目前在三代核电安全端焊接接头中广泛应用的、具有更佳性能的52M镍基合金作为填充金属的异种金属焊接件的研究;缺乏国产化的异种金属焊接接头的研究;另外,采用热丝堆焊工艺可以显著提高焊接效率,但是也尚未见相关的研究报道。

本工作对带有热丝隔离层的核电安全端异种金属焊接接头SA508/52Mb/52Mw/316LN的微观结构、局部力学性能及不同部位的应力腐蚀敏感性进行详细的研究,分析了焊接件微观结构对力学性能及应力腐蚀敏感性的影响,并提前确定此焊接接头的薄弱环节,为焊接工艺的评定及未来核电站实际运行过程中的重点监测部位提供了实验依据。

1 实验方法

本工作的研究对象为以镍基合金52M为填充金属,以低合金钢SA508 Gr.3 Cl.2 (简写为SA508)及奥氏体不锈钢SA 336 Gr F316LN (简写为316LN)为母材的国产核电安全端异种金属焊接接头。各种材料的化学成分如表1所示。焊接工艺流程为:首先采用钨极氩弧焊(GTAW)工艺在SA508管嘴堆焊隔离层,焊丝为直径1.2 mm的52M,堆焊时焊丝要进行预热;隔离层堆焊完成后进行焊后热处理以消除焊接残余应力;再次采用GTAW工艺,以直径为0.9 mm的52M为填充金属将带有隔离层的管嘴与安全端过渡管(316LN)进行对接焊;对接焊完成后不再进行焊后热处理。在本文中,隔离层及对接焊缝区分别记为52Mb和52Mw。焊接件的局部切割件及焊缝截面示意图如图1a和b所示。

表1 焊接件不同部位主要化学成分
Table 1 Chemical composition of materials in dissimilar metal weld joint (DMWJ) (mass fraction / %)
Material C Si Mn Cr Ni S P Fe N Mo
316LN 0.014 0.624 1.576 17.34 10.84 <0.001 0.026 Bal. 0.116 2.210
SA508 0.170 0.210 1.360 0.16 0.80 0.001 0.006 Bal. - 0.490
52Mb 0.019 0.110 0.810 29.77 59.20 <0.0005 0.003 8.73 0.006 0.008
52Mw 0.023 0.110 0.900 29.76 58.80 <0.0005 0.003 8.74 0.006 0.100
Material Co Cu Al Ti Cb(Nb)+Ta V B Zr Other
316LN <0.050 - - - - - - - -
SA508 - 0.04 - - - 0.005 - - -
52Mb 0.010 0.03 0.11 0.17 0.89 - 0.0007 0.010 <0.500
52Mw 0.006 0.02 0.11 0.19 0.89 - 0.0005 0.003 <0.494

Note: 52Mb—Inconel filler metal 52M used for buttering, 52Mw—Inconel filler metal 52M used for welding

表1 焊接件不同部位主要化学成分

Table 1 Chemical composition of materials in dissimilar metal weld joint (DMWJ) (mass fraction / %)

为了便于金相试样的制备,采用线切割工艺将焊缝切片沿厚度方向(径向)分为6块,分别记为H1~H6,如图1b所示。将线切割的试样首先利用水砂纸逐级打磨至2000号,然后机械抛光至2.5 µm,刻蚀后进行金相组织观察。由于材料不同,选用不同的刻蚀液:SA508低合金钢用4% (体积分数)硝酸酒精进行刻蚀;隔离层、对接焊缝区及316LN由16 g FeCl3+80 mL HCl+2 mL HNO3+11 mL H2O 配置的溶液进行刻蚀。利用 Observer. Z1m 光学显微镜(OM)对焊接接头不同部位的金相组织进行观察分析。利用配有能谱仪(EDS)的XL30 场发射环境扫描电镜(ESEM)分析了SA508/52Mb及52Mw/316LN熔合线界面处主要合金元素的分布。应用飞行时间二次离子质谱ToF-SIMS分析SA508/52Mb界面处C元素的分布情况,采用液态金属铋离子枪,加速电压为30 keV,控制软件及分析软件均为Surface Lab 6。

图1 焊接实物图和焊接件截面示意图及取样示意图

Fig.1 Photograph of the safe-end DMWJ (a) and schematic of the cross-section of the DMWJ and positions for micro-hardness testing, metallographic and EBSD observation (b) (unit: mm)

采用MHVD-1000AP显微硬度计测量了焊接件的外侧、对接焊缝底焊位置处及内侧的硬度变化,测量位置分别如图1b中的直线L1、L2及L3。加载载荷为200 g,保荷时间为15 s。

用ESEM的背散射电子衍射(EBSD)附件,分别对H1、H4及H6试样的52Mw/316LN界面附近的母材及焊缝区的晶粒取向、晶界类型以及残余应变(用kernel average misoritation (KAM)值表示[16])的分布进行了分析。每个试样上选取11个不同的区域进行EBSD扫描,以每个区域的中线与熔合线之间的距离来定义此区域距熔合线的距离,扫描步长为 3.5 µm,用 OIM (orientation imaging microscopy)软件对获得的数据进行分析。选取H4试样的SA508/52Mb界面进行EBSD观察,扫描步长为 0.5 µm。

采用小尺寸拉伸试样及带有缺口的棒状试样来研究焊接件不同部位的力学性能及应力腐蚀敏感性,其尺寸示意图分别如图2及3所示。采用线切割的方式在焊接接头的不同部位一共切取51个小尺寸拉伸试样,分别命名为T1~T51,它们在焊接接头中的位置如图4所示。拉伸实验在室温下进行,拉伸速率为5 µm/s。根据相关的文献[4,12]报道,熔合线附近部位或者焊缝区一般为应力腐蚀最为敏感的区域,所以选取了4个位置进行研究:S1试样的缺口位于SA508/52Mb界面;S2试样的缺口位于52Mw/316LN界面;S3试样的缺口位于对接焊底焊52Mw/52Mb界面附近的52Mb中;S4试样的缺口位于52Mb/52Mw界面。慢应变速率拉伸实验是在配有动态循环水系统的高温高压腐蚀测试系统中完成的,测试环境为含有1500 mg/L B及2.3 mg/L Li的模拟核电一回路水溶液,温度为325 ℃,初始氧含量为2 mg/L,应变速率为4×10-6 s-1

图2 小尺寸拉伸试样尺寸示意图

Fig.2 Schematic of the small-sized flat tensile sample used for the local mechanical property test (δ—thickness, unit: mm)

图3 慢应变速率拉伸实验试样尺寸示意图

Fig.3 Schematic of U-notched round-bar sample used for slow strain rate test (unit: mm)

图4 小尺寸拉伸试样在焊接件中的位置示意图

Fig.4 Schematic of locations of the 51 small-sized flat tensile samples in the DMWJ (unit: mm)

