Weld Metal Region Research Articles

Study on crack growth path of API-5L X80 welded joints based on inhomogeneous mechanical properties

This study analyses the effect of inhomogeneous mechanical properties on the stress-strain field of crack tips and crack growth path at different locations of API-5L X80 welded joints. First, the inhomogeneous mechanical properties and microstructure of welded joints were analyzed in detail in this paper. Then, the ABAQUS subroutine USDFID was developed to ensure the continuity variation of the mechanical properties of the material in this region. Finally, the cracks in the welded joints were analysed using the extended finite element method (XFEM). The findings demonstrate that the inhomogeneous mechanical properties of welded joints give rise to crack growth path that are deflected towards the weld metal region, exhibiting the greatest rate of growth in this region.

Mechanical and Metallurgical behavior of Manual Metal Arc Welded P91 Ferritic Martensitic Steel Joints

In the nuclear power plant industry, Cr-Mo ferritic steels are indispensable due to their high-temperature tensile strength, creep strength, and resistance to stress corrosion cracking. This study evaluated and analyzed the mechanical properties and metallurgical behavior of indigenously developed filler (over matched with base metal) manual metal arc welded Cr-Mo ferritic steel (hereafter referred as P91 steel). The analysis revealed that the ultimate tensile properties of the weld joint exceed those of the unwelded metal, being 6% higher than the base metal. Consequently, the joint efficiency for the weld joint is 106%. However, the impact toughness of the weld pad is significantly lower compared to the unwelded metal, nearly 2.5 times less than the base metal. The weld metal region’s microstructure is characterized by untempered lath martensite pinned with dense dislocations, which is attributed to rapid cooling from the liquidus range. Furthermore, a distinct Heat-Affected Zone (HAZ) was identified adjacent to the weld metal region, which results from the elevated temperature experienced in this area.

Enhancing NDE Reliability for Grade 91 Steel Welds: Ultrasonic Imaging and Microstructural Correlations

Ensuring the integrity of Grade 91 (9Cr-1Mo-V) steel welds is vital for the safe and reliable operation of fossil fuel–fired and nuclear power plants. This study applies an imaging technique for the ultrasonic characterization of two Grade 91 steel welds created with cold metal transfer and flux-cored arc welding processes. Ultrasonic immersion testing in the through-transmission configuration was employed to generate shear waves, which helped identify the weld metal, heat-affected zone, and base metal regions. These weld microstructures were also correlated to their ultrasonic images using metallography, ultrasonic amplitude, hardness measurements, and grain size. The findings from this study can assist practitioners in developing new nondestructive evaluation technologies, improving the inspection reliability of creep strength–enhanced ferritic steel welds by potentially identifying weld microstructure regions susceptible to creep-type failures.

Effect of PWHT on microstructure and mechanical properties of welded joint of a new Fe-Ni-based superalloy

In this study, the effect post weld heat treatment (PWHT) on microstructural evolution and mechanical properties of weld metal of HT700P alloy with Inconel 617 filler metal was investigated. After welding, columnar grain, dendritic microstructure and segregation of Mo, Co and Ti elements related to the equilibrium distribution coefficient were observed in the weld metal. The PWHT was conducted in both direct-aging treatment with different time and solution treatment with different cooling methods. The results showed that direct-aging treatment had a limit impact on homogenization of weld metal, while microstructural homogenization and elemental segregation were significantly improved after solution treatment and massive M6C and M23C6 carbides precipitated at the grain boundaries and interdendritic region of weld metal. It was found that the elongation at 700 °C of welded joints after solution treatment was relatively increased, which was related to the decrease of residual stress and the precipitation of Mo-rich carbides at the grain boundaries. The fracture occurred in the weld metal presented interdendritic fracture characteristics, and the fracture mechanism for stress relaxation cracking was the strong grain interior induced by the precipitation of γ′ phase, leading to the stress concentration and nucleation of cavities at grain boundaries. After solution treatment, the creep lifetime of furnace cooling was much greater than that of water quenching. The superior creep properties of furnace-cooled joint could be attributed to the lower residual stress and the migration of carbon element in the fusion line, which inhibited the nucleation and aggregation of cavities in the partially melted zone.

