316NG焊接接头在含氯离子的高温高压水中应力腐蚀开裂研究

通过慢应变速率拉伸(SSRT)试验和高温电化学相结合的方法,研究外加电位对奥氏体不锈钢316NG焊接接头在含氯离子的高温高压水中应力腐蚀开裂(SCC)倾向的影响。试验结果表明:退火态316NG焊接接头SCC敏感性随外加电极电位升高而增大,且存在一个介于+50~+100 mV[相对标准氢电极(vs.SHE)]之间的SCC临界电位;低于该电位时,SCC敏感性较小,无明显沿晶开裂,仅断口边缘处存在少量穿晶开裂,随电极电位变化不明显;高于该临界电位时,SCC敏感性急剧增加,并出现明显的沿晶开裂。此外,高温Ar和腐蚀性低(电极电位≤50 mV)的环境中,焊接接头的断裂为力学主导的塑形开裂,其与焊接接头的硬度分布密切相关,硬度越低,越容易断裂;强腐蚀性(电极电位50 mV)环境中,焊接接头的断裂为腐蚀主导的脆性开裂;显然,焊缝及热影响区的SCC敏感性高于母材。     

国产690合金与321不锈钢异种金属焊缝的应力腐蚀行为研究

利用慢应变速率试验,采用非标准的漏斗状试样,对国产690合金与321不锈钢异种金属焊接部位(包括690合金热影响区、焊缝、321不锈钢热影响区)在100 mg/L Cl~(-1)除O_2条件下和100 mg/L Cl~(-1)饱和O_2条件下的应力腐蚀行为进行研究。并通过慢应变速率应力-位移曲线和断口形貌对微观组织、氯离子、氧含量对于材料的应力腐蚀(SCC)的影响进行分析。结果表明:690合金热影响区在100 mg/L Cl~(-1)除O_2条件下不易发生SCC,在100 mg/L Cl~(-1)饱和O_2条件下表现出一定的SCC倾向;321不锈钢热影响区在2种条件下均表现出明显的SCC倾向;690合金热影响区的粗大晶粒不利于塑性变形的晶粒间相互协调,导致了热影响区SCC的倾向增大。     

Cl~-和Cu~(2+)对国产690合金应力腐蚀性能的影响

采用慢应变速率拉伸(SSRT)的方法,研究国产690合金分别在316℃含100 mg/L Cl-、1000 mg/L Cu2+以及100 mg/L Cl-与1000 mg/L Cu2+混合溶液中的应力腐蚀行为。通过激光共聚焦显微镜和扫描电镜观察断裂后试样的表面、断口以及纵剖面的微观形貌。结果表明:国产690合金在同时含Cl-与Cu2+的溶液中具有较高的应力腐蚀敏感性;断裂试样表面出现了大量的腐蚀坑,断口呈明显的脆性断裂特征,且在表面蚀坑的底部发现了沿晶应力腐蚀裂纹的萌生。由此证明Cl-与Cu2+对国产690合金应力腐蚀的协同作用,并初步探讨了其作用机理。     

固溶态控氮不锈钢在高温水中的应力腐蚀破裂

采用高温电化学测控和慢应变拉伸实验方法,研究了304NG超低碳控氮不锈钢（固溶态）在250℃高温水中的应力腐蚀破裂（SCC）与电极电位和水中Cl^-浓度的关系,并与316LN控氮不锈钢对比：结果表明,在不同环境下两种不锈钢的SCC敏感性随电极电位的升高而升高,并且存在一个临界电位Ec,当电位高于该Ec时,才发生SCC。该临界电位Ec随水中Cl^-浓度升高而下降,即发生SCC的环境范围扩大。304NG钢在含5mg／L Cl^-的250℃高温水中的Ec处于0～＋200mV标准氢电极（SHE）之间,更高时发生穿晶型SCC,表明该Cl^-浓度下只有在高氧（高电位）环境中才能发生SCC。当Cl^-浓度升高到50mg／L时,Ec降到-700mV（SHE）以下,表明该浓度下即使完好除氧（低电位）也可能发生SCC。316LN的SCC抗力高于304NG,其在含5mg／L Cl^-的高温水中的Ec位于＋300～＋400mV（SHE）之间,主要是穿晶型SCC。     