2 实验结果及讨论
2.1 金相组织观察

焊接件母材SA508、316LN、隔离层及对接焊缝区的金相组织分别如图5a~d所示。SA508低合金钢为回火贝氏体组织。仔细观察,又可区分为浅色的低C区及深色的高C区,表明C在SA508中的分布是不均匀的(图5a)。316LN为含有大量孪晶界的等轴奥氏体组织,在晶粒内部及晶界上有少量黑色点状的夹杂(图5b)。隔离层52Mb及对接焊缝区52Mw均为柱状奥氏体组织(图5c和d)。不同的是,隔离层中晶粒水平生长,对接焊缝区的晶粒则接近于竖直生长。这主要是由于在焊熔池凝固的过程中,晶粒沿着具有最大的温度梯度的方向(散热最快的方向)生长导致的。在隔离层中,某些不同焊接道次之间还存在一层细晶区(图5c)。

图5 SA508母材、316LN母材、 52Mb和 52Mw 的OM像

Fig.5 OM images of SA508 (a), 316LN (b), 52Mb (c) and 52Mw (d)

图6a和b分别为SA508/52Mb和52Mw/316LN界面的金相组织。在SA508/52Mb熔合线附近由于稀释作用而形成宽度约为20 μm 的灰色区域。在52Mb侧观察到大量的与熔合线接近平行的II型晶界以及连接II型晶界与熔合线的I型晶界。需要指出的是,虽然在整个焊接件截面厚度方向SA508/52Mb界面附近的52Mb侧存在大量的I型晶界与II型晶界,但是II型晶界并不是连续的。文献[18]中认为I型晶界主要是由于外延生长引起的。II型晶界的形成主要有2种解释,一种观点认为这主要取决于焊缝凝固过程中凝固方式的改变。在A588/309L界面处II型晶界的形成与熔池中凝固方式的转变有关,在熔化界面处A588基体为bcc结构,靠近熔化界面处液态金属凝固为铁素体(bcc结构),随着距熔化界面距离的增加,奥氏体稳定化元素含量增加,凝固方式发生变化,形成奥氏体(fcc结构),凝固方式的改变导致II型晶界的产生,其与熔合线的距离决定于成分浓度变化的快慢[19]。另一种观点则认为主要形成于晶界的迁移。Nelson等[18]在研究70Ni/30Cu/纯Fe焊接界面时,认为II型晶界是焊接冷却阶段固态晶界在奥氏体化温度内迁移的结果。此外,研究[20]表明,在450 ℃下进行老化处理的时间越长,II型晶界越多,II型晶界与熔合线之间的距离越大,这也是晶界迁移引起的。因此,观察到的52M侧的II型晶界的形成更可能是γ-fcc晶界在奥氏体温度范围的迁移而形成的。

图6 SA508/52Mb界面和52Mw/316LN界面的OM像

Fig.6 OM images of SA508/52Mb interface (a) and 52Mw/316LN interface (b)

进一步分析52Mw/316LN界面的金相组织(图6b)可知,焊接熔池边界处结晶过程为明显的外延生长,取向与316LN母材半熔化的原始晶粒的取向一致。这主要是因为52M与316L均为奥氏体组织,具有相同的晶体结构,半熔化的母材晶粒与熔池中的液态金属几乎完全润湿,液体金属原子排列在半熔化的母材晶粒上而不改变原有的晶粒取向[21]。在熔化边界外,晶粒的生长为竞争生长,沿着垂直于熔池边界的方向生长,因为这个方向上的温度梯度最大、散热最快。当晶粒的取向与温度梯度最大方向一致时晶粒的生长速度最快,形成粗大的柱状树枝晶,当晶粒的取向与温度梯度最大的方向不一致时晶粒的生长速度小,遇到已经形成的其它晶粒后,生长停止[6]。在界面附近存在一宽度不均匀的未混合区。熔化的316LN母材与熔化的镍基合金52M在没有进行充分的混合的情况下就冷凝固,形成未混合区。

图7为图6a中SA508低合金钢的热影响区组织。SA508低合金钢热影响区的宽度为2~3 mm。随着距熔合线距离的增加存在明显的组织过渡:粗大的铁素体+少量碳化物(粗晶区,图7a)→贝氏体+细晶马氏体(细晶区,图7b~f)→铁素体+贝氏体+马氏体(部分相变区,图7g)→贝氏体(母材,图7h)。这主要是由于焊接热影响区受热时升温速率快、高温停留时间短、冷却速率快,焊接条件下的组织转变不同于等温转变,导致组织极其复杂。同时各种组织相互掺杂,很难确定各组织所占的分数。在紧邻熔合线的部位还观察到明显的贫C区(图7a中白亮区域),且贫C区的宽度分布不均匀。凸出到焊缝中的部位具有更多的C扩散通道,凸出到焊缝中的部位也就具有更宽的贫C区[6]

图7 SA508低合金钢热影响区的OM像

Fig.7 OM images of the microstructure transition in the SA508 heat affected zone (HAZ) (Figs.7a~h are higher magnification images of the microstructure transition: coarse ferrite+small amounts of carbides (coarse-grained region, Fig.7a)→bainite+fine martensite (fine-grained region, Figs.7b~f)→ferrite+martensite+bainite (partially transformed region, Fig.7g)→bainite (base metal, Fig.7h))

图8为316LN热影响区的金相组织。与远离焊缝的母材组织相似(图5b),为奥氏体组织,且不存在明显的晶粒长大。因而焊接过程对316LN侧的宏观组织影响不大。

图8 316LN热影响区的OM像

Fig.8 OM image of the microstructure transition in the 316LN HAZ

2.2 界面处成分分布

用较高的倍数对SA508/52Mb界面附近进行更加细致的观察,发现熔合线附近存在2种组织形态。一种如图9所示(与图7a类似),在熔合线附近没有发现马氏体区,成分由SA508到52M在界面处突变,几乎不存在成分过渡区。另一种如图10所示,在熔合线附近存在马氏体区(AB之间),马氏体区的成分介于SA508与52M之间而更接近于SA508;成分由SA508到52M存在较宽的过渡区。关于马氏体的形成可以用Schaeffler图进行解释[6,7]

图9 无马氏体区的SA508/52Mb界面形貌及主要金属元素分布

Fig.9 Morphology of SA508/52Mb interface without a martensite zone (a) and EDS analysis along the line in Fig.9a (b)

图10 带有马氏体区的SA508/52Mb界面形貌及主要金属元素分布

Fig.10 Morphology of SA508/52Mb interface with a martensite zone (a) and EDS analysis along the line in Fig.10a (b)