Effect of hydrogen on the pitting corrosion of 2205 duplex stainless steel welded joints

The effect of hydrogen on the pitting susceptibility of 2205 duplex stainless steel welded joints was investigated using metallographic characterisation, electrochemical techniques and immersion experiments. The results show that the diffusible hydrogen content in the welded joints is approximately twice that of base material (BM). Hydrogen significantly decreases the pitting potential (Epit) of the welded joints, while increasing the carrier density in the passive film, suggesting that hydrogen weakens the protective properties of the passive film and increases the pitting susceptibility of the welded joints. According to immersion experiments, the pitting susceptibility of the welded joint from high to low is heat-affected zone (HAZ), BM and weld metal (WM), the HAZ tends to be more prone to the formation of larger and more stable pits, and the number of inclusions at the HAZ is significantly greater than that at the BM and WM. For BM under hydrogen charging, austenite phase is usually corroded first, because hydrogen tends to be enriched in the austenitic phase of BM. However, in some regions of WM, ferrite phase corroded first, which is related to the enrichment of hydrogen in the ferrite in the weld area.

A Phenomenological Model for Creep and Creep-Fatigue Crack Growth Rate Behavior in Ferritic Steels

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.

Microstructural features and hot tensile behaviour of tungsten inert gas welded TP321 nuclear tubes at elevated temperatures

Austenitic stainless steel (ASS) tubes, specifically TP321, are widely used in nuclear reactors and heat exchangers due to their excellent strength, toughness, and corrosion at ambient and elevated temperatures. The microstructure observed in the TP321 weldment reveals the existence of equiaxed dendrites, along with delta ferrite and TiC precipitates within the austenitic matrix. Electron backscatter diffraction (EBSD) scans confirm differences in texture, grain size, and grain misorientation distribution in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weldment. The EBSD texture at the WM zone is strong in the 〈0 0 1〉 fiber direction, consistent with the face-centered cubic structure of the TP321. Hot tensile tests were conducted on BM and WM samples at 450 °C, 550 °C, 650 °C, 700 °C, 850 °C, and 900 °C using a Gleeble 3500 simulator. The yield strength (YS), ultimate tensile strength (UTS), and percentage elongation (EL) exhibited a decreasing trend with the rise in test temperature. The strain hardening capacity (Hc) of both BM and WM decreases with increasing test temperatures due to enhanced dislocation movement. The strain hardening exponent, ‘n’ was determined for BM and WM through suitable fitting of stress–strain curves using the Hollomon equation and depicted a decreasing trend with rising test temperatures. The fractured samples are subjected to scanning electron microscope (SEM) analysis to investigate the failure mechanism. Fractography of the fractured samples reveal the presence of micro voids and dimples, supporting the ductile nature of the rupture.

Identifying the influence of transient thermal tensioning treatments on minimizing distortion and improving fatigue behavior of steel welded

Due to the cost efficiency of welding repairs, the use of transient thermal tensioning (TTT) has begun to be applied to minimize distortion and residual stresses, particularly on thin plates. However, it requires a long preheating time especially on large structures, so that the efficiency of welding process cannot be maximized. Application of TTT treatment using flame heater on TTT treatment which require no preheating time so that welding efficiency can be increased. The aims of this study are to investigate the TTT treatment in reducing distortion, investigate the effect of TTT treatment on tensile strength and hardness, investigate the microstructure and its effect on tensile strength and hardness, investigated the effect of TTT treatment on fatigue crack growth rate. In this research, TTT treatment was performed by flame heating on the both side of weld line integrated in welding process. Temperature in both side plates were controlled and measured using thermocouple. The tests on the weld joints were carried out including distortion measurement, microstructure examination, hardness measurement, tensile test and fatigue test. Results showed that the TTT (60) treatment is the most effective in decreasing the longitudinal distortion which placing the flame heating a 60 mm behind welding torch. It tends to increase the tensile strength of weld metal supported by its increasing hardness. The increase in the percentage of the acicular ferrite phase is linearly related to the tensile strength and hardness of the weld joint. The fatigue behavior could be improved by TTT treatment (60) which is associated with the effect of decreasing residual stress in the weld metal region. This treatment is the best parameter in an effort to increase the welding efficiency of the TTT method

Investigation on the impact of B4C content in AA5052 based metal matrix composites on the mechanical properties of welded joints

In this research, the tungsten inert gas welding was used to join together metal matrix composites (MMCs) reinforced in different percentages with B4C. The welding settings were maintained at a constant value for all specimen situations. Thermal ageing was used to achieve maximum age-hardening in the welded composites. Misorientation and energy storage are the microscopic features of aged composites that distinguish them from composites that have not been aged. Tensile strength and hardness both increased with increasing B4C concentration, but at opposite rates, suggesting that hardness resulted from energy storage while tensile strength resulted from misorientation. Age hardening as measured by electron backscatter diffraction (EBSD) occurs due to a active recovery of the weld metal (WM) region following a reorganization in displacement structures caused by the precipitation of magnesium silicate or an increase in boron carbide content. The healing process is primarily driven by the action of precipitation strengthening, rather than the impact of obliteration of displacements, as evidenced by the correlation between tensile strength and reduction in misorientation. Adding B4C to aluminium metal matrix composites before welding causes the structural changes that need to be characterized. The welding process's structural changes are analysed, and connections are made to the welded junctions' mechanical behaviour. The study's findings suggest that the microstructure's misorientation distribution is as crucial as the average amount of misorientation in determining a material's strength.

