Electrochemical impedance spectroscopy study on the corrosion of the weld zone of 3Cr steel welded joints in CO_2 environments

The welded joints of 3Cr pipeline steel were fabricated with commercial welding wire using the gas tungsten arc welding （GTAW） technique. Potentiodynamic polarization curves, linear polarization resistance （LPR）, electrochemical impedance spectroscopy （EIS）, scan- ning electron microscopy （SEM）, and energy-dispersive spectrometry （EDS） were used to investigate the corrosion resistance and the growth of a corrosion film on the weld zone （WZ）. The changes in electrochemical characteristics of the film were obtained through fitting of the EIS data. The results showed that the average corrosion rate of the WZ in CO2 environments first increased, then fluctuated, and finally de- creased gradually. The formation of the film on the WZ was divided into three stages： dynamic adsorption, incomplete-coverage layer forma- tion, and integral layer formation.

Improving the fatigue property of welded joints for AZ31 magnesium alloy by ultrasonic peening treatment

The fatigue property of AZ31 magnesium alloy and its TIG welded joints were investigated. The ultrasonic peening treatment （UPT） was used to improve the fatigue property of the TIG welded joints, which was treated at the weld toe by the UPT process. The test results show that the fatigue strength of the base metal of AZ31 magnesium alloys is 57.8 MPa, and those of the fillet joint and the transverse cross joint are respectively 20. 0 MPa and 17.2 MPa at 2 × 10^6 cycles. The fatigue strengths of two kinds of welded joints treated by the UPT are respectively 30. 3 MPa and 24. 7 MPa, which have been improved by 51.5% and 43.6%, respectively. The fatigue life of the fillet joint specimens is prolonged by about 2. 74 times and the fatigue life of the transverse cross joint specimens is prolonged by about 1.05 times when the stress range is at 40. 0 MPa.

EFFECT OF MEAN STRESS ON FATIGUE PERFORMANCE OF WELDED JOINTS TREATED BY UPT

The fatigue contrast tests of unload longitudinal direction comer joints asoriginal welded and treated by ultrasonic peening of Q235B in various stress ratio are directed. Theimprovements of fatigue performance of unload longitudinal direction corner joints resulted byultrasonic peening are studied. The effect pattern of stress ratio on fatigue performance of weldedjoints that are treated by ultrasonic peening is studied. As tests results indicate that: ① In thecondition of stress ratio .R=-l, the fatigue strength 'of specimen treated by ultrasonic peening isincreased by 165 percent of that of the original welded specimen. And the fatigue life of specimentreated by ultrasonic peening is as much as 75 approx 210 times of that of the latter. When R=0.1,the fatigue strength is increased by 87 percent and the fatigue life is extended by 21 approx 29times. When R=-0.5, the fatigue strength is increased by 123 percent and the fatigue life isextended by 42 approx 59 times. When R=0.45, the fatigue strength is increased by 51 percent and thefatigue life is extended by 3 approx 14 times. ② If the welded joints are treated by ultrasonicpeening, the fatigue strength is no longer independent on the applied mean stress. The more thestress ratio R, the less the fatigue stress range which can be sustained by the joints is. ③Whether the high value residual stress is in the joints or not, the dead load portion of the appliedload must be considered in the design of the joints which should be treated by ultrasonic peening.

Effects of laser heat treatment on the fracture morphologies of X80 pipeline steel welded joints by stress corrosion

The surfaces of X80 pipeline steel welded joints were processed with a CO2 laser, and the effects of laser heat treatment （LHT） on H2S stress corrosion in the National Association of Corrosion Engineers （NACE） solution were analyzed by a slow strain rate test. The fracture morphologies and chemical components of corrosive products before and after LHT were analyzed by scanning electron microscopy and energy-dispersive spectroscopy, respectively, and the mechanism of LHT on stress corrosion cracking was discussed. Results showed that the fracture for welded joints was brittle in its original state, while it was transformed to a ductile fracture after LHT. The tendencies of hydrogen-induced corrosion were reduced, and the stress corrosion sensitivity index decreased from 35.2% to 25.3%, indicating that the stress corrosion resistance of X80 pipeline steel welded joints has been improved by LHT.

