Base Metal Zone Research Articles

Impact fracture energy of steel welded connections in fire and post-fire

Charpy impact tests were conducted on steel welded connections at elevated temperatures and at the ambient temperature after heating and cooling processes. The elevated temperature tests were conducted to examine the Charpy impact energies under the blue brittleness temperature (100–300 °C) of steel. V-notch specimens quarried from steel base metal, weld metal, and heat-affected zone of the welded connection were heated and subjected to Charpy impact tests. In the ambient temperature tests after the heating and cooling processes, the heated specimens were either air cooled in a furnace or quenched with water. The test results show that the Charpy impact energies do not decrease in the blue brittleness temperature, and the quenched welded connections demonstrated very low Charpy impact energies.

Mechanical and fracture properties of aluminium cylinders manufactured by orbital friction stir welding

AbstractButt welding of AA6063 aluminium cylindrical shells was performed using the orbital friction stir welding (FSW) method. Tool rotation speed and orbital speed (i.e., traverse speed of rotating cylinder during welding) were considered as variable, and the strength and the mechanical properties including tensile strength, microhardness, mode I fracture energy and mode I crack growth behaviour of manufactured cylinders were investigated experimentally. A novel and subsized test specimen was designed and manufactured for fracture testing of specimens extracted from both base metal and weld zone region of cylinders. The initial precrack was introduced along (i) the tool penetration through the pipe thickness (i.e., T‐direction) and (ii) along the tool travelling direction (i.e., L‐direction). It was found that the crack growth resistance and fracture energy values of FSW samples are greater than the corresponding values of base aluminium material along both “L”‐ and “T”‐directions. Also, the fracture resistance value in T‐direction was higher than the L‐direction for the whole tested FSW samples with different welding speeds.

Characterization and mechanical behavior of AISI 316L/Incoloy 825 dissimilar welds processed by friction stir welding

This work explored the feasibility of friction stir welding between Incoloy 825 Ni-based superalloy and AISI 316 L stainless steel. Microstructure, microtexture, and mechanical properties of the weldments were investigated as well. By using a range of FSW parameters including the rotation speed (ω) of 400-500 rpm and the travel speed (v) of 18-24 mm/min, sound welds were obtained. Optical and electron microscopy examinations revealed that the formation of equiaxed recrystallized austenite and ferrite grains in the stir zones led to an increase in microharness of the welds. Elongated grains with uncompleted recrystallization forms were observed in thermo-mechanically affected zones, too. According to the results of electron backscatter diffraction analysis, a fiber texture with {111} ǁ welding direction relationship for ferrite phase formed in AISI 316 L base metal and stir zones while the orientations of austenite grains in the stir zones were highly dependent on the ω/v ratio. Upon tensile testing, the samples underwent ductile fracture mode, and the tensile strength of the weldments was similar to that of the weaker base metal (i.e. AISI 316 L). Based on the obtained results, the welding conditions including ω =500 rpm and v = 24 mm/min by positioning the stainless steel on the advancing side were recommended.

The Effect of Stand - Off Distance on the Mechanical Properties of TIG Butt Joints of 316 Stainless Steel

The objective of this study is to investigate and determine the effect of the stand-off distance (distance between the welded plates) on the mechanical properties of the Tungsten Inert Gas (TIG) welded joints. Butt TIG welding was performed for 316 stainless steel (SS) by using different pre welding stand-off distances with fixing the other parameters (thickness of welded plates, voltage, current, groove shape, and scanning speed). The influence of the stand-off distance parameter was examined by using tensile test, hardness test in the three different regions (base metal, heat effected zone, and molten zone), non-destructive testing (including visual inspection, liquid pentrant and X – ray) and microscopic examinations. Results show that the stand-off distance is one of the most important geometrical parameters of the Butt welded joints to end by good mechanical properties. It is found that the optimum stand-off distance was about 1 mm (shows the highest hardness results), but still there were some defects in some spots in the molten metal zone which caused a decrease in the hardness values in these locations. The 2 mm stand-off distance shows reasonable results, and the worst case was recorded for the 0 mm stand-off distance condition. Generally the hardness values of the heat affected zone in all conditions were the highest when compared to both metal welding zone and the base metal zone.

