Weld Cracking Research Articles

Failure Analysis of Cracking on the Outer Pipe of 12Cr1MoV Water Wall in Thermal Power Plants

In order to identify the main causes of cracking and leakage of 12Cr1MoV water wall tubes outside the furnace in thermal power plants, this paper uses macroscopic morphology observation, stereomicroscope observation, alloy composition analysis, Brinell hardness test, metallographic structure inspection, and other means to detect and analyze the failed water wall tube. The alloy composition content of the water wall tubes submitted for inspection is within the standard requirements, and the Brinell hardness value of the water wall tubes also meets the requirements of relevant standards, The matrix metallographic structure of the water wall tube is normal. The initial leakage point is the cracking of the fillet weld at the end of the water wall tube fin, while other leakage points are caused by the blowing of the medium leaked from the initial leakage point. The initial leakage point is formed by the fatigue crack initiated at the fillet weld of the fin end and the corrosion crack on the inner wall extending towards each other. It is recommended to further inspect whether there is still cracking at the end of adjacent water wall tube fins to prevent similar cracking from happening again. Due to the frequent changes in the “peak shaving” period and amplitude in recent years, it is necessary to design the peak shaving period or amplitude reasonably to ensure the safe operation of the boiler.

Failure analysis of a loader hydraulic cylinder and its end cap structure improvement

With the rapid development in the manufacturing sector, lightweight manufacturing has become an inevitable trend. However, achieving lightweight engineering machinery requires the structural redesign of most internal components, including the hydraulic cylinders in hydraulic systems. When evaluating the lifespan of redesigned loader hydraulic cylinders using simulated pulse experiments, it was evident that the fillet weld between the connecting end cap and cylinder barrel failed and fractured before reaching its full lifespan. Consequently, in order to assess the failure conditions of the loader hydraulic cylinder, the generation and extension of the failure cracks were explored using methods such as macroscopic fracturing, hardness testing, metallographic analysis, inclusion component analysis, and finite element analysis. During this process, the end cap structure of hydraulic cylinder was improved using finite element analysis. The results showed that the presence of numerous coarse spherical carbides at the tip of the weld cracks and the formation of dislocation tangles around the carbides, resulting in high stress concentrations and accelerated fatigue crack propagation. Additionally, the specific welding-joint structure of the hydraulic cylinder, resulting in high stress concentrations and accelerated fatigue crack initiation at the weld. After improving the hydraulic-cylinder end cap structure using finite element analysis, the fatigue life of the hydraulic cylinder was improved. This work provides a theoretical basis for the design and manufacture of lightweight and high-life hydraulic cylinders.

Solidification and Liquation Cracking in Welds of High Entropy CoCrFeNiCux Alloys.

High entropy CoCrFeNiCux alloys with a Cu molar ratio of x ≈ 0, 0.5, 1, 1.5 and 2 were arc welded. Solidification cracking occurred in the fusion zones of alloys with x ≈ 0.5, 1 and 1.5. Cu-rich material was observed around cracks, increasing in quantity with increasing Cu content. Liquation cracking occurred in the partially melted zone next to the fusion zone, and it propagated into the fusion zone as solidification cracking. A recently proposed index for the susceptibility to solidification cracking was tried, i.e., |dT/d(fS)1/2| near (fS)1/2 = 1, where T is temperature and fS the solid fraction. The index was higher in alloys with x ≈ 0.5, 1.0 and 1.5, consistent with the solidification cracking observed.

Dramatic improvement of the strength of laser welded joints of Nb521 to GH3128 by adding pure copper as an interlayer

