An investigation on the performance of stress corrosion cracking in aluminum-copper alloy welded joint

With the resistance to stress corrosion of the base metal as a reference, the contrast result of stress corrosion cracking （ SCC） susceptibility of aluminum-copper alloy 2219 and 2014 welded joints under different welding processes （ VP-TIG welding, HF-TIG hybrid welding, laser-TIG hybrid welding and laser HF-TIG hybrid welding） is obtained via the slow strain rate testing （ SSRT） , scanning electron microscope （ SEM） and microstructure observation auxiliary technologies. Test results show that the joints of aluminum alloy 2219, welded by hybrid welding processes, have superior resistance to stress corrosion compared to those welded by the VP-TIG welding process in varying degrees, especially, the joint welded by the laser HF-TIG hybrid welding process, where the resistance to stress corrosion is almost the same as that of the base material. However, the HF or laser hybrid welding effect is not significant under the same welding conditions for welded joints of aluminum alloy 2014.

The Cracking Induced by Oxidation-Hydriding in Welding Joints of Zircaloy-4 Plates

The welding joints of Zircaloy 4 plates obtained by diffusion welding at 800°C under pressure in vacuum were cracked during autoclave tests at 400°C superheated steam after exposure longer than 150 days. The section of specimens was examined by optical microscopy and the composition at the tips of cracking was analyzed by electron microprobe. The result shows that the combination of oxidation and hydriding induced cracking is responsible for this failure of the welding joints.

Formation mechanism of transverse crack in repair welding for thick plate of Q345 high strength steel

Transverse cracks occur usually in repair welding for thick plate of high strength steel. It needs multiple times of repair welding. The quality of production and deliver deadline will be influenced. Therefore, it is very significant to investigate the cause and control of transverse crack in repair welding. In this paper, both ends restraint crack experiment is developed to produce delay transverse crack for high strength steel. Metallographic results show that four types of cracks are found in repair welding metal zone and heat affected zone. Large chevron transverse cracks are found in repair welding zone. Lots of micro transverse cracks are found in inter-layer repair welding metal zone, root HAZ and two ends of repair welding individually. The distribution character and formation mechanism of the transverse crack are further analyzed through hardness testing and residual stress measurement.

A J-INTEGRAL ANALYSIS OF CRACKS IN OVERMATCHED WELDED JOINTS

The effects of material inhomogeneity and weld width of the overmatched welded joints with a longitudinal crack or flaw upon the J-interal,a characterizing parameter for elastoplastic fracture,have been studied.The overmatched weld is idealized to be a parallelly cracked and sandwiched hard layer,the yieht stress of which is higher than that of the base metal.Numeri- cal solutions were obtained by using elastoplastic finite element method under the assumption of plane stress.The resuits show that the values of J-integral are remarkably affected by the material inhomogeneity ant the weld width especially when the nominal stress or nominal strain is comparatively high,which,therfore,should be taken into account when evaluating the crack driving force for an overmatched joint.

Effects of strength matching and crack depth on the fracture parameters for welded joints

Fracture parameters of welded joints with different strength matching and crack depth in weld metal are investigated by using the methods of elastoplastic finite element analysis and three point bend specimen test. The results show that for shallow crack, the plastic zone turns large in loading process, and the fracture toughness turns high. The extent of the plastic zone of overmatched joint is larger than that of undermatched joint because it will extends to parent metal from the weld metal in loading process for the same CTOD value. The plastic zone of undermatched joint is restricted within the weld, and the size of that is small. Overmatched joint shows the fracture behaviour of shallow crack may more easily than the undermatched joint, while the two sorts of joint specimens have the same crack depth. Therefore, the fracture-resistant capability of overmatched weld is better than that of undermatched weld. when the toughness of weld metals is similar for both overmatched and undermatched joints.

