Weld Cracking Research Articles

Application of the total focusing method for quantitative nondestructive testing of anisotropic welds with ultrasound

Abstract In this paper, we report on the application of the ultrasonic Total Focusing Method for quantitative imaging of cracks in austenitic welds. Imaging inspection procedures provide more information than punctual measurements. In addition, an image provides more easy access for most human viewers. In ultrasonic testing, array transducers have played an important role in imaging for many years. The Full Matrix Capture technique in combination with Total Focusing is one of the newer methods with decisive advantages. In combination with fast electronics, computer technology and image processing, it now enables tomographic imaging in real time. The functionality of the method and the technical requirements are explained here. Austenitic weld seams are considered as examples for anisotropic materials, which can be examined with this method. An important requirement for quantitative measurement of defect size in anisotropic materials is the knowledge of the ultrasonic phase and group velocities of the inspected volume. The Gradient Elastic Constant Descent Method can deliver unknown parameters, which are needed to calculate these velocities.

Features of electron beam welding of molybdenum alloy ZrMo2А

ABSTRACT The paper demonstrates the positive experience of fighting hot cracks in electron beam welding of molybdenum alloy СМ2А by double passage by an electron beam, which led to a decrease in the content of harmful impurities in the seam. The structure of the compounds obtained is shown, and their hardness and the content of harmful impurities are measured.

Structure, Mechanical Properties, and Stressed State of Weld Joints of the Tial(Nb, Cr, Zr) Intermetallic System

Structural features, mechanical properties, and stressed state of welded joints of the TiAl system intermetallic compound (Nb, Cr, Zr) after electron-beam welding and subsequent electron-beam local heat treatment are determined. It was shown that stresses generated immediately after welding contributed to the appearance of cold cracks in the weld. Residual stresses in welded joints were determined using the technology and compact equipment based on the method of electronic speckle interferometry created at the Paton Electric Welding Institute of the National Academy of Sciences of Ukraine. It was found that after electron-beam welding, due to rapid cooling of the weld metal, phase transformations occurred, which deteriorated its mechanical characteristics. A method was developed for the local heat treatment of a welded joint by an electron beam immediately after the end of the welding process. Parameters of the local heat treatment process can significantly reduce the cooling rate, maintaining the temperature of the welded joint for 5 min at the level of 900°C. This can significantly reduce the level of residual stresses in the weld and contribute to the formation of a favorable three-component structure: γ-phase, (γ + α2)-phase and β-phase, which increases the weld ductility. A study of mechanical properties during the compression test of the weld showed that after local heat treatment, the yield strength of the weld was improved, while the tensile strength and degree of deformation under compression increased significantly. The proposed local heat treatment is effective and economically justified, which allows to reduce residual welding stresses by 30%, thus preventing the formation of cracks in welds, as well an improving structural and mechanical characteristics.

Role of liquid backfilling in reducing solidification cracking in aluminium welds

ABSTRACTThe solidification cracking susceptibility of aluminium alloys 2024, 2219, 6061 and 7075 was evaluated recently by the transverse motion weldability test (the lower sheet moved in the transverse direction of lap welding to cause cracking). The tested welds were analysed in the present study. Their transverse cross-sections showed wide backfilled cracks in 2219 (least susceptible) and narrow open cracks in 6061 (most susceptible). Their fracture surfaces showed continuous eutectic in 2219 and discontinuous eutectic in 6061, consistent with the less backfilling in 6061. T was plotted against (fS)1/2 (T: temperature; fS: fraction solid) to explain the differences. The critical local strain rate near the crack was estimated, significantly lower in 6061 than 2219 and consistent with the higher crack susceptibility of 6061.

Investigation on solidification cracks in pulsed laser spot welding of an AZ31 magnesium alloy

A direction-dependent model of solidification crack initiation and propagation is established to analyze the influence of the melt pool shape on the distribution of solidification cracks in pulsed laser spot welding. The formation direction of the initial crack is related to the variation of the solidification parameters. When the aspect ratio of the spot welds with a default Rectangular pulse is greater than 0.3 and less than 0.8, an initial crack is formed in the 45° direction of the mushy zone, and then the crack propagates along the grain boundary. That is, the solidification cracks are distributed in the direction of 45° on both sides of the surface center of the welding cross-section. When the aspect ratio of the spot welds is less than 0.3 or greater than 0.8, an initial crack is formed in the 90° direction of the mushy zone, and then the solidification crack is vertically distributed at the center of the welding cross-section. The Step-Down-10ms-0.3P pulse can significantly reduce the solidification rate of the melt pool interface, preventing the formation of initial cracks in the sensitive directions of the spot welds with an aspect ratio less than 0.8, thereby eliminating solidification cracks.