Weld cracks and low weld strength of Nb521 to GH3128 dissimilar metal welded joint are key factors influencing their weldability. To obtain favorable crack-free laser welded joints, this research adopted copper as an interlayer to research the microstructure and properties of laser direct welded and laser offset welded joints of Nb521 to GH3128 superalloy. Compared to previous research results, the tensile strength of laser direct welded joints is enhanced by 2.5 folds. There is a melting reaction layer at the position where the center welded joints are adjacent to the Nb521 base metal, and Ni3Nb and Laves phases are present in the layer. The copper solid solution is located in the center of fusion zone, and Ni3Nb-Laves hard particles are found. The phases near the side of GH3128 superalloy base metal are Ni solid solution, Cu solid solution, Ni3Nb/Cr2Nb hard particles and high melting point alloy mixture. Macrosegregation at the laser offset welding fusion zone is more serious. The phases near the Nb521 base metal are Nb solid solution, Cu solid solution, Ni3Nb and Cu/Cr eutectic structure. At the center of the fusion zone, Cu solid solution, Ni3Nb and Cu/Cr eutectic structure are observed. The phases near GH3128 base metal are Ni solid solution, Cr2Nb phase and Cu solid solution. Mean Vickers hardness values of the direct welded fusion zone is 271.7 ± 129.8 HV, which reduces mismatching of hardness of the joints to a certain degree. As there are Laves phase and Ni3Nb brittle phase in the melting reaction layer adjacent to Nb521 base metal, the microhardness value is up to 589.1 HV. The average microhardness of the offset welded fusion zone is 449.1 ± 217.4 HV, which is higher than the hardness of Nb521 and GH3128 base metals, due to increasing number of brittle phases in the joints. Strength values of the direct welded and offset welded joints are 418.7 ± 49.8 MPa and 364.3 ± 24.5 MPa respectively, which are separately 86.0% and 74.8% of Nb521 base metal. Fractures are all found to occur at the fusion zone on the side of Nb521. The direct welded joints exhibit mixed fracture characteristics with dimples and cleavage facets on the fracture, while the offset welded joints are characterized by brittle fracture.

Effect of process parameters on crack characteristics in laser welding of Al2O3 ceramic

The crack of ceramic weld is a worldwide problem for brittle material, high energy laser beam is expected to solve this problem. In this paper, the crack of fiber laser welding of Al2O3 ceramics was studied. The weld crack rate was used to characterize the crack condition of weld, and the influences of laser power, welding speed and defocusing distance on crack characteristics were carried out. The results showed that Al2O3 ceramics weld has obvious crack tendency, and the cracks mainly appeared on the weld center line. When the crack appeared on the weld center line, there was crack-free on the base metal. When the defocusing distance increased from +3 mm to +20 mm, the number of cracks gradually decreased. When the defocusing distance was greater than +17 mm, cracks-free appeared on the weld and base metal. Abaqus software was used to simulate the relationship between crack and stress based on thermal elastoplastic theory. The high crack areas, few crack areas and free crack areas were divided according to the maximum principal stress value. No matter what welding conditions, as long as the maximum principal stress was less than 1576 MPa, there was crack-free on the weld and base metal.

Fatigue weld crack detection using distributed fiber optic strain sensing

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for early detection and location of fatigue cracks in welds in steel tubular test specimens. To do so, we have subjected welded tubular specimens instrumented with surface-mounted optical fiber sensors to resonant bending load, and we have measured the strain distributions in the test samples continuously and without any interruption of the test throughout its whole duration, with a 2.6 mm spatial resolution and a 1 με strain resolution. We show that the fatigue cracks, which initiate from the inner surface of the wall of the specimens, can be successfully detected and located in real time using our measurement method. We conclude that the detection of the crack initiation may provide relevant information serving the estimation of the remaining lifetime of the component.

Research Summary of Jacket Cleaning and Detection Robot

In order to solve the jacket platform in the sea level for a long time by the platform pressure, seawater erosion, marine biological corrosion and other factors and the emergence of weld cracks, depressions, deformation, fracture and other defects, it is necessary to regularly carry out cleaning and testing to ensure the stability of the jacket structure. At present, the cleaning and detection of offshore platform jackets are mostly carried out by divers diving into the water. The operation is difficult, inefficient, costly, and greatly affected by the weather. When cleaning operations are carried out in areas with poor water quality, it will pose a threat to the lives of divers. Therefore, it is necessary to develop a new type of jacket cleaning and detection robot structure that can carry cleaning and detection devices instead of manual operation.

