新锆合金碘致应力腐蚀断口研究

利用扫描电镜和EDXA对新锆合金碘致应力腐蚀的断口进行了形貌观察和成分分析。在断口表面发现腐蚀产物、二次裂纹、沿晶开裂和穿晶准解理开裂等应力腐蚀断裂特征,并观察到锆合金碘致应力腐蚀的“沟槽”特征。起裂区为沿晶开裂,在裂纹扩展阶段,开裂以穿晶为主。断口上腐蚀产物的成分主要是氧和锆,局部准解理开裂区域可检测到碘。去应力退火试样上发现了平行轧面的深沟,且沿晶开裂不明显。试验温度升高,断口上的腐蚀产物增多,而且沿晶开裂更容易。碘分压不仅影响腐蚀产物层的厚度,而且碘分压较高时沿晶开裂容易发生。     

氯和氧对304N在高温水中应力腐蚀开裂的影响

采用慢应变速率拉伸试验（SSRT）与断口形貌分析技术研究304N不锈钢（固溶退火态）在300 ℃高温水中的应力腐蚀行为与机理。结果表明：304N在300 ℃高温水中的最大抗拉强度、延伸率以及断裂吸收能随Cl^-浓度的增大显著降低;随氧浓度的急剧降低而显著增大;304N在高温水中发生应力腐蚀开裂（SCC）主要为穿晶型;随Cl^-浓度增加,304N的应力腐蚀敏感性也迅速增加,在含50 mg/L Cl^-的空气饱和高温水环境中,试样断口形貌表现为完全脆断;在溶解氧浓度急剧降低时,氯致应力腐蚀开裂的敏感性大幅降低,表明溶解氧对304N在高温水中的氯致应力腐蚀开裂具有明显的促进作用。     

6XN不锈钢与825合金的高周疲劳行为研究

采用旋转弯曲疲劳方法完成了6XN不锈钢与825合金在室温和550℃空气中的高周疲劳试验。结果表明:室温时,6XN不锈钢的耐久极限应力大于825合金的耐久极限应力,与静强度顺序一致;550℃高温时,试样氧化速率增加,材料疲劳寿命降低,6XN不锈钢的疲劳寿命下降趋势较大,耐久极限应力接近825合金的耐久极限应力;6XN不锈钢对温度更敏感,而825合金对应力循环次数更敏感;与经验公式比较,2种材料具有较好的抗高周疲劳性能;疲劳过程为裂纹源产生、扩展和断裂,最后断裂区具有韧窝特征。     

镍基合金825在超临界水中的腐蚀性能研究

研究了镍基合金825在550 ℃/25 MPa、600 ℃/25 MPa和650 ℃/25 MPa超临界水（SCW）中的应力腐蚀开裂敏感性,以及在超临界650 ℃/25 MPa、次临界290 ℃/15.2 MPa水中的均匀腐蚀性能。通过慢应变速率拉伸实验得到了相应的应力-应变曲线,结果表明,随温度的升高,825的机械强度和塑性逐渐下降;实验后试样的SEM图像表明,825在3种工况下的应力腐蚀开裂倾向大小关系为600 ℃＞550 ℃＞650 ℃。825在SCW条件下的腐蚀实验表明,其腐蚀增重大致符合幂函数生长规律;而其在次临界条件下的腐蚀增重变化却呈现出先减后增的特征。     

周向拉伸条件下的N36锆合金包壳管碘致应力腐蚀开裂

350℃下,对N36锆合金包壳管环形试样施加周向拉伸载荷,绘制碘浓度分别为102、103、104 Pa下的应力-延伸率曲线,研究包壳材料在碘气体(I2)环境下的碘致应力腐蚀开裂(I-SCC)情况。应力-延伸率曲线能够衡量锆合金包壳管发生I-SCC开裂的敏感性:当I-SCC发生时,在曲线中表现为没有明显的颈缩阶段。在应力-延伸率曲线上可以找出I-SCC裂纹萌生、扩展的相应发生阶段。I-SCC降低材料的延伸率和断裂能量,降低程度均随着碘浓度的升高而增加。I-SCC环形试样在周向拉伸过程中产生平行于断口的裂纹沟槽,裂纹起始阶段均为沿晶扩展形式。     

AL6XN超级奥氏体钢的高温蠕变及疲劳行为研究

研究了AL6XN超级奥氏体钢在650～750℃和120～220MPa应力水平下的高温蠕变特性,以及300和600℃不同恒应变幅值条件下的疲劳特性。结果表明,AL6XN具有优越的高温蠕变抗力,其蠕变激活能Q为327kJ/mol,蠕变应力指数为5.23。结合变形微结构观察结果表明,AL6XN的蠕变机制为位错攀移和滑移机制。在一定应变量下,AL6XN在600℃时疲劳试验的应力水平高于300℃的应力水平,同时随着应变量的增加和温度的升高,其疲劳寿命显著降低;600℃疲劳试验后仅形成位错缠结,疲劳裂纹扩展断口存在典型的疲劳辉纹,无明显二次裂纹。以上结果表明,AL6XN疲劳裂纹扩展行为与其动态应变时效有关。     

