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Residual stresses significantly influence the integrity and performance of welded structures. Accurate measurement and understanding of these residual stresses are essential to ensure the reliability and safety of welded parts. There are several methods used to measure residual stresses in welds, including the Center Hole Drilling (CHD) technique, Deep-Hole Drilling (DHD) technique, Contour method (CM), and Block Removal, Splitting, and Layering (BRSL) technique. Additionally, non-destructive measurement procedures like X-ray diffraction (XRD), neutron diffraction (ND), and synchrotron X-rays are used for analyzing residual stresses in welded components. The wire-cut electrical discharge machining technique is frequently utilized for small, precise cuts. Wire-cut electrical discharge machines are typically used to cut materials with hard structures and delicate geometries. Since this machining method is non-conventional, there is no contact between the tool (wire) and the workpiece, which is its main advantage. This gives it an advantage over other machining procedures and makes the procedure competent for machining even weakly structured and delicate materials. The function of wire electrical discharge machine in cutting has been discussed in this study to measure residual stresses in weldments by the contour method. [ABSTRACT FROM AUTHOR]

Prompted by the unexpected observation of the pitting of the weldments of a highly corrosion- and pitting-resistant duplex stainless steel, SAF2507, in chloride solutions with nitride addition, the pitting and corrosion resistance of SAF2507 and its weldments were investigated in chloride solutions with and without different levels of nitrite. The Incoloy 825 and 316L austenitic stainless steels were included for the purpose of developing a comparative appreciation. The microstructures of the weldments were characterised, and 316L showed a profound influence of nitrite addition in inhibiting pitting, while 'meta-stable' pitting transients that were clearly visible in the chloride solution without nitrite were absent when nitrite was added. Both the parent metal and the weldment of SAF2507 had similar pitting potential (Ep) in 0.1 M NaCl without nitrite, which was the highest Ep among the three alloys tested. Additions of nitrite at low concentrations had an inhibitive effect on pitting, whereas higher nitrite contents had a deleterious effect on pitting resistance. On the other hand, Incoloy 825 showed a trend of Ep ennoblement with an increasing nitrite content of 0.1 M NaCl, and the weldment underwent greater ennoblement. Moreover, 316L showed a trend similar to Incoloy 825; however, the Ep ennoblements were significantly more pronounced for both the weldment and the base metal of 316L. [ABSTRACT FROM AUTHOR]

A model to rationalize the effects of test temperature and microstructural variables on the creep crack growth (CCG) and creep-fatigue crack growth (CFCG) rates in ferritic steels is described. The model predicts that as the average spacing between grain boundary particles that initiate creep cavities decrease, the CCG and CFCG rates increase. Further, the CCG data at several temperatures collapse into a single trend when a temperature compensated CCG rate derived from the model is used. The CCG and CFCG behavior measured at different temperatures is used to assess the effects of variables such as the differences between the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions. The model is demonstrated for Grade 22 and Grade 91 steels using data from literature. It is shown that differences between the CCG behavior of the Grade 22 steel in new and ex-service conditions are negligible in the BM and WM regions but not in the HAZ region. The CCG behavior of the Grade 91 steels can be separated into creep-ductile and creep-brittle regions. The creep-brittle tendency is linked to the presence of excess trace element concentrations in the material chemistry. Significant differences found in the CCG rates between the BM, WM, and HAZ regions of the Grade 91 steel are explained. [ABSTRACT FROM AUTHOR]

Prompted by the unexpected observation of the pitting of the weldments of a highly corrosion- and pitting-resistant duplex stainless steel, SAF2507, in chloride solutions with nitride addition, the pitting and corrosion resistance of SAF2507 and its weldments were investigated in chloride solutions with and without different levels of nitrite. The Incoloy 825 and 316L austenitic stainless steels were included for the purpose of developing a comparative appreciation. The microstructures of the weldments were characterised, and 316L showed a profound influence of nitrite addition in inhibiting pitting, while ‘meta-stable’ pitting transients that were clearly visible in the chloride solution without nitrite were absent when nitrite was added. Both the parent metal and the weldment of SAF2507 had similar pitting potential (Ep) in 0.1 M NaCl without nitrite, which was the highest Ep among the three alloys tested. Additions of nitrite at low concentrations had an inhibitive effect on pitting, whereas higher nitrite contents had a deleterious effect on pitting resistance. On the other hand, Incoloy 825 showed a trend of Ep ennoblement with an increasing nitrite content of 0.1 M NaCl, and the weldment underwent greater ennoblement. Moreover, 316L showed a trend similar to Incoloy 825; however, the Ep ennoblements were significantly more pronounced for both the weldment and the base metal of 316L.

The present work discusses the macrostructure, microstructure, and microhardness of dissimilar metal weld joints between AISI 316L SS-DSS 2205 developed by gas tungsten arc welding (GTAW) welding. Post-weld heat treatments (PWHT) were carried out at temperatures of 750°C for 24 h and 850°C for 0.5 h. The macrostructure and microstructure of the welding joints at different welding conditions were observed by scanning electron microscope. The macrostructure study results indicated that all the welded joints were fully penetrated and free from defects. However, the microstructure investigation revealed a solid-state transformation of ferrite into grain boundary austenite, widmanstatten austenite, intergranular austenite, and partially transformed austenite. The post-weld heat treatment processes enhanced the precipitation of the sigma phase in the ferrite matrix. Secondly, the microhardness across and along the weld bead has been evaluated, and the results are compared. It was observed that the microhardness of the heat-affected zone of DSS 2205 is found to be higher as compared to a heat-affected zone of AISI 316L, the weld metal zone, and base metals at all welding currents (i.e. 90 A, 109 A, and 132 A). The microhardness values decrease when transverse from the face of the weld toward the root pass. Microhardness values at the fill (top) pass of the weld zone at 90 A, 109 A, and 132 A were 268.3 HV, 257 HV, and 255 HV respectively. It was found that the microhardness has improved significantly after post-weld heat treatments and was highest for 750°C/24 h. It may be due to higher holding time which led to the involvement of carbides. [ABSTRACT FROM AUTHOR]

