Numerical Investigation on Fracture Initiation Properties of Interface Crack in Dissimilar Steel Welded Joints

Fracture toughness property is of significant importance when evaluating structural safety.The current research of fracture toughness mainly focused on crack in homogeneous material and experimental results.When the crack is located in a welded joint with high-gradient microstructure and mechanical property distribution,it becomes difficult to evaluate the fracture toughness behavior since the stress distribution may be affected by various factors.In recent years,numerical method has become an ideal approach to reveal the essence and mechanism of fracture toughness behavior.This study focuses on the crack initiation behavior and driving force at different interfaces in dissimilar steel welded joints.The stress and strain fields around the crack tip lying at the interfaces of ductile-ductile,ductile-brittle and brittle-brittle materials are analyzed by the numerical simulation.For the interface of ductile-ductile materials,the strain concentration on the softer material side is responsible for ductile fracture initiation.For the ductile-brittle interface,the shielding effect of the ductile material plays an important role in decreasing the fracture driving force on the brittle material side.In the case of brittle-brittle interface,a careful matching is required,because the strength mismatch decreases the fracture driving force in one side,whereas the driving force in another side is increased.The results are deemed to offer support for the safety assessment of welded structures.

Microstructures and Plane Energy Spectra of X80 Pipeline Steel Welded Joints by Submerged Arc Automatic Welding

X80 pipeline steel was welded with submerged arc automatic welding, the microstructures, cavity sizes, fusion depths and plane scanning of chemical elements in the welded zone, fusion zone, heat affected zone and base steel were observed with OM（optical microscope） and SEM（scanning electron microscope）, respectively. The experimental results show that there is main acicular ferrite in the base steel and welded zone, the microscopic structure of fusion zone is a blocked bainite, and the heat affected zone is composed of multilateral ferrite and pearlite. M-A unit of the welded zone is the main factor to strengthen the welded zone, composed of acicular ferrites. The percentage of cavities in the welded joint is less than that in the base steel, which is beneficial to increasing its mechanical performance and corrosion resistance. The fusion depth in the fusion zone and welded zone is 101.13 μm and 115.85 μm, respectively, and the distribution of chemical elements in the welded zone is uniform, no enrichment phenomena.

Activated flux tungsten inert gas welding of 8 mm-thick AISI 304 austenitic stainless steel

AISI 304 stainless steel plates were welded with activated flux tungsten inert gas(A-TIG) method by utilizing self-developed activated flux. It is indicated from the experimental results that for 8 mm-thick AISI 304 stainless steel plate, weld joint of full penetration and one-side welding with good weld appearance can be obtained in a single pass without groove preparation by utilizing A-TIG welding. Moreover, activated flux powders do not cause significant effect on the microstructure of TIG weld and the mechanical properties of A-TIG weld joints are also superior to those of C-TIG(conventional TIG) welding.

Investigation on Electron Beam Welded Copper to AISI 316 Stainless Steel Joints

Joints of copper and stainless steels are used in a er ospace applications. Production of these joints by fusion welding faces many dif ficulties. This may be due to the differences in their physical, metallurgical a nd mechanical properties. Electron Beam Welding (EBW) process has been found to be especially well suited in this area. Selection of the appropriate welding par ameters needs thorough investigations. These parameters include: preheat tempera ture (℃), welding current (I w), focusing current (I F), welding spee d (V), height between the gun and workpiece surface (H), scan width (S w) and shift distance (S). The present work aims firstly, setting the pr oper welding conditions to get sound joint between commercially pure copper (C10 200) and AISI 316 stainless steel plates 8 mm thickness. Secondly, investigate t he effect of Electron Beam (EB) shift, single-sided and double-sided welds on the mechanical, metallurgical and chemical properties of the weld bead. Due to t he high difference in thermal conductivity between copper and stainless steel, E lectron Beam (EB) was shifted towards copper with different values. These values were ranged from 0.3 to 0.9 mm in welding without preheating of copper plate an d from 0.1 to 0.4 mm with preheating. Number of joints were welded using variabl e EBW parameters in view to obtain the sound weld bead. These parameters are as follows: gradual reduction I w=51 to 49 mA, I F=845 mA, V=8 mm/sec , H=130 mm, S w=500 μm and S=0.4 mm. The investigation has shown t hat, the copper (C10200) plate must be preheated to get sound welded joint with AISI 316 stainless steel using the EBW process. The tensile fracture in all wel ded samples occurred in copper plate away from the weld bead. This reflects that the weld bead tensile strength is greater than the copper strength. The EB shif t has slight effect on hardness distribution through weld bead. The hardness val ue (H v) reduces in gradual manner from stainless steel hardness to copper one. The EB shift distance has no significant effect on the impact toughness.

Interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints during aging

The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints.The results showed that not only a band of granular Cr_(23)C_(6)carbides were formed along the fusion boundary in the ferritic steel during aging,but also a large number of granular or plate-like Cr_(23)C_(6)carbides,which have a cube-cube orientation relationship with the matrix,were also precipitated on the weld metal side of the fu-sion boundary,making this zone be etched more easily than the other zone and become a dark etched band.Stacking faults were found in some Cr_(23)C_(6)carbides.In the as-welded state,deformation twins were observed in the weld metal with a fully austenitic structure.The peak micro-hardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased signific-antly.Based on the experimental results,a mechanism of premature failures of the joints was proposed.

Fatigue life evaluation of girth butt weld within welded cast steel joints based on the extrapolation notch stress method

The fatigue life evaluation of the girth butt weld within the welded cast steel joint was studied based on the extrapolation notch stress method.Firstly,the mesh sensitivity of the finite element model of the welded cast steel joint was analyzed to determine the optimal mesh size.Based on the stress field analysis of the finite element model of the welded cast steel joint at the weld toe and weld root,the sharp model of the extrapolation notch stress method was applied to derive the effective notch stress of the rounded model belonging to the effective notch stress method,in which the key problem is to calculate the extrapolation point C,and the extrapolation point C has an exponential function relationship with the geometric parameters of the welded cast steel joint.By setting different values of geometric parameters,the corresponding value of parameter C is calculated,and then the functional relationship between the extrapolation point C and the geometric parameters can be obtained by the multiple linear regression analysis.Meanwhile,the fatigue life evaluation of the girth butt weld within welded cast steel joints based on the effective notch stress was performed according to the guideline recommended by the IIW(International Institute of Welding).The results indicate that the extrapolation notch stress method can effectively simplify the process of calculating the effective notch stress and accurately evaluate the fatigue life of the girth butt weld within welded cast steel joints.

Possibility of Inducing Compressive Residual Stresses in Welded Joints of SS400 Steels

Since the welded constructions produce easily stress corrosion cracking (SCC) or fatigue disruption in corrosive medium or under ripple load, two methods inducing compressive stress on structural surfaces by anti-welding-heating treatment (AWHT) and explosion treatment (ET) are presented. The results show that they are good ways to resisting SCC on the welded SS400 steel or other components.

Multiaxial Fatigue Analysis on Reeled Deepwater Steel Catenary Risers with Girth Weld Defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers（SCRs） using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis（FEA）, we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.

Study on an Explosion Treated Steel Weld Metal with Prepared Crack

A softening zone with recrystallized grain around the prepared crack tip in the shock waves treated C-Mn steel weld metal was observed. It is suggested that a dynamic recovery occurred around the prepared crack tip even at a low explosion pressure (5 GPa) because of the stress and strain concentration at the crack tip when shock waves pass through. This result supports the previous work on the improved mechanical properties of a shock treated welded joint with residual crack from the viewpoint of microstructure.

Microstructure in the Weld Metal of Austenitic-Pearlitic Dissimilar Steels and Diffusion of Element in the Fusion Zone

Microstructure and alloy element distribution in the welded joint between austenitic stainless steel (1Cr18Ni9Ti) and pearlitic heat-resistant steel (1Cr5Mo) were researched by means of light microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructure, divisions of the fusion zone and elemental diffusion distributions in the welded joints were investigated. Furthermore, solidification microstructure and S-ferrite distribution in the weld metal of these steels are also discussed.

Calculation of AF transformation kinetics in HSLA steel weld

It is of great importance for obtaining the perfect welding properties to control the acicular ferrite (AF) transformation behavior reasonably in steel weld. AF continuous transformation kinetics in the HSLA steel weld was calculated and modeled based on the direct growth on the inclusions inert interface. The simulation results are coincident with the experimental value well.

