Dissimilar material welding and assessing reliability of super alloy for green and high efficiency thermal power plant

This paper studies the prediction of fatigue and corrosion fatigue lives using neural network and accelerated life methods for dissimilar material weld between Alloy617 and 12Cr steel. First, dissimilar material welding between Alloy617 and 12Cr steel was performed using buttering technology. The fatigue and corrosion fatigue strengths, and electrochemical corrosion susceptibility of dissimilar material weld were assessed. After that, on the basis of obtained data, fatigue life and corrosion fatigue life of dissimilar material weld were predicted using the neural network and accelerated life test methods. The predicted results showed good agreement with the actual fatigue and corrosion fatigue lives. Especially, the results of the neural network prediction were more accurate than those of the accelerated life method.     

Assessing mechanical properties of the dissimilar metal welding between P92 steels and alloy 617 at high temperature

In this study, a new welding technology of dissimilar materials, Cr-based P92 steels and Ni-based Alloy 617 is introduced and demonstrated to investigate its reliability. Firstly, multi-pass dissimilar metal welding between P92 steel and Alloy 617 was performed using DCEN TIG welding technology, buttering welding technique and a narrow gap groove. After welding, in order to understand characteristics of the dissimilar metal welds, metallurgical micro-structures analysis by optical observation and static tensile strength assessment of the dissimilar welded joints were conducted at 700°C.     

Local fracture properties and dissimilar weld integrity in nuclear power plants

In this paper, the local fracture properties in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316 L stainless steel in nuclear power plants were investigated by using the single-edge notched bend (SENB) specimens, and their use in integrity assessment of DMWJ structures was analyzed. The results show that the local fracture resistance in the DMWJ is determined by local fracture mechanism, and which is mainly related to the microstructures and local strength mismatches of materials at the crack locations. The initial cracks always grow towards the materials with lower strength, and the crack path deviation is mainly controlled by the local strength mismatch. If the local fracture properties could not be used for cracks in the heat affected zones (HAZs), interface and near interface zones, the use of the fracture properties (J-resistance curves) of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture properties in the integrity assessment of DMWJs.     

基于BS7910含缺陷X80钢焊接管道的安全评定

根据英国标准BS7448,制备带预制疲劳裂纹的三点弯曲(three point bent,TPB)标准试样,对X80管线钢焊缝、热影响区和母材进行0℃断裂韧度试验,根据CTOD(crack tip opening displacement)试验结果,结合材料的力学性能、载荷条件,采用英国BS7910结构完整性评定方法的1A、2A评定曲线,针对焊缝内部的深埋裂纹,对X80钢管道焊缝进行安全评定。通过迭代计算确定含有深埋裂纹X80钢管道焊缝中的容许裂纹尺寸。     

焊后热处理对2A14高强铝合金电子束焊接头组织及力学性能的影响

轻质化的需求使得人们把关注的焦点集中于轻质材料,高强铝合金作为钢结构材料的最佳替代品,受到越来越广泛的关注,利用电子束焊接高强铝合金,为获得性能优良的2A14高强铝合金电子束焊接接头,采用焊后热处理,通过组织观察(光学显微镜和扫描电镜)、维氏硬度测试、接头拉伸性能测试等方法研究焊后热处理对2A14电子束焊接接头显微组织和性能的影响。结果表明,通过焊后热处理,焊缝中心原晶界分布的网状共晶组织回溶于基体组织中消失,焊缝内部析出大量弥散强化项,基体强化效果增强,显微硬度显著升高,由焊态下低于母材硬度直接升高至超过母材硬度。接头抗拉强度由原来的355MPa提高到465 MPa,超过了母材强度。接头断裂均发生在焊缝,由断口分析发现热处理后接头韧性增强,韧窝深度增加,且有第二相质点出现。     

静轴肩FSW铝合金6061-T4板的T形接头组织特征及动静载特性研究

利用自主开发的静止轴肩焊接工具及工装,在不同的焊接参数下均获得到了外观成形良好、无焊接缺陷的T形接头,并对接头内部成形、显微组织、硬度、静载强度和疲劳性能及疲劳失效机制进行了研究。研究结果表明铝合金静止轴肩搅拌摩擦焊T性接头内部质量良好,无缺陷,接头表面存在表面超细晶区,且焊核区内部由于流动状态不同导致组织及硬度存在一定差异,T形接头底板及筋板硬度较母材有不同程度的降低,底板和筋板拉伸试验均断裂于接头热影响区,在优化的参数下底板和筋板方向的接头系数均高于0.7,在2×10⁶疲劳寿命下的特征疲劳强度可达101.0 MPa,远高于IIW建议的设计准则。焊接速度对接头疲劳性能及疲劳失效机制影响较大,疲劳裂纹萌生于T形接头底板和筋板过渡处,低焊速时断裂机制为穿晶断裂,高焊速时由于晶界稳定性差,断口呈现穿晶断裂和晶间断裂的混合形貌。完成了铝合金T形接头的无缺陷焊接及组织性能研究,为铝合金静止轴肩搅拌摩擦焊T形接头在新型轻量化航天器密封舱结构中的应用提供了技术支撑。     