Morphology Study of the Corrosion Rate on Weld Joint of Double Side Friction Stir Welding Aluminum Alloy AA6061

DSFSW welding uses the rotating of a tool in the workpiece to create heat owing to friction between workpiece and tool to be connected. Corrosion is influenced by the presence of high heat owing to friction and a non-uniform cooling rate. The heat transforms the metal in the Heat Affected Zone (HAZ) and Weld Metal (WM) region, which can result in flaws such porosity, kissing bond, fractures, voids (from welding penetration), and flash as well as changes to the microstructure. Changes in the microstructure of welded joints can affect the resistance of welded joints to corrosion. The purpose of this study is to experimentally analyze the main causes of failure of AA6061 DSFSW joints based on welding temperature, weld defects, microstructure, corrosion rate and morphology of the corroded surface. Temperature testing using thermocouple to analyze the temperature, welding joint defects using DSLR camera and radiography test, microstructure using optical microscope, corrosion rate using AUTOLAB PGSTAT and morphology of corroded surface using SEM. The temperature analysis results show that the advancing side has a higher temperature than the retreating side, due to friction between the tool and base metal accompanied by the opposite welding direction. Visual inspection shows that all specimens and welding positions produce flash that is quite rough on the top (1G) and bottom (4G) surfaces and radiographic test results show incomplete fusion in 4 specimens. Microstructure shows a change in shape and size resulting in recrystallization in the form of fine grains. The highest corrosion rate is found in specimen B 1G welding position of 0.63856mm/year and the lowest corrosion rate in specimen A of 0.058567mm/year. SEM test results show the type of corrosion that occurs in DSFSW welding joints is pitting corrosion.

Investigating the impact of weld dilution and local mismatch on the low-cycle fatigue failure of Alloy 182 dissimilar weld transition under intermediate isothermal condition

Dissimilar welded joints are the critical region that determines the operation lifetime of pressure vessels under severe cyclic deformation. The failure susceptibility of dissimilar welded joints is attributed to local strength mismatch and heterogeneous microstructures. This study evaluated the isothermal low-cycle fatigue performance of a dissimilar welded joint between nickel-based Alloy 182 and 1.25Cr-0.5Mo steel at 250 °C. During low-cycle fatigue tests, the Alloy 182 weld metal exhibited significant cyclic hardening over the 1.25Cr-0.5Mo steel base metal. The local strain variations in the weld transition were measured using the digital image correlation (DIC) technique to reveal the impact of material strength and hardening mismatch upon the cyclic behavior within the weld transition. Microhardness measurements of as-welded and cyclically deformed weld transition regions were performed to assess the hardening due to cyclic deformation. The WT failures occurred in the weld metal region adjacent to the fusion boundary. In addition, microstructural characterization shows that the crack initiation occurred within the weld dilution region, and the short crack growth was affected by microstructural heterogeneity in the weld metal.

Evaluation of hardness and impact toughness of drawn arc stud weldments on different structural steel grades

The drawn arc stud welding (DASW) procedure is usually qualified by visual examinations and practical destructive tests of the joint according to international standards. An assessment of the material properties in specific weld zone regions is not requested under consideration of their limited areas or the general joint dimension although the press welding process implies a complex metallurgy. Nonetheless, there is a need of knowledge regarding the heterogeneous material properties in the weld zone, especially when welding on base materials with specified strength and toughness behavior that might be significantly diminished. In this study, the local strength and toughness properties in welded joints of non-alloyed steel studs (Ø20 mm) on plates of different generic structural steel grades are investigated by hardness test and Charpy impact testing. A non-alloyed normalized structural steel (nominal yield strength 355 MPa) and a thermomechanically rolled fine grain structural steel of same strength as well as a quenched and tempered structural steel with high nominal yield strength of 690 MPa were examined. In the weld metal region, relatively similar hardness and low impact energy were determined despite of the different base plate steel grades. The base plate heat-affected zones suffer a strong embrittlement due to the DASW thermocycle.