Asset management strategy for HAZ cracking caused by sigma-phase and creep embrittlement in 304H stainless steel piping

This study aims to develop an asset management strategy to address weld cracking on 304H austenitic stainless-steel piping welded with 308H filler metal. The 304H NPS 16 outlet line from the primary reformer fired heater to the primary reformer which operates in high temperature service at around 620 °C was the principal case study. A metallurgical evaluation characterized the failure as sigma-phase and creep embrittlement in the heat-affected zone; however, sigma-phase embrittlement was the more predominant damage mechanism. Sigma-phase is the most common intermetallic phase precipitating in austenitic stainless steels. The cracks due to embrittlement were exacerbated by a significant amount of sigma-phase formation, which initiated at the fusion line and propagated at the coarse-grained heat-affected zone which were interconnected with creep voids. Welding procedures were developed using two alternative filler metals, ER16-8-2 and ER19-10H, as a repair option and for new fabrication. The philosophy was to reduce the sigma-phase formation by controlling the heat input; thereby limiting the ferrite; and the probability for embrittlement. The results from the mechanical and metallurgical examination of the welded specimens proved that the ER16-8-2 filler metal possessed superior resistance to sigma-phase formation. Acoustic emission testing and a combination of field metallographic replication and ferrite testing were identified as the most effective predictive technologies to detect the incipient cracking.

Seismic behaviour of combined steel framed-tube substructure with replaceable shear links

A combined steel framed-tube structure with replaceable shear links (SFTS-links) can make plasticity isolated to links for rapid replacement of damaged links and improve the applicability of SFTS-links with enhanced ductility in high-seismic zones. In this paper, the seismic performance of thirty-storey SFTS-links and SFTS was compared by elasto-plastic time history and nonlinear static analyses. A 2/3-scale double half-storey, one-bay substructure was tested under cyclic loading in two stages. The substructure was repaired by replacing the damaged link in Stage I. Then the substructure restored to its original position was loaded to destruction in Stage II. Analysis of the substructure was conducted for two stages. The results indicate that ductility of SFTS-links is 34% higher than that of SFTS; the link can be replaced even at the story drift angle of 0.43%; replacing the damaged link can substantially improve the performance of the damaged structure; the ductile factor and the maximum viscous damping ratio of the specimen are 3.92, 0.39, respectively, and the maximum rotation angle and overstrength factor of the link are 0.147 rad, 1.54, respectively; the failure mode of the link is that expansion of the weld crack at the chamfer position of stiffeners causes tearing of link web and fracture of stiffeners. A numerical model is developed and verified with the test results. With increasing link length, initial stiffness, peak strength and cumulative energy dissipation decrease. It is recommended that the length ratio of the link should be 0.93–1.26 in SFTS-links.

Effects of Filler Metals on Heat-Affected Zone Cracking in IN-939 Superalloy Gas-Tungsten-Arc Welds

Filler metals play an important role in reducing the weld cracking in nickel-based superalloys. In order to improve the welding conditions of IN939 superalloy, a wide range of solid solutions and age-hardenable filler metals were investigated in this study. Five solid solutions (HAYNES 230, IN625, IN617, HASTELLOY X, HAYNES 25) and two precipitation-strengthened alloys (IN718, HAYNES C-263) were used as filler metal to weld IN939 superalloy via gas-tungsten-arc welding. Microstructural studies were performed using optical microscopy and field-emission scanning electron microscopy. The results revealed that IN939 alloy is susceptible to liquation cracking in the heat-affected zone. In addition, the primary γ′ particles had grown into “ogdoadically diced cubes” of about of 2 μm in side length. The microstructure of the weld pool made with various filler metals was observed to be made up of fine spherical γ′ particles with a diameter of about 0.2 μm. The study of mechanical properties and thermodynamic behavior of the weld showed that the filler metals with lower concentrations of (Al + Ti + Nb + Ta + Mo + W) than the base metal can effectively attenuate the PWHT cracking. We found that optimal IN939 alloy weld performance can be achieved using HAYNES C-263 as filler metal, followed by IN617 and then IN625.