图11为52Mw/316LN界面成分分析结果。52Mw侧Ni和Cr的含量明显高于316LN中Ni和Cr的含量,而Fe的分布则完全相反。在熔合线附近焊缝侧存在成分稀释区(AB之间),在该区域内Ni、Cr和Fe的含量介于52M和316LN的化学成分之间。另外在,在BC之间存在一区域,其成分与316LN类似,可能是熔化的316LN或半熔化的

316LN在熔池中搅拌力的作用下进入焊缝,在没有与熔化的52M进行充分的混合前发生凝固,就形成了这一区域。

2.3 EBSD观察

H1、H4及H6试样52Mw/316LN界面区域的EBSD结果相似,图12a是H1样品的具体分析结果。熔合线两侧的晶粒反极图(IPF,图12a)表明,熔合线界面附近316LN与邻近的52M晶粒取向相近,进一步证实对接焊缝中熔合线附近由外延生长形成。在316LN侧,晶粒的取向随机分布,没有发现有择优取向现象;随着距熔合线距离的增加晶粒尺寸没有明显的变化,这与金相观察的结果一致,焊接过程对316LN侧的金相组织影响较小。研究[18]表明,在焊缝中应该存在择优取向,因为凝固过程中晶粒的生长方向为最大散热方向。但是,由于焊缝中晶粒尺寸过大,扫描范围内晶粒的个数较少,无法准确完成织构分析。

图11 52Mw/316LN界面形貌及主要金属元素分布

Fig.11 Morphology of 52Mw/316LN interface (a) and EDS analysis along the line in Fig.11a (b)

图12b和c分别是H1样品界面两侧区域的残余应变分布图和晶界类型分布图,将图12b及H4和H6样品中的残余应变以及图12c中H1样品的不同类型晶界数量分数按照距离熔合线的距离进行统计分析,结果分别如图12d和e所示。在熔合线附近存在较大的残余应变,特别是熔合线附近316LN侧。随着距离熔合线距离的增加,316L侧残余应变逐渐降低。残余应变主要分布在熔合线位置处,这是因为距离熔合线较近的区域在焊接过程中存在高的峰值温度及较快的升温和降温速率,而随着距熔合线距离的增加,峰值温度降低,升、降温速率降低[22]。在52M一侧,残余应变的分布很不均匀,无明显的规律。在熔合线两侧,较大的残余应变主要分布在界面位置处。H1、H4及H6试样52Mw/316LN界面两侧残余应变的最大值均出现在界面附近的316LN侧(图12d)。对比3个试样,处在对接焊底焊位置处的H4试样的残余应变远高于其余2个试样的残余应变,因为这个位置受到的焊接热循环次数最多[23],受到的约束最大。

图12 H1、H4及H6试样EBSD分析

Fig.12 Inverse pole figures (IPFs) (a), kernel average misorientation (KAM) maps (b), grain boundary character distribution (GBCD) maps (c) as a function of the distance from the fusion boundary (x) of 316LN and 52M in sample H1, KAM as a function of the distance from the 52Mw/316LN interface in samples H1, H4 and H6 (d), and the number fractions of low angle boundary (LAB), coincidence site lattice (CSL) boundary and random high angle grain boundary (RGB) as a function of the distance from the 52Mw/316LN interface in sample H1 (e)

根据两相邻晶粒间的取向差可以将晶界分为3种:小角度晶界(LAB, 5°<θ<15°),重位点阵(CSL)晶界(3≤Σ≤29)及随机大角度晶界(RGB,15°<θ<180°),其中Σ为CSL点阵单晶胞与实际点阵单晶胞的比值[9]。在52M侧(图12c),主要为随机大角度晶界并伴随有小角度晶界;在316LN侧(图12c及e),随着距离熔合线距离的增加,随机大角度晶界的数量分数越来越小,小角度晶界的数量分数越来越小,CSL晶界的数量分数越来越大。

图13a~d分别是H4试样SA508/52Mb界面的IQ (image quality)图、IPF图、KAM图及相分布图。可见,在界面处没有发现明显的I型晶界及II型晶界,不存在外延生长现象,界面附近也不存在马氏体区。52M中残余应变分布不均匀,SA508组织为bcc结构,52M为fcc结构。

图13 H4试样SA508/52Mb界面的图像质量(image quality, IQ)图、IPF、KAM图及相分布图

Fig.13 IQ map (a), IPF (b), KAM map (c) and phase distribution (d) of the SA508/52Mb interface in sampe H4

2.4 显微硬度测试

图14~16分别是图1b中L1、L2和L3 3个位置的显微硬度测量结果。结果表明,SA508低合金钢基体内、外壁的显微硬度约为210 HV0.2,心部显微硬度略低且起伏较大,约为200 HV0.2。316LN不锈钢基体内、外壁的显微硬度约为195 HV0.2,心部显微硬度约为180 HV0.2。3个位置的硬度分布存在相同的变化趋势:SA508低合金钢及316LN不锈钢的热影响区的显微硬度高于母材的显微硬度;最高硬度出现在SA508/52Mb界面附近的52Mb中,而最低硬度出现在SA508/52Mb界面附近的SA508热影响区的脱碳层中,如图14~16中SA508/52Mb界面的方框所示;在52Mb中,随着距离SA508/52Mb界面距离的增加显微硬度增大;52Mb及52Mw中显微硬度分布不均,起伏较大;在316LN热影响区中最邻近52Mw/316LN界面处一定范围内会出现一定的回火软化。3个位置的显微硬度分布不同的是,在L1处52Mw的显微硬度略小于52Mb,在L3处52Mw与52Mb的硬度相差不大,在L2处52Mw的硬度大于52Mb的硬度并且大于其余2个部位的52Mw的硬度。

图14 沿图1b中L1线的显微硬度分布及界面处显微硬度压痕

Fig.14 Microhardness distribution along the DMWJ across L1 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

图15 沿图1b中L2线的显微硬度分布及界面处显微硬度压痕

Fig.15 Microhardness distribution along the DMWJ across L2 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

图16 沿图1b中L3线的显微硬度分布及界面处显微硬度压痕

Fig.16 Microhardness distribution along the DMWJ across L3 (as labeled in Fig.1b) (a) and the indentations in the interface (b, c)

应用ToF-SIMS研究了SA508/52Mb界面的C元素分布,如图17所示。发现在界面处52Mb侧存在宽度为25 µm的富C区,起到了强烈的固溶强化的作用,使得在此区域出现显微硬度的最大值。在SA508侧C元素的含量低,衬度不明显,为贫C区,使得此区域出现显微硬度的最低值。富C区及贫C区的形成都是焊接过程及热处理过程中C元素迁移的结果。C迁移的驱动力为C浓度梯度,更准确的说是低含Cr量的低合金钢与高含Cr量的填充金属之间的C活度梯度[24]。316LN中,热影响区的硬度高于基体的硬度主要是因为热影响区的残余应变高于基体的残余应变,如图12d所示。在隔离层及对接焊缝区,残余应变的分布不均匀,起伏较大,导致在隔离层及对接焊缝区的显微硬度起伏较大。在对接焊的底焊位置处,由于52Mw具有高的残余应变,使得此处的显微硬度明显高于52Mb的显微硬度。

