Cracks In Welded Joints Research Articles

Microstructure evolution and crack prevention in TiC nanoparticles-enhanced 2195 Al[sbnd]Li alloy joint fabricated by coaxial fiber-diode hybrid laser wire-feeding welding

High hot crack susceptibility (HCS) in laser-welded 2195 AlLi alloy poses a considerable challenge that restricts its widespread application. Nonetheless, the utilization of advanced laser beam shaping technology and nano-technology presents novel approaches to modify the melting and solidification procedures, enabling precise control over microstructure and crack. Herein, a crack-free and well-formed weld seam (WS) of 2195 AlLi alloy was successfully achieved by employing a coaxial “Gussies + Flat top” hybrid laser source and introducing TiC nanoparticles. The contrast experiments were performed to comprehensively investigate the effect of TiC nanoparticles on the microstructure evolution and hot cracks in welded joints of 2195 AlLi alloy. The results indicated that the crack propagation occurs linearly along the grain boundaries of columnar dendrites and laterally along the grain boundaries of equiaxed dendrite utilizing the conventional AlCu filling wire. With the assistance of TiC nanoparticles, the crack-free WS with homogeneously equiaxed dendrites is facilitated. The elimination of cracks by introducing TiC nanoparticles is attributed to the ameliorative liquid feeding channels, grain refinement, as well as the the pinning effect of TiC nanoparticles.

Deep learning-based acoustic emission data clustering for crack evaluation of welded joints in field bridges

To advance the intelligent operation and maintenance of bridges, a deep learning-based acoustic emission (AE) data clustering framework was developed for evaluating fatigue cracks in welded joints under conditions of operational noise interference and complex damage mechanisms. Specifically, a convolutional autoencoder (CAE) model was implemented to extract damage-sensitive features from AE wavelet images. Additionally, a physics-guided single-and-cross-case strategy using Gaussian mixture models (GMMs) was presented to diagnose overlapping microscopic noise and damage mechanisms across different cases with various crack lengths. Field tests demonstrated the efficiency of the proposed framework to distinguish AE data induced by noise, crack propagation, surface fretting, and impact, enabling accurate identification of no-damage, minor-damage, and serious-damage cases according to their characteristic mechanisms. Future work will incorporate long-term monitoring data from additional cases to further refine the damage quantification and enhance the overall robustness.

Generation and propagation mechanism of cracks in welded joints of thin-walled capsules during hot isostatic pressing

The quality of the capsule has an important effect on the quality of hot isostatic pressing (HIP) products. The reliability of the capsule is a key factor in determining the success or failure of HIP. The capsule is mainly formed through a sheet welding process. In order to ensure the reliability of the weld in HIP, stainless steel with better high-temperature and high-pressure performance than Q235 steel needs to be used. In this case, the fusion line becomes a dissimilar-material interface, which is more prone to fusion-line failure. This paper focuses on examining and studying the formation mechanism of fusion line cracks in welded joints during HIP. The results show that, first, the increase of the HIP temperature weakens the strength near the fusion line significantly. Second, the higher the HIP temperature, the more severe the grain coarsening, and the grain size along the radial direction of the capsule exhibits a monotonic change. Third, since the HIP process changes the micro-zone grain orientation of mild steel, the distribution characteristics of ferrite and pearlite change as well. when the temperature and pressure are higher, the texture still exhibits a strong and complex distribution.

Coupled propagation behavior of multiple fatigue cracks in welded joints of steel-bridge

Fatigue cracking in welded joints is a critical problem in bridge engineering, especially for aging long-span steel bridges. The crack size and density are increasing gradually with the bridge aging process, where the crack interaction will accelerate the crack propagation or even short cracks merging into a long crack. However, the mechanism underlying the coupled propagation behavior of multiple cracks remains unclear. In this study, the coupling effect of multiple cracks was investigated based on numerical simulations. Subsequently, the numerical result was verified by a full-scale segmental fatigue experiment. In order to overcome the time-consuming iterative computations, the back propagation (BP) neural network was utilized to predict the effective coupling spacing between multiple cracks. Finally, a new crack reconstruction method was proposed to simplify crack merging processes. Results indicate that the coupled propagation behavior is significantly impacted by the effective crack spacing. The coupling effect is more significant at the near-end point compared to other feature points. In addition, the cracks with similar sizes have greater coupled effect, which is the worst-case scenario for the multiple fatigue crack problem. The experimental study demonstrates that the growth rate of the near-end point is 1.53 times that of the far-end point and pre-made cracks merge together successfully. By utilizing the BP neural network, the relative error of predicted effective spacing of two cracks is 3.52%. Compared to existing design specifications, the new reconstruction method provides a more reliable result for the fatigue life prediction of the welding seam with multiple fatigue cracks.