The Measurement to the Pillar Fillet Weld of the Differential Settlement Spherical Tank by the ACFM

Present the method that using the ACFM to test the spherical tank which had appeared the differential settlement. First introduced the principle and the system composition of the ACFM, by using which, the pillar fillet weld of the LPG spherical tank had been tested, and then the quantitative evaluation of the weld cracks had been get out. After analyzed deeply, the reliability of the ACFM when testing the pillar fillet weld of the spherical tank had been proved, in additionally, the causes of how the cracks come out had been given, too.

Noise Reduction of Welding Crack AE Signal Based on EMD and Wavelet Packet.

The acoustic emission (AE) signal collected by a sensor in the welding process has an overlapping frequency band and weak characteristics under a complex noise background. It is difficult for the wavelet noise reduction method, with single basis function, to effectively match the different characteristic information of the welding crack AE signal. Taking into account the adaptive decomposition characteristics of Empirical Mode Decomposition (EMD), a novel wavelet packet noise reduction method for welding AE signal was proposed. The welding AE signal was adaptively decomposed into several Intrinsic Mode Functions (IMFs) by the EMD. The effective IMFs were selected by the frequency distribution characteristics of the welding crack AE signal. A wavelet packet, with a specific basis function, was subsequently performed on the effective IMFs, which were reconstructed to be the welding crack AE signal. The simulated and experimental results indicated that the proposed method can effectively achieve noise reduction of the welding crack AE signal, which provided a mean for structure crack detection in the welding process.

Advanced microstructural characterisation of cast ATI 718Plus\xae\u2014effect of homogenisation heat treatments on secondary phases and repair welding behaviour

The influence of base metal conditions on the weld cracking behaviour of cast ATI 718Plus® is investigated by comparing 4 h and 24 h dwell time pseudo-hip homogenisation heat treatments at 1120, 1160 and 1190 °C with the as-cast microstructure. Scanning electron microscopy (SEM), X-ray diffraction (XRD) on electrolytically extracted powder and transmission electron microscopy (TEM) were used to identify Nb-rich secondary phases in interdendritic areas as the C14 Laves phase and Nb(Ti) MC-type carbides. All homogenisation heat treatments but the 1120 °C 4-h condition dissolve the Laves phase. A repair welding operation was simulated by linear groove multi-pass manual gas tungsten arc welding (GTAW). The Laves phase containing microstructures resulted in lower total crack length for heat affected zone cracking. Constitutional liquation of Nb(Ti) MC-type carbides is observed as a liquation mechanism in Laves-free microstructure, while thick liquid film formation due to the Laves eutectic melting could reduce the formation of weld cracks in microstructures containing the Laves phase.

Investigating delayed cracking behaviour in laser welds of high strength steel sheets using an X-ray transmission in-situ observation system

ABSTRACTWe have developed a technique for observing the weld cracking behaviour in laser welds of high strength steel sheets, using an X-ray transmission in-situ observation system. Weld cracking in the lap joints of this type of steel could be clearly seen, demonstrating that this was, in fact, delayed cracking initiated from the solidification crack at the weld end. It was also discovered that the delayed cracking behaviour such as cracking modes, incubation periods and crack growth rates, varied according to the carbon content in the steel and welding length. The former influenced the hardness of the weld bead, while the latter affected solidification cracking sizes and residual stress. The results of this crack propagation are useful not only for developing laser welding methods of high strength steel but also for additive manufacturing, aimed at mass production.

Solidification Crack Evolution in High-Strength Steel Welding Using the Extended Finite Element Method.

