Experimental Investigation of Welding Technology Effects on ASTM A516 Grade 70 Welding Joint Microstructure, Corrosion Resistance, and Mechanical Properties

Joint configurations were prepared by abrasive cutting HSLA steel plates with 5 mm thickness to specified dimensions (100 mm × 150 mm), as seen in Figure 1(a) and (b). Figure 2 shows the parent metal microstructure, which consists of ferrite and pearlite minor amount. ESAB Electrodes, Ltd. offered GMAW process filler wire and SMAW electrode. The electrodes utilized in this experiment were E8018 for Shielded Metal Arc Welding (SMAW) and ER70S-6, MIG electrodes designed to weld alloy steels using metal inert gas (MIG) welding technology. The chemical composition of the welding electrodes and the parent metal is shown in Tables 1 and 2.

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Figure 1. Welding joint configuration and dimensions (a) Width and length and (b) Width and thickness

Table 1. A516 grade G chemical composition wt. %

Table 2. ESAB E8018 SMAW electrode and ER70S-6 MIG electrode rode chemical composition wt. %

All the welding parameters and conditions that went into making the joints are shown in Table 3. Tensile test specimens were produced in accordance with ASTM E8M-04 standards. Electromechanical controlled universal testing equipment (UNITEK-94100) made by FIE Bluestar in India, with 100 kN tensile strength employed for this purpose.

The authors averaged the results from two specimens tested for each condition. Charpy impact specimens are used to assess weld metal impact toughness, and as a result, they are machined with a notch in the middle. Due to the plate's thinness, sub-size specimens were made. The impact tests were carried out at room temperature using a pendulum-type impact testing machine from Enkay, India. The absorbed energy quantity in the test was documented. The impact material toughness is defined as the absorbed energy. The hardness distribution across the welded joint and mid-thickness region was measured with Vickers' microhardness tester (Make: Shimadzu, Japan, Model: HMV-2T) under a 0.5 N force. Using emery papers of varying grades, the specimen was sectioned to the necessary size from the joint, which contained the weld metal, the HAZ, and the base metal areas, in preparation for metallographic analysis. Last but not least, a disc polishing machine was used to apply diamond compound with a 1 mm particle size. In order to expose the joints' microstructural characteristics, the specimens were etched using 2% Nital solution. An optical microscope (Make: MEJI, Japan; Model: MIL-7100) equipped with an imaging reagent used for microstructural analysis.

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Figure 2. Parent metal microstructure, optical micrographs, and SEM

Table 3. SMAW and GMAW parameters

The welding parameters chosen in the paper, from current and voltage to travel speed, electrode type, and heat input, were selected for easy and controlled comparison between Shielded Metal Arc Welding (SMAW) using E8018 electrodes, and Gas Metal Arc Welding (GMAW) using ER70S-6 wire, for making Grade 70 (ASTM A516) steel. Nevertheless, these conditions do not have a huge justification, as admitted by the authors, and require more elaborate interpretation to explain the rationale behind them and the implications for the comparative results. The process parameters are chosen carefully to provide a desired compromise among heat input, cooling rate, and metallurgical transformations. In comparison, a travel speed of 140 mm/min and a current of 125 A in the SMAW mode yield an input heat of 17.86 kJ/mm^-1, which will speed up slower cooling and coarse-grained microstructure formation. This simulates the thermal conditions of manual welding, where much less control can be exerted over the heat source. The low hydrogen content of E8018 electrodes reduces hydrogen-induced cracking, which is a problem with pressure vessel steels such as A516. Conversely, the GMAW process uses a higher current value of 240 A at a voltage of 30 V and a faster traveling speed (270 mm/min) and a CO₂ shielding gas, which results in a nominal heat input of 22.67 kJ/mm³ with better heat concentration and control. Continuous wire feeding utilizing ER70S-6 wire promotes penetration depth, minimizes droplet transfer, and creates a better microstructure. This can be attributed to the faster self-quenching effect of a more efficient gaseous shielding. Thus, parameter selection was not random, but, on the contrary, it was chosen to have a representative of the typical operative envelopes for each process, and keep the joint geometry equal. The forced conditions have a strong impact on the fusion zone (FZ) and HAZ in both modalities. The combination of higher arc energy and comparatively poor heat control in SMAW results in a broader HAZ of about 19mm and a course, marginally grained structure of ferrite and upper bainite, and martensite. These structures present a stress-hardened microstructure (hardness values of the order of 520HV at the weld center), which is related to the loss of toughness and corrosion resistance due to the presence of residual stresses and precipitation of carbides in such zones. In the GMAW configuration, the controlled arc and shielding gas show a narrower HAZ of about 14 mm and fine bainitic/martensitic microstructures. This results in reduced cooling time and oxygen pick-up, smaller dendrites, and an improved bead profile, which increases mechanical homogeneity and corrosion behavior.