Effect of Dynamic Preheating on the Thermal Behavior and Mechanical Properties of Laser-Welded Joints.

The high cooling rate and temperature gradient caused by the rapid heating and cooling characteristics of laser welding (LW) leads to excessive thermal stress and even cracks in welded joints. In order to solve these problems, a dynamic preheating method that uses hybrid laser arc welding to add an auxiliary heat source (arc) to LW was proposed. The finite element model was deployed to investigate the effect of dynamic preheating on the thermal behavior of LW. The accuracy of the heat transfer model was verified experimentally. Hardness and tensile testing of the welded joint were conducted. The results show that using the appropriate current leads to a significantly reduced cooling rate and temperature gradient, which are conducive to improving the hardness and mechanical properties of welded joints. The yield strength of welded joints with a 20 A current for dynamic preheating is increased from 477.0 to 564.3 MPa compared with that of LW. Therefore, the use of dynamic preheating to reduce the temperature gradient is helpful in reducing thermal stress and improving the tensile properties of the joint. These results can provide new ideas for welding processes.

Study on Susceptibility and Mechanism of Reheat Cracking in Welded Joint of Type 347 Austenitic Stainless Steel

The reheat cracking susceptibility of welded joints has been evaluated by 900°C high-temperature constant load test and 750°C–900°C high-temperature slow strain rate tensile (SSRT) test on welded joints of TP347 steel, and the influencing factors and mechanism of reheat cracking in welded joints have been further investigated by microcosmic characterization techniques of SEM and EDS, etc. The result shows the critical fracture stress of TP347 steel welded joints at 900°C is 24 MPa, which is much lower than 80% of the high-temperature yield strength of base material, indicating that the reheat crack of the welded joint is highly sensitive at this temperature; the average reduction of area (RoA) of welded joints obtained by 750°C–900°C SSRT test at each temperature is below 20%, which means all welded joints have a reheat cracking susceptibility in this temperature range, and the reheat cracking susceptibility increases with the rising of temperature. Based on study and analysis using microcosmic methods of optical microscope, scanning electron microscope, and energy spectrum analysis, it is found that welded joints of TP347 steel have a high reheat cracking susceptibility because the high temperature accelerates the diffusion of Cr, Nb, and impurity elements to the grain boundary, and a large quantity of carbides and low-melting-point eutectics are collected and precipitated at the grain boundary to reduce the plasticity of the grain boundary. Since there is a high welding residual stress in welded joints, cavities are formed under the action of the stress at a high temperature, and the merging of cavities causes the formation of cracks which extend along the grain boundary.

The impact of flow-accelerated corrosion on the crack developing in welded joints of NPP equipment and pipelines

Thermal mechanical equipment or the pipelines of the second circuit of a VVER type NPP are mainly subject to two degradation mechanisms that reduce the strength of structural elements: flow-accelerated corrosion and fatigue damage. As a rule, the aforementioned degradation mechanisms bear no combined impact as they affect different zones of the object. However, welded joints are subjected to the impact of both mechanisms which results in initiation and development of cracks in welded joints and in their local thinning due to the low content of alloying elements or due to hydrodynamic features of the flows arising in back of taps, throttling devices, cone transitions and similar pre-connected sections. The goal of the study is to elucidate the effect of flow-accelerated corrosion on the development of fatigue cracks in welded joints. We used the formulas of fracture mechanics to describe the time-dependent undergrowth of cracks and empirical data to describe the dependence of flow-accelerated corrosion on the operation time and postulated residual defect structure. The proposed approach provided estimation the effect of flow-accelerated corrosion on the rate of crack developing in the equipment or pipeline. It is shown that flow-accelerated corrosion can both reduce and significantly increase the crack growth rate. The distribution of defects versus the operating time is obtained, taking into account the entire set of possible missed defects. The distribution obtained provides for more correct assessing of the reliability and safety of welded joints of the NPP equipment and pipelines.