图17 SA508/52Mb界面的SEM像及C元素分布

Fig.17 SEM image of the SA508/52Mb interface (a) and C distribution (b)

2.5 焊接件不同部位力学性能测试

该焊接件不同部位的屈服强度、抗拉强度及断裂应变的测量结果如图18所示。不同部位的强度变化规律与显微硬度类似。SA508及316LN热影响区的强度均高于基体,在SA508热影响区中细晶区的强度最高,在316LN热影响区中随着距熔合线距离的增大屈服强度逐渐降低,抗拉强度变化不大;52Mw的强度略高于52Mb的强度;在52Mb中,随着距SA508/52Mb界面距离的增大,屈服强度逐渐增大。在SA508 (包含热影响区)中,断裂应变变化不大;在316LN中,热影响区的断裂应变明显低于基体的断裂应变;52Mb的平均断裂应变高于52Mw的平均断裂应变,且52Mb中的断裂应变起伏变化明显高于52Mw;在52Mb及52Mw中均出现断裂应变极低的位置。

图18 焊接件不同部位室温下屈服强度、抗拉强度及断裂应变

Fig.18 Yield strength, ultimate strength and fracture strain of all the 51 samples across the DMWJ at room temperature (as shown in Fig.4)

在SA508热影响区的细晶区,由于细晶强化作用而具有较高的强度;在316L热影响区中由于存在较高的残余应变而使其具有较高的强度;52Mw中柱状晶的长度方向与拉伸方向接近一致,52Mb中柱状晶的长度方向与拉伸方向接近垂直,导致两者之间强度的差异;研究过程中发现在52Mb及52Mw中均存在焊接缺陷,52Mb及52Mw中断裂应变极低的位置可能与这些区域组织中随机分布的焊接缺陷,如微观焊接热裂纹等有关。焊接件不同位置的力学性能差异主要取决于不同部位的微观结构差异。

2.6 焊接件不同部位慢应变速率拉伸实验

焊接件不同部位在模拟核电高温高压水环境中的慢应变速率拉伸实验结果如图19所示。通过断裂时的伸长量来评价焊接件不同部位的应力腐蚀敏感性,发现不同部位应力腐蚀敏感性由高到低为:S1 (SA508/52Mb)>S2 (52Mw/316LN)>S4 (52Mb/52Mw)>S3 (近52Mw/52Mb)。

图19 焊接件不同部位在模拟核电高温高压水环境中慢应变速率拉伸实验结果

Fig.19 Stress-extension curves of local areas in the DMWJ obtained by slow strain rate test (SSRT) conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃

Hou等[4]和Chung等[9]研究发现,I型晶界及II型晶界主要为随机大角度晶界,晶界上有大量的富Cr碳化物,富Cr碳化物周围存在贫Cr区,因而其抗晶间腐蚀及应力腐蚀的能力低,易成为应力腐蚀裂纹萌生的区域,也易成为应力腐蚀裂纹的扩展通道。在SA508/52Mb界面附近的52Mb中存在大量的对应力腐蚀敏感的I型晶界与II型晶界,导致此界面处具有最高的应力腐蚀敏感性。此外,有研究[25]也发现硬度与SCC存在联系,在研究冷加工对不锈钢SCC的影响时,发现20%的冷加工会使其硬度达到200 HV以上,穿晶裂纹起始于表面硬化区,当裂纹扩展出表面硬化层时转为沿晶开裂。界面处52Mb侧在焊接件中具有最高的硬度,也加剧了此区域的应力腐蚀敏感性。

焊接时焊件受到不均匀的加热并使焊缝熔化,与熔池相邻的母材的受热膨胀则会受到周围冷态材料的制约,产生不均匀的压缩塑性变形,在随后的冷却过程中这部分材料同样会受到周围金属的制约而不能自由收缩,并在一定程度上受到拉伸而卸载,与此同时,熔池凝固,焊缝金属的冷却收缩也因为受到制约而产生收缩拉应力及变形,形成焊接残余应力/应变[26]。焊接残余应力/应变与冷加工相似,都会促进SCC的发生,加速裂纹扩展[27]。Andresen[28]用EBSD技术研究焊接部位的残余应变,认为焊接引起的残余应变的峰值相当于室温下20%的应变,而且残余应变与残余应力一样,对焊缝及母材的热影响区的应力腐蚀行为具有非常重要的影响,使不锈钢焊缝及焊接热影响区具有高的应力腐蚀裂纹扩展速率。

此外,许多研究[29,30]已经表明SCC裂纹主要沿着随机大角度晶界扩展,说明大角度晶界是三类晶界中抗腐蚀、抗开裂性能最差的。研究[31~33]表明,在304不锈钢、600合金、15%应变的316L不锈钢、25%应变的316L不锈钢、15%应变的690合金及25%应变的690合金中,随着小角度晶界及CSL晶界数量的增多,应力腐蚀裂纹的数量/长度减小,说明随着材料中小角度晶界及CSL晶界数量的增加,材料的抗SCC性能增加。在316LN中,CSL晶界主要由Σ3晶界构成(>95%)。文献[31]发现,Σ3晶界也会发生应力腐蚀开裂现象,主要是由于这些Σ3晶界与理想的Σ3晶界结构存在较大的偏差角。在316LN的热影响区中,Σ3晶界与理想的Σ3晶界的偏差角随距离熔合线距离的增大而减小,如图20所示,说明热影响区中不仅Σ3晶界数量少而且抗应力腐蚀能力也比基体中的Σ3晶界低。

图20 H1试样中316LN侧距熔合线不同距离位置处Σ3晶界与理想的Σ3晶界偏差角分布图

Fig.20 Distributions of angular deviations from the ideal Σ3 misorientation as a function of the distance from the fusion boundary in 316LN of sample H1

因此,316LN热影响区中高的残余应变及显微硬度、高的随机大角度晶界数量分数使得其具有高的应力腐蚀敏感性,也就使得S2 (52Mw/316LN)试样具有较高的应力腐蚀敏感性,且断口位于316LN的热影响区中。

3 结论

(1) SA508母材为贝氏体组织,316LN为具有大量孪晶的奥氏体组织,52Mb及52Mw为柱状奥氏体组织。在SA508/52Mb界面处的52Mb中具有大量的对应力腐蚀敏感的I型晶界及II型晶界,导致此界面具有最高的应力腐蚀敏感性。