Effect of Nd:YAG pulsed laser welding process on the liquation and strain-age cracking in GTD-111 superalloy

The weldability of GTD-111 nickel-based superalloy by Nd:YAG pulse laser with an average power of 250 W was studied using several pre- and post-weld heat treatment cycle, and the characteristics of liquation, solidification and strain-age cracks were also investigated. The results revealed that liquation cracks in the GTD-111 Nickel-Based Cast Superalloy were associated with the constitutional liquation of γ′ particle, MC carbides, inter-dendritic γ-γ׳ eutectic and melting of Cr-rich boride. Also, cracking during welding occurred at the places that the concentration of Al and Ti is high. Crack-free laser welds were observed in the MT1 conditions (1200 ℃ for 2 h) owing to dissolution of deleterious phases before welding. γ′ phase had little effect on the incidence of cracking in the MT1 condition. An analysis of the microstructures indicated that the cracking was caused primarily by liquation in the as-welded condition and was exacerbated by post-weld heat treatment cracking during the subsequent heat treatment. aging heat treatment of samples which were undergone aging heat treatment before welding, resulted in the formation of strain-age cracking due to γʹ and γ-γ׳ eutectic phases precipitation. The results of micro-hardness indicated that the hardness of fusion zone (FZ) was higher than that of HAZ and base metal zone (BMZ). The liquation cracking in the heat affected zone (HAZ) was observed to be affected by the hardness of base-alloy.

Study on the Prediction Method of the Ultra-Low-Cycle Fatigue Damage of Steel

Cyclic void growth model (CVGM) and continuum damage mechanics (CDM) model are suitable for predicting the damage of ultra-low-cycle fatigue (ULCF) theoretically. However, studies on the prediction of ultra-low-cycle fatigue (ULCF) damage is lacking. To determine which method is better, we used the two methods to predict the damage of ULCF. Firstly, uniaxial tensile and large strain cycle tests were performed on the base metal, weld metal and heat-affected zone and the material parameters were calibrated respectively. The uniaxial plastic strain threshold and toughness parameter of weld metal were minimum, and the dispersion was maximum. The finite element models of the base metal and weld specimens were established based on the calibrated parameters, and the ULCF damage was predicted. Compared with the CVGM model, the CDM model can predict the fatigue life and the relationships among the fatigue and fracture lives, the post-fracture path and the number of cycles to initial damage. The parameter calibration is simple. CDM is superior to CVGM in predicting the ULCF damage of steel and its weld joints.

The determination of the weakest zone and the effects of the weakest zone on the impact toughness of the 12Cr2Mo1R welded joint

For the actual welded joints, determination of the weakest zone in the welded and the effects of the weakest zone is very important in characterization of the welded joints. In this paper, microstructure feature and charpy toughness of the 12Cr2Mo1R for pressure vessels steel welded joints were systematically investigated. The standard Charpy V-notch impact test specimens were prepared on the base metal, weld metal and various zones of the heat affected zone. The weakest zone of the 12Cr2Mo1R welded joint and its effects on the impact toughness of the welded joint were studied. It shows that at −30 °C, the weakest zone of the welded joint is the coarse-grained heat affected zone (CGHAZ), which impact toughness is 65 J, and its microstructure is composed of a large number of coarse granular bainite and a few blocks ferrite. With the fraction of CGHAZ on the ligament at the notch tip increases, the impact toughness decreases. The closer the CGHAZ location from the notch is, the less the impact toughness of the sample is. Above two aspects fully reflect the effect of the weakest zone on the toughness of the joint. This work reveals the effect of fraction of weakest zone in front of the notch tip on the impact toughness of the 12Cr2Mo1R welded joints.

A Study on Fatigue Crack Propagation for Friction Stir Welded Plate of 7N01 Al-Zn-Mg Alloy by EBSD.

EBSD (electron backscattered diffraction) was used to study the fatigue crack propagation mechanism in a friction stir welding joint of a 15 mm-thick 7N01 aluminum alloy plate. Crack tips with detailed features were clearly characterized by EBSD images. The plastic zone caused by crack was small in the stir zone. Due to the fine grain strengthening in the stir zone, there were several lattice distortion regions which were observed in the BC (band contrast) map but disappeared in the SEI (secondary electron image). In the stir zone, fatigue crack tends to awake and grow along grain boundaries, and propagate with little deformation of the grains. When the crack tries to grow across a boundary, the deformation of the plastic zone at the crack tip shows little correlation to the cyclic loading direction. However, the plastic zone in base metal, the rolled plate, is large and continuous, and no obvious lattice distortion region was found. According to Schmidt factor, the base metal near crack is fully deformed, lots of low angle boundaries parallel to the cyclic force can be observed. The base metal showed a better ability for fatigue crack propagation resistance.