Investigation of electrode and hydrogen induced crack in welded joint of dissimilar steel

The welded joint of dissimilar heat-resisting steels 20Crl2MoV (F12)and 12 Cr2MoWVTiB(102)generally works around 600°C.In this paper three kinds of ferritic electrodes are used for testing.They are R817 high- strength electrode(CrllMoVNi),R347 low-strength electrode(Cr2MoVWB) and newly-developed R507MoNb medium-strength electrode.The study on the influence of those three different electrodes on carbon migration,HIC and hy- drogen diffusion shows that medium-strength electrodes can well control the carbon migration,and that the tendency to HIC in the joint formed by R817 is smaller than that by R347 instead.Considering the effect of weld metal transfor- mation on the restraint stress and hydrogen concentration of a joint,the hydro- gen distribution in the heat-affected zone(HAZ)is calculated by using finite ele- ment method(FEM)with stress and strain changing,and so the effect of the transformation behaviour on HIC is revealed.In addition,newly-developed R507MoNb electrodes,tested the elevated-temperature property,oxidation re- sistance and creep rupture strength,have fulfilled the technical standards con- cerned and passed the examination of on-the-spot operation.

Fatigue Test of Homemade Z Direction Steel Welded T Tubular Joints

This paper introduces the process and result of fatigue test of steel (Z direction steel) welded T tubular joints used in offshore engineering. Detailed measurement of stress concentration factor, stress distribution, fatigue life and crack development has been performed. Through analysis, an empirical formula of stress concentration factor for T tubular joints, fatigue S-N curve and crack propagation rule are obtained.

Spall Strength of Resistance Spot Weld for QP Steel

The spall tests under the plane tensile pulses for resistance spot weld （RSW） of QP980 steel are performed by using a gun system. The velocity histories of free surfaces of the RSWare measured with the laser velocity interferometer system for any reflector. The recovered specimens are investigated with an Olympus GX71 metallographic microscope and a scanning electron microscope （SEM）. The measured velocity histories are explained and used to evaluate the tension stresses in the RSW applying the characteristic theory and the assumption of Gathers. The spall strength （1977 2784MPa） of the RSW for 0,P980 steel is determined based on the measured and simulated velocity histories. The spall mechanism of the RSW is brittle fracture in view of the SEM investigation of the recovered specimen. The micrographs of the as-received QP980 steel, the initial and recovered RSW of this steel for the spall test are compared to reveal the microstructure evolution during the welding and spall process. It is indicated that during the welding thermal cycle, the local martensitic phase transformation is dependent on the location within the fusion zone and the heat affected zone. It is presented that the transformation at high strain rate may be cancelled by other phenomenon while the evolution of weld defects is obvious during the spall process. It may be the stress triaxiality and strain rate effect of the RSW strength or the dynamic load-carrying capacity of the RSW structure that the spall strength of the RSW for QP980 steel is much higher than the uniaxial compression yield strength （1200 MPa） of the rnartensite phase in 0,P980 steel. Due to the weld defects in the center of the I^SW, the spall strength of the RSW should be less than the conventional spall strength or the dynamic load-carrying capacity of condensed structure.

Microstructure and properties of pure iron/copper composite cladding layers on carbon steel

In the present study, pure iron/copper composite metal cladding was deposited onto carbon steel by tungsten inert gas welding. The study focused on interfacial morphological, microstructural, and mechanical analyses of the composite cladding layers. Iron liquid–solid-phase zones were formed at copper/steel and iron interfaces because of the melting of the steel substrate and iron. Iron concentrated in the copper cladding layer was observed to exhibit belt, globule, and dendrite morphologies. The appearance of iron-rich globules indicated the occurrence of liquid phase separation（LPS） prior to solidification, and iron-rich dendrites crystallized without the occurrence of LPS. The maximum microhardness of the iron/steel interface was lower than that of the copper/steel interface because of the diffusion of elemental carbon. All samples fractured in the cladding layers. Because of a relatively lower strength of the copper layer, a short plateau region appeared when shear movement was from copper to iron.