HG80钢及其焊接接头的疲劳裂纹扩展

为适应大型工程机械焊接用钢需要，对低合金高强度钢HG80及其焊接接头的疲劳裂纹扩展速率进行了研究。结果表明，疲劳裂纹扩展速率对钢板的轧制方向不敏感，焊缝及热影响区疲劳裂纹扩展速率明显低于基材。在排除焊接残余应力导致的裂纹闭合效应后，焊缝及热影响区疲劳裂纹扩展速率与基材相当。     

焊后热处理对13MnNiMoR钢焊接接头在服役环境下SCC敏感性的影响

通过慢应变速率拉伸试验（SSRT）,对比研究了经过焊后热处理和未经过焊后热处理的13MnNiMoR钢焊接接头在250℃下、不同pH值的锅炉水环境中的应力腐蚀开裂敏感性。结果表明：在相同应变速率、相同温度下,经过焊后热处理和未经过焊后热处理的13MnNiMoR钢焊接接头,应力腐蚀开裂敏感性有明显的区别。经过热处理的13MnNiMoR焊接接头应力腐蚀开裂敏感性随着pH值的增大而减小,相反,未经过热处理的接头,其敏感性随着pH值的增大而增大。研究结果表明,合理的焊后热处理能够显著降低焊接接头的应力腐蚀开裂敏感性。     

The effect of hydrogen gas environment on fretting fatigue strength of materials used for hydrogen utilization machines

The objective of this study is the characterization of the fretting fatigue strength in a hydrogen gas environment. The test materials were a low alloy steel SCM435H, super alloy A286 and two kinds of austenitic stainless steels, SUS304 and SUS316L. The test was performed in hydrogen gas at 0.12 MPa absolute pressure. The purity of the hydrogen gas was 99.9999%. The fretting fatigue limit was defined by the fretting fatigue strength at 30 million cycles. For all materials, the fretting fatigue strength in the hydrogen gas environment increased in the short-life region. However, the fretting fatigue strength in the hydrogen gas environment decreased in the long-life region when exceeding 10 million cycles except for SCM435H, while there was no reduction in the fretting fatigue strength in air between 10 and 30 million cycles. The reduction rate was 18% for A286, 24% for SUS304 and 7% for SUS316L. The tangential force coefficient in the hydrogen gas environment increased when compared to that in air. It can be estimated that this increase is one of the causes of the reduced fretting fatigue strength found in a hydrogen gas environment. In order to discuss the extension of the fretting fatigue life in hydrogen gas observed at the stress level above the fretting fatigue limit in air, continuous measurement of the fretting fatigue crack propagation was performed in a hydrogen gas environment using the direct current potential drop method. As a result, it was found that the extension of the fretting fatigue life was caused by the delay in the start of the stable crack propagation.     

Fatigue strength depending on position of cracks for weldments

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.     

Preliminary application of the draft code case for alloy 617 for a high temperature component

The ASME draft Code Case for Alloy 617 was developed in the late 1980s for the design of very-high-temperature gas cooled reactors. The draft Code Case was patterned after the ASME Code Section III Subsection NH and was intended to cover Ni-Cr-Co-Mo Alloy 617 to 982°C (1800°F). But the draft Code Case is still in an incomplete status, lacking necessary material properties and design data. In this study, a preliminary evaluation on the creep-fatigue damage for a high temperature hot duct pipe structure has been carried out according to the draft Code Case. The evaluation procedures and results according to the draft Code Case for Alloy 617 material were compared with those of the ASME Subsection NH and RCC-MR for Alloy 800H material. It was shown that many data including material properties, fatigue and creep data should be supplemented for the draft Code Case. However, when the evaluation results on the creep-fatigue damage according to the draft Code Case, ASME-NH and RCC-MR were compared based on the preliminary evaluation, it was shown that the Alloy 617 results from the draft Code Case tended to be more resistant to the creep damage while less resistant to the fatigue damage than those from the ASME-NH and RCC-MR.     

Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates

In this study, AA7075-O and AA7075-T6 Al alloy plates were friction stir butt welded using two sets of weld parameters in order to investigate the effects of temper condition prior to joining and post weld heat treatment on microstructure and mechanical properties of the joints. Another goal of the work is to determine the possibility of restoring the loss of strength in the joint area experienced in welding of this alloy in the age-hardened condition by subsequent heat treatment. The study revealed that the friction stir welding resulted in a strength undermatching when the alloy joined in T6 condition while a significant strength overmatching was obtained in the joints produced in O temper condition. The post weld heat treatment led to a significant recovery in the strength of the joints produced in T6 condition, thus a significant increase in the joint performance. On the other hand, it led to a decrease in the joint performance value of the joints produced in O temper condition although the strength was increased. Furthermore, the post weld heat treatment resulted in abnormal grain growth in the joint area the degree of which depends on weld parameters used and the prior temper condition.     

Assessment of the sulfide corrosion fatigue strength for a multi-pass welded A106 Gr B steel pipe below the low SSCC limit

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. Welding processes happen to produce residual stresses and change the metal structure as a result of the large nonlinear thermal loading that is created by a moving heat source. The fusion welding process generates formidable welding residual stresses and metallurgical change, which increase the crack driving force and reduce the resistance to the brittle fracture as well as the environmental fracture. This is a serious problem with many alloys as well as the A106 Gr B steel pipe. This pipe that is used in petrochemical and heavy chemical plants either degrades due to corrosive environments, e.g., chlorides and sulfides, and/or become damaged during service due to the various corrosion damage mechanisms. Thus, in this study, after numerically and experimentally analyzing the welding residual stress of a multi-pass welded A106 Gr B steel pipe, the sulfide stress corrosion cracking (SSCC) characteristics were assessed in a 3.5 wt.% NaCl solution that was saturated with H₂S gas at room temperature on the basis of NACE TM 0177-90. The specimens used are of two kinds: un-notched and notched. Then, the sulfide corrosion fatigue (SCF) strength for the un-notched specimen was assessed below the low SSCC limit that was previously obtained from the SSCC tests for the notched specimen. From the results, in terms of the SSCC and SCF, all the specimens failed at the heat-affected zone, where a high welding residual stress is distributed. It was found that the low SSCC limit of un-notched specimens (σ_{SSCCun-notched}) was 46% (230 MPa) of the ultimate tensile strength (σ_U=502 MPa) of a multi-pass welded A106 Gr B steel pipe, and the notched specimens (σ_SSCCnotched) had 40% (200 MPa) of the ultimate tensile strength. Thus, it was determined that σ_{SSCCun-notched} was 13% lower than σ_SSCCnotched. Further, the sulfide corrosion fatigue limit (σ_{SCFun-notched}) was 32% (160 MPa) of the ultimate tensile strength of welded specimens. This σ_{SCF un-notched} was 20% lower than σ_SSCCnotched.     

Determination of Elastoplastic Mechanical Properties of the Weld and Heat Affected Zone Metals in Tailor-Welded Blanks by Nanoindentation Test

The elastoplastic mechanical properties of the weld and heat affected zone metals have comparatively major impact on the forming process of tailor-welded blanks. A few scholars investigated the elastoplastic mechanical properties of the weld and heat affected zone, but they only simply assumed that it was a uniform distribution elastoplastic material different from the base materials. Four types of tailor-welded blanks which consist of ST12 and 304 stainless steel plates are selected as the research objects, the elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals are obtained based on the nanoindentation tests, and the Erichsen cupping tests are conducted by combining numerical simulation with physical experiment. The nanoindentation tests results demonstrate that the elastoplastic mechanical properties of the weld and heat affected zone metals are not only different from the base materials, but also varying between the weld metals and the heat affected zone metals. Comparing the Erichsen cupping test resulted from numerical with that from experimental method, it is found that the numerical value of Erichsen cupping test which consider the elastoplastic mechanical properties of the weld and heat affected zone metals have a good agreement with the experimental result, and the relative error is only 4.8%. The proposed research provides good solutions for the inhomogeneous elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals, and improves the control performance of tailor-welded blanks forming accuracy.     

Effect of Lack of Penetration on the fatigue strength of high strength steel butt weld

Fatigue tests are performed to evaluate the fatigue strength of high strength steel containing partial penetration butt weld and full penetration butt weld. The influence of the unwelded ligament (Lack of Penetration) in the partial penetration welds on the fatigue life is analyzed for various LOP sizes. For full penetration welds, the fatigue crack initiated at the weld toe and propagated to the HAZ. For partial penetration welds, however, the fatigue cracks initiated at the LOP section and propagated to the weld metal (or weld toe) for the considered LOP sizes (from 2mm to 4mm) reducing the fatigue strength. Consequently, the increament of the LOP size yield in the fatigue life degradation by some extent.     