(2) SA508热影响区存在明显的组织过渡;316LN热影响区中随着离熔合线距离的增加,CSL的数量分数逐渐增大,Σ3晶界与理想的Σ3晶界的偏差角减小,残余应变逐渐减小,残余应变的最高值出现在对接焊底焊位置处的316LN热影响区中,316LN的热影响区也具有较高的应力腐蚀敏感性;52M (包含52Mb和52Mw)为柱状奥氏体组织,堆焊层(52Mb)中不同焊道间存在细晶区,52M中的残余应变分布不均匀,残余应变主要集中在晶界附近,晶界主要为随机大角度晶界。

(3) 在SA508/52Mb界面及52Mw/316LN界面处均存在明显的成分过渡;在SA508/52Mb界面附近SA508侧存在贫C区,在52Mb侧存在富C区。

(4) 整个焊缝区由一侧母材到另一侧母材,显微硬度、强度及断裂应变存在明显的变化。对于硬度分布而言,显微硬度变化最剧烈的位置发生在SA508/52Mb界面附近,且此界面附近的52Mb具有最高的硬度,此界面附近的SA508脱C区具有最低的硬度。强度的变化趋势与硬度的变化趋势类似。一般强度高的地方断裂应变低。焊接件不同位置的性能差异主要取决于不同部位的微观结构(包含组织、成分等)差异。

The authors have declared that no competing interests exist.