A review porosity in aluminum welding

Aluminum is a material that is often used in the transportation industry because it has a low density, high strength, and good corrosion resistance. The transport structure requires connection to achieve a form as designed. The joint process usually uses a welding process because welding has good metallurgical properties, superior mechanical properties, and low cost. Aluminum welding generally has disadvantages. One of these weaknesses is the formation of porosity, which will affect the welded joint’s mechanical properties, so the discussion of porosity in aluminum welded joints is critical. This paper attempts to provide an overview of the formation mechanisms and types of porosity defects in aluminum alloys’ welding method. A brief description of the effect of the environment on porosity defects, the causes of porosity formation, the impact of porosity, and how to reduce porosity in aluminum welding will be discussed. In conclusion, the formation of porosity in the welding process is due to dissolved hydrogen in the weld metal, air/bubbles from the welding process, filler/filler wire, and the environment welding influence. The impact of porosity can cause cracks in welded joints, decreased ductility, and failure of fatigue. This study’s results can be used as a basis for analyzing the relationship between the welding environmental parameters and the welding defect porosity.

Results of the experimental study of the fatigue characteristics of the repaired welded joint of the marine stationary oil and gas producing platform

Offshore stationary platforms are actively exploited in the world. The lifespan of some of these platforms exceeds 20 years. Such a significant period of operation leads to the fact that the platforms are exposed to various loads for a long time, causing alternating stresses and, as a result, fatigue damage, including fatigue cracks in welded joints and the base metal of the platform. When fatigue cracks are identified, defect joints are repaired commonly using welding techniques. However, the duration of the subsequent operation of such repaired welded joints is not currently known. The authors conducted an extensive experimental study to understand the life span of welding repair technologies. This approach was for the first time used to repair the welded joint of a platform located in the White Tiger. White Tiger is the Vietnamese offshore oil field, located 200 km east of Ho Chi Minh City on the shelf of the South China Sea. The result of this study was the fatigue diagram and its equation, which were first obtained by the authors.

The analysis of the causes of efficiency decrease in the metal of liquefied gas pipeline under the conditions of corrosive environmental exposure

Migration of gas and oil production facilities to remote northern regions of the country, increase in the unit capacity of the equipment set the task of increasing the service life and reliability of equipment for the extraction and transportation of gaseous and liquid media. One of the main reasons for the accelerated failure of equipment is the corrosion effect of transported media. In this work, the effect of corrosion under stress on the performance of the main metal and welded joints of stainless pipes of austenitic-grade chromium-nickel steels has been studied. It is shown that the rate of the anodic process increases as the level of plastic deformation increases and the rate of growth depends on the degree of imperfection of the structure of the material. It has been established that the heat-affected zones of assembly welded joints of austenitic steels are the most prone to corrosion cracking, and the requirements for restoring the structure of heat treatment of the weld are not fulfilled. Refusal of heat treatment leads to the appearance and growth of carbide particles in the structure of the heat-affected zone, which reduces almost by a factor of two, from 270 to 150 MPa, the level of stresses, leading to the occurrence of corrosion cracking in welded joints.

Experimental investigation on the stress corrosion cracking of FV520B welded joint in natural gas environment with ECP and SSRT

For the welded joint specimens of the steel FV520B used in natural gas impellers, the uniform experimental method is applied in the experiment of electrochemical corrosion potential (ECP) under a certain concentration of H2S and CO2, temperature and pressure to get some test data including electrochemical corrosion potential, current density, Tafel slope and polarization resistance. In addition, the stress corrosion susceptibility index has been measured by the experimental of slow strain rate tensile (SSRT). Combined with observation of crack and fracture morphology, the stress corrosion cracking mechanism of the martensitic stainless steel FV520B welded joints in the service environment is explored. The experimental results show that there are obvious tendency of stress corrosion cracking in welded joints and the relatively significant effects of temperature and CO2 concentration on corrosion current density and stress corrosion susceptibility. In the environment of 98 °C, 6 MPa, 30% H2S and 65% CO2, the stress corrosion susceptibility of welded joint specimens is the highest, and the fracture surfaces display brittle fracture behavior.