A model to rationalize the effects of test temperature and microstructural variables on the creep crack growth (CCG) and creep-fatigue crack growth (CFCG) rates in ferritic steels is described. The model predicts that as the average spacing between grain boundary particles that initiate creep cavities decrease, the CCG and CFCG rates increase. Further, the CCG data at several temperatures collapse into a single trend when a temperature compensated CCG rate derived from the model is used. The CCG and CFCG behavior measured at different temperatures is used to assess the effects of variables such as the differences between the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions. The model is demonstrated for Grade 22 and Grade 91 steels using data from literature. It is shown that differences between the CCG behavior of the Grade 22 steel in new and ex-service conditions are negligible in the BM and WM regions but not in the HAZ region. The CCG behavior of the Grade 91 steels can be separated into creep-ductile and creep-brittle regions. The creep-brittle tendency is linked to the presence of excess trace element concentrations in the material chemistry. Significant differences found in the CCG rates between the BM, WM, and HAZ regions of the Grade 91 steel are explained.

Mechanical‐technological behaviour of ETFE foils and their welded connections The thermoplastic fluoropolymer ethylene/tetrafluoroethylene, ETFE, has been increasingly used in membrane structures in the construction industry since the 1980s. The plastic ETFE exhibits a nonlinear, viscoelastic material behaviour that has not yet been fully investigated. However, when building membrane structures with ETFE foils, knowledge of the nonlinear, viscoelastic material behaviour is essential in order to be able to build wide, open and at the same time safe and economical structures. The material behaviour can be divided into short‐term tensile behaviour and long‐term behaviour, both under monoaxial and biaxial stresses. For the assembly of the surface structures, welding of the individual cut length is essential, since the basic material is only available in limited product widths. So far, there have only been a few studies on the load‐bearing behaviour of welded ETFE foils. This article provides a brief overview of the material behaviour of ETFE foils in short‐term tensile tests as well as long‐term tests in monoaxial and biaxial stress conditions. In addition, material models are presented which describe the material behaviour in the above‐mentioned tests or stress‐states. Furthermore, first insights into the fracture behaviour of welded ETFE foils are given. [ABSTRACT FROM AUTHOR]

The industrial use of ultrasonic flaw classification using neural networks in weldments must overcome many challenges. A major constraint is the use of numerous systems, including a combination of transducers and equipment. This causes high complexity in the datasets used in the training of neural networks, which decreases performance. In this study, the performance of a neural network was enhanced using signal processing on an ultrasonic weldment flaw dataset to achieve system invariance. The dataset contained 5839 ultrasonic flaw signals collected by various types of transducers connected to KrautKramer USN60. Every signal in the dataset was from 45 FlawTech/Sonaspection weldment specimens with five types of flaw: crack, lack of fusion, slag inclusion, porosity, and incomplete penetration. The neural network used in this study is a residual neural network with 19 layers. The performance evaluation of the same network structure showed that the original database can achieve 62.17% ± 4.13% accuracy, and that the invariant database using the system invariant method can achieve 91.45% ± 1.77% accuracy. The results demonstrate that using a system invariant method for ultrasonic flaw classification in weldments can improve the performance of a neural network with a highly complex dataset. [ABSTRACT FROM AUTHOR]

In an attempt to provide a more flexible means of achieving friction stir welding (FSW) of Aluminum without the use of expensive FSW machine tool, which are not readily available. This study developed a mobile fixture system (MFS) for FSW on a Vertical milling Machine Tool (VMMT). A conceptual design of a fixture system with a moving work table, which provides for transverse feed (movement) of the workpieces during FSW, was generated. The detailed design of the components of the MFS was done using existing mechanical design formulae. Subsequently, the design was fabricated and evaluated. An ATmega 328P Arduino Uno microcontroller was used to design a control system to automate the MFS worktable movement. Results revealed that the MFS worked smoothly during FSW of AA 1100 materials. The efficiency of its motion accuracy was estimated as 87.2%. Also, the trend of the tensile strength and the hardness value as well as the joint efficiencies of the AA 1100 weldments produced using the developed MFS agreed with existing studies. The MFS can be used for FSW of Aluminum materials on VMMT as well as on pillar drilling machine tool. [ABSTRACT FROM AUTHOR]

Effects of welding parameters on the characterization and mechanical properties of steel 37 weldments were investigated on a single V-shaped groove weld joint. These parameters are; type of electrode, welding current, welding speed, and type of used arc welding and plate thickness. Radiography, metallography and mechanical testing were carried out for the steel 37 weldments. Results indicated that as the heat input increased, the welding current and plate thickness increased but the welding speed decreased. The grains of the base metal, heat affected zoon (HAZ), root and cap were changed in size and phases in these locations. Radiography showed that no defects were found. The average hardness values were 173 HB at base metal, 175 HB at the HAZ, 180 HB at the cap and 133 HB at the root, respectively. Yield strength, ultimate tensile strength and elongation of the welded steel 37 were in the range between 374 and 425 MPa, between 542 and 606 MPa and in the range between 34.6 and 37.5 %, respectively. Such values tend to increase with increasing the current and heat input and decreasing the plate thickness. The chemical and mechanical properties of the welded electrodes proved to influence the efficiency of the welding process. The best welding results were obtained with current 75 A at the root, 130 A at the cap and welding speed of 1.3 mm/s in the root and 2 mm/s in the cap.