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The microstructure of an SA508–309L/308L–316L domestic dissimilar metal welded safe-end joint was characterized in this work by optical microscopy, scanning electron microscopy (with electron back scattering diffraction) and micro-hardness testing. Epitaxial growth and competitive growth are evident in the 308L–316L fusion boundary regions. A martensite layer, carbon-depleted zones, and type-II and type-I boundaries are found in the SA508–309L fusion boundary regions, while only martensite and austenite mixed zones are observed in the SA508–308L fusion boundary regions. The microstructure near the fusion boundary and the microstructure transition in the SA508 heat affected zone are quite complex. Both for SA508–309L/308L and 308L–316L, the highest residual strain is located on the outside of the weldment. The residual strain and the grain boundary character distribution change with increasing distance from the fusion boundary in the heat affected zone of 316L. Micro-hardness measurements also reveal non-uniform mechanical properties across the weldment.
DOI:10.1016/j.matchar.2014.08.023      URL     [本文引用:4]
[7] Wang S Y, Ding J, Ming H L, et al.Characterization of low alloy ferritic steel-Ni base alloy dissimilar metal weld interface by SPM techniques, SEM/EDS, TEM/EDS and SVET[J]. Mater. Charact., 2015, 100: 50
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[8] Hou J, Peng Q J, Takeda Y, et al.Microstructure and mechanical property of the fusion boundary region in an alloy 182-low alloy steel dissimilar weld joint[J]. J. Mater. Sci., 2010, 45: 5332
Characterizations of the microstructure and mechanical property of the fusion boundary region of an Alloy 182-A533B low alloy steel (LAS) dissimilar weld joint were conducted. The existence of type-II
DOI:10.1007/s10853-010-4581-6      URL     [本文引用:0]
[9] Chung W C, Huang J Y, Tsay L W, et al.Microstructure and stress corrosion cracking behavior of the weld metal in alloy 52-A508 dissimilar welds[J]. Mater. Trans., 2011, 52: 12
ABSTRACT In the nuclear power industry, dissimilar metal welding is widely used for joining low alloy steel to austenite stainless steel components with nickel-base filler metals. In this study, attention was paid to the weld metal in multi-pass Alloy 52-A508 dissimilar welds. An approximately 2 mm wide transition zone was observed that consisted of a martensitic layer (10-20um) along the weld interface and the austenite phase region with varying degrees of dilution. After post-weld heat treatment, the microstructures near the weld interface consisted of martensite, carbides and Type II boundaries. The presence of Type II boundaries significantly reduced the resistance to stress corrosion cracking (SCC) and formed intergranular cracking under simulated reactor coolant conditions. Constant extension rate tensile (CERT) tests were performed on the notched tensile specimens in 300掳C water at two extension rates, 3 脳 10-4 and 1 脳 10-6 mm/s. A fast CERT test can be regarded to have no contribution of corrosion, and its results can be used as standards for comparison. In the slow CERT tests, the ductility losses of round-bar specimens with a circumferential notch at various regions in the weld metal were ranked accordingly. The relative susceptibility to SCC in terms of the ductility loss in increasing order of severity was as follows: the undiluted weld metal, the transition zone and the weld interface. SEM fractographic observations were consistent with the SCC results, i.e., an increased ductility loss or SCC susceptibility was associated with more brittle fractures.
DOI:10.2320/matertrans.M2010294      URL     [本文引用:3]
[10] Hou J, Peng Q J, Shoji T, et al.Study of microstructure and stress corrosion cracking behavior in welding transition zone of Ni-based alloys[J]. Acta Metall. Sin., 2010, 46: 1258
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(侯娟, 彭群家, 庄子哲雄等. 镍基合金焊接过渡区微观结构及应力腐蚀行为研究[J]. 金属学报, 2010, 46: 1258)
利用微观分析手段深入分析了镍基合金690-52同种金属焊接和镍基182合金-A533B低合金钢异种金属焊接的焊接过渡区微观结构, 定量测量了同种金属焊接热影响区(HAZ)残余应变分布, 用带缝隙的弯曲横梁样品模拟了异种金属焊接过渡区在高温含氧水中的应力腐蚀开裂(SCC)行为. 结果表明, 同种金属焊接过渡区的HAZ具有残余应变峰值、敏感的晶界微观结构, 因而导致最高的SCC敏感性; 异种金属焊接过渡区具有复杂的微观结构和成分分布, 典型特征是靠近熔接线(FB)的熔合区(FZ)内形成平行于FB的type-II和连接FB与type-II的type-I晶界,type-II具有高的SCC敏感性和裂纹扩展速率, type-I引导裂纹向FB扩展, 裂纹到达FB后发生点蚀钝化停止扩展, FB起阻碍裂纹进一步向低合金钢扩展的作用.
DOI:10.3724/SP.J.1037.2010.00236      Magsci     URL    
[11] Ding J, Zhang Z M, Wang J Q, et al.Micro-characterization of dissimilar metal weld joint for connecting pipe-nozzle to safe-end in generation III nuclear power plant[J]. Acta Metall. Sin., 2015, 51: 425
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(丁杰, 张志明, 王俭秋. 三代核电接管安全端异种金属焊接接头的显微表征[J]. 金属学报, 2015, 51: 425)
利用OM, TEM, SEM, 显微硬度仪, AFM, 磁力显微镜(MFM)和扫描Kelvin探针(SKPFM)等微观分析手段, 分析了先进压水堆核电站反应堆压力容器安全端异种金属焊接接头低合金钢A508/镍基焊料52M/奥氏体不锈钢316L的金相组织、显微硬度、主要合金元素、晶界类型以及残余应变的分布, 并对比了整个焊接接头不同厚度上的组织和性能. 结果表明, 焊缝厚度方向上组织和硬度没有显著差别, 底焊位置出现一层未熔焊料形成的细小等轴晶, 在316L母材热影响区(HAZ)内残余应变较焊接件其它位置高, 熔合线附近具有复杂的微观结构、显微硬度、晶界类型、元素成分和残余应变分布. TEM和MFM分析表明, 母材316L基体内有富Cr, Mo元素的颗粒状析出相, SKPFM的结果显示该析出相Volta电势较基体更负, 因而更不耐腐蚀.
DOI:10.11900/0412.1961.2014.00299      Magsci     URL    
[12] Ming H L, Zhang Z M, Xiu P Y, et al.Microstructure, Residual strain and stress corrosion cracking behavior in 316L heat-affected zone[J]. Acta Metall. Sin.(Engl. Lett.), 2016, 29: 848
DOI:10.1007/s40195-016-0461-7      URL     [本文引用:2]
[13] Li G F, Congleton J.Stress corrosion cracking of a low alloy steel to stainless steel transition weld in PWR primary waters at 292 ℃[J]. Corros. Sci., 2000, 42: 1005
The effects of electrode potential and sulphate ( ja:math ) content in the water on the stress corrosion cracking (SCC) of a low alloy steel — austenitic stainless steel transition weld, A508-309L/308L, in pressurised water reactor (PWR) primary side waters at 292°C have been studied using slow strain rate testing (SSRT). The weld was post-weld-heat-treated at 620°C for 20 h before testing. Results showed that the transition zone in the weld had a higher susceptibility to SCC than either the bulk stainless steel or the bulk low alloy steel. The SCC in the transition zone was mainly intergranular in the austenitic layer, but transgranular cracking occurred at the interface and in the low alloy steel. The minimum potential for SCC, ja:math , in each water used was higher than the free corrosion potential range of 61880 to 61660 mV (SHE) and the susceptibility to SCC increased with increasing electrode potential. In sulphate doped waters, crack growth rates >2 × 10 616 mm/s occurred at high applied potentials in the low alloy steel and/or in the austenitic layer but some less severe cracking occurred at the interface. Contamination of the water with ja:math increased the SCC susceptibility by both decreasing the minimum potentials for cracking and increasing the crack growth rate. However, the data suggest that transition welds should be immune from SCC in typical PWR primary side coolant water at 292°C even in the unlikely event that a break in the stainless steel cladding allowed access of the cooling water to the transition joint area.
DOI:10.1016/S0010-938X(99)00131-6      URL     [本文引用:1]
[14] Li G F, Li G J, Fang K W, et al.Stress corrosion cracking behavior of dissimilar metal weld A508/52M/316L in high temperature water environment[J]. Acta Metall. Sin., 2011, 47: 797
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(李光福, 李冠军, 方可伟. 异材焊接件A508/52M/316L在高温水环境中的应力腐蚀破裂[J]. 金属学报, 2011, 47: 797)
采用慢应变速率实验(SSRT)方法, 研究了先进的异材焊接件A508/52M/316L在模拟压水堆一回路290 ℃高温水环境中的应力腐蚀破裂(SCC)特性. 实验在-780 mV至+200 mV范围的电位下进行, 模拟一回路水化学从低O含H的理想低电位状态到溶解O$_{2}$明显超标的高电位状态的服役环境. 该焊接件显微组织和化学成分分布较复杂, 显著的变化发生在A508/52M和52M/316L 2个界面附近. 在SSRT拉伸试样的典型位置处加工了同样尺寸的尖锐缺口, 以模拟应力集中和加速实验, 并比较这些典型位置的SCC 敏感性. 结果表明, 当电位位于-780 mV至-300 mV范围时, SSRT试样总是以韧性断裂形式在镍基合金焊缝中部发生断裂. 当电位升到-200 mV至+200 mV范围时, 试样发生显著的SCC脆断, A508/52M界面区周围是该焊接件最脆弱的部位, 在该界面和附近的A508热影响区发生穿晶应力腐蚀破裂(TGSCC), 在紧邻界面的镍基合金焊缝薄层发生沿晶应力腐蚀破裂(IGSCC). 讨论了破裂机理和实验结果的工程意义.
DOI:10.3724/SP.J.1037.2011.00316      Magsci     URL    
[15] Deng D, Murakawa H.Numerical simulation of temperature field and residual stress in multi-pass welds in stainless steel pipe and comparison with experimental measurements[J]. Comput. Mater. Sci., 2006, 37: 269