Correlation between Tensile Deformation Behavior and Microstructural Morphology of Nuclear Grade Austenitic Stainless Steel Weld Joints using Infrared Thermography Technique

Tensile deformation behavior of nuclear grade Austenitic Stainless Steel (SS) and its weld joints fabricated by Gas Tungsten Arc Welding (GTAW) and Activated flux Gas Tungsten Arc Welding (AGTAW) processes were studied and correlated with relevant microstructural morphologies using Infrared Thermography (IRT) technique. The microstructure of base metal showed a complete austenite phase. GTAW Fusion Zone (FZ) exhibited both primary ferrite and primary austenite mode of solidification. Meantime, AGTAW FZ exhibited only primary austenite mode of solidification A strain rate of 4.4x10-4 s-1 was used during the tensile test of the base metal and weld samples. The failure locations of the base metal, GTAW and AGTAW samples were noticed at the center of the gauge portion, the base metal side away from Fusion Line (FL) and Heat Affected Zone (HAZ) respectively. Temperature variations of the base metal and weld zones were recorded in the form of thermograms using the IR camera at the different stages of the tensile deformation. During deformation study, peak temperatures of 39.2 oC, 38.8 oC and 34 oC were observed at the base metal, GTAW and AGTAW samples respectively. The lesser peak temperature of the AGTAW sample compared to the base metal and GTAW samples indicated that the AGTAW sample undergone lesser deformation. Moreover, tensile deformation behaviors of the base metal and weld samples were correlated with their microstructural morphologies using corresponding temperature curves.

Shielded Metal Arc and Thermite Welding Effect to Residual Stress, Hardness and Crack of R54-Rail Weld Joint

The heating and cooling at the end of the welding process can cause residual stresses that are permanent and remain in the welded joint. This study aims to evaluate the magnitude and direction of residual stresses on the base metal and heat-affected zone of rail joints welded by the manual shielded metal arc and thermite welding. This research supports the feasibility of welding for rail. The material used in this study is the R-54 rail type, and the procedure used two rail samples of one meter long each, welded using manual shielded metal arc welding and thermite welding. The base metal and heat-affected zone of the welded joints were scanned with neutron ray diffraction. The scan produces a spectrum pattern and reveals the direction of the residual stress along with it. We found the strain value contained in both types of welded joints by looking at the microstrain values, which we obtained using the Bragg equation. The results show that the magnitude and direction of the residual stress produced by manual shielded metal arc welding and thermite welding are not the same. Thermite welding produces lower residual stress (lower crack susceptibility) than manual shielded metal arc welding. The melt's freezing starts from the edge to the center of the weld to create random residual stresses. The residual stress results of both the manual shielded metal arc welding and thermite welding are still below the yield strength of the base metal.

Experimental Study of Microstructures in Bias Weld of Coiled Tubing Steel Strip With Multi-Frequency Eddy Current Testing

Coiled tubing technology is widely used in drilling and repairing operations, and the quality of bias weld directly determines the fatigue life of the coiled tubing. The microstructures consistency between the bias weld and the base metal determines that whether the fatigue resistance of the weld reaches the level of the base metal. In this paper, multi-frequency eddy current testing technique is used to detect the reactance of the bias weld zone and the base material zone within coiled tubing steel strip, which can be used to determine the microstructure consistency between the base metal area and the weld area, and in turn to improve the welding technology and heat treatment process. In this work, CT100 and CT90 materials with the bias weld for coiled tubing were used as the test samples, and the on-line real-time non-destructive testing technology was used to judge whether welding technology and heat treatment technology have been improved. The basis for determination is whether the mechanical properties and fatigue properties of the welding site reach the base material level. The weld zone and base metal zone can be identified by analyzing the characteristic cloud map of the detection signal, and then the quality of partial weld can be evaluated according to the difference of microstructure between them, which is of guiding significance for quality control and improvement of coiled tubing production process.

Corrosion behavior and mechanical properties of cold metal transfer welded dissimilar AA7075-AA5754 alloys

Cold metal transfer (CMT) welding is a brand-new arc welding technique which shows adequate results for welding of thin sheets and dissimilar materials. Corrosion behavior of dissimilar aluminum joints should be determined in terms of predicting the effect of welding process on the possible failures in their constructions caused by corrosive agents. The present study investigates the effect of heat input on mechanical properties and corrosion rate of AA5754-AA7075 joints welded by CMT using ER5356 filler wire. Pore formation was observed not only in the weld metal but also in the partially melted zone of AA7075 base metal due to the vaporization of zinc. Increased heat input caused over aging and zinc vaporization in AA7075 base metal, and grain coarsening in AA5754 base metal consequently decreased the tensile strength. The average tensile strength of AA7075-AA5754 joints varies between 235 and 240 MPa. The ductile fracture occurred at the AA5754 base metal side in all samples. Pitting was observed as the dominant corrosion mechanism. Corrosion resistance tended to increase with increasing heat input. Heat input values between 95 and 110 J/mm are recommended for the optimization of corrosion resistance and strength.