Ultimately, results obtained from the experimental setup highlight process-driven heat flow and metallurgical transformations as the main source of the differences in performance. Thus, they confirm the correctness of the selected parameters that dictate the cooling behavior and the subsequent structure-property relationships in welded A516 Grade 70 joints.

4.1 Macrostructure observation

The weld center line (WCL), coarse-grain heat-affected zone (CGHAZ), and fine-grain heat-affected zone (FGHAZ) can be seen in the macrograph of SMAW and GMAW joints separately. Because of the high heat input and subsequent delayed cooling rate in the SMAW process, the resulting bead structure was coarser. Another factor contributing to this expansive FZ is the broader arc column (Figure 3(a)). In comparison to the SMAW method, the energy density of the GMAW process is much higher. This process's strong self-quenching rates unquestionably contribute to the rapid cooling rates. Figure 3(b) shows that this could be because of the small FZ. The manual control of arc and electrode in SMAW results in increased heat input that creates substantial HAZ modifications. GMAW maintains specific heat control throughout the weld process, which generates uniform microstructural distribution across the entire weld area.

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Figure 3. SMAW and GMAW weld cross-section macrographs

The continuous feeding mechanism of GMAW, together with shielding gas, produces smoother and more uniform beads than SMAW welds. Due to its ability to regulate the weld pool, GMAW produces higher-quality welds with uniform results than SMAW welds do because of their irregular bead shapes. The amount of spatter and porosity produced in SMAW welds through electrode flux exceeds that of GMAW, making it an advantageous option for precise welding requirements [22].

4.2 Optical microstructure observation

The steel alloy joint's welded region develops Fusion Zone (FZ), Heat Affected Zone (HAZ), and Base Metal (BM) microstructures when using the provided parameters. Multiple textural phases will form in each area due to the weld-specific thermal treatment of these regions. Figure 4(a), (b), and (c) show an A516 GMAW joint with a three-zone microstructure. Figure 4(a) shows the fusion zone combines base material elements with filler material parts from ER70S-6 until solidification occurs. The fusion zone stands as the fundamental area since it determines the properties and strength of the welded joint. Figure 4(a) reveals the fusion zone with various microscopic textures along with different phases, which result from multiple affecting factors such as cooling speeds and welding heat inputs, and alloy composition. A complete description follows regarding the microstructural elements found in fusion zones. Dendritic crystal formation can be observed in Figure 4(a), during which the solidification process is mostly present in the core area of the fusion zone. Dendrites manifest themselves through branching geological formations that often appear during metal solidification processes. Dendrite growth in aligned shapes with heat flow direction (columnar) and randomly oriented dendrites (equiaxed) can also be observed. This microstructure evidence that the heat dissipation rate controls dendrite formation patterns within fusion zone areas. Faster cooling rates (usually at the edges of the fusion zone near the heat-affected zone) lead to finer dendritic branches [23]. During solidification of fusion zones, a cooling pattern results in the creation of large, coarse dendrites that develop within its central region, as seen in Figure 4(a). Various grains appear throughout the fusion zone because of the uneven cooling process that occurs across its width.

Figure 4(a) reveals that no solidification crack appears in this zone. However, the fusion zone may exhibit solidification cracks, particularly when the weld pool cools with low ductility and high stress levels in its center area. The high thermal gradient produces micro-cracks oriented in the direction of solidification [24]. The micrographic picture shows ferrite as the primary phase in the fusion zone of low alloy steels when welded with ER70S-6 filler material, while the welding material cools down. The weld inclusions absorb ductility from ferrite, even though ferrite exists as a weak material individually. Upon the cooling rate becoming slower, the formation of austenite structures that adopt a face-centered cubic arrangement can be noticed. Austenite formation during welding becomes stable in the fusion area when ER70S-6 electrode compositions introduce manganese, silicon, and carbon elements. The solid part exhibits better ductility while providing enhanced strength properties to certain alloys, although it demonstrates embrittlement under fast cooling methods that generate martensitic formation. Martensite phase forms are noticed in specific regions of the fusion zone, where rapid cooling occurs around fusion edges. The rapid cooling or “quenching” of austenite leads to hard, brittle martensite phase formation. Microscopic observation reveals needle-like structures in the appearance of this material. The formation of martensite depends on the carbon concentration and the speed of cooling down the material. Alloy steels contain a martensitic phase, which delivers strong strength characteristics yet leads to diminished material toughness [25]. The region between austenitic and martensitic zones allows bainite to develop as a microstructural component. Bainite presents itself as a microstructure that emerges during intermediate cooling speeds, which delivers a beneficial blend of toughness and strength attributes. A microstructure comprising fine ferrite and cementite exists in this phase, with microscopy showing feathery or acicular features.