High-strength steel suffers from an increasing susceptibility to solidification cracking in welding due to increasing carbon equivalents. However, the cracking mechanism is not fully clear for a confidently completely crack-free welding process. To present a full, direct knowledge of fracture behavior in high-strength steel welding, a three-dimensional (3-D) modeling method is developed using the extended finite element method (XFEM). The XFEM model and fracture loads are linked with the full model and the output of the thermo-mechanical finite element method (TM-FEM), respectively. Solidification cracks in welds are predicted to initiate at the upper tip at the current cross-section, propagate upward to and then through the upper weld surface, thereby propagating the lower crack tip down to the bottom until the final failure. This behavior indicates that solidification cracking is preferred on the upper weld surface, which has higher weld stress introduced by thermal contraction and solidification shrinkage. The modeling results show good agreement with the solidification crack fractography and in situ observations. Further XFEM results show that the initial defects that exhibit higher susceptibility to solidification cracking are those that are vertical to the weld plate plane, open to the current cross-section and concentratedly distributed compared to tilted, closed and dispersedly distributed ones, respectively.

An experimental study on eccentrically braced beam-through steel frames with replaceable shear links

Prefabricated tension-only concentrically braced beam-through frames (BTFs) have received popularity for low-rise steel buildings in low to moderate seismic regions. However, the deteriorating pinched hysteretic behaviour of the tension-only concentric braces limits the energy dissipation capability of such systems under earthquakes. This study presents a modified eccentrically braced lateral force-resisting system aiming to enhance the energy dissipation capability of the BTFs. Meanwhile, seismic resilience is promoted with the utilization of replaceable shear link beams acting as structural fuses. The structural configuration and the basic design concept of the proposed eccentrically braced BTFs is first introduced. The essential mechanical characteristics of this bracing system are subsequently described with a simplified theoretical model. Seven full-scale eccentrically braced BTF specimens are subsequently tested, where the effects of link length, bracing angle, and link section are examined. The test results show that short links tend to improve the stiffness, load resistance and ductility of the BTF due to the exhibited shear-dominant behaviour. An increase in bracing angle leads to increased lateral stiffness and load resistance but at the cost of reduced ductility. Reducing the link section helps delay the development of weld cracks and results in better ductility of the structural fuse. The test results also warn that weld defects and inappropriate design of the link beam would induce brittle weld fractures at the ends of the link and lead to unsatisfactory ductility.

Identification of Tiny Surface Cracks in a Rugged Weld by Signal Gradient Algorithm using the ACFM Technique.

It is still a big challenge to identify tiny surface cracks in a rugged weld due to the lift-off variations using the nondestructive testing (NDT) method. In this paper, the signal gradient algorithm is presented to identify the tiny surface crack in the rugged weld using the alternating current field measurement (ACFM) technique. The ACFM simulation model and testing system was set up to obtain the insensitive signal to the lift-off variations. The signal gradient algorithm was presented to process the insensitive signal for the identification of the tiny surface crack in the rugged weld. The results show that the Bz signal is the insensitive signal to lift-off variations caused by the rugged weld. The signal to noise ratio (SNR) of the crack identification signal was greatly improved by the signal gradient algorithm, and a tiny surface crack can be identified effectively in the weld and the heat affected zone (HAZ).

Fatigue life assessment of the structure with widespread damage exposed to high temperature

The exhaust nozzle represents a part of a jet engine which is crucial to its overall performance, since it accelerates the flow of hot gases out of the engine and creates a thrust. During its long service, it is exposed to elevated temperatures and high compressive and hoop stresses, which makes the cracks’ occurrence in the exhaust nozzle’s inner sleeve almost unavoidable. The aim of this paper is to analyze and numerically estimate the fatigue life of inner sleeve in the presence of widespread damage. When damage is detected, two repair methods are employed: welding of single cracks (manual TIG welding) and the use of welded patches. Using numerical simulations based on extended finite element method (XFEM) and finite element method (FEM), fatigue life of repaired inner sleeves were estimated. Based on the comparison of the results obtained in simulations, it was shown that the repair method with welded patches provides longer fatigue life of widespread damaged nozzle than the other with welding of cracks.