Microstructures of 2219 twin wire welded joints

With thick plates of 2219 high-strength alloy, the microstructures of welded joints with twin wire MIG welding were analyzed. Experimental results show that no hot crack was found in the weld due to discontinuous distribution of cocrystallization with low melting temperature, but porosity is serious in the first weld seam that is mainly composed of equiaxial grains with uneven sizes. As the poor position of the whole welded joint, fusion zone has big and coarse grains, uneven microstructures ; In quenching zone, there exist a lot of soaked microstructures that cocrystallizntion with low melting temperature solute into matrix, thus strengthening the metal in this zone; In excessive aging zone, much more phases that distribute evenly will be separated from the matrix; Ontside this zone, properties and microstructures of the metal are basically similar to matrix due to the relatively low temperature or unaffected heat in the zone during welding.

Changing Rule of Carbon-Enriched Zone and Diffusion Behavior of Carbon in Aging 0Cr6Mn13Ni10MoTi/1Cr5Mo Dissimilar Welded Joints

The microstructures, the changing rule of carbon-enriched zone, the diffusion behaviors of elements C and Cr, and thecarbide type of 0Cr6Mn13Ni10MoTi/1Cr5Mo dissimilar welded joints after aging at 500℃ for various times and afterlong-term service in technical practice were investigated by using the optical microscopy electron probe microanalysis,scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that in aging0Cr6Mn13Ni10MoTi/1Cr5Mo dissimilar welded joints, the main carbides are M_3C and a few carbides are M_7C_3 andM_(23)C_6. The M_3C carbide decomposition and dissolution with increasing aging time or aging temperature and theanti-diffusion of C and Cr cause the decrease and disappearance of the carbon-enriched zone. The results are differentfrom those of the A302/1Cr5Mo dissimilar welded joints in previous studies.

EFFECT OF CARBON MIGRATION ON CREEP PROPERTIES OF Cr5Mo DISSIMILAR WELDED JOINTS WITH Ni-BASED AND AUSTENITIC WELD METAL

In this paper, the effect of carbon migration on creep properties of Cr5Mo dissimilar welded joints with Ni-based (Inconel 182) and Cr23Nil3 (A302) austenitic weld metal was investigated. Carbon migration near the weld metal/ferritic steel interface of Cr5Mo dissimilar welded joints was analyzed by aging method. Local creep deformations of the dissimilar welded joints were measured by a long-term local creep deformation measuring technique. The creep rupture testing was performed for Cr5Mo dissimilar welded joints with Inconel 182 and A302 weld metal. The research results show that the maximum creep strain rate occurs in the decarburized zone located on heat affect zone (HAZ) of Cr5Mo ferritic steel. The creep rupture life of Cr5Mo dissimilar welded joints with A 302 weld metal decreases due to carbon migration and is about 50% of that welded with Inconel 182 weld metal.

An investigation on the performance of stress corrosion cracking in aluminum-copper alloy welded joint

With the resistance to stress corrosion of the base metal as a reference, the contrast result of stress corrosion cracking （ SCC） susceptibility of aluminum-copper alloy 2219 and 2014 welded joints under different welding processes （ VP-TIG welding, HF-TIG hybrid welding, laser-TIG hybrid welding and laser HF-TIG hybrid welding） is obtained via the slow strain rate testing （ SSRT） , scanning electron microscope （ SEM） and microstructure observation auxiliary technologies. Test results show that the joints of aluminum alloy 2219, welded by hybrid welding processes, have superior resistance to stress corrosion compared to those welded by the VP-TIG welding process in varying degrees, especially, the joint welded by the laser HF-TIG hybrid welding process, where the resistance to stress corrosion is almost the same as that of the base material. However, the HF or laser hybrid welding effect is not significant under the same welding conditions for welded joints of aluminum alloy 2014.