The industrial use of ultrasonic flaw classification using neural networks in weldments must overcome many challenges. A major constraint is the use of numerous systems, including a combination of transducers and equipment. This causes high complexity in the datasets used in the training of neural networks, which decreases performance. In this study, the performance of a neural network was enhanced using signal processing on an ultrasonic weldment flaw dataset to achieve system invariance. The dataset contained 5839 ultrasonic flaw signals collected by various types of transducers connected to KrautKramer USN60. Every signal in the dataset was from 45 FlawTech/Sonaspection weldment specimens with five types of flaw: crack, lack of fusion, slag inclusion, porosity, and incomplete penetration. The neural network used in this study is a residual neural network with 19 layers. The performance evaluation of the same network structure showed that the original database can achieve 62.17% ± 4.13% accuracy, and that the invariant database using the system invariant method can achieve 91.45% ± 1.77% accuracy. The results demonstrate that using a system invariant method for ultrasonic flaw classification in weldments can improve the performance of a neural network with a highly complex dataset.

Metal magnetic memory (MMM) testing method has been proved to be a valid approach to monitor early damage and predict fatigue life, but there is no systematic description and comparison about the characteristics of MMM signals for welding and non-welding specimens with different materials under tension-compression fatigue. Thus, the fatigue tests for Q345B and Q345qC welding and non-welding specimens were carried out and the MMM testing was done synchronously. Then, based on the normal components of MMM signals, Hp(y), and its variations, ΔHp(y), the slope of ΔHp(y) fitting curves, KC, and the average of ΔHp(y) signals on the whole testing line, ΔHp(y)ave, were extracted as the characteristic parameters for Q345B and Q345qC non-welding specimens, respectively. Additionally, the gradient of “peak–vale” on Hp(y) signals curves, k, and the average of Hp(y) signals in the scope of welding zone, Hp(y)ave, were extracted as the characteristic parameters for Q345B and Q345qC welding specimens, respectively. Some conclusions can be drawn by comparison: the variation laws of Hp(y) signals with cyclic loads are related to the material. The characteristic parameters for welding and non-welding specimens with the same material have a certain similarity. Then, the failure of non-welding steel can be warned early by KC or ΔHp(y)ave when the fatigue life has 20% left, but the damage degree of welding specimen should begin to be valued highly once the k or Hp(y)ave is decreasing. Finally, the fatigue life of welding and non-welding specimens with different materials can be evaluated effectively by using corresponding magnetic characteristic parameters. [ABSTRACT FROM AUTHOR]

In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised. [ABSTRACT FROM AUTHOR]

A multiaxial, physically based, continuum damage mechanics methodology for creep of welded 9Cr steels is presented, incorporating a multiple precipitate-type state variable, which simulates the effects of strain- and temperature-induced coarsening kinematics. Precipitate volume fraction and initial diameter for carbide and carbo-nitride precipitate types are key microstructural variables controlling time to failure in the model. The heat-affected zone material is simulated explicitly utilising measured microstructural data, allowing detailed investigation of failure mechanisms. Failure is shown to be controlled by a combination of microstructural degradation and Kachanov-type damage for the formation and growth of creep cavities. Comparisons with experimental data demonstrate the accuracy of this model for P91 material. [ABSTRACT FROM AUTHOR]

S355 structural steel is commonly used in fabrication of offshore structures including offshore wind turbine monopiles. Knowledge of mechanical and fracture properties in S355 weldments and the level of scatter in these properties is extremely important for ensuring the integrity of such structures through engineering critical assessment. An interlaboratory test programme was created to characterise the mechanical and fracture properties of S355 weldments, including the base metal, heat‐affected zone, and the weld metal, extensively. Charpy impact tests, chemical composition analysis, hardness tests, tensile tests, and fracture toughness tests have been performed on specimens extracted from each of the 3 material microstructures. The experimental test results from this project are presented in this paper, and their importance in structural integrity assessment of offshore wind turbine monopiles has been discussed. The results have shown a decreasing trend in the Charpy impact energy and Jmax values with an increase in yield stress from base metal to heat‐affected zone to weld metal. Moreover, the JIC fracture toughness value in the heat‐affected zone and weld metal is on average around 60% above and 40% below the base metal value, respectively. In addition, the average Charpy impact energy value in the heat‐affected zone and weld metal is around 5% and 30% below the base metal value, respectively. The effects of mechanical and fracture properties on the critical crack size estimates have been investigated, and the results are discussed concerning the impact of material properties on structural design and integrity assessment of monopiles. [ABSTRACT FROM AUTHOR]

The paper provides an application of the IBESS approach to the investigation of the influence of various parameters of the global and local weld geometry as well as material defects on the fatigue strength of weldments. For this purpose, the global weld parameters, such as the weld toe radius, the flank angle, the excess weld metal, local secondary notches (in the present study as a measure of surface imperfections) and inclusions sizes have been determined as statistical distributions for different joint types and geometries and two steels of different strengths. The results are in line with literature data and reveal the potential of the theoretical approach to predict the correct trends. The combination with an advanced weld quality system has been demonstrated to be possible. [ABSTRACT FROM AUTHOR]