FATIGUE CRACK GSOWTH AND CRACK CLOSURE OF SIMULATED OVER-MATCHED WELDED JOINTS

Mismatching is one of the basic characteristics of welded joints.Overmatched welded joint with a crack in hard weld is simplified as a cracked body of CCP type SHS (a hard bar sandwiched between two soft material with a crier perallel to the interface ) in this paper.Effects of mechanical heterogeneity on fatigue crack growth and crack closure of this model are studied by experimental and elastic - plastic finite element methods. It is found that, residual plastic deformation remaining in the wake of a growing crack tip, which cases crack closure,is affected the plastic deformation in adjacent soft mate- rial near by the crack tip. Both the shape of crack tip plastic zone and stress distribution ahead of crack tip in heterogeneous cracked body are different from that of homogeneous one. Crack closure (open- ing) loads are estimated by investigating the changes of crack tip opening displacement during fatigue loading, and a new model of crack oening process is presented. Because of the difference in the effect of mechanical heterogeneity on displacement of non - fatigue crack and fatigue crack, conventional CTOD can not reflect the real deformation at crack tip of SHS specimens.

Cracking Phenomenon in Spot Welded Joints of Austenitic Stainless Steel

The spot welds nugget cracking of austenitic stainless steel at temperatures between 700°C - 1010°C was investigated. Traditionally, the cracks have been observed around the spot nugget in welded temperature. Actually, these cracks are developed due to incomplete melting and inappropriate electrode pressure, which causes an expulsion of molten metal. These cracks start to grow and cause either the interface or plug fracture according to the loading type. In this work, the micro-cracks in the weld nugget were indicated for this type of steel at elevated temperature. Cracks appear in a certain range of temperature;about 700°C - 750°C. The cracks like defect and cavitations were presented. According to the fracture mechanics point of view, these cracks reduce the mechanical strength. Therefore, these cracks have to be taken into account with a certain precaution. Moreover, considering the working temperature and reducing the element may develop ferrite particles.

Fatigue behavior and life prediction of A7N01 aluminium alloy welded joint

Fatigue characteristics of A7N01 aluminium alloy welded joint were investigated and a fatigue crack initiation life-based model was proposed. The difference of fatigue crack initiation life among base metal, weld metal and heat affected zone (HAZ) is slight. Furthermore, the ratio of fatigue crack initiation life (Ni) to fatigue life to failure(Nf) is a material dependent parameter, 26.32%, 40.21% and 60.67% for base metal, HAZ and weld metal, respectively. Total fatigue life predicted using the presented model is in good agreement with the experimental data and that using Basquin’s model. The observation results of fatigue fracture surfaces, using scanning electron microscope (SEM), demonstrate that fatigue crack initiates from smooth surface due to welding process for weld metal, blowhole in HAZ causes fatigue crack initiation, and the crushed second phase particles play an important part in fatigue crack initiation in base metal.

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.

Creep fracture parameters of polyethylene pipe welded joints

Double edge notched tensile （DENT） tests were carried out to investigate the slow crack growth （SCG） behavior of PESO welded joints. Efforts were made to acquire creep fracture parameters which can characterize creep resistance. Results show that creep compliance indicates an ascending trend with the increase of applied stress. A global creep load parameter is obtained. It has a linear relationship with fkacture time in logarithmic coordinate system and helps to predict the fracture time at any applied load.

STUDY ON DYNAMIC J-INTEGRAL OF MECHANICAL HETEROGENEOUS WELDED JOINT

Welded joint is a mechanical heterogeneous body, and mechanical heterogeneity has great effect on dynamic fracture behaviour of welded joints. In the present investigation, dynamic response curve and dynamic J-integral of practical undermatched welded joint and whole base and whole weld three-point-bend (TPB) models containing longitudinal crack are com- puted. Dynamic J-integral is evaluated using virtual crack extension (VCE) method and the computation is performed using MARC finite element code. Because of the effect of inertia, dynamic load response curve of computed model waves periodically. Dynamic J-integral evaluated by VCE method is path independent. The effect of inertia has little influence on dynamic J-integral curve. The value of dynamic J-integral of undermatched welded joint is lower than that of whole base metal and higher than that of whole weld metal. The results establish the foundation of safety evaluation for dynamic loaded welded structures.