Microstructure and mechanical properties of wire-filled tungsten argon arc welded joints for LA141 magnesium-lithium-aluminum alloy

Wire-filled tungsten argon arc welding was carried out on LA141 alloy plates with specially prepared homogenous welding wires in an argon gas protection device. The microstructure and mechanical properties of joints were described and analyzed. The results show that the apparent quality and formability of welding seams are excellent, the lithium burning loss of joints is controlled effectively. The composition and microstructure of joints are relatively uniform which can be divided into base metal zone, heat-affected zone, transition zone and fusion zone, the transition zone plays a key role in the joint performance which be reflected in the smooth transition distribution of joints hardness and the improvement of the consistency for joints structure and performance. After post-weld heat treatment, the comprehensive properties of welded plates are further improved with a tensile strength of 124 MPa and an elongation of 16 %, the hardness distribution becomes more even.

Stress corrosion property of 304 stainless steel and Q345 steel laser-MAG hybrid welded joints

The stress corrosion cracking (SCC) property of laser-MAG hybrid welded 304 stainless steel and Q345 steel was evaluated through cycle-immersion testing in 3.5 wt.% NaCl solution. The average SCC crack propagation rate of different zones under different initial stress intensity factors was calculated, and the SCC fracture and crack propagation path were observed. The microstructure and mechanical properties of the weld joint have also been examined. The result indicates that the fusion zone (FZ) is extremely prone to SCC. The average SCC crack propagation rate in FZ is [Formula: see text] mm/h, while no obvious SCC was found in the base metal (BM) and heat-affected zone (HAZ). The steel BM and HAZ may also suffer SCC, but not as fast as in FZ. Grooves caused by SCC were found on the fracture surface with a large amount of corrosion products accumulated close to the interface between the pre-crack section and SCC section. Crystallized-sugar-shaped pattern was found on the SCC zone of FZ. Crack jumping, deflection and crack closure occurred in the crack propagation path. Martensite on the FZ was considered to be the major reason that the FZ has a higher SCC propagation rate.

Propagation characteristics of ultrasonic weld-guided waves in Friction stir welding joint of same material

Friction stir welding (FSW) is an important technology for manufacturing large-scale aluminum alloy propellant tank. Due to stress corrosion and cyclic loads, the key FSW joints require online monitoring to ensure the structural integrity and service safety of long-term storage propellant tanks. FSW joints in the propellant tank are regarded as a type of circumferential or longitudinal long and narrow region. In order to detect the flaws with high efficiency and fewer sensors, the propagation characteristics of ultrasonic guided waves in the FSW joint of same material is investigated in this paper. The weld of a FSW joint is characterized by concave cross-sectional shape and different microstructure-mechanical parameters. The micro-structure, micro-hardness, and Young′s modulus of a real FSW joint are analyzed, and a two-dimensional semi-analytical finite-element (SAFE) method is employed to study the effects of different parameters on the modal characteristics of weld-guided waves in the FSW joint. In the studied fundamental modes (symmetric (S0), anti-symmetric (A0), and shear-horizontal (SH0)), an almost non-leaky A0-like weld-guided wave at a certain frequency range from 100 kHz to 210 kHz is discovered in the welded zone of a specific FSW model and shows a potential for long-distance detection. Parametric simulation results show that A0-like, SH0-like and S0-like modes at 120 kHz always exist when the weld width is changed while the moduli of the welded zone and base metal zone are maintained the same. Additionally, the simulations demonstrate that some weld-guided waves only exist if the modulus value of the welded zone is lower than that of the base metal zone when the cross section is geometrically continuous (i.e. the shoulder plunge depth is zero). Comparing with weld-guided waves affected by weld width, the weld-guided waves affected by the modulus change shows less obvious energy leakage during propagation. The experiments are conducted to validate the existence of A0-like weld-guided mode with a primary energy trapping effect.