ДОСЛІДЖЕННЯ ВПЛИВУ ГЕТЕРОГЕННОСТІ ЗВАРНИХ ШВІВ НА РЕСУРС ТРИМАЛЬНИХ СИСТЕМ ПРИЧЕПА

The paper considered the issues of the influence of structural heterogeneity of welds on the life of the trailer design. Mainly in the works on welded structures, the effect of stress concentration during welding on the strength and crack resistance of the weld is considered and the possibilities of reducing temperature differences and deformations, which create the main threat of microcracks, are examined. One of the essential features of welded joints is the structural-mechanical inhomogeneity of the metal. The nature and properties of this heterogeneity as a factor in the destruction of welded joints is not fully determined. Therefore, the object of this study is the structural heterogeneity of welds of the supporting structure of the trailer BMZ-887.General questions are considered regarding the trailer designs and physical properties of welded joints and the peculiarities of heterogeneity of the heat-affected zone or near the suture zone. For the study, we used the following methods: tensometric research, quantitative metallography. The paper presents experimental data that were obtained using the above mentioned methods, and a comparative analysis of these data was carried out. The relationship between the numerical values of the parameters of the weld heterogeneity and the propagation velocity of small and main fatigue cracks in welded joints is established.The numerical values of the heterogeneity parameters and the propagation speeds of fatigue cracks in the welded joints of the following types were obtained: the propagation speed of small cracks decreases slightly with increasing diameter of the second phase particles; particle diameter does not significantly affect the propagation velocity of trunk cracks; the effect of concentration (distance between particles) for small cracks is not significant; for trunk cracks with increasing particle density, the rate of propagation of cracks is somewhat reduced.

Determination the maximum load capacity of the welded structure of the transport carriage in operation

The article presents a calculation procedure for determining the maximum permissible load on the welded structure of the transport carriage. During the transport carriages operation inside the production hall there was no breakage of the welding joints. The use of carriages outside the hall and with an uneven surface began to create cracks and fractures in the weld joints. The values of vertical accelerations that were generated by running on uneven surfaces were determined by experimental measurement. To avoid cracks in welded joints, the maximum permissible load value of the transport carriage was subsequently determined by calculation using FEM.

A Basic Study on Brittle Crack Propagation Path with Ultra Large Steel Plate Weld Joints

As the recession in shipbuilding industry became longer, domestic shipbuilders focused on the need for high value added and high technology, and strived to build safer vessels. There are LNG carriers and container ships that require representative technologies. Both ship types are rapidly increasing ship size in the 2000s. Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. Studies on unstable fracture have mainly been conducted to evaluate the brittle crack stopping properties of the high strength steel welded joints through large scale fracture tests, and studies on the difficulty of stopping brittle cracks in welded joints. It is known that BCA (Brittle Crack Arrest) steel has recently been developed to prevent unstable fracture if cracks propagate toward the BCA steel even if brittle cracks occur. Therefore, it is considered that the ship can be dried more easily if a technology capable of inducing the brittle crack to the BCA steel is developed. In this study, we describe a technique for securing unsafe fracture safety by inducing cracks toward the BCA steel, which has excellent brittle crack stopping ability, regardless of the welding process when brittle cracks occur in the superstructure of super large container ships. Key words: Brittl crak arrestability, Crack arrest toughness, Thick steel plate, FCAW, ESSO test, Welding residual stress

Improved Formula for the Stress Intensity Factor of Semi-Elliptical Surface Cracks in Welded Joints under Bending Stress.