Design of orthogonal eddy-current probe for weld crack detection of carbon-steel plate

Due to uneven surface and lift-off effect, it is difficult to detect weld crack by eddy-current testing. A new orthogonal eddy-current probe for weld crack detection of carbon-steel plate was designed in this paper. Based on COMSOL Multiphysics, the influence of scanning angle on detection sensitivity of the probe was compared firstly. Then, the effects of coil width, coil side length, detection coil height, and lift-off distance on detection sensitivity of the probe were studied, respectively. Finally, the test piece of carbon-steel plate weld with crack, and the physical probe used to verify the crack detection effect were made. The experimental results show that the weld crack of carbon-steel plate with length × width × depth of 20.0 mm × 0.3 mm × 1 mm can be effectively identified, and the lift-off noise can be effectively suppressed by the method presented in this paper. At the same time, the signal-to-noise ratio of the probe keeps constant in the lift-off distance range of 0.3 mm–3.0 mm.

Varieties and kinetics of fatigue cracking of fixed offshore platforms for oil and gas production

The article investigates the different types of cracks and kinetics of propagation of cracks in main structural components and welds of the support blocks of fixed offshore platforms. Describes the process of metal fatigue for offshore installations. The article provides methods for calculating of the stress intensity factor and describes the methods of calculation for some cases. Based on the analysis of scientific studies, the author provides methods allowing to calculate two parameters describing the kinetics of crack propagation in relation to the elements and welded joints of offshore fixed platforms, namely the methods of calculation of threshold and critical stress intensity factor. On the basis of the conducted research the author constructed the kinetic chart of the development of fatigue cracks in relation to the elements and welded joints of the supporting blocks of fixed offshore platforms. In addition, the author gives the equation in which it is possible to determine the growth rate of fatigue cracks in various areas of the kinetic diagram.

Effect of Load Ratio on the Mechanical Field around Crack Front of Dissimilar Metal Weld Joints

Stress corrosion cracking (SCC) of the dissimilar metal weld (DMW) joints in nuclear power equipment has been paid more attention in recent years. Besides the geometric size, load level, multiaxial stress state and mechanical heterogeneity, the relaxation and redistribution of residual stress make it very difficult to directly study the influence of residual stresses on the actual welded components. To study the effect of load ratio on residual stress around the crack front in the weld zone of DMW joints, the distributions of residual stress and stress triaxiality under different load ratios are simulated and discussed. The analyses show that the stress becomes larger with the increasing load ratio. The stress is tensile stress near the crack front. While the stress becomes compressive stress when it is far away from the crack front. The external load rotio has great effect on the stress around the circumferential direction at the crack front. The higher load ratio are more likely to cause the crack propagation. In the region of crack angle θ=80-120 °, the load ratio has little effect on the stress triaxiality.

Experimental Study of Laser Beam Welding Process Parameters on AISI 4130-309 Joint Strength

Abstract Laser Beam Welding (LBW) is extensively being utilized in manufacturing processes to join dissimilar metals and alloy steels because its special advantages of controlled heating, Low Heat Affected Zone (HAZ) and smaller weld bead formation. However, it is viewed as a strange strategy with applications normally and constrained to welding of thick plates of metals. With a new generation of high power lasers, there has been a renewed interest in thick section LBW (also known as Keyhole Laser Welding (KLW)). KLW makes the process a possibility for industrial applications dealing with thick metals for welding in power plants, pipelines, offshore structures, shipbuilding. The advantages provided by such LBW, is suitable for joining at high process speed, low heat input, and to achieve high productivity, leading to significantly reduction in process costs. LBW of dissimilar metals such as alloy steel and stainless steel is still a challenging task, particularly due to the formation of martensitic formation in HAZ, brittle phases in the weld and solidification cracking in the fusion zone. Such issues can significantly deteriorate strength of the welded joint. Therefore, the aim of the present work is to examine the basic phenomena that happen inside the different weld zones and their impact on weld quality in the wake of LBW joints. Taguchi L25 is chosen and experiments are conducted with LBW considering 2 mm thick dissimilar metals, i.e. AISI 4130 and AISI 309 stainless steel by varying Laser Power, Welding Speed, Beam Angle, Focal Point Position and Focal Length. The experimental output results that are measured for the mechanical properties like Ultimate Tensile Strength and Impact Strength. The Analysis of Variance (ANOVA) is carried out to obtain the influence of the process parameters and statistical evolution of the results. Firefly optimization algorithm is used to determine the optimal combination of the process parameters of the LBW