Joints properties in rotary friction welding of AA5083-H112 aluminium alloy

Rotary friction welded joints of AA5083-H112 aluminium alloy rods with diameter of 15 mm and length of 50 mm were produced at various rotational speeds. The current research is aimed to understand the static and fatigue properties of welding joints. Several experiments were carried out including macrostructure and microstructure observations, hardness measurements, tensile testing, residual stress measurements by neutron diffraction, rotary bending fatigue tests and fractography. The results showed that the static fracture occurred at the base metal away from the weld metal region for all weld joints under study with the average ultimate tensile strength of 272.7 MPa. In this condition, the joint efficiency was higher than 100%. It was found that the high strength of the weld joints was associated with fine-grained microstructure. On the other hand, fatigue property of the weld was influenced by both welding microstructure and residual stress. This argument was supported by the facts that the initial crack occurred at HAZ at which the residual stress was tensile and then followed by fatigue crack growth from HAZ towards TMAZ.

Effect of the notch location on the Charpy-V toughness results for robotic flux-cored arc welded multipass joints

Abstract In this study, the effect of the notch locations on the Charpy-V toughness values of the all-welded joint obtained using robotic flux-cored arc welding was investigated with respect to microstructures at the notch locations. Charpy impact tests were performed through the thickness with notch location at the centerline as well as off-set regions of the weld metal in addition to the microhardness measurements conducted. The detailed weld metal characterization was conducted using a stereo microscope, optical microscope, and scanning electron microscope at the same location where the Charpy tests and microhardness tests were performed. The sub-zero impact toughness test results indicated that the columnar weld metal regions exhibited low toughness values while the centerline microstructure consisting of mainly reheated regions displayed much higher toughness values even at the test temperature of −60 °C, satisfying the toughness requirement of the requested 47 J value. It is concluded that a small variation of the through-thickness notch position may result in different toughness values for the same weld metal. On this basis, the notching procedure of the Charpy-V samples for the multi-pass weld metal should be conducted with care and obtained results should be explained with respective notch position and microstructure.

Metallurgical, mechanical and corrosion characteristics of vibration assisted gas metal arc AA6061-T6 welded joints

Microstructure, mechanical properties and corrosion performance of gas metal arc AA6061-T6 aluminum alloy welded joints under the effect of mechanical vibration during welding have been investigated. The vibration frequencies, f used in this study were in the range of 0-500 Hz. Subsequently, several experiments were performed including microstructural observation, microhardness measurements, tensile tests, fatigue crack growth tests combined with fractography study and electrochemical polarization tests in 3.5% NaCl solution. Results indicated that the application of mechanical vibration increased tensile strength of the welded joints up to 83.3 % higher than the weld without vibration achieved at a frequency of 300 Hz. It was found that the strengthening mechanisms in the vibrated weld joints occurred mainly by the grain refinement of columnar dendritic and equiaxed dendritic microstructures in the weld metal region. Such microstructures could contribute to fatigue crack growth inhibition in the weld metal. Based on electrochemical corrosion tests, the vibration assisted weld joints were found to have better corrosion resistance than as-welded weld joint. The improved corrosion performance was likely attributable to suppression of intermetallic compound formation such as Mg2Si by mechanical vibration during weld solidification.

Performance and Characteristics of Stationary Arc and Rotating Arc-Gas Metal Arc Welded DMR 249 Naval Grade Steel Joints

Abstract In this investigation, 12-mm thickness naval grade high-strength low-alloy steel plates were welded by rotating arc-gas metal arc welding (RA-GMAW) and stationary arc-gas metal arc welding (SA-GMAW) processes. The main objective of this work was to carry out the comparative analysis of mechanical properties and microstructural characteristics of the joints fabricated by the outlined welding processes. From this experimental work, it is found that the ultimate tensile strength, yield strength, and impact toughness of the RA-GMAW joint are 15 % higher than those of the SA-GMAW joint. This is because of the formation of a higher volume percentage of acicular ferrite microstructure and island martensite/austenite constituent in the weld metal region of the RA-GMAW joint. Moreover, from the productivity point of view, the number of passes in the RA-GMAW process was minimized to three (from six passes), and the width of the heat-affected zone was also reduced by 45 % to SA-GMAW processes. Overall, the RA-GMAW joints are found to have superior mechanical properties and microstructural characteristics compared to SA-GMAW joints.