In present research article, microstructure evolution in P91 steel and their weldments are reviewed in as-virgin and heat treatment and creep exposure condition. The thermal stability of P91 steel is derived from solid solution strengthening, sub-grain hardening and precipitation hardening. The initial microstructure plays an important role in deciding the mechanical properties of P91 steel and their weldment in long-term ageing and creep exposure condition. Effects of various alloying elements present in P91 steel and their related phase have also been discussed in details. The role of grain coarsening, Cr/Fe ratio, lath widening and dislocation density on creep rupture life of base metal and weldments are discussed. The combined effects of lath martensitic microstructure, residual stress and diffusible hydrogen content on performance of P91 steel material are also discussed. [ABSTRACT FROM AUTHOR]

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments' creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band. [ABSTRACT FROM AUTHOR]

Creep strength enhanced ferritic/martensitic 9Cr-1Mo-V-Nb (P91) steel is also designated as ASTM A335 used for out-of-core and in-core (piping, cladding, ducts, wrappers, and pressure vessel) of Gen IV reactors. In present investigation, the dissimilar weld joint of P91 and P92 steel were made using the autogenous tungsten inert gas (TIG) welding with single pass, double side pass and multi-pass gas tungsten arc (GTA) welding with filler wire. Microstructure evolution in sub-zones and mechanical properties of dissimilar welded joints were studied in as-welded and post weld heat treatment (PWHT) condition. Formation of δ-ferrite patches in weld fusion zone and heat affected zones (HAZs) and their influence on the mechanical behaviour of the welded joints were also studied. Presence of higher content of ferrite stabilizer in P92 steel have resulted the formation of δ-ferrite patches in weld fusion zone as well as HAZs. The δ-ferrite was observed in autogenous TIG welds joints. The δ-ferrite patches were formed in as-welded condition and remained in the microstructure after the PWHT. The δ-ferrite patches leads to reduction in Charpy toughness of autogenous TIG welds joint and also lower down the average hardness of weld fusion zone. Peak hardness and poor impact toughness were observed for autogenous TIG welds joint as compared to GTA welds. For microstructure characterization, field-emission scanning electron microscope (FESEM) with energy dispersive spectroscopy (EDS) and optical microscope were utilized. [ABSTRACT FROM AUTHOR]

Creep strength enhanced ferritic/martensitic P91 steel is considered as a candidate material for the reactor pressure vessels and reactor internals of Very High Temperature Reactor (VHTR). Heterogeneous microstructure formation across the P91 weldments lead to premature Type IV cracking and makes the weldability of P91 steel a serious issue. The present research work describes the effect of normalizing and tempering (N&T) treatment on microstructure evolution in various zones of gas tungsten arc welded (GTAW) P91 pipe weldments. For N&T treatment, P91 pipe weldments were subjected to various normalizing (950–1150 °C) and tempering (730–800 °C) temperature. The effect of varying heat treatment on tensile properties and hardness of P91 pipe weldments were studied for V-groove and narrow-groove weld designs. The effect of increase in normalizing temperature (fixed tempering temperature) resulted in increase in strength and hardness, while increase in tempering temperature (fixed normalizing temperature) resulted in the decrease in strength and hardness of P91 steel weldments. The better combination of strength, ductility and microstructure were obtained for the maximum normalizing temperature of 1050 °C and tempering temperature of 760 °C. [ABSTRACT FROM AUTHOR]

9%Ni steel was recently used for the first time in the field of injection unit (IU) for the injection of CO2 into oil wells. Because such steels are operated in H2S medium and are susceptible to sulfide stress cracking, their pipes are cladded with Ni-based superalloy 625 by using gas tungsten arc welding with a hot wire to prevent this phenomenon from occurring. The transition zone of substrate/clad can have high hardness and low toughness, and promote failure of the component during service; therefore, it is very important to know its characteristics. In this work, this transition zone was analyzed through optical and scanning electron microscopy with energy dispersive X-ray spectrometry and electron backscatter diffraction, as well as Vickers microhardness, shear and bend tests. Metallographic analysis identified type I and II boundaries with distinct chemical gradients, MC-type carbides, Laves/γ eutectics, peninsulas macrosegregation, crystallographic texture close to in the clads, residual strain, and drop of microhardness across the transition zone. The clads were approved in the shear and bend tests. This work proposes a new type II boundary formation mechanism in dissimilar welds of steels that do not exhibit the allotropic transformation δ → γ during the welding thermal cycle and do not experience a change in the solidification mode.

Metal magnetic memory testing is a novel method for evaluating the fatigue damage of ferromagnetic weldments. To promote a further understanding of the impact of fatigue loads on magnetic memory signals of weldments, the normal component of magnetic signals,Hp(y), on the centre-notched welding specimens were measured during dynamic bending fatigue tests. The variation of peaks–troughs differential value ΔHp(y) of magnetic signals and its maximum gradientKmaxat the welded joint were also investigated. Results indicated that the ΔHp(y) on scanning line near the notch increased sharply when stress concentration and crack initiation appeared in the specimens. The microstructures on scanning lines were also examined. It shows that there was an increase in intragranular slips with the increase in loading cycles, and the change in crystallographic textures and movement of dislocations contributed to the magnetic signals varying significantly. The maximum gradientKmaxwas found to increase exponentially with the increase in crack length, which is well coincident with the analytic result of the magnetic dipole model. The research provides a potential possibility of quantitative evaluation on stress concentration and crack in weldments under dynamic bending loads. [ABSTRACT FROM AUTHOR]