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve the fatigue performance of welded joints.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

Effect of Scandium on Corrosion Resistance of Welded Joint ofAl-6Mg-Zr Alloy

The corrosion resistance of welded joints of Al-6Mg-Sc-Zr alloy was studied by neutral salt spray and exfoliation corrosion methods. The microstructure of welded joints was investigated by using optical microscope and transmission electron micrograph (TEM). It is demonstrated that the welded joints of Al-6Mg-Sc-Zr alloy are more corrosion resistance, comparing with Al-6Mg-Zr alloy. The addition of scandium in the alloy results in (Al_3Sc, Zr) particles, potently refined grains and restrained recrystallization process. The formation of homogeneous, discontinuous distribution of β-phase in welded joints improves the corrosion resistance of welded joints of Al-Mg-Zr alloy with high level content of magnesium.

A NOVEL METHOD FOR EVALUATING PLANE STRESS DYNAMIC FRACTURE TOUGHNESS OF 0Cr18Ni10Ti STAINLESS STEEL WELDED JOINTS

A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional （3D） transient numerical simulations were performed by a finite element （FE） computer program. Macroscopical plastic deformation was observed at the loading and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Therefore, elustic-viscoplustic analysis was performed on the specimen by adopting the Johnson-Cook （J-C） model to describe the rate-dependent plastic flow behavior of the material. The material heterogeneity in the mismatched welded joints, induced by the difference in the base metal （BM） and the weld metal （WM） in yield stress, has also been taken into consideration by using the J-C models separately. Good accordance was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 108 MPa.m^1/2.s^-1.

Evaluation on Fracture Toughness at Dynamic Loading for Welded Joint Based on the Local Approach

The changes in mechanical properties and fracture toughness by dynamic loading were investigated with experiments. The parameter R, which can reflect the effect of the loading rate and the temperature rising during the high loading rate, could be employed to describe the constituent relation for the typical structure steel and its weld metal. The dynamic loading effect on the stress/strain fields and the temperature variation in the vicinity of the crack tip was analyzed by the finite element method, the dynamic fracture behavior was evaluated based on the local approach. It has been found that the Weibull stress is an effective fracture parameter, independent of the temperature and the loading rate.

Study on the Basic Mechanical Behaviors of High Density Polyethylene Electrofusion Welded Joints at Different Temperature

The basic mechanical behaviors of high density polyethylene electrofusion welded joint at different temperatures were studied by using differently designed specimens in this paper. The results show that the strength of weld bonding plane is higher than that of the pipe and socket materials at room temperature. In order to get the shear strength of electrofusion welded joint, the effective bond lengths were reduced by cutting artificial groove through the socket. The effective bonding length of welded joint to get the shear strength is decreased with decreasing testing temperature. The shear strength and the sensibility to sharp notch of HDPE material increased with decreasing temperature.

Microstructure and mechanical property of resistance spot welded joint of aluminum alloy to high strength steel with especial electrodes

Dissimilar material joining of 6008 aluminum alloy to H220 YD galvanized high strength steel was performed by resistance spot welding with especial electrodes that were a flat tip electrode against the steel surface and a domed tip electrode upon the aluminum alloy surface. An intermetallic compound layer composed of Fe2Al5 and FeAl3 was formed at the steel/ aluminum interface in the welded joint. The thickness of the intermetallic compound layer increased with increasing welding current and welding time, and the maximum thickness being 7. 0 μm was obtained at 25 kA and 300 ms. The weld nugget diameter and tensile shear load of the welded joint had increased tendencies first with increasing welding current （ 18 -22 kA） and welding time （ 50 - 300 ms）, then changed little with further increasing welding current （ 22 - 25 kA） and welding time （300 -400 ms）. The maximum tensile shear load reached 5.4 kN at 22 kA and 300 ms. The welded joint fractured through brittle intermetallic compound layer and aluminum alloy nugget.