新锆合金碘致应力腐蚀性能研究

研究了再结晶状态的相同成分，不同织构取向的两块新锆合金板材在不同的实验条件下的抗碘致应力腐蚀性能。用直流电位降法动态监测裂纹的长度并对试样进行了织构的测定、第二相的观察和断口分析结果表明，在实验条件下，N18-2(L-T)抗碘致应力腐蚀的能力优于N18-1(T—L)断口形貌表明，在应力腐蚀裂纹的初始扩展阶段，断口沿晶开裂；在裂纹的稳态扩展阶段，以穿晶准解理扩展为主。     

核级316L不锈钢管道射线插塞孔开裂原因分析

为查明某核电厂核级316L奥氏体不锈钢管道射线插塞孔裂纹显示的成因,对含插塞孔不锈钢管段的宏/微观形貌、化学成分、力学性能、维氏硬度、断口形貌、腐蚀产物、应力分布等进行了分析。结果表明：裂纹以沿晶方式扩展,断口呈冰糖块脆性断裂花样并伴有大量氧化腐蚀产物,属于典型的压水堆一回路水介质条件下由插塞孔局部应变-硬化导致的晶间应力腐蚀开裂。引起应变-硬化的主要原因是插塞孔和插塞的过盈配合以及射线插塞孔密封焊缝焊接残余应力过高。建议加强在役机组同类结构的检查,减少新建机组类似结构的使用。     

核电厂余热排出管道射线插塞孔失效原因分析

为研究反应堆厂房余热排出管道射线插塞孔失效原因,开展了材料组织及性能分析、裂纹及断口形貌分析以及模拟计算分析。结果表明,插塞孔是因裂纹导致失效;裂纹扩展方式为沿晶扩展,裂纹断口发生腐蚀,近插塞孔螺纹处裂纹腐蚀产物最多;部分裂纹发生了局部应变-硬化,螺纹处外表面的硬度最高并沿径向由外到内逐渐降低;插塞孔失效性质为晶间应力腐蚀开裂,受制造、安装及焊接修复的叠加影响所致。     

温度对Zr-Sn-Nb合金致应力碘腐蚀开裂的影响

研究了N18和N36锆合金在不同温度下的碘致应力腐蚀开裂(SCC)行为,用扫描电镜进行了断口分析.结果表明,对于再结晶状态的锆合金,随着试验温度升高,KISCC(临界应力强度因子)降低,裂纹萌生所需的应力降低,裂纹萌生所需的时间也变短;对于去应力状态的N18合金,试验温度从300℃增加到350℃,KISCC基本不变,裂纹萌生所需的应力降低,裂纹萌生所需的时间也变短;随着试验温度升高,断口上的腐蚀产物增多.     

Stress corrosion cracking behavior of 310S in supercritical water with different oxygen concentrations

The effect of dissolved oxygen(DO) on the stress corrosion cracking(SCC) of 310 S in supercritical water was investigated using slow-strain-rate tensile tests.The tensile properties, fracture morphology, and distribution of the chemical composition of the oxide were analyzed to evaluate the SCC susceptibility of 310 S. The results showed that the rupture elongation decreased significantly as the degree of DO increased. A brittle fracture mode was observed on the fracture surface, and only intergranular cracking was observed on the surface of the gauge section, regardless of the DO. Cracks were widely distributed on the gauge surface near the fracture surface.Oxides were observed in the cracks with two-layered structures, i.e., a Cr-rich inner oxide layer and an Fe-rich outer oxide layer.     

Corrosion Resistance of Fe-Al-Alloy-Coated Ferritic/Martensitic Steel under Bending Stress in High-Temperature Lead-Bismuth Eutectic

The corrosion test was performed for ferritic/martensitic steel HCM12A with and without Fe-Al alloy coating in LBE at temperatures of 550 and 650°C under loading for an immersion time up to 500 h. After the corrosion test at 550°C for 500 h, both of the uncoated and Fe-Al-coated HCM12A showed a good corrosion resistance without the influence of the tensile stress on the LBE corrosion. On the other hand, after the corrosion test at 650°C, the Fe-Al coating layer on the specimen surface exhibited no LBE dissolution corrosion because of the formation of a stable oxide protection film on the coating layer surface, although the coating layer cracked. The LBE penetrated into the cracks and corroded the base metal and the precoating layer. The uncoated HCM12A exhibited the double oxide layers of FeO and Fe- Cr spinel. The FeO was damaged in the bent zone by the stress, and the Fe-Cr spinel layer was not destroyed by the influence of cracking and the tensile stress. The cracking and stress did not have a large influence on the overall oxidation corrosion rate.     