In this work, the morphology, phase composition, and corrosion properties of microarc oxidized (MAO) gas tungsten arc (GTA) weldments of AZ31 alloy were investigated. Autogenous gas tungsten arc welds were made as full penetration bead-on-plate welding under the alternating-current mode. A uniform oxide layer was developed on the surface of the specimens with MAO treatment in silicate-based alkaline electrolytes for different oxidation times. The corrosion behavior of the samples was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. The oxide film improved the corrosion resistance substantially compared to the uncoated specimens. The sample coated for 10 min exhibited better corrosion properties. The corrosion resistance of the coatings was concluded to strongly depend on the morphology, whereas the phase composition and thickness were concluded to only slightly affect the corrosion resistance. [ABSTRACT FROM AUTHOR]

The majority of fatigue crack growth (FCG) data sets available on steels in air and seawater environments are several decades old and may not be appropriate for structural integrity assessment of offshore wind turbine foundations, which are fabricated using contemporary materials and welding technologies. Therefore, the SLIC joint industry project was formed to investigate the fatigue crack initiation and growth behaviour in offshore wind welded steel foundations. The FCG test data from the SLIC inter-laboratory (round robin) test programme have been analysed using a new proposed shape function solution and the results are presented and discussed. The obtained FCG trends in air and seawater environments have been compared with the recommended trends available in standards. The Paris-law constants and Δ K th values obtained from this programme can be used for defect assessment and remaining life prediction of offshore monopile weldments in air and seawater environments. The results from the SLIC project show that for a given value of Δ K the fatigue crack growth rate, da/dN , is on average around 2 times higher in seawater compared to air for the base metal and weldments. This factor of 2 in the seawater environment is almost half of the crack acceleration factor recommended by standards. [ABSTRACT FROM AUTHOR]

Alloy 800H is currently under consideration to be used in a very high temperature reactor system. This paper describes the experimental low-cycle fatigue study on Alloy 800H base metal and weldments in the air with fully-reversed strain control at 700 °C. The recommended modern Alloy 800H weldments were fabricated by gas tungsten arc welding. The observed fatigue life is found to be comparable between base metal and weldments at this temperature, indicating the welding technique can compete with base material. The influence of hold time was further investigated at the maximum tensile strain of 0.6% total strain range condition for 60 s, and it is found to reduce the fatigue life. For most of the tests, the base metal revealed an initial cyclic hardening stress response, otherwise, the weldment was entirely softened with cycles. The addition of hold time is also found to induce an unequal cyclic stress response in both materials. Accordingly, for continuous low-cycle fatigue tests, fatigue cracks were initiated and propagated in a classical transgranular fracture mode. However, the hold time affected the mixed-mode fracture mode by means of a transition from transgranular to intergranular fracture.

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.

Nel presente articolo viene proposta una nuova metodologia di progettazione a fatica, basata sull’utilizzo del metodo delle Curve di Wöhler Modificate, per la previsione della vita a fatica di giunzioni saldate, sia in acciaio che in alluminio, soggette a carichi multiassiali ad ampiezza variabile. In particolare, il criterio delle Curve di Wöhler Modificate è stato applicato determinando l’orientazione del piano critico mediante il Metodo della Massima Varianza, ovvero definendo il piano critico come quello contenente la direzione che sperimenta la massima varianza della tensione tangenziale risolta. L’accuratezza della metodologia di progettazione a fatica proposta nella presente memoria è stata valutata mediante due serie di dati sperimentali di letteratura ottenute sollecitando, sia ad ampiezza costante che variabile, giunti saldati tubo-piastra in acciaio e lega di alluminio con carichi di flesso/torsione in fase e sfasati di 90°. Il criterio delle Curve di Wöhler Modificate, applicato in concomitanza con il Metodo della Massima Varianza, si è dimostrato capace di fornisce stime accurate della durata a fatica anche in presenza di sollecitazioni multiassiali ad ampiezza variabile, e questo sia quando applicato in termini di tensioni nominali che in termini di tensioni di “hot-spot”.

During the last decade, the demand for natural gas has steadily increased for the prevention of environmental pollution. For this reason, many liquefied natural gas (LNG) carriers have been manufactured. Since one of the most important issues in the design of LNG carriers is to guarantee structural safety, the use of low-temperature materials is increasing. Among commonly employed low-temperature materials, nickel steel has many benefits such as good strength and outstanding corrosion resistance. Accordingly, nickel steels are one of the most commonly used low-temperature steels for LNG storage tanks. However, the study of fracture toughness with various welding consumables of 7% nickel alloy steel is insufficient for ensuring the structural safety of LNG storage tanks. Therefore, the aim of this study was to evaluate fracture toughness of several different weldments for 7% nickel alloy steels. The weldment of 7% nickel alloy steel was fabricated by tungsten inert gas (TIG), flux cored arc welding (FCAW), and gas metal arc welding (GMAW). In order to assess the material performance of the weldments at low temperature, fracture toughness such as crack tip opening displacement (CTOD) and the absorbed impact energy of weldments were compared with those of 9% nickel steel weldments. [ABSTRACT FROM AUTHOR]

Oxidation behaviour of weldments at elevated temperature has become an object of scientific investigation. Weldments were prepared using shielded metal arc welding and tungsten inert gas processes to weld together 1Cr-0.5Mo (T11) boiler tube steels. This paper reports the oxidation behaviour of welded and unwelded 1Cr-0.5Mo (T11) boiler tube steel specimens after exposure to air at 900 °C under cyclic condition. The thermogravimetric technique was used to establish kinetics of oxidation. X-ray diffraction and scanning electron microscopy/energy-dispersive analysis techniques were used to analyse the oxidation products. The unwelded steel showed a higher oxidation rate (in terms of weight gain) than that of welded steels. [ABSTRACT FROM AUTHOR]