Welded joints are prone to fatigue cracking with the existence of welding defects and bending stress. Fracture mechanics is a useful approach in which the fatigue life of the welded joint can be predicted. The key challenge of such predictions using fracture mechanics is how to accurately calculate the stress intensity factor (SIF). An empirical formula for calculating the SIF of welded joints under bending stress was developed by Baik, Yamada and Ishikawa based on the hybrid method. However, when calculating the SIF of a semi-elliptical crack, this study found that the accuracy of the Baik-Yamada formula was poor when comparing the benchmark results, experimental data and numerical results. The reasons for the reduced accuracy of the Baik-Yamada formula were identified and discussed in this paper. Furthermore, a new correction factor was developed and added to the Baik-Yamada formula by using theoretical analysis and numerical regression. Finally, the predictions using the modified Baik-Yamada formula were compared with the benchmark results, experimental data and numerical results. It was found that the accuracy of the modified Baik-Yamada formula was greatly improved. Therefore, it is proposed that this modified formula is used to conveniently and accurately calculate the SIF of semi-elliptical cracks in welded joints under bending stress.

Characteristics of Welding Crack Defects and Failure Mode in Resistance Spot Welding of DP780 Steel

The mechanical properties of welded joints in resistance spot welding of DP780 steel were tested, and three different types of welding cracks in welded joints were investigated by optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Finally, the failure mode of the welded joints in shear tensile test was discussed. It is found the shear tensile strength of welded joints can be greatly improved by adding preheating current or tempering current. The surface crack in welded joint is intergranular fracture, while the inner crack in welded joint is transgranular fracture, and the surface crack on the edge of the electrode imprint can be improved by adding preheating current or tempering current. The traditional failure mode criterion advised by American Welding Society is no longer suitable for DP780 spot welds and the critical nugget size suggested by Pouranvari is overestimated.

Formation of lack of fusion defects and hot cracks in welding steel structures with a corrosion-resisting zinc coating

The conditions of formation of lack of fusion defects and hot cracks in welded joints in welding steel structures with zinc coatings are investigated. The results show that the probability of formation of lack of fusion defects in the metal of the welded joints produced in CO2 mechanized welding with a consumable electrode increases with increase of the thickness of the zinc coating and is caused by the additional consumption of the heat of the welding arc for evaporation of zinc. To prevent the formation of lack of fusion defects in the metal of these welded joints, it is recommended to correct the welding conditions taking the thickness of the zinc coating into account.

Hydrogen-assisted cold cracking in welded joints of press-hardened 22MnB5

Over the last decade, press hardening has become all pervasive for automotive body-in-white design. Press-hardened boron micro-alloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The mechanical properties of the welded joints are influenced by the hydrogen content in the base metal as well as the weld metal. Hydrogen absorption may result from the press-hardening process, as well as the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper focuses on the determination of the diffusible hydrogen content in the base metal after the press-hardening process, as well as in gas metal arc (GMA) welded joints of 22MnB5. Hydrogen source during press hardening is the humidity at the austenitizing temperature in the furnace atmosphere. During welding, for this study hydrogen was deliberately introduced by hydrogenous fluids on the sheet surface. The diffusible hydrogen content in the base metal and the weld metal was quantified by thermal desorption mass spectroscopy (TDMS) technique. The influence of the diffusible hydrogen on the mechanical properties of the welds was determined by four-point-bend testing. The time to fracture was identified using acoustic emission technique.

Investigation of Static and Dynamic Characteristics of Complex Thin-Walled Shell Structure with Cracks

A numerical approach to investigation of static and dynamic characteristics of fuel reservoir and surge-protection capacity with cracks in welded joints of their walls has been proposed. Mathematical modeling of the nonlinear behavior of the complex shell structure has been performed under the action of static vertical loading using the software complex of finite element analysis. The models of cracks with different length within vertical and horizontal welded joints of shell walls have been developed. The influence of cracks on the stress-strain state and stability of the fuel reservoir as well as the surge-protection capacity has been investigated. The dynamic characteristics of the structure have been determined, the influence of cracks on the frequencies and the modes of their natural vibrations has been evaluated.

Investigation of the mechanisms and causes of cracking in welded joints in 20 K steel (P265GH)

The results of investigation of the reasons for the formation of cracks in welded joints in 20 K steel are presented. The nature of failure was determined by the methods of vacuum thermal etching in metallographic and fractographic studies of fracture surfaces. The intragranular nature of failure by the ‘cold cracking’ mechanism is described.