The PC-GTAW of Ti–6Al–4V Thin Sheets and Its Effects on Mechanical and Microstructural Properties

In this study, annealed Ti–6Al–4V alloy sheets with a thickness of 1 mm and with a butt-joint design were welded using gas tungsten arc welding without using filler metal. The aim of this study was to compare the specimens of the welding process with different current ratios (Ib/Ip) to find sample with high tensile strength and toughness. The welded specimens with optimal pulsed current ratios (Ib/Ip = 50, 75%), which made a basket-shaped structure with appropriate distribution alongside Widmanstatten layer phases, resulted in a soft fracture in the base metal zone as well as high strength in the tensile test.

Effect of Post-Weld Heat Treatment on the Microstructure of Plasma Arc Welded DP600 Steel

The effect of isothermal post-weld heat treatment on keyhole plasma arc welded DP600 steel was investigated using optical microscopy, scanning electron microscopy and analytical transmission electron microscopy. The isothermal heat treatment had no effect on the allotriomorphic, Widmanstatten and acicular ferrite in the fusion zone (FZ). The martensite lath structure showed complete recovery in FZ, base metal (BM) and heat-affected zone (HAZ) with the formation of coarse cementite particles of up to 200 nm size in subcritical HAZ. The coarse grain HAZ showed little change in bainite and had tempered martensite and transformed retained austenite. The fine grain HAZ consisted of bainite, tempered martensite and coarsened preexisting cementite. The recovered BM had tempered martensite and reduced dislocation density. Hardness was found to be the lowest in subcritical HAZ, which is attributed to complete tempering of martensite, formation of large cementite particles and disappearance of dislocations.

Effect of cathodic potential on stress corrosion cracking behavior of different heat-affected zone microstructures of E690 steel in artificial seawater

In this work, the stress corrosion cracking (SCC) behavior of E690 steel base metal (BM) and different heat-affected zone (HAZ) microstructures, i.e., coarse grain HAZ (CGHAZ), fine grain HAZ (FGHAZ), and intercritical HAZ (ICHAZ), was investigated at different cathodic potentials in artificial seawater by slow strain rate tensile tests, scanning electron microscopy and electron back-scattered diffraction measurements. The results show that the HAZ microstructures and BM exhibit different SCC susceptibilities: FGHAZ < ICHAZ < BM < CGHAZ, which are controlled by anodic dissolution (AD) at the open circuit potential. With the cathodic potential equaling to −750 mV, the SCC susceptibility of the four microstructures increases because of the synergistic effect of AD and weak hydrogen embrittlement (HE). At -850 mV, AD is inhibited, and the SCC susceptibility of BM decreases, while the SCC susceptibility of the HAZ microstructures increases. At a potential below -850 mV, the SCC susceptibility of the four microstructures gradually increases because of the augment of HE, and the SCC susceptibility of the HAZ microstructures is higher than that of BM. The distinction reveals that the HAZ microstructures have the greater HE susceptibility than BM.

Microstructre and mechanical properties of Mg/Al joints by pulse MIG assisted induction pre-heating with composite interlayers

Butt Joints of AZ31 magnesium alloys and Al1060 aluminium plates were produced by conventional MIG welding process with composite interlayers composed of zinc, copper and aluminium foil and induction pre-heating method was used to promote the melting degree of the high melting interlayers, which achieved the connection of dissimilar materials effectively by comparison via SEM and tensile test. The results indicated that crack-free Al/Mg butt joints were obtained. The analysis of the microstructure of the joints showed that the phase composition at the interface between base metal and fusion zone related to the sequence of the different foils. The tensile strength of the joints could reach 21 MPa and the fracture located at the interfacial zone which was mainly composed of brittle Al-Mg compounds.

Microstructure of Mg96Zn2Y2 alloy joint formed by resistance spot welding

Mg96Zn2Y2 alloy sheets were welded by employing a resistance spot welding technique. The microstructures of the joint were observed and analyzed using scanning electron microscopy and transmission electron microscopy to investigate the difference between it and those of the base material. The joint comprises a base metal zone, heat-affected zone, fusion zone, and nugget. In the heat-affected zone, the dissolution of secondary phases occurred locally. In the fusion zone, the α-Mg grains were coarsened; and the secondary phase was transformed into a network distribution. In the nugget, the α-Mg grains were approximately 30 μm in diameter; and most of the secondary phase possessed a long-period stacking-ordered structure of 18 periods rhombohedral lattice. The results reveal that the number of secondary phases and Mg3Zn3Y2 compounds formed in the nugget were smaller than those observed in the base metal.