超高强度钢十亿周疲劳研究

利用20kHz压电超声疲劳试验技术，对六种抗拉强度高于1500MPa的低合金钢进行测试。结果表明，这些材料并不存在传统规范中所谓的“疲劳极限”，它们在超过10^7，甚至10^9应力循环后仍然继续发生破坏，而且其S－N曲线表现出“阶梯”型特征。研究还发现，这些材料都存在一个分界应力幅值区间（或应力循环数区间，大约在10^6到10^7的平台内），裂纹萌生由试件表面向其内部转换。实验证明了十亿周级超高周疲劳裂纹由试件内部萌生的机理。     

搅拌摩擦修复2A12铝合金的疲劳性能和裂纹扩展行为

对含预制裂纹的2A12铝合金板进行搅拌摩擦修复试验,并对修复后的试样进行热处理。对修复试样与修复后热处理试样分别进行疲劳寿命与裂纹扩展试验,研究其疲劳性能的变化。结果表明:热处理可使修复试样疲劳寿命延长34.79%,修复试样与修复后热处理试样分别达到母材寿命的36.98%和49.89%,裂纹扩展速率均比母材的快,但修复后热处理试样裂纹扩展速率较修复试样低。疲劳断口显示,母材裂纹源多萌生于表面或亚表面夹杂相,修复试样与修复后热处理试样裂纹源多萌生于修复区和母材的界面,且裂纹源通常不止一处。稳定扩展区修复试样的修复区没有河流花样的解理面,而热处理后试样出现与母材类似的致密紧凑疲劳条带。修复试样瞬断区中由大量细小的等轴韧窝组成,而修复后热处理试样韧窝大小不等,但分布均匀。     

铝合金/黄铜异种金属电弧熔钎焊

采用Zn-15%Al实芯焊丝开展了铝合金/黄铜TIG熔钎焊搭接试验,并对接头力学性能、显微组织和界面层成分进行了测试分析。测试结果发现,焊态下,接头抗拉强度为164MPa;热处理后,接头抗拉强度为160MPa。分析结果表明,热处理后焊缝中的枝晶组织长大,部分柱状晶从界面层脱落进入焊缝,恶化了接头性能。界面层中的金属间化合物主要由CuZn5相组成,以柱状晶向焊缝生长。     

新旧输油管道材料力学性能对比研究

管道在经过长期服役以后,对其整体强度的变化情况进行分析研究是管道研究领域的一个重要课题。管道整体强度的降低涉及到诸多方面原因,其中在内外荷载、环境腐蚀等的共同作用下,其材料的力学性能会发生变化,强度降低是很重要的一个方面。因此对长期服役管道材料进行力学性能研究显得十分迫切。通过对材质均为16Mn,分别截取服役近30年的某管线与尚未投入使用的新管线的管道材料,进行拉伸、冲击、疲劳等试验,比较分析了两种材料的屈服极限、强度极限、冲击韧性等力学性能指标,发现16Mn管道材料力学性能稳定,在经过长期服役后其强度无明显降低。     

Welding of thin sheet of Al alloy (6082) by using Vario wire DC P-GMAW

This paper describes an investigation on the micro-structure, weld bead geometry, dilution rate and mechanical properties of the butt and overlap weld joints of 1-mm-thick 6082 aluminium alloy sheet. Weld joints were produced with the help of a variant of gas metal arc welding (GMAW) process, i.e. direct current-pulsed GMAW (DC P-GMAW), using a Vario wire. The capability of the new process has been assessed in terms of dilution, weld bead geometry, mechanical properties and porosity. The welding results with this new process showed good process stability in the welding of thin sheets of aluminium, while weld mismatch was found to increase with an increase in heat input. Weld bead geometry parameters such as weld size, throat and weld convexity increases with the increase in heat input. The dilution in case of lap joints (10–25%) was less than that of butt joints (60–80%). The increase in factor Φ (summarizing the effect of pulse parameters) increases the form factor and lowers the toe angle. Mechanical properties of the welds are poor as the tensile strength of 6082 alloy welds was around 150 MPa, and the percent elongation was about 1.3%, and it was primarily due to high porosity. Porosity (%) in weld joints was found in the range of 0.33–11.59%. The porosity is a major issue with DC P-GMAW welds.