A computational procedure is presented for analyzing temperature fields and residual stress states in multi-pass welds in SUS304 stainless steel pipe. Based on the ABAQUS software, uncoupled thermal-mechanical three-dimensional (3-D) and two-dimensional (2-D) finite element models are developed. The finite element models are employed to evaluate the transient temperature and the residual stress fields during welding. Firstly, a 3-D model is developed to simulate the temperature fields and welding residual stresses. Secondly, based on the characteristics of the temperature fields and the welding residual stress fields, a 2-D axisymmetric model is also developed. The simulated result shows that the 2-D axisymmetric model can be effectively used to simulate the thermal cycles and the welding residual stresses for SUS304 stainless steel pipe. Using the 2-D model, a large amount of computational time can be saved. In this study, experiments are also carried out to verify the effectiveness of the proposed numerical models. The results of both 3-D model and 2-D model are in very good with the experimental measurements.
DOI:10.1016/j.commatsci.2005.07.007      URL     [本文引用:1]
[16] Lim Y S, Kim H P, Cho H D, et al.Microscopic examination of an alloy 600/182 weld[J]. Mater. Charact., 2009, 60: 1496
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[17] Ming H L, Zhu R L, Zhang Z M, et al.Microstructure, local mechanical properties and stress corrosion cracking susceptibility of an SA508-52M-316LN safe-end dissimilar metal weld joint by GTAW[J]. Mater. Sci. Eng., 2016, A669: 279
Abstract The microstructure, local mechanical properties and local stress corrosion cracking susceptibility of an SA508-52M-316LN domestic dissimilar metal welded safe-end joint used for AP1000 nuclear power plant prepared by automatic gas tungsten arc welding was studied in this work by optical microscopy, scanning electron microscopy (with electron back scattering diffraction and an energy dispersive X-ray spectroscopy system), micro-hardness testing, local mechanical tensile testing and local slow strain rate tests. The micro-hardness, local mechanical properties and stress corrosion cracking susceptibility across this dissimilar metal weld joint vary because of the complex microstructure across the fusion area and the dramatic chemical composition change across the fusion lines. Briefly, Type I boundaries and Type II boundaries exist in 52 Mb near the SA508-52Mb interface, a microstructure transition was found in SA508 heat affected zone, the residual strain and grain boundary character distribution changes as a function of the distance from the fusion boundary in 316 LN heat affected zone, micro-hardness distribution and local mechanical properties along the DMWJ are heterogeneous, and 52Mw-316LN interface has the highest SCC susceptibility in this DMWJ while 316LN base metal has the lowest one.
DOI:10.1016/j.msea.2016.05.101      URL     [本文引用:1]
[18] Nelson T W, Lippold J C, Mills M J.Nature and evolution of the fusion boundary in ferritic-austenitic dissimilar metal welds-Part 2: on-cooling transformations[J]. Weld. J., 2000, 79: 267s
Microstructural evolution at the fusion boundary in dissimilar welds between ferritic and austenitic alloys can significantly influence both the weldability and service behavior of the dissimilar combination. A fundamental investigation was undertaken to characterize fusion boundary microstructure and to better understand the nature and character of boundaries that are associated with cracking in dissimilar welds. In a previous paper, the evolution of the fusion boundary during the onset of solidification was discussed. In this paper, the nature and evolution of the fusion boundary and surrounding regions in dissimilar metal welds during subsequent on-cooling transformations in the fusion zone and heat-affected zone (HAZ) will be discussed. A model system consisting of a high-purity iron base metal and 70Ni-30Cu (AWS A5.14 ERNiCu-7) filler metal was used to study this behavior. Using this simple Fe-Ni-Cu system, fusion boundary microstructures were developed that were analogous to those observed in more complex engineering systems. Transmission electron diffraction analysis and orientation imaging microscopy (OIM) revealed the orientation relationships between adjacent HAZ and weld metal grains at the fusion boundary were different than the cube-on-cube relationship normally observed in similar metal welds. The room temperature fusion boundary in the system studied exhibited grain boundary misorientations consistent with common FCC/BCC relationships, i.e., Bain, Kurdjumov-Sachs and Nishyama-Wassermann. A theory describing the evolution of the fusion boundary is proposed and the nature and character of the Type II grain boundary is described.
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[19] Wu Y, Patchett B M.Formation of crack-susceptible structures of weld overlay of corrosion resistant alloys[J]. Mater. Perform.: Sulphur Energy, 1992, 32: 83
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[20] Yoo S C, Choi K J, Bahn C B, et al.Effects of thermal aging on the microstructure of Type-II boundaries in dissimilar metal weld joints[J]. J. Nucl. Mater., 2015, 459: 5
ABSTRACT In order to investigate the effects of long-term thermal aging on the microstructural evolution of Type-II boundary regions in the weld metal of Alloy 152, a representative dissimilar metal weld was fabricated from Alloy 690, Alloy 152, and A533 Gr.B. This mock-up was thermally aged at 450 掳C to accelerate the effects of thermal aging in a nuclear power plant operation condition (320 掳C). The microstructure of the Type-II boundary region of the weld root, which is parallel to and within 100 渭m of the fusion boundary and known to be more susceptible to material degradation, was then characterized after different aging times using a scanning electron microscope equipped with an energy dispersive X-ray spectroscope for micro-compositional analysis, electron backscattered diffraction detector for grain and grain boundary orientation analysis, and a nanoindenter for measurement of mechanical properties. Through this, it was found that a steep compositional gradient and high grain average misorientation is created in the narrow zone between the Type-II and fusion boundaries, while the concentration of chromium and number of low-angle grain boundaries increases with aging time. A high average hardness was also observed in the same region of the dissimilar metal welds, with hardness peaking with thermal aging simulating an operational time of 15 years.
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[21] Kou S.Welding Metallurgy[M]. 2nd Ed., Hoboken, New Jersey: John Wiley & Sons Inc., 2003: 170
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[22] Srinivasan P B, Muthupandi V, Dietzel W, et al.An assessment of impact strength and corrosion behaviour of shielded metal arc welded dissimilar weldments between UNS 31803 and IS 2062 steels[J]. Mater. Des., 2006, 27: 182
The joining of duplex stainless steel (DSS) to carbon steel (CS) was attempted by shielded metal arc welding, with E2209 and E309 electrodes. The hardness and impact strength of the weld metal produced with E2209 electrodes were found to be better than that obtained with E309. Though the general corrosion resistance of the weld metal produced with E309 was superior in 1M NaCl solution, they exhibited a higher pitting susceptibility in this test environment. The passivation behaviour of the weld metal with E2209 was observed to be on par with that of the duplex stainless steel base material (DSSBM) in 1M H 2 SO 4 solution; however, in terms of pitting resistance in 1M NaCl solution both weld metals were inferior to the DSSBM. Though E309 electrodes are widely employed for producing dissimilar weld joints, based on the observations in the current work, it is concluded that the E2209 electrode is the most suitable consumable for joining DSS to CS.
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[23] Qiao D X, Zhang W, Pan T Y, et al.Evaluation of residual plastic strain distribution in dissimilar metal weld by hardness mapping[J]. Sci. Technol. Weld. Joi., 2013, 18: 624