During the cooling of the molten weld pool, alloying elements carbon, manganese, and silicon tend to distribute unevenly. During welding solidification processes, the distribution of alloying elements through the material results in areas with distinct phase compositions as well as mechanical properties. Weld zones containing phase boundaries and minor structural variations appear because of molten weld pool solidification. Between large dendrites, dispersed liquid and solid phases at sites which will become fully solidified as the process completes. Interdendritic segregation zones develop because of the higher concentration of alloying elements that build up at interdendritic boundaries [26]. Figure 4(b) illustrates the coarse-grained heat-affected zone (CGHAZ) in the A516 joint. The temperature level in this area triggers specific metallurgical transformations because of intense thermal exposure. Cooled too quickly before transformation, the previous austenite grains have become either martensite or bainite and appear as bright regions. The large polygonal grains show evidence of coarsened martensitic structure, which developed from thermal peak exposure-caused grain growth. The CGHAZ heats beyond the upper critical (Ac₃) limit, thus causing the total transformation of Austenite. Grains grow into extensive sizes because both intense heat exposure and minimal presence of nucleation locations exist. Rapid welding cooling leads austenite to transform into martensite or bainite according to alloy composition and cooling speed. The dark parts in the structure normally form as etch-reactive products along either grain boundaries or within grain boundaries. Dark structures appear in the metal due to both high dislocation density and strong etch response of martensite laths/plates [27]. Ferrite in its Widmanstätten crystal structure or bainite structure forms when cooling occurs at a moderate pace. No carbides together with retained austenite appear as precipitates next to grain boundaries. The presence of C, Cr, Mo, Mn, or Ni in steels leads to carbide formation as M₂₃C₆, which appears as boundary-decorating particles and microscopically small precipitates. The combination of martensite within large grains generated material hardness in (CGHAZ) will cause brittleness, and the weldment displays its maximum tendency to develop cracks in this zone.

Results from Figure 4(b) show that austenitization occurs when the temperature reaches above Ac, and all ferrite and pearlite structures transform into austenite. The elimination of fine precipitates at temperatures above 1100–1200℃ starts abnormal grain growth. Complete homogenization of the metal does not take place because microsegregation still exists throughout the structure. Joints with thick sections created through SMAW typically cool at rates from 50 to 100℃/sec, yet failure to pass the martensite start temperature (Ms~400–500℃). Lath martensite develops inside coarse grains because of displacive transformation [28]. Enthalpy equilibrium at grain boundaries leads to carbide formation, particularly when welding materials include Cr, Mo, and V. Thick weldments retain heat that causes minor martensite tempering, which decreases hardness without affecting strength [29].

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Figure 4. SMAW with E8018 electrode A516 grade G welding joint (a) Fusion zone (FZ), (b) Coarse-grained HAZ (CGHAZ), and (c) Fine-grained HAZ (FGHAZ)

Figure 4(c) illustrates the FGHAZ microstructure. The FGHAZ receives heat exposure at temperatures slightly over the Ac₃ (~900–1050℃), resulting in total austenitization while the exposure time remains shorter than CGHAZ. The lower peak temperature effectively hinders grain enlargement. Better mechanical characteristics developed because the material retained its fine, small prior austenite grains. When cooled down, the refined austenite forms martensite together with bainite and ferrite, depending on cooling rate, combined with alloy content and nucleation site presence [30]. The changed ferritic or martensitic structure produces bright areas that create small, evenly shaped or slightly stretched grains with light markings due to their few dislocations. The likely phases are tempered martensite or lower bainite because they originate from fine-grained austenite. A polygonal formation of ferrite occurred because it formed through diffusion processes when the cooling rate permitted diffusional transformation. The austenite grains stay small because of reduced peak temperatures, together with accelerated cooling processes. The internal strain of fine martensite in CGHAZ remains lower than that of lath martensite and results in decreased etching. Ferrite crystal formation relies on diffusion-controlled transformation at a suitable cooling rate. The dark regions inside the material structure contain either martensite laths or bainite plates and fine carbides [31]. The material displays dark appearances due to the etching process that affects zones containing numerous dislocations and tiny carbide particles under stress or composition differences. The process of undercooled austenite yields lath martensite, which contains an elevated quantity of dislocations. The acicular ferrite plates forming bainite at the upper parts contain carbides that separate themselves along the laths. The formation of carbides such as M₂₃C₆ and Fe₃C, as well as intragranular carbides, occurs during the process of cooling the system [32]. The phase transformation sequence involved in forming FGHAZ begins with Austenitization, which occurs above Ac₃ (~900–950℃) It transforms ferrite/pearlite into austenite, but the brief waiting period minimizes grain size refinement of austenitic grains before rapid air-cooling triggers martensitic transformations. Rapid cooling through air leads to the transformation into martensite within refined grains.