某些加工因素对改性800合金碱性应力腐蚀破裂敏感性的影响

通过显微组织检验、拉伸试验、应力分析、C形环应力腐蚀试验、俄歇电子能谱(AES)分析、腐蚀形貌观察,研究了某些加工因素(冷加工和热处理)对国产改性800合金(800M)在沸腾的50%NaOH+0.3%SiO2+0.3%Na2S2O3溶液中的应力腐蚀破裂(SCC)敏感性的影响.冷加工拉伸25%的800M合金管的晶粒变形拉长,屈服强度和抗拉强度增大而延伸率降低,管的残余应力增加,SCC敏感性增加.对冷变形800M合金管热处理,随温度的升高,晶粒逐渐等轴化并变大,延伸率增大,屈服强度、抗拉强度和残余拉应力和SCC敏感性降低.AES分析表明,800M合金在沸腾碱液中恒电位极化后的表面膜是富Ni 而贫Fe 、Cr的.C形环试样碱性SCC裂纹在点蚀坑起源萌生,裂纹沿晶界扩展.     

PWR核电站蒸汽发生器传热管和主管道的应力腐蚀破裂研究

用慢应变速率试验(SSRT)、恒载荷试验(CLT)和低周循环载荷试验方法研究以秦山和大亚湾核电站安全为目的的有关压力边界管道破裂始发事件应力腐蚀破裂(SCC)的行为,为评价管道的结构完整性提供支持性实验数据。研究的材料有核等级主管道焊接热影响区(WHAZ)316不锈钢(SS),核等级蒸汽发生器(SG)传热管材Incoloy-800、Inconel-600、Inconel-690和321SS。研究的影响因素包括材料冶金、表面喷丸处理、载荷、应变速率、循环载荷以及水化学条件对SCC的影响规律。     

Inconel 718合金压紧弹簧应力腐蚀开裂的应力阈值研究

采用适当的热处理时效工艺,可以避免板状压紧弹簧的Inconel 718镍基合金在晶界上产生有害的δ相,从而对压水堆一回路水质条件具有较低的应力腐蚀开裂敏感性。为防止镍基合金发生应力腐蚀开裂(SCC),其承受的应力不应超过该合金屈服强度的75%~80%。对于采用Inconel 718材料的板状压紧弹簧,其设计应力阈值设为931 MPa(135 ksi)在合理范围内,且其数值并不保守。     

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。     

焊接残余应力对316LN不锈钢应力腐蚀裂纹扩展速率的影响

核电站不锈钢管道焊接过程中引入的残余应力对焊接接头的应力腐蚀开裂性能有较大影响。本文针对一AP1000主管道316LN不锈钢焊接模拟件进行残余应力分析和应力腐蚀裂纹扩展速率测量,得到了焊后原始状态和去应力热处理状态的焊接热影响区材料在高温高压水中的应力腐蚀裂纹扩展速率。实验结果表明,焊接残余应力明显提高了热影响区的应力腐蚀裂纹扩展速率,且在含氢的压水堆一回路正常水化学下焊接残余应力的影响更加显著。     

核级304L不锈钢钎焊接头组织及耐腐蚀性能研究

核级304L不锈钢与BNi-7钎料真空钎焊接头存在晶间腐蚀行为,但工艺与钎缝耐腐蚀性能的关系尚未得到充分研究。为充分评估压水堆燃料组件结构件中不锈钢真空钎焊接头的晶间腐蚀和应力腐蚀敏感性,降低腐蚀失效风险,采用定量金相方法分析了钎缝中的化合物相含量,采用硫酸-硫酸铁法和双环动电位再活化(DL-EPR)法评价了钎缝耐晶间腐蚀性能,并采用高温高压水应力腐蚀裂纹扩展试验评价了钎缝的耐应力腐蚀性能。结果表明,钎缝中化合物相含量越高,耐晶间腐蚀性能越好。且钎缝在高温高压水中存在明显的应力腐蚀开裂行为,但其与钎焊工艺的关系尚需进一步试验研究。     

Environmentally-assisted cracking behaviour in the transition region of an Alloy182/SA 508 Cl.2 dissimilar metal weld joint in simulated boiling water reactor normal water chemistry environment