ABSTRACT The knowledge of residual plastic strains is a prerequisite for studying the stress corrosion cracking in dissimilar metal welds common to nuclear power plant structures. In this work, the distribution of residual equivalent plastic strains in a multipass dissimilar metal weld composed of nickel alloy 82 and austenitic stainless steel 304L is evaluated quantitatively through microhardness mapping. The contribution to hardness from the plastic strain (workhardening) is separated from that from the chemistry variation in the dissimilar metal weld. It is found that high equivalent plastic strains are predominately accumulated in the buttering layer, the root pass and the heat affected zone, which experience multiple welding thermal cycles. The final cap passes, experiencing only one or two welding thermal cycles, exhibit less plastic strain accumulation. Moreover, the experimental residual plastic strains are compared with those predicted using an existing weld thermomechanical model with two different strain hardening rules. The importance of considering the dynamic strain hardening recovery due to high temperature exposure in welding is discussed for the accurate simulation of weld residual stresses and plastic strains. Finally, the experimental result reveals that the typical post-buttering heat treatment for residual stress relief may not completely eliminate the residual plastic strains in the buttering layer.
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[24] Bhaduri A K, Venkadesan S, Rodriguez P, et al.Transition metal joints for steam generators-an overview [J]. Int. J. Press. Vessels Pip., 1994, 58: 251
ABSTRACT The transition metal joint (TMJ) between an austenitic stainless steel and a chromium-molybdenum (Cr-Mo) ferritic steel used widely in steam generators of power plants has for a long time presented problems relating to premature failures in service. The direct (bimetallic) TMJ presently in use is designed for a service life of about 200,000 h; but such TMJs with iron-base weld metals have been failing in service within about one-third of their design lifetime, while their counterparts with nickel-base weld metals fail within about one-half of their design lifetime. The causes for such premature service failures of these TMJs are discussed in detail, leading to the development of improved TMJs. One of the improved TMJs with a trimetallic configuration of austenitic stainless steel/Alloy 800/Cr-Mo ferritic steel is discussed in detail, covering its development, characterisation and evaluation. Accelerated performance tests in the laboratory have indicated a four-fold improvement in the service life of the TMJ with this trimetallic configuration compared to the bimetallic configuration. The metallurgical details of these studies are also discussed in this paper.
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[25] Kuniya J, Masaoka I, Sasaki R.Effect of cold work on the stress corrosion cracking of nonsensitized AISI 304 stainless steel in high-temperature oxygenated water[J]. Corrosion, 1988, 44: 21
Abstract The effect of cold work on the stress corrosion cracking (SCC) of solution annealed (nonsensitized) AISI 304 stainless steel (SS) in 288 C oxygenated pure water was studied utilizing creviced bent beam tests. The SCC susceptibility increased with an increased degree of cold work, especially when above 40%. The relationship between the SCC susceptibility and changes in metallurgical properties was examined to clarify the fundamental factors for SCC. The SCC susceptibility is closely related to the existence of crevices, stress, dissolved oxygen, deformation-induced martensite, and work hardening. The role of hydrogen in cracking is also considered.
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[27] Zhang L T, Wang J Q.Stress corrosion crack propagation behavior of domestic forged nuclear grade 316L stainless steel in high temperature and high pressure water[J]. Acta Metall. Sin., 2013, 49: 911
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(张利涛, 王俭秋. 国产锻造态核级管材316L不锈钢在高温高压水中的应力腐蚀裂纹扩展行为[J]. 金属学报, 2013, 49: 911)
<p>采用直流电位降<span>(DCPD)方法, 实现了模拟核电站高温高压水环境中的国产锻造态核级管材316L不锈钢应力腐蚀裂纹扩展速率的实时检测.断口观察表明, 国产316L不锈钢在高温高压水环境中表现出明显的沿晶应力腐蚀开裂(IGSCC)行为.降低溶解O含量, 增加溶解H含量能够显著降低应力腐蚀裂纹扩展速率;梯形波载荷下的裂纹扩展速率大于恒载荷下的扩展速率, 裂纹扩展速率随着</span>(<em>t</em><sub>fl</sub>=<em>t</em><sub>rs</sub>)/<em>t</em><sub>h</sub>比值的增加而升高<span>.</span></p>
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[28] Andresen P L.Suzhou international seminar on welding and non-destructive examination in nuclear power plants, Suzhou, China, 2009 (CD-ROM)
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high-temperature water; assisted cracking; alloy 690tt; pure water; growth; behavior; dependence; chemistry; coolant; model
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[30] Hu C L, Xia S, Li H, et al.Effect of grain boundary network on the intergranular stress corrosion cracking of 304 stainless steel[J]. Acta Metall. Sin., 2011, 47: 939
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通过晶界工程 (GBE)处理, 可使304不锈钢样品中的低<em>&Sigma;</em>CSL晶界比例提高到70%(Palumbo-Aust标准)以上 , 同时形成了大尺寸的&ldquo;互有<em>&Sigma;</em>3$<em><sup>n</sup></em>取向关系晶粒的团簇&rdquo;显微组织. 采用C型环样品恒定加载方法, 在pH值为2.0的沸腾20%NaCl酸化溶液中进行应力腐蚀实验. GBE样品在平均浸泡472 h后出现应力腐蚀裂纹, SEM, EBSD和OM分析表明, 应力腐蚀开裂(SCC)为沿晶开裂(IGSCC)和穿晶开裂(TGSCC)的混合型. 而未经GBE处理的样品在平均浸泡192 h后出现多条应力腐蚀主裂纹, 且多为沿晶界裂纹. 经过GBE处理的样品中大尺寸的晶粒团簇及大量相互连接的<em>&Sigma;</em>3-<em>&Sigma;</em>3-<em>&Sigma;</em>9和 <em>&Sigma;</em>3-<em>&Sigma;</em>9-<em>&Sigma;</em>27等<em>&Sigma;</em>3<em><sup>n</sup></em>类型的三叉界角, 阻碍了IGSCC裂纹的扩展, 从而提高了304不锈钢样品的抗IGSCC性能.
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[31] Gertsman V Y, Bruemmer S M.Study of grain boundary character along intergranular stress corrosion crack paths in austenitic alloys[J]. Acta Mater., 2001, 49: 1589
Abstract Samples of austenitic stainless alloys were examined by means of scanning and transmission electron microscopy. Misorientations were measured by electron backscattered diffraction. Grain boundary distributions were analyzed with special emphasis on the grain boundary character along intergranular stress corrosion cracks and at crack arrest points. It was established that only coherent twin Σ3 boundaries could be considered as “special” ones with regard to crack resistance. However, it is possible that twin interactions with random grain boundaries may inhibit crack propagation. The results suggest that other factors besides geometrical ones play an important role in the intergranular stress corrosion cracking of commercial alloys.
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[32] Tan L, Allen T R, Busby J T.Grain boundary engineering for structure materials of nuclear reactors[J]. J. Nucl. Mater., 2013, 441: 661
Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic鈥搈artensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys.
DOI:10.1016/j.jnucmat.2013.03.050      URL     [本文引用:0]
[33] West E A, Was G S.IGSCC of grain boundary engineered 316L and 690 in supercritical water[J]. J. Nucl. Mater., 2009, 392: 264
This study evaluated the influence of a high fraction of special grain boundaries on the intergranular stress corrosion cracking susceptibility of 316L stainless steel and nickel base alloy 690 in supercritical water. By thermomechanically processing the alloys to create specimens with largely different special boundary fractions, it was possible to isolate the effects of the grain boundary structure on the intergranular stress corrosion cracking behavior. Constant extension rate tensile experiments were performed in 50002°C deaerated supercritical water, and SEM analysis of the cracking behavior was performed on the gage surfaces of the specimens. Results indicate that the fraction of cracked grain boundary length in the specimens with higher fractions of special boundaries is reduced for 316L and 690 by factors of 9 and 5 at 15% strain, and 3 and 2 at 25% strain, respectively. This reduction is due to the special boundaries, which at 25% strain have a frequency of cracking that is 9–18 times lower than that for a random high angle boundary.
DOI:10.1016/j.jnucmat.2009.03.008      URL     [本文引用:1]
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关键词(key words)
异种金属焊接
微观结构
局部力学性能
应力腐蚀敏感性
残余应变

dissimilar metal
weld joint,
microstructure,
local mechanical property...
stress corrosion cracking...
residual strain

作者
明洪亮
张志明
王俭秋
韩恩厚
苏明星

MING Hongliang
ZHANG Zhiming
WANG Jianqiu
HAN En-Hou
SU Mingxing