Acta Metallurgica Sinica, 2017, 53(1): 57-69
doi: 10.11900/0412.1961.2016.00135

Microstructure and Local Properties of a Domestic Safe-End Dissimilar Metal Weld Joint by Using Hot-Wire GTAW

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

1 实验方法

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

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)

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

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

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

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

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

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))

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

2.2 界面处成分分布

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

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

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。

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

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)

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

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

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

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

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

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)

2.6 焊接件不同部位慢应变速率拉伸实验

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。

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

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 heterogeneous mechanical properties are mainly related to the local microstructures of the DMWJ. The complex microstructures in the interface regions between materials were mainly caused by the heat flow and element migration during welding process. The local mechanical properties and their mismatches may have significant effect on crack-tip fracture mechanics parameter, plastic deformation behavior, local fracture resistance and crack growth behavior. Therefore, they need to be obtained and used in the integrity assessment of theDMWJs.      URL     [本文引用:1] [3] Lu Z P, Shoji T, Yamazaki S, et al.Characterization of microstructure, local deformation and microchemistry in Alloy 600 heat-affected zone and stress corrosion cracking in high temperature water[J]. Corros. 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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.      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 [本文引用:1] [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      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. 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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.      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. 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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.      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 [本文引用:2] [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.      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.      URL     [本文引用:2] [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 [本文引用:1] [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.      URL     [本文引用:1] [21] Kou S.Welding Metallurgy[M]. 2nd Ed., Hoboken, New Jersey: John Wiley & Sons Inc., 2003: 170 [本文引用:1] [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.      URL     [本文引用:1] [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.      URL     [本文引用:1] [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.      URL     [本文引用:1] [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.      URL     [本文引用:1] [26] Fang H Y.Welding Structural [M]. Beijing: Mechanical Industry Press, 2008: 56 [本文引用:1] (方洪渊. 焊接结构学 [M]. 北京: 机械工业出版社, 2008: 56) [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 [本文引用:1] (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)

[28] Andresen P L.Suzhou international seminar on welding and non-destructive examination in nuclear power plants, Suzhou, China, 2009 (CD-ROM) [本文引用:1] [29] Zhang L T, Wang J Q.Effect of dissolved oxygen content on stress corrosion cracking of a cold worked 316L stainless steel in simulated pressurized water reactor primary water environment[J]. J. Nucl. Mater., 2014, 446: 15 high-temperature water; assisted cracking; alloy 690tt; pure water; growth; behavior; dependence; chemistry; coolant; model      URL     [本文引用:1] [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 [本文引用:1] (胡长亮, 夏爽, 李慧等. 晶界网络特征对304不锈钢晶间应力腐蚀开裂的影响[J]. 金属学报, 2011, 47: 939) 通过晶界工程 (GBE)处理, 可使304不锈钢样品中的低ΣCSL晶界比例提高到70%(Palumbo-Aust标准)以上 , 同时形成了大尺寸的“互有Σ3\$n取向关系晶粒的团簇”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Σ3-Σ3-Σ9和 Σ3-Σ9-Σ27等Σ3n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. [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.      URL     [本文引用:1] [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.      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.      URL     [本文引用:1]

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dissimilar metal
weld joint,
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residual strain

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