In order to better understand the high temperature low cycle fatigue behavior of Alloy 617 weldments, this work focuses on the comparative study of the low cycle fatigue behavior of Alloy 617 base metal and weldments, made from automated gas tungsten arc welding with Alloy 617 filler wire. Low cycle fatigue tests were carried out by a series of fully reversed strain-controls (strain ratio, Rε = -1), i.e., 0.6%, 0.9%, 1.2% and 1.5% at a high temperature of 900 °C and a constant strain rate of 10-3/s. At all the testing conditions, the weldment specimens showed lower fatigue lives compared with the base metal due to their microstructural heterogeneities. The effect of very high temperature deformation behavior regarding cyclic stress response varied as a complex function of material property and total strain range. The Alloy 617 base weldments showed some cyclic hardening as a function of total strain range. However, the Alloy 617 base metal showed some cyclic softening induced by solute drag creep during low cycle fatigue. An analysis of the low cycle fatigue data based on a Coffin-Manson relationship was carried out. Fracture surface characterizations were performed on selected fractured specimens using standard metallographic techniques. [ABSTRACT FROM AUTHOR]

The study is devoted to geometrically non-linear modelling of viscoplastic structures with residual stresses. We advocate and develop a special approach to residual stresses based on the transition between reference configurations. The finite strain kinematics of the viscoplastic material is modelled by the multiplicative decomposition of the deformation gradient tensor. Numerical algorithms originally developed for unstressed materials are extended to materials with pre-stresses. Owing to the weak invariance of constitutive equations, the incorporation of pre-stresses happens without additional costs. Thus, the advocated approach is especially efficient. A novel experimental/theoretical method for assessment of residual stresses in welded structures is presented; the method combines advantages of purely experimental and theoretical approaches. To demonstrate the applicability of the proposed procedure, we simulate plate welding. As an example we show that the procedure allows extrapolation of the field of residual stresses away from the measurement points. As another example, we address the reduction of weldment-related residual stresses by mechanical treatment. • An F0-approach to residual stresses is extended to cover volume changes. • The approach enables experimental/theoretical analysis of residual stresses. • The numerical schemes benefit from the weak invariance of constitutive equations. • Transformation rules for initial conditions, stresses and material tangent are given. • For a welded plate, the residual stresses and operational loads are simulated. [ABSTRACT FROM AUTHOR]

In this study, a solution containing ammonium fluoride (NH4F) and nitric acid (HNO3) was used as an alternative to the conventional highly toxic pickling solution HF/HNO3 for pickling weldments of selected stainless steels including Type 316 stainless steel (UNS S31600), duplex stainless steel 2205 (UNS S32205), and super duplex stainless steel 2507 (UNS S32750). Electrochemical and surface analytical methods were used to understand the effects of pickling on the stainless steel weldments. Cyclic potentiodynamic polarization (CPP) test results indicated that the restoration of passivity of stainless steel weldments could be achieved by pickling the weldments in both HF/HNO3 solution and NH4F/HNO3 solutions. Scanning electron microscopy observation of the UNS S32750 weldment surface revealed that both the HF/HNO3 solution and the NH4F/HNO3 solution could remove the heat tint on the weldment. X-ray photoelectron spectroscopy analysis indicated that treatment in these two pickling solutions produced passive films with similar characteristics. Thus, this work suggests that the NH4F/HNO3 solution is a promising alternative to HF/HNO3 solution for the pickling of stainless steel weldments, and that the CPP test approach can be used in conjunction with surface analytical methods for further development of safer and environmentally friendly pickling solutions. [ABSTRACT FROM AUTHOR]

In a number of previous papers, the authors have proposed a model for fracture mechanics based prediction of the S-N characteristics of metallic components with large microstructural defects. Here, an extension to materials that do not show large defects onto the fracture surfaces is provided. In such cases, an approach based on a so-called cyclic R-curve analysis is proposed for the determination of the initial flaw size, which has to be used in the calculation of fatigue crack propagation. The principle is explained and demonstrated by a first application to a welded joint. [ABSTRACT FROM AUTHOR]

• Investigation of mechanical and fracture properties of WAAM Cr-Mo Steel Weldments. • Specimen orientation influence the elastic plastic fracture toughness (J-integral). • Lower Impact toughness and Tensile strength was observed in longitudinal specimens. • Higher Impact toughness and Tensile strength was observed in transverse specimens. • Important parameters affect the fracture behavior of WAAM fabricated steel. This paper discusses the additive welding of Cr-Mo steel weldment carried out with Cold Metal Transfer Technology (CMT). The mechanical characterization including fracture behaviour and metallography of the Wire Arc Additive Manufacture (WAAM) weldments in the As Welded (AW) and Post Weld Heat Treated (PWHT) conditions were investigated. The longitudinal and transverse orientations parallel to building and deposition directions respectively were found to influence the fracture behavior of weldments. The specimens of longitudinal direction showed lower toughness and tensile strength when compared to the specimens of transverse direction both in the case of AW and PWHT weldments. Orientation was also observed to influence the elastic–plastic fracture toughness (J integral) values both in the as welded and heat-treated specimens. Fracture topographical analysis of specimens carried out by Scanning Electron Microscope (SEM) in association with Energy Dispersive Spectroscopy (EDS) showed the presence of soft ductile features and Mn deposition respectively in the specimens of transverse direction and justified the findings. [ABSTRACT FROM AUTHOR]