Stress corrosion cracking susceptibility of austenitic stainless steels in supercritical water conditions

The presented paper summarizes the results of general corrosion and stress corrosion cracking (SCC) susceptibility tests in supercritical water (SCW), studied for austenitic stainless steel 316L, with the aim to identify maximum SCW temperature usability and specific failure mechanisms prevailing during slow strain-rate tensile (SSRT) tests in ultra-pure demineralized SCW solution with controlled oxygen content. The general corrosion tests clearly revealed the applicability of austenitic stainless steel in SCW to be limited to 550 °C as maximum temperature as oxidation rates of austenitic stainless steels 316L increase dramatically above 550 °C. The SSRT tests were performed using a step-motor controlled loading device in an autoclave at 550 °C SCW. Besides the strain rate (resp. crosshead speed), the oxygen content was varied in the series of tests. The obtained results showed that even at the lowest strain rate, a serious increase of SCC susceptibility, as typically characterized by IGSCC crack growth, was not observed. The fractography confirmed that failure was due to a combination of transgranular SCC and transgranular ductile fracture. Based on fractographic findings a phenomenological map describing the SCC regime of SSRT test parameters could be proposed for AISI 316L.     

HR3C在超临界水中的腐蚀性能研究

通过高压釜浸泡实验研究了超级奥氏体不锈钢HR3C在循环的超临界水（SCW）中的均匀腐蚀性能,实验温度分别为550、600、650 ℃,压力为25 MPa,并对实验后试样生成的氧化膜进行了SEM、EDS和XRD分析。实验结果显示,HR3C在SCW环境中的氧化腐蚀速率随着温度的升高而增大,1 500 h后650 ℃ SCW环境下材料的腐蚀增重约为550 ℃时的2倍。材料表面生成的氧化膜主要成分为FeCr₂O₃、Fe₃O₄和Fe₂O₃,内层氧化膜富Cr而外层氧化膜富Fe。     

N18锆合金的低周疲劳行为

在不同试验温度（室温～500℃）下,对N18合金进行了低周疲劳试验。试验结果表明：室温～300℃温区,合金表现为明显的循环软化;400、450℃时,合金逐渐呈现循环硬化,450℃时其硬化现象更为明显;500℃时则主要表现为循环饱和。随着温度的升高,疲劳寿命先增加后降低,300℃时疲劳寿命最高。低应变幅下,温度对疲劳寿命的影响更明显。通过疲劳断口SEM分析,室温下疲劳起源于单个裂纹源,疲劳裂纹扩展阶段的微观特征主要是疲劳条纹,局部区域出现轮胎状花样。在高温下为多裂纹源,大量二次裂纹的存在是高温疲劳断口的主要特征。     

304NG在超临界水中的腐蚀增重随温度的异常关系

研究了奥氏体不锈钢304NG在550、600和650℃超临界水环境下的腐蚀行为。采用扫描显微镜、X射线能谱仪、X射线衍射仪分析了氧化膜的腐蚀形貌、组织结构和成分分布。实验结果表明,试样在3种不同温度下经1000h腐蚀实验后的增重均符合幂函数规律,但650℃时的腐蚀增重与600℃时的相比大幅下降,其主要原因为在较高温时,Cr的扩散速度快,试样表面氧化膜能够维持保护性从而使疖状腐蚀分布数量减少所致。     

Study on Incipient Cracks at Inner Surface of Cladding Liner after High Power Irradiation Test

10x10 BWR segmented rods with a burnup of about 30 MWd/kgU were subjected to high power irradiation tests in the R2 reactor. Metallography on the cladding showed small incipient cracks with a depth of 10mm at the inner surface of the cladding liner after the irradiation tests. In order to investigate the incipient cracks, several post irradiation examinations were performed. Local micro-hardness of the liner was measured with an indentation force of 1 g. The results showed that the hardness was significantly increased at the inner surface facing the fuel. Electron probe micro-analysis (EPMA) of the liner gave an elemental profile showing an increased amount of fission products at the inner surface and the profile agreed with the micro-hardness profile. Scanning electron microscopy showed propagation of the incipient cracks along grain boundaries. Elemental mapping with EPMA showed concentrated cadmium at the incipient cracks. Considering these observed facts, the cause of the crack formation could be stress corrosion cracking (SCC). Applied hoop stress history to the liner during the irradiation test was calculated with a fuel performance analysis code. Based on the results of the observation and calculation, the process of the crack formation and propagation was discussed.     

A study on the mechanism of iodine-induced stress-corrosion cracking of Zircaloy-4

The iodine-induced stress-corrosion cracking (SCC) of Zircaloy-4 plate specimens has been studied by the constant elongation rate test (CERT) and U-bend test methods. In order to systematically evaluate the effects of stress states on the SCC behavior, four kinds of specimens were prepared from as-annealed Zircaloy-4 plate specimens. The uniaxial tensile and single edge-notched (SEN) specimens fractured in the ductile manner, not by SCC. However, SCC resulted in the plane strain tensile specimens and deep-notched U-bend specimens. As the strain rate was lowered, the susceptibility to SCC increased. The stress-corrosion (SC) cracks propagated by the trans-granular brittle fracture mode for as-annealed specimens, whereas by the inter- and trans-granular mixed fracture mode for reannealed specimens. The SCC processes of Zircaloy-4 in iodine-gas are discussed in terms of effects of stress states, elongation rates and yield stresses. The present experimental results suggest that the iodine-induced SCC processes are based upon the iodine-diffusion model.