The stress corrosion cracking (SCC) and corrosion fatigue behaviour perpendicular and parallel to the fusion line in the transition region between the Alloy 182 Nickel-base weld metal and the adjacent SA 508 Cl.2 low-alloy reactor pressure vessel (RPV) steel of a simulated dissimilar metal weld joint was investigated under boiling water reactor normal water chemistry conditions. A special emphasis was placed to the question whether a fast growing interdendritic SCC crack in the highly susceptible Alloy 182 weld metal can easily cross the fusion line and significantly propagate into the adjacent low-alloy RPV steel. Cessation of interdendritic SCC crack growth was observed in high-purity or sulphate-containing oxygenated water under constant or periodical partial unloading conditions for those parts of the crack front, which reached the fusion line. In chloride containing water, on the other hand, the interdendritic SCC crack in the Alloy 182 weld metal very easily crossed the fusion line and further propagated with a very high rate as a transgranular crack into the heat-affected zone and base metal of the adjacent low-alloy steel. The observed SCC cracking behaviour at the interface correlates excellently with the field experience of such dissimilar metal weld joints, where SCC cracking was usually confined to the Alloy 182 weld metal.     

Evaluation of stainless steel pipe cracking: Causes and fixes

This paper discusses (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) intergranular crack growth rate measurements, (3) finite-element studies of the residual stresses produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360° part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on intergranular crack growth behavior in large diameter piping weldments. The studies on the effect of impurities on IGSCC of Type 304 stainless steel show a strong synergistic interaction between dissolved oxygen and impurity concentration of the water. Low carbon stainless steel (Type 316NG) appear resistant to IGSCC even in impurity environments. However, they can become susceptible to transgranular SCC with low levels of sulfate or chloride present in the environment. The finite-element calculations show that IHSI and the weld overlay produce compressive residual stresses on the inner surface, and that the stresses at the crack tip remain compressive under design loads at least for shallow cracks.     

316LN在模拟压水堆一回路异常水化学条件下的应力腐蚀敏感性研究

采用慢应变速率拉伸试验（SSRT）法研究了氯离子(Cl^-)和溶解氧对316LN在模拟压水堆（PWR）一回路异常工况下的应力腐蚀开裂（SCC）的影响。结果表明：在饱和氧条件下,随着Cl^-浓度的增大,试样的延伸率降低,SCC敏感指数增大,当Cl^-浓度大于1 mg/L后开始萌生SCC裂纹;溶液除氧后,即使Cl^-浓度增大到10 mg/L也不会引发SCC裂纹。316LN的SCC裂纹萌生于表面的缺陷或贫Cr区,呈穿晶型向基体内扩展。     

AL-6XN合金在超临界水中的应力腐蚀研究

采用慢拉伸试验研究了AL-6XN合金在550~650℃、25 MPa超临界水中的应力腐蚀行为,使用扫描电镜观察了材料断口形貌与标距面裂纹分布。结果显示:550℃试验条件下材料表现为穿晶开裂,标距面裂纹很多且分布较均匀,试样边角处出现扇形河流花样;650℃试验条件下材料表现为沿晶开裂,断口呈现典型的冰糖状,标距面裂纹数量大幅减少且集中于断口附近,相应的延伸率和断面收缩率也大幅降低。质子辐照对AL-6XN宏观力学性能影响不大,但导致试样断口的扇形花样(起裂源)数量增加,解理台阶宽度增大。以上结果表明,AL-6XN在超临界工况下具有严重的应力腐蚀开裂敏感性,升高温度和质子辐照会明显提高材料的应力腐蚀敏感性。     

Intergranular stress corrosion cracking behavior of niobium-added Type 308 stainless steel weld overlay metal in a simulated BWR environment

Type 308 stainless steel weld metal as an internal cladding of reactor pressure vessels for boiling water reactors is subject to postweld heat treatment during fabrication and can suffer sensitization depending on carbon and ferrite contents. This sensitization can be avoided by using niobium-added Type 308 weld metal (specified as Type 308 NbL) which was developed for one-layer overlay welding application. In the present study, stress corrosion cracking (SCC) behavior of heat-treated Types 308 and 308NbL weld metals in oxygenated high temperature pure water was evaluated by slow strain rate test and U-bend tests with and without crevice. Every test showed that Type 308NbL weld metals were highly resistant to SCC compared to ordinary Type 308 weld metals. In single U-bend test, one-layer overlay weld metals of Type 308NbL produced by electroslag welding process using wide strip electrodes were crack free over 23,000 h. The U-bend test data of ordinary Type 308 weld metals were successfully analyzed by an SCC reaction model. Using this analysis, the SCC life margin for Type 308NbL over ordinary Type 308 weld metals, expressed as a ratio of respective times to SCC initiation, was estimated to be about 36.     