In the present work, the influence of welding stainless steel on the microstructural development and associated mechanical performance were investigated. Gas tungsten arc welding (GTAW) process was used to weld 6 mm thick AISI 304L stainless steel using three levels of welding arc energy inputs. Different weld thermal cycles that resulted due to different combinations of heat input used caused significant microstructural changes in the weldments, which consequently affected the tensile and impact properties of these joints. Mechanical and material characterization of these joints have revealed that the weld metal under low welding heat input condition possessed relatively fine columnar dendritic microstructure which resulted in high microhardness and impact toughness of this zone. For every joint, the weld metal exhibited superior mechanical strength (transverse tensile strength, elongation, and Charpy impact energy) than the corresponding heat-affected zone. This work establishes that GTAW process when used for manufacturing operations involving AISI 304L stainless steel fabrications is capable of giving superior mechanical properties provided parametric combinations leading to low welding heat input are used. [ABSTRACT FROM PUBLISHER]

Fracture toughness testing was conducted on standard single-edge notched bend bar specimens of base and weld metal. The material was the SAF 2906 super duplex stainless steel. The aim was to evaluate the susceptibility for brittle failure at sub-zero temperatures for the base and weld metal. The base metal was tested between −103 and −60°C and was evaluated according to the crack-tip opening displacement method. The fracture event at and below −80°C can be described as ductile until critical cleavage initiation occurs, which caused unstable failure of the specimen. The welding method used was submerged arc welding with a 7wt% nickel filler metal. The welded specimens were post-weld heat treated (PWHT) at 1100°C for 20min and then quenched. Energy-dispersive X-ray spectroscopy analysis showed that during PWHT substitutional element partitioning occurred which resulted in decreased nickel content in the ferrite. The PWHT weld metal specimens were tested at −72°C. The fracture sequence was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture. [ABSTRACT FROM AUTHOR]

The influence of the weld metal chemistry on the susceptibility of AISI 444 ferritic stainless steel (FSS) weldment to stress corrosion cracking (SCC) in hot chloride was investigated by constant load tests and metallographic examination. Two types of filler metal of austenitic stainless steel (E316L and E309L) were used in order to produce fusion zones of different chemical compositions. The SCC test results showed that the interface between the fusion zone (FZ) and the heat affected zone (HAZ) was the most susceptible region to SCC. Results also showed that the AISI 444 stainless steel weldment with E309L weld metal presented the best SSC resistance. Microstructural examinations indicated that the cracks initiated in the weld metal and propagated to the HAZ of the AISI 444 FSS, where the fracture occurred and it was observed a considerable amount of precipitates. Additionally, the higher SCC resistance of the AISI 444 FSS weldment with E309L weld metal may be attributed to the presence of a discontinuous delta-ferrite network in its microstructure, which acted as a barrier to cracks propagation from the fusion zone to the HAZ/fusion zone interface of AISI 444 FSS. Fractrography analyses showed that the transgranular quasi-cleavage fracture mode was predominant in the AISI 444 weldment with E316L weld metal and the mixed fracture mode was the predominant in the AISI 444 weldment with E309L weld metal. [ABSTRACT FROM AUTHOR]

Abstract: Determination of initiation fracture toughness of ductile materials is a challenging task due to the ambiguity associated with the identification of the point of departure of the initial linear region of the fracture resistance curve. Acoustic emission (AE) is a technique that is capable of directly indicating the crack initiation point during fracture toughness tests using a single specimen. The objective of this investigation is to illustrate some results related to monotonic and cyclic fracture behaviour of AISI 304LN weldments estimated by this ‘combined type’ experiment. The use of cyclic J-integral tests has been proposed to simulate the deleterious effect of periodic load reversals during monotonic loading. A comparative analysis of all these data leads to the conclusion that the fracture toughness of a material determined from AE parameters is generally conservative compared to that obtained by conventional techniques. [ABSTRACT FROM AUTHOR]

The improvement of tensile and creep rupture strength, achieved by the development of martensitic-ferritic 9-12%Cr creep-resistant steels, known as creep strength enhanced ferritic (CSEF) steels, is attractive to designers who want to take advantage of them to improve power plant efficiency and reduce component wall thickness. However, although parent alloy developments have been making good progress, weldment performance has been of increasing concern. In particular, the cross-weld creep performance for CSEF steels is an issue, with Type IV cracking characterised by rupture in the outer region of the heat affected zone (HAZ) and low strain to failure being the typical features of failure in cross-weld tests. In this investigation, narrow gap TIG welding, flux-cored arc welding (FCAW) and reduced pressure electron beam (RPEB) welding were employed to produce a series of butt welds in a new European steel -FB2- a wrought, boron-containing 9%Cr steel. These welds were subjected to metallurgical examination and mechanical testing, including toughness and cross-weld creep rupture tests. Results are compared with the well known steel grades 91 and 92. Welding process was found to have a marked effect on weld metal toughness, but not on the long term cross-weld creep rupture performance. FB2 weldments were found to out-perform those of grades 91 and 92 in long term creep tests, but they did suffer from type IV cracking and reduced rupture strength, compared with the parent steel. The paper draws conclusions on the performance of the welded joints, and makes recommendations for further work and possible application within the power industry. [ABSTRACT FROM AUTHOR]

Methods for evaluating the creep toughness parameter, , are reviewed and data are determined for a ferritic P22 steel from creep crack growth tests on compact tension, C(T), specimens of homogenous parent material (PM) and heterogeneous specimen weldments at 565 °C and compared to similar tests on austenitic type 316H stainless steel at 550 °C. Appropriate relations describing the time dependency of are determined accounting for data scatter. Considerable differences are observed in the form of the data and the time-dependent failure assessment diagrams (TDFADs) for both the 316H and P22 steel. The TDFAD for P22 shows a strong time dependency, but is insensitive to time for 316H. Creep crack initiation (CCI) time predictions are obtained using the TDFAD approach and compared to experimental results from C(T) specimens and feature components. The TDFAD based on parent material properties can be used to obtain conservative predictions of CCI on weldments. Conservative predictions are almost always obtained when lower bound values are employed. Long-term test are generally more relevant to industrial component lifetimes. The different trends between long- and short-term CCI time and growth data indicate that additional long-term test are required to further validate the procedure to predict the lifetimes of high temperature components. [ABSTRACT FROM AUTHOR]