Principle of Mangling Correction of Plate-welding Section Steel and its Application

Thanks to the fantastic development of power, transportation, chemical engineering, architecture and other fields, there has been an increasing demand for new type of section steel with large sections used for construction frame, such as type-H, type-T, and type-C section steel. New types of section steel usually are long and with large cross section. Therefore, for the sake of the cost, steel plate welding structure is used home and abroad. During the welding process, welding distortion seems unavoidable. Presently, proper welding machinery, technique and standards are adopted to minimize the distortion. Angular distortion is corrected with oil press and angular distortion corrector, while horizontal distortions like sickle curve and arch are mainly done with flame. Flame correction is of great labor intensity and is also time and material consuming. Besides, the correction is affected by the operators’ skill and mood, which, to some extent, limits the productivity. In this paper, a new technique of section steel distortion correction with high efficiency and adaptability and yet low investment and cost will be proposed for the development of new-type section steel. The correction principles and capability calculation should be done some research on, which will be introduced. The practice done on section steel with a wing edge of 40～60 mm, proves satisfactory: compared with oil-press correction, continual mangling correction saves equipment investment by three quarters, installed capacity by three quarters and investment by three quarters and with a much higher efficiency. Particularly in the distortion correction of heavy welding structures, mangling correction stands out from the crowd. Mangling correction proves to be the best solution to the continual horizontal distortion correction of welding section steel.

Microstructure and Fracture Morphology in the Welding Zone of T91 Heat-resisting Steel Used in Power Station

Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding was researched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimental results indicated that microstructure of T91 weld metal was austenite + a little amount of S ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6 blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. There existed no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of $63 mmx5 mm, when increasing welding heat input (E) from 4.8 kJ/cm to 12 5 kJ/cm, fracture morphology in the fusion zone and the HAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about 9.8 kJ/cm is suitable in the welding of T91 heat-resistant steel.

INVESTIGATION ON BRITTLE FRACTUREPROPAGATION－ARRESTING CHARACTERISTICS OF STEEL PIPE

In order to study crack propagation-arresting characteristics of steel pipe construction, a new test installation was designed. The experiments of 20 steel pipes, pipes with longitudinally weld, pipes with circumferential weld and steel pipe with sleeve were conducted. The testing results indi- cated that circumferential weld in pipe with overrnatching weld had beneficial effect on crack prop- agation arrcsting propcrtics and the mechanical split sleeve can be used as crack arrester to prevent crack propagation.

Automated identification of steel weld defects,a convolutional neural network improved machine learning approach

This paper proposes a machine-learning-based methodology to automatically classify different types of steel weld defects,including lack of the fusion,porosity,slag inclusion,and the qualified(no defects)cases.This methodology solves the shortcomings of existing detection methods,such as expensive equipment,complicated operation and inability to detect internal defects.The study first collected percussed data from welded steel members with or without weld defects.Then,three methods,the Mel frequency cepstral coefficients,short-time Fourier transform(STFT),and continuous wavelet transform were implemented and compared to explore the most appropriate features for classification of weld statuses.Classic and convolutional neural network-enhanced algorithms were used to classify,the extracted features.Furthermore,experiments were designed and performed to validate the proposed method.Results showed that STFT achieved higher accuracies(up to 96.63%on average)in the weld status classification.The convolutional neural network-enhanced support vector machine(SVM)outperformed six other algorithms with an average accuracy of 95.8%.In addition,random forest and SVM were efficient approaches with a balanced trade-off between the accuracies and the computational efforts.

CO_(2) corrosion of X80 steel welded joints under micro-turbulence induced by welding reinforcement height

Corrosion failure accidents owing to flow erosion and pipeline corrosion frequently occur during transportation.The welding reinforcement height(WRH)can induce locally micro-turbulent flow field,which aggravates local corrosion of welded joints.A high wall shear stress(WsS)experimental setup was established to conduct the online electrochemical corrosion test.The influence of WRH sizes on local corrosion of welded joints was studied at different flow rates.The electrochemical signals of the local corrosion of X80 welded joints at different flow rates were monitored in real time using electrochemical impedance spectroscopy and wire beam microelectrode.In addition,the corrosion products composition and properties were analyzed.The results show that the micro-turbulent flow fields induced by the WRHs can enhance ion mass transfer near the welded joints.The corrosion products on the WRH surface also present different microscopic morphologies at different flow rates.In strong flow fields,the locally enhanced wsS can peel off the dense corrosion product partially,leading to the electrochemical distribution of large cathode and small anode,which accelerates the occurrence and development processes of the local corrosion of welded joints.The scientific guidelines for the corrosion protection of long-distance oil and gas pipelines can be potentially provided.