Tailoring the weld microstructure to prevent solidification cracking in remote laser welding of AA6005 aluminium alloys using adjustable ringmode beam

Solidification cracking is a prevalent defect in welding aluminium alloys which can be effectively mitigated by employing grain refinement strategies. This paper investigates the feasibility of tailoring the weld zone grain structure of 6xxx series aluminium alloys using an adjustable ring mode (ARM) laser beam to reduce the centreline cracking susceptibility. An in-process monitoring approach, comprising a high-speed camera and an ad-hoc Digital Image Correlation, was adopted to examine the strain development during laser welding on a self-restraint test rig. Results revealed that transverse strain in the weld centre exhibits a parabola-like relationship with the core/ring power ratio, with the minimum strain determined at the power ratio of ∼1.5, regardless of the total power employed. The microstructural analysis demonstrated that increasing the core/ring power ratio can lead to the refinement of columnar and equiaxed grains. Furthermore, the formation of secondary equiaxed grains is promoted when the power ratio increases, and this is proved to be related to the characteristic of the ARM laser by the like-for-like comparison. Overall, a fine-tuned power ratio of 1.5 is determined in the studied material and welding configuration for achieving a centreline crack free weld, benefiting from the improved microstructure and reduced thermal strain.

Prevention of cold cracking in armour steel welding

Prevention of cold cracking in armour steel welding

Stress prediction of heated crude oil pipeline in permafrost region via fully coupled heat-moisture-stress numerical simulation and SVM algorithm

Large diameter crude oil transmission pipelines in unstable permafrost areas have been put into operation in China in recent dozen years. The perennial positive temperature operation of buried heating transmission pipelines leads to freeze–thaw of the soil around pipelines, and the axial stresses generated by thawing settlement of permafrost may induce fracture failure of weld crack. However, there are many shortcomings in existing frozen soil thawing settlement analysis and pipeline stress assessment methods, which makes it difficult to accurately analyze and predict the axial additional stress of pipelines induced by thawing settlement. Based on heat conduction and permafrost thawing consolidation settlement theory, numerical inversion analysis of centrifuge test was conducted to verify the accuracy of the finite element method. The key influencing factors such as wall thickness, insulation layer thickness, buried depth of pipe, and length of thawing settlement zone on the axial stress and strain state of buried pipeline under the condition of double pipe operation of Mo-da line were investigated. The results show that: Pipeline with an insulation layer, larger wall thickness and smaller buried depth is beneficial to improve the bearing capacity of buried pipeline under thawing settlement of permafrost. The axial stress varies nonlinearly with the increase of the thawing settlement zone length. Based on support vector regression (SVR) model, the axial stress prediction model of pipe crown and invert at different positions in the thaw settlement area was proposed and verified by field monitoring data of buried pipeline. The relative error between the prediction results and the field monitoring results is less than 15 %. This study can provide some theoretical support for the structural design of pipelines in thaw settlement area.

Ultimate Shear Strength of Pitted Plates with Crack Using Finite Element Method

_ Ultimate strength of thin steel plates was studied under shear loading when pitting corrosion and cracking occur simultaneously. Nonlinear finite element method was used and the effects of different geometrical parameters, such as crack position, crack length, pit depth, pit diameter, number of the pits, and the inclination angle of the crack, were investigated. It was found that the degree of the pitting (DOP) significantly reduces the ultimate shear strength of the pitted plates when the corrosion is low. For high amount of corrosion, the reduction of the ultimate shear strength remains the same regardless of the amount of the DOP. In plates with short cracks, the crack length has no influence on the reduction of the ultimate shear strength. Also, it was found that the position of the crack is more effective than the length of the crack on the reduction of the ultimate shear strength of the cracked plates. In the cracked-pitted plates, there was no interaction between crack and pit for extremes value of DOP. For low value of DOP, the ultimate shear strength of the cracked-pitted plate was the same as the cracked plate, and for large value of DOP, it is the same as pitted plate. Introduction According to a study performed between 2002 and 2008 by American Bureau of Shipping (ABS) (“Review of Current Practices of Fracture” 2012), majority of reported fractures of the ships were structural failures, such as weld defect, buckling, fracture, and cracking. Cracks and corrosion are two more common defects, which are the main reasons of the failure of the aged steel structures. Plates and stiffened plates are the main structural elements in many structures, including airplanes, bridges, offshore structures, and ships. Ultimate strength evaluation of these structural elements is crucial for rational design of structures, such as Load Resistance Factor Design (LRFD) methods in civil engineering and Ultimate Strength (UTS) evaluation in ship structure. Many studies are carried out to evaluate ultimate strength of intact and damaged plates/ stiffened plates under in-plane compressive stress. Ranji (2012) studied ultimate strength of plate with general corrosion under in-plane compression. He proposed a spectrum for random generation of corroded surface and ultimate strength of corroded plate was evaluated using computer code ANSYS. Ranji and Zarookian (2015) analyzed ultimate strength of cracked stiffened plates with cracks having different stiffeners numerically.