Stress corrosion behavior of pipeline steel(X52)welded joint

The resistance of pipeline steel joint welded by submerged arc welding to stress corrosion cracking (SCC) at its different regions in boiling mixed nitrate solution and in H,S saturated solution at room temperature is examined by the constant-load tension method. The result shows that when tested in nitrate, the weld metal may be the weakest link of the joint if the welding material doesn't match well. The resistance to SCC of the joint in H2S is very low, especially at the bond and coarse grain zone and is closely dependent on the metallurgical quality of base metal.

Fatigue Crack Growth Behavior of Different Zones in an Annealed Automatic Gas Tungsten Arc Weld Joint of TA16 and TC4 Titanium Alloys

Based on the investigated microstructure of different zones in the annealed automatic gas tungsten arc weld joint of TA16 and TC4 titanium alloys,the mechanical property of them was assessed under fatigue crack growth rate tests.For evaluation of fatigue crack growth rate,three points bending specimens were used.The correlation between the range of stress intensity factor and crack growth rate was determined in different zones of the annealed weld joint.Fatigue crack growth rates were obviously different in different zones of weld joint of dissimilar titanium alloys,due to their different microstructures.Scanning electron microscope examinations were conducted on the fracture surface in order to determine the relevant fracture mechanisms and crack growth mechanisms with respect to the details of microstructure.

Cryo-Cracking方法及其在焊接接头蠕变断裂失效分析中的应用

介绍了 Cryo- Cracking方法及其在分析和评定高温下长期服役的焊管焊接接头蠕变失效中的应用特点。这种方法使用的试样易于截取 ,制样工艺过程简单 ,对制样者的技艺无特殊要求 ,不易产生因试样的制备而引起的误解。通过扫描电镜断口形貌的分析 ,结合金相定量分析方法 ,从分析断口的二维图象获得的数据 ,可以定量地反映在役构件蠕变失效的程度。实践表明 ,Cryo- Cracking方法是一种简单实用的焊接接头蠕变失效分析方法。

Effect of Weld Parameters on Effective Notch Stress at Weld Root and Toe of Load Carrying Cruciform Joints

Cruciform joints in ships are prone to fatigue damage and the determination of type of weld plays a significant role in thefatigue design of the joint. In this paper, the effect of weld geometry on fatigue failure of load carrying cruciform joints inships is investigated using Effective Notch Stress (ENS) approach. A fictitious notch of 1 mm radius is introduced at theweld root and toe and fatigue stress is evaluated. The effect of weld leg length (l) and weld penetration depth (p) on ENS atweld root and toe are determined. The critical weld leg length (lcr) at which fatigue failure transitions from weld root toweld toe is investigated. An approximation formula for determination of the critical weld leg length considering weldpenetration depth (p) is proposed.

Simulation-Based Optimization of Aspect Ratio in Tungsten Inert Gas Welding

The shape of the fusion zone after weld in terms of its width-to-depth ratio is known as the aspect ratio, large aspect ratios in welded joints usually results in cracks formation during solidification of the weld;it also results in tensile residual stresses at the fusion zone. In this study, central composite design matrix was employed using Design Expert 7.01 software to optimize the aspect ratio of mild steel welded joint. A total of 20 sets of experiments were produced;the weld specimen was mild steel plate measuring 60 mm × 40 mm × 10 mm. TIG welding machine with 100% Argon Shielding Gas was used for this experiment and at the end of the experiment, an optimum weld aspect ratio of 0.646 was achieved using current of 140 amp, voltage of 25 volt and gas flow rate of 15 L/min. This value of 0.646 is expected to contain the minimum adequate molten metal just enough to make the desired bead penetration to form good aspect ratio at a minimum cost with appropriate weld quality and productivity. This would help minimize the formation of cracks after weld.

Study on fracture mechanics parameters of surface crack in overmatched weldments

In this paper, the J integral of surface cracks in overmatched weldments with different matching ratio has been investigated using direct evaluation method. The influence of matching ratio of joints on the crack driving forces (J integral, crack mouth opening displacement CMOD) and strain distributions along the contour have been discussed in detail. The results indicate that the comprehensive effects of crack position and the matching ratio of welded joint dominate the distribution of strain along the contour. Moreover, the crack driving force increases with the decrease of the matching ratios in overmatched weldments.