Effects of filler material selection on the microstructural, mechanical and corrosion properties of TIG welded AISI/SAE 304L stainless steel sheets and rings

Abstract Austenitic stainless steels are mainly preferred especially for resistance to aggressive oxidizing medias and high temperature applications such as equipments and mechanical parts which are used in defense and conventional industries. In this study; 3 mm thick 304L austenitic stainless steel sheets and rings are joined to each other by using Tungsten Inert Gas welding method under pure argon shielding gas with ER316L and ER2209 filler metals. Weld metals and heat-affected zones of welded joints were examined by metallurgical and scanning electron microscopes. Microhardness, tensile and Charpy impact tests of weld regions are investigated. It has been determined that the filler metals have dominantly changed the microstructure of weld metals. The microhardness values of the welded samples joined with ER2209 filler metal was lower than the sample joined with ER316L filler metal in weld metal regions. Besides, corrosion tests indicated that the corrosion rates of welded samples joined with ER2209 filler metal is lower than the samples joined with ER316L filler metal.

Deformation and damage assessment in type 316 LN stainless steel weld joint under isothermal and thermomechanical cyclic loading

Present investigation is aimed at understanding the deformation behavior and the development of damage in a type 316 LN austenitic stainless steel (SS) weld joint (WJ) under isothermal low cycle fatigue (IF) and thermomechanical fatigue (TMF). In-phase (IP) and out-of-phase (OP) TMF tests were carried out by maintaining a phasing relation of 0° and 180° respectively, between the mechanical strain and temperature cycles. Dynamic strain ageing (DSA) was found to exert a strong influence on the cyclic stress response (CSR) of the joint under both IF and TMF cycling. The CSR was observed to be higher under TMF compared to IF cycling due to the activation of additional hardening mechanisms in the former tests. The difference in fatigue life under TMF and IF is rationalized based on the development of deformation and damage through optical and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) studies. A clear demarcation of the strain distribution in the base metal (BM) and weld metal (WM) region of the joint is achieved through detailed EBSD analysis of the tested specimens. Cyclic plastic deformation and the associated development of damage take place independently in the BM and the WM parts of the joint, competing to cause the failure. The build-up of damage under IF and TMF cycling is dictated by a combination of DSA and δ-ferrite transformation, depending on the applied strain amplitude and the strain-temperature phasing employed. The observed life variations have been rationalized in terms of the substructural evolution and fracture behavior under different testing conditions.

Effect of failure modes on ballistic performance of Gas Metal Arc welded dissimilar armour steel joints

In this study, Ultra-high Hard Armor (UHA) steel is welded with Rolled Homogenous Armor (RHA) steels using Gas Metal Arc Welding (GMAW) process with Austenitic Stainless Steel (ASS), Duplex Stainless Steel (DSS), and Low Hydrogen Ferritic (LHF) fillers. Ballistic resistance against Weld Metal (WM) is determined using 7.62 × 39 mm BALL projectile. Three new modes of failure were identified from the ballistic testing (i) Finer Wear Debris with Cleavage (FWDC) (ii) Coarser Wear Debris with Cleavage (CWDC) (iii) Cleavage failure (C), based on the fractured surface. All joints stopped the ball projectile at the weld metal region without perforation. However, the joint fabricated using ASS electrodes shows superior ballistic performance than other joints with Cleavage mode of failure (C) mainly due to the absence of wear debris. ASS weldment (5% Delta ferrite and 95% austenite) performs better in ballistic resistance because of the plastic deformation along with energy absorption properties of the austenite phase than DSS (69 Delta ferrite and 31% austenite) and LHF (100% ferrite). Migration of Chromium (Cr) from the weld metal to base metal results in an unmixed zone and martensitic band in the HAZ of ASS joint.

The effects of post weld heat treatment for welded high-Mn austenitic steels using the submerged arc welding method

The high-Mn austenitic steels exhibit fully austenitic structure at room temperatures while showing twinning mechanisms during deformations. In the SAW welding process, the microstructure and mechanical properties of heat affected zone (HAZ) and weld metal (WM) are affected by the welding heat causing potential degradation of mechanical properties due to the segregation and depletion of Mn in HAZ and WM regions. In this study, post weld heat treatment (PWHT) was performed at 800 °C, 900 °C and 1000 °C to improve the mechanical properties of welded high-Mn austenitic steels. We examined electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) analysis to investigate the relationship between microstructure and mechanical properties after heat treatment. In particular, mechanisms of impact toughness improvement was identified by analyzing the microstructure and deformation behavior such as mechanical twin and grain size through the instrumented Charpy impact test and EBSD analysis.