BWR pipe crack and weld clad overlay studies

Several topics pertaining to the problem of stress corrosion cracking (SCC) of piping in boiling water reactors are addressed in this paper: (1) the effects of impurities, dissolved oxygen content, and strain rate on susceptibility of SCC of “Nuclear Grade” Type 316NG and sensitized Type 304 stainless steel, (2) finite-element analyses and experimental measurement of residual stresses in weldments with weld overlays, and (3) analysis of field components to assess effectiveness of in-service inspection techniques and the in-reactor performance of weld overlays. Several anion impurities including sulfates, chlorides, nitrates, borates, and carbonates were studied under both near neutral and slightly acidic conditions. At the low impurity concentrations expected in reactor coolant systems (<0.1 ppm), the sulfur species appear to be the most deleterious. They promote intergranular SCC in sensitized stainless steel and transgranular SCC in the low-carbon “Nuclear Grade” stainless steel. Correlations between experimental data and a phenomenological model that describes the effect of strain rate on SCC are presented. Measurements of the residual stresses produced by weld overlays confirm that the process is very effective in producing compressive stresses on the inner surface of the weldment. Examination of a weld overlay removed from service suggests that no additional throughwall crack growth had occurred after application of the overlay.     

贯穿件J形坡口焊接残余应力分析

核电站反应堆压力容器（RPV）顶盖控制棒驱动机构（CRDM）管座J形坡口焊缝在一回路高温高压水环境下存在应力腐蚀开裂（SCC）的风险，而焊接残余应力是SCC的主要驱动力。使用二维轴对称模型有限元方法对CRDM中心管座J形坡口进行焊接残余应力分析。为了探索一种简单、高效和保守的方法，研究了热源简化、焊缝形状简化、屈服强度、相变和强化行为对焊接残余应力的影响。结果表明:双椭球热源与均匀热源得到的残余应力结果基本一致；焊缝形状由鱼鳞状简化为方块模型对焊接残余应力结果影响不大，但是与合并焊道的结果相差较大；采用低屈服强度得到的残余应力结果并不保守；在ANSYS软件中，固液相变对残余应力结果影响不大；等向强化模型的结果比随动强化模型的结果保守；在工程上，建议采用均匀热源、方块焊道模型和等向强化模型进行焊接模拟。      

Separation of Pu-238 and Pu-242 from Irradiated Am-241

Failure analysis was made on samples taken from type 304 stainless steel piping systems (core spray, unloading and feed water pipes) that had cracked in service. In the core spray pipe, large cracks including one penetrating through the wall were found in the upper half of the pipe wall, within the heat-affected zone of the weld joint between the pipe and the nozzle safe end of the reactor pressure vessel. These cracks were of intergranular type, also small transgranular cracks were found beyond the heat-affected zone. Strong correlation was established between the intergranular cracking and severe sensitization in the heat-affected zone.

In the unloading pipe, the region close to the weld joint contained cracks similar to those in the core spray pipe. The feed water pipe, in contrast to the foregoing cases, contained numerous shallow transgranular cracks both within and outside the heat-affected zone.     

Corrosion of Ti-STS dissimilar joints brazed by a Ag interlayer and Ag-Cu-(Pd) alloy fillers

Corrosion behavior of dissimilar brazed joints between titanium Gr. 2 (Ti) and S31254 stainless steel (STS) was investigated. For the study, a Ag interlayer and two Ag-base eutectic alloys, 72Ag-28Cu and 66.2Ag-25.8Cu-8Pd (wt.%), were introduced as a diffusion control layer and fillers, respectively, between the base materials. The joints commonly had a layered structure of Ti(base)/TiAg/Ag solid solution/STS(base), but the one brazed by the Ag-Cu-Pd filler was slightly alloyed with the noble Pd elements over the Ag-rich solid solution region. A series of corrosion test experiments in a sea water revealed that a corrosion of TiAg layer and a stress-induced cracking at the TiAg/Ag solid solution interface were dominant due to a galvanic attack, but notably the Ti-STS dissimilar joint’s resistance to corrosion was significantly improved by alloying the Pd in the joint. The corrosion behavior of such dissimilar metal joints was discussed based on galvanic corrosion effect.