This paper summarises an attempt of devising a new engineering approach, based on the so-called Modified Wöhler Curve Method, suitable for estimating fatigue lifetime of both steel and aluminium welded connections subjected to variable amplitude multiaxial fatigue loading. The Maximum Variance Method is used to determine the orientation of the critical plane and the cycle counting is directly performed in terms of shear stress resolved along the maximum variance direction. The accuracy of the proposed methodology was checked by using two different datasets taken from the literature and generated by testing both steel and aluminium tube-to-plate welded connections under in-phase and 90° out-of-phase variable amplitude bending and torsion. The proposed fatigue life assessment technique was seen to be highly accurate, resulting in estimates falling within the scatter bands of the curves used to calibrate the Modified Wöhler Curve Method itself. This seems to strongly support the idea that the proposed approach can be considered as an effective engineering tool, capable of performing multiaxial fatigue assessment under variable amplitude loading by fully complying with recommendations of the available standard codes. [ABSTRACT FROM AUTHOR]

Abstract: The present paper reviews the different procedures to be followed to use the Modified Wöhler Curve Method (MWCM) to estimate fatigue lifetime of steel and aluminium welded joints subjected to multiaxial cyclic loading. In more detail, the MWCM is a conventional critical plane approach which postulates that fatigue damage reaches its maximum value on the plane experiencing the maximum shear stress range. According to the above assumption, the complexity of the stress field acting on the fatigue process zone is directly taken into account through the ratio between the ranges of the normal and shear stress relative to the critical plane, being such a ratio sensitive to the degree of non-proportionality of the applied loading but not to the presence of superimposed static stresses. Thanks to its peculiar features, our multiaxial fatigue criterion can be used to estimate fatigue damage in weldments by applying it in terms of either nominal, hot-spot or local stresses. The accuracy of the above three ways of applying the MWCM was checked by using several experimental results taken from the literature and generated by testing steel and aluminium welded details characterised by different geometries and subjected to different cyclic loading paths. Such a validation exercise allowed us to prove that the MWCM is a powerful engineering tool capable of highly accurate estimates, and it holds true independently of the definition adopted to calculate the stress quantities needed for its in-field application. This suggests that our fatigue life estimation technique can be considered as an efficient alternative to other existing linear-elastic approaches, allowing an adequate margin of safety to always be reached when designing welded components against fatigue. [Copyright &y& Elsevier]

This paper reports creep crack growth modelling of Grade 91 vessel weldments at high temperature using a modified ductility-based continuum damage mechanics (CDM) model, which is based on the Kachanov-Rabotnov CDM and utilizing the concept of ductility exhaustion. The proposed model holds a key advantage over existing models in that it requires fewer material constants to be identified and calibrated. The new modified model was implemented into a user-defined subroutine in ABAQUS and then used to predict the creep crack growth of Grade 91 vessel weldments. Creep crack initiation and growth in the vessels were predicted to occur in the heat-affected zones of the weldments associated with the end closures, which is in good agreement with the corresponding vessel tests. Further, the predicted creep rupture lives of the notched bars and the vessels using the proposed model correlated reasonably well against the experimental results. This suggests that the modified ductility-based damage model can be used for creep crack growth and life prediction for high temperature structures with confidence. [Display omitted] • A modified ductility-based continuum damage mechanics (CDM) model is proposed to simulate creep crack growth. • The proposed approach can capture the influence of material creep ductility on creep damage and life. • An inverse optimization algorithm is developed to obtain the material parameters of the new model. • The proposed model can satisfactorily predict creep damage and failure for high temperature structures. • An improved understanding into Type IV failure in CSEF steel weldment is gained. [ABSTRACT FROM AUTHOR]

[Display omitted] • Modified Tanaka-Mura model for cyclic softening of P91 steel. • Physically-based yield strength and unified cyclic viscoplasticity models. • Application to high temperature low cycle fatigue of P91 cross-weld test specimen. • Successfully predicts detrimental effect of welding on fatigue crack initiation. • Packet size identified as critical length-scale for slip band formation. A methodology is presented for physically-based prediction of high temperature fatigue crack initiation in 9Cr steels, with a focus on material inhomogeneity due to welding-induced metallurgical transformations. A modified form of the Tanaka-Mura model for slip band formation under cyclically-softening conditions is implemented in conjunction with a physically-based unified cyclic viscoplasticity constitutive model. The physically-based constitutive model accounts for the key strengthening mechanisms, including precipitate hardening and hierarchical grain boundary strengthening, successfully predicting cyclic softening in 9Cr steels. A five-material, finite element model of a P91 cross-weld test specimen, calibrated using the physically-based yield strength and constitutive models, successfully predicts the measured detrimental effect of welding on high temperature low-cycle fatigue crack initiation for P91 cross weld tests, via the modified Tanaka-Mura model. A key finding of the current work is the requirement to adopt an energy-based Tanaka-Mura method to account for cyclic softening in 9Cr steels, with packet size as the critical length-scale for slip band formation. The work demonstrates the significant effect of highly flexible operation on the service life of welded connections. [ABSTRACT FROM AUTHOR]