Non-destructive evaluation of weld defect with coating using electromagnetic induction thermography

ABSTRACT The weld inspection process under coating is complex and labour-dependent. Therefore, it is of great engineering significance to study weld defect inspection. Based on eddy current pulsed thermography (ECPT) method, this work theoretically analysed the mechanism of weld defect detection undercoating and proposed an inductor suitable for defect detection under weld coating to achieve non-contact, rapid and visual detection results. The simulation model for weld defect detection under coating was constructed, and the effect of coating on weld crack inspection was investigated. However, weld defect test blocks with different coating, thicknesses were verified by experiment, and the thermal characteristics of the circular hole area were analysed. The results demonstrate that the proposed method is effective in weld defect detection, which provides a promising way to extend the application of ECPT to weld defect inspection.

Analytical study on rationalize and reliability improvement of maintenance against stress corrosion cracking in stainless steel welds of lwr primary system

Analytical study on rationalize and reliability improvement of maintenance against stress corrosion cracking in stainless steel welds of lwr primary system

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Experimental and numerical study on the prefabricated double L-shaped beam-column joint with triangular stiffener

This study proposes an innovative prefabricated double L-shaped beam-column joint with triangular stiffener (double L-shaped joint) for low-rise residential buildings. The double L-shaped joint consists of the L-shaped connector with triangular stiffener, web side plate connection, H-shaped beam, and concrete-filled lightweight steel tube (CFLST) column or square hollow steel tube (SHS) column. Twelve double L-shaped joints are manufactured and experimented. The design parameters include the column types (CFLST or SHS), triangular stiffener height, perforated end plate (with or without), and the web side plate connection (with or without). In addition, the failure modes, hysteretic performances, ultimate capacity, stiffness degradation, deformation capacity, energy dissipation, and strain responses of joint specimens are analyzed. The experimental results show that the failure modes include steel tube local buckling, weld cracking, and bolts fracture in the area of the L-shaped connector. The ultimate moment of the CFLST column joints rises from 4.53% to 13.05% compared to the SHS column joints, while the initial stiffness increases from 2.16% to 26.46%. The perforated end plate grows at the ultimate moment, but the ductility index reduces. The initial stiffness and deformation capacity of the specimens without a web side plate connection decreases significantly. The finite element model is developed, and the numerical simulation is conducted. The experimental results are consistent with failure modes and bending moment versus rotation angle curves obtioned by numerical simulation.

ELASTIC-WAVE SCATTERING BY A PLANE CRACK: FLAW DETECTION APPLICATION

The 3D problem of elastic wave scattering by a crack in the form of a half-plane in an elastic medium is considered. It is noted that the previously published solutions do not fully cover the issues relevant to ultrasonic flaw detection, for example, to detect transverse cracks in welds. The proposed solution is made by the Wiener–Hopf tecnique. It is shown that scattered longitudinal and transverse waves can be generally written in the closed form. Some features of the identification of the tip of transverse cracks are noted, including scattering features during sonication of cracks along their surface and the influence of critical angles.

Strain Prediction Using Deep Learning during Solidification Crack Initiation and Growth in Laser Beam Welding of Thin Metal Sheets

The strain field can reflect the initiation time of solidification cracks during the welding process. The traditional strain measurement is to first obtain the displacement field through digital image correlation (DIC) or optical flow and then calculate the strain field. The main disadvantage is that the calculation takes a long time, limiting its suitability to real-time applications. Recently, convolutional neural networks (CNNs) have made impressive achievements in computer vision. To build a good prediction model, the network structure and dataset are two key factors. In this paper, we first create the training and test sets containing welding cracks using the controlled tensile weldability (CTW) test and obtain the real strain fields through the Lucas–Kanade algorithm. Then, two new networks using ResNet and DenseNet as encoders are developed for strain prediction, called StrainNetR and StrainNetD. The results show that the average endpoint error (AEE) of the two networks on our test set is about 0.04, close to the real strain value. The computation time could be reduced to the millisecond level, which would greatly improve efficiency.