基于等效裂纹法的铝合金T形焊接接头疲劳寿命评估

针对焊接缺陷的多样性、复杂性及随机分散性等特征,提出一种以一组高应力疲劳试验结果作为参考点的等效裂纹法开展焊接结构的疲劳寿命研究。以疲劳试验结果作为参考,假设一个完整板中的初始裂纹等效焊接结构中的缺陷及应力集中效应,基于线弹性断裂力学和Paris公式计算等效裂纹扩展寿命作为焊接结构的疲劳失效寿命。提出基于等效裂纹法的结构疲劳寿命计算流程;开展铝合金T形焊接接头的疲劳试验以及拟合P-S-N曲线;基于T形焊接接头的焊根失效模式采用等效裂纹方法预测T形接头的计算P-S-N曲线,并与试验拟合曲线进行对比,相对误差为10%,验证了该方法的可行性与有效性;最后,通过一组变幅载荷谱疲劳试验与预测P-S-N曲线对比,相对误差均小于15%,同时也验证了该方法对于变幅载荷的适用性,实现了基于一组疲劳试验结果预测全范围T形焊接接头疲劳寿命的评估。以较少的测试和成本实现确定的疲劳寿命,简化了概率疲劳寿命的评估过程,该方法可以推广于其他焊接接头寿命评估中。

蒸汽发生器异种钢焊接接头疲劳裂纹扩展研究

钠冷快堆是我国发展第4代先进核能系统的主力堆型,蒸汽发生器是钠冷快堆重要部件.为了保证蒸汽发生器的安全性和可靠性,对异种钢焊接接头进行疲劳裂纹扩展行为研究.异种钢焊接接头共有6个不同微区,分别为F22母材、F22侧热影响区、隔离层、堆焊层、对接焊缝及316H母材.对异种钢焊接接头进行硬度测试、显微组织观察、疲劳裂纹扩展试验,利用光学显微镜和扫描电子显微镜(SEM)进行微观组织观察.F22母材组织主要为铁素体、珠光体和贝氏体,F22侧热影响区组织主要为铁素体和珠光体,隔离层、堆焊层、对接焊缝主要组织为奥氏体和δ铁素体,316H母材组织主要为奥氏体.F22母材的显微硬度最小,隔离层显微硬度最大,堆焊层、对接焊缝、316H母材的显微硬度逐渐下降,F22侧热影响区显微硬度变化较大.在低ΔK时,F22母材的疲劳裂纹扩展速率最快,隔离层、堆焊层、对接焊缝、316H母材的疲劳裂纹扩展速率逐渐变缓,随着ΔK的增加,隔离层的疲劳裂纹扩展速率变化最快,这是因为隔离层裂纹逐渐向F22母材侧偏转,使得其抵抗疲劳裂纹扩展能力逐渐降低.δ铁素体会影响隔离层、堆焊层、对接焊缝的抗疲劳裂纹扩展能力.F22母材和F22侧热影响区的疲劳裂纹断口撕裂最严重,隔离层、堆焊层、对接焊缝的断口平坦度逐渐增加,316H母材的疲劳裂纹断口形貌最平坦,与抗疲劳性能相对应.

焊接残余应力对钢桥面疲劳性能的影响与处理措施

为改善正交异性钢桥面的抗疲劳性能,对钢桥面进行退火处理以降低关键焊缝的焊接残余应力。首先系统性地回顾和探讨钢桥面焊接残余应力分布模式,同时就残余应力对疲劳裂纹发展的影响进行了文献分析。随后,归纳和总结了国内外主流焊接残余应力消除方法,并开展对比分析。借鉴压力容器制造方法,建议在正交异性钢桥面制造中引入退火处理工艺以降低焊接残余应力。研究聚焦钢桥面中疲劳问题较突出的顶板与U肋连接焊缝,设计了16个(其中9个退火处理,7未作处理)局部足尺单U肋试件进行残余应力测试与疲劳试验。试验结果表明,退火处理后焊缝残余应力实测值降低约80%,疲劳强度提高约23%。可以看出,退火处理可以大幅降低焊接残余应力,从而有效提升钢桥面抗疲劳性能。