Quantitative Metal Magnetic Memory Reliability Modeling for Welded Joints

Metal magnetic memory（MMM） testing has been widely used to detect welded joints. However, load levels, environmental magnetic field, and measurement noises make the MMM data dispersive and bring difficulty to quantitative evaluation. In order to promote the development of quantitative MMM reliability assessment, a new MMM model is presented for welded joints. Steel Q235 welded specimens are tested along the longitudinal and horizontal lines by TSC-2M-8 instrument in the tensile fatigue experiments. The X-ray testing is carried out synchronously to verify the MMM results. It is found that MMM testing can detect the hidden crack earlier than X-ray testing. Moreover, the MMM gradient vector sum K_（vs） is sensitive to the damage degree, especially at early and hidden damage stages. Considering the dispersion of MMM data, the K_（vs） statistical law is investigated, which shows that K_（vs） obeys Gaussian distribution. So K_（vs） is the suitable MMM parameter to establish reliability model of welded joints. At last, the original quantitative MMM reliability model is first presented based on the improved stress strength interference theory. It is shown that the reliability degree R gradually decreases with the decreasing of the residual life ratio T, and the maximal error between prediction reliability degree R_1 and verification reliability degree R_2 is 9.15%. This presented method provides a novel tool of reliability testing and evaluating in practical engineering for welded joints.

Nanoindentation Characterization of Creep-fatigue Interaction on Local Creep Behavior of P92 Steel Welded Joint

In modern fossil and nuclear power plants,the components are subjected to creep,fatigue,and creep-fatigue(CF)due to frequent start-up and shut-down operations at high temperatures.The CF interaction on the in-service P92 steel welded joint was investigated by strain-controlled CF tests with different dwell times of 30,120,300,600 and 900 s at 650℃.Based on the observations of the fracture surface by scanning electron microscope(SEM),the character-istic microstructure of fatigue-induced damage was found for the CF specimens with short dwell times(30 and 120 s).The hardness,elastic modulus and creep deformation near the fracture edges of four typical CF specimens with 30,120,600 and 900 s dwell times were measured by nanoindentation.Compared to specimens with post-weld heat treatment(PWHT),lower hardness and creep strength were found for all CF specimens.In addition,significant reduc-tions in hardness,elastic modulus,and creep strength were measured near the fracture edges for the CF specimens with short dwell times compared to the PWHT specimens.Compared to PWHT specimens(0.007),the increased strain rate sensitivities(SRS)of 0.010 to 0.17 were estimated from secondary creep.The increased values of SRS indicate that the room temperature creeps behavior is strongly affected by the decrease in dislocation density after the CF tests.

Improved Fatigue Behavior of Pipeline Steel Welded Joint by Surface Mechanical Attrition Treatment(SMAT)

A pipeline steel X80 with welded joint was subjected to surface mechanical attrition treatment （SMAT）. After SMAT, a nanostructure surface layer with an average grain size of about 10 nm was formed in the treated sample, and the fatigue limit of the welded joint was elevated by about 13% relative to the untreated joints. In the low and the high amplitude stress regimes, both fatigue strength and fatigue life were enhanced. Formation of the nanostructured surface layer played more important role in the enhanced fatigue behavior than that of residual stress induced by the SMAT.

Tensile properties and fracture surface of 07MnNiCrMoVDR steel welded joint at low temperature

The tensile properties and fracture surface of 07MnNiCrMoVDR steel welded joint at low temperature have been studied by universal testing machine and scanning electron microscope. The results show that the tensile properties of 07MnNiCrMoVDR steel welded joint are greatly affected by temperature. Tensile strength and yield strength of 07MnNiCrMoVDR steel welded joint increase, but elongation and reduction of area decrease with temperature decreasing. The macro-fracture of 07 MnNiCrMoVDR steel welded joint exhibits that the shear lip is not significant and micro-fracture makes up of dimpled fracture and tear fracture, and dimple becomes tiny and uniform with temperature decreasing.

Creep fracture parameters of polyethylene pipe welded joints

Double edge notched tensile （DENT） tests were carried out to investigate the slow crack growth （SCG） behavior of PESO welded joints. Efforts were made to acquire creep fracture parameters which can characterize creep resistance. Results show that creep compliance indicates an ascending trend with the increase of applied stress. A global creep load parameter is obtained. It has a linear relationship with fkacture time in logarithmic coordinate system and helps to predict the fracture time at any applied load.