Research on magnetic signal of cracks in girth weld of ferromagnetic pipeline

The crack of pipeline girth weld is one of the most common and dangerous defects in girth weld. According to the magnetic dipole theory model, the magnetic signal model of pipeline girth weld cracks is established. By means of numerical simulation, the distribution characteristics of magnetic signals of crack defects in girth welds and the variation rules of magnetic signals of different flaw sizes and lifting heights of girth welds were studied. The simulation results show that the normal component Hy appears zero crossing and changes direction, and the tangential component Hx appears extreme value upward. Finally, the magnetic signal model of crack defects in pipeline girth weld was verified through field engineering test. The actual magnetic induction intensity detected at the flaw of pipeline girth weld was basically consistent with the theoretical calculation results, which verified the accuracy of the model.

Study on the Localization of Defects in Typical Steel Butt Welds Considering the Effect of Residual Stress

When using magnetic memory detection technology to locate weld cracks and porous defects, the traditional zero-point polarity theory leads to misjudgments in defect location and difficulty in distinguishing between the residual stress and the magnetic signals generated by defects due to the influence of external noise and residual stress. Therefore, this paper considers the different mechanisms of magnetic signal generation in areas where crack- and porosity-type defects and residual stresses are located and discusses research focused on the detection of weld defect localization considering the influence of residual stresses. Using the mechanism of magnetic signal generation as a starting point, the three-dimensional magnetic modulus gradient polarity determination method is proposed to distinguish residual stress and defects’ magnetic signals. Through the COMSOL simulation of a welding defect’s finite element magnetic signal, the resulting magnetic signal is converted into a characteristic determination formula for characterization. To verify the accuracy of the simulated characterization, the 3D magnetic signal is extracted and verified manually. Finally, a double orthogonal wavelet transform is introduced to eliminate the random noise in the gradient of the three-dimensional magnetic modulus. The results show that the theoretical analysis, numerical simulation, and experimental results agree with each other. The three-dimensional magnetic modulus gradient values of cracks and pores are much higher than that of the defect-free residual stress area. The three-dimensional magnetic gradient modulus can locate defects and characterize the lengths of defects. The dual orthogonal wavelet eliminates noise interference while improving the accuracy of locating three-dimensional magnetic modulus gradient defects.

Prediction of the influence of welding metal composition on solidification cracking of laser welded aluminum alloy

The crack length of laser welding 6082 aluminum alloy with three types of 5000 series aluminum alloy welding materials was compared by fishbone test method. It was found that ER5A06 welding material corresponds to the shortest crack length, indicating that the welding material composition has an impact on the welding crack and the crack susceptibility could be controlled by adjusting the welding material composition. Combined with SEM and EBSD, the crack morphology was observed and determined to be solidification cracking. Through EDS, EPMA and thermodynamic calculation, it is found that the main elements affecting solidification cracking are Al, Mg, Si and Cu. The cooling rate of the fusion zone was calculated according to the secondary dendrite arm spacing and welding composition. Compared with casting and arc welding, the laser welding aluminum alloy fusion zone has rapid cooling rate. Due to the rapid cooling rate, the T-fs curves of Al-Mg, Al-Si and Al-Cu binary alloys were calculated by commercial thermodynamic software Thermo-Calc. Furthermore, the cracking susceptibility index (CSI) of binary aluminum alloys were also calculated to determine the element content range of the cracking. To understand the element interaction, both the machine learning and computational thermodynamic methods were employed to predict the influence of multi-component welding metal on solidification cracking susceptibility. Based on the calculation results of solidification cracking susceptibility for binary aluminum alloys, a data set containing 45 groups of aluminum alloy components was established and set as the input dataset. The CSI values corresponding to the compositions in the data set were set as the target and the relationship “welding metal composition - CSI” was founded. Based on the Kriging model, the influence of welding metal composition on solidification cracking susceptibility and the components of most crack susceptible were obtained, which provides a basis for welding materials design.