The cooling rate, together with alloy content, determines whether upper bainite will form in specific areas. The material develops tempered martensite after transformation because of the remaining heat [33]. Grain boundary ferrite develops as a result of temperatures that exceed the Ms threshold during transformation. Carbides prefer to form at grain boundaries or within laths, in particular when Cr or Mo-rich alloys are present. The metallurgical Table 4 shows how the optical micrograph features relate to phase transformations that occur throughout each area of the SMAW-welded joint between A516 grade G materials made with E8018 electrode. Several microstructurally distinct areas form during fusion welding when different thermal gradients operate, resulting in special transformation processes and specific grain shapes accompanied by certain phases. These microstructural zones enable the exact interpretation of observed physical characteristics that relate to metallurgical importance because they clearly differentiate between the fusion zone and both the CGHAZ and the FGHAZ regions [34]. Structure–property relationships in welded joints become understandable through systematic mapping because the method enables improved mechanical performance optimization along with the prediction of potential failure zones, including brittle fracture and hydrogen-assisted cracking, and aids in the development of post-weld heat treatment strategies. The creation of this table represents an essential requirement for descriptive characterization as well as a predictive method for process improvements and structural reliability enhancement.

4.3 Fusion zone SEM analyses

4.3.1 SMAW with E8018 fusion zone SEM analyses

The study of welding fusion zone microstructures for examining mechanical behavior needs the use of scanning electron microscopy (SEM). Regular optical microscopy falls short of SEM in identifying bainite, martensite, ferrite, and retained austenite simultaneously at the same level. SEM serves three functions: revealing solidification patterns while investigating grain structures and scanning for flaws that include inclusions and micro-cracks. Microstructural information serves to evaluate weld performance for both inspection and post-weld heat treatment decision-making and quality control purposes.

Table 4. Microstructural features across weld zones

A noticeable coarse grain inside the E8018 fusion zone exists within the SMAW structure of A516. High welding heat temperatures lead to the direct formation of this structure. When the welding pool reaches its melting point and then cools rapidly, it results in A516 forming large, non-uniform column-shaped grains. Several different-sized grains can be observed in the fusion zone according to this presentation. SEM morphology demonstrates that both martensite and bainite exist within the examined area. Fast cooling of the fusion zone leads to the development of these phases. The quick solidification of molten metals does not allow carbon atoms to move, therefore creating hard but brittle martensite phases. Bainite emerges through temperature variations at different zones in material structures, and it demonstrates higher strength than martensite under specific temperature conditions. The microstructure of martensite shows black needles, while bainite contains light-colored plates. During solidification, the fusion zone reveals a complicated texture from the various-sized grains and phases that developed because cooling rates differed throughout the process. In Figure 5(a), elongated grains establish that the material adopted columnar growth patterns, which align with the heat flow direction while solidifying. This occurrence is normal within the fusion zone because cooling rates vary from place to place. The micrograph shows high-magnification martensite and bainite structures that formed because the cooling rate was sufficiently fast. Rapid cooling of the weld pool mainly determines its texture properties.

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Figure 5. Fusion zone SEM (a) SMAW with E8018 SEM and (b) GMAW with ER70S-6 SEM

The molten state temperature gradient drives solidification, which starts from the weld pool edges before reaching the center and produces coarse-grained structures together with martensite in specific areas and bainite in others, depending on local cooling rates. The E8018 electrode influences the microstructure formation through its selected alloying elements, which impact cooling processes during solidification. The electrode produces welds with both high strength and toughness, which enables the formation of martensitic and bainitic phases within the fusion zone. During Shielded Metal Arc Welding, the welder must pay close attention to the heat input since this factor significantly influences the welding outcome. The alloy structure becomes more extensive when the heat input runs high, but remains fine when the heat input stays low. Fast cooling conditions support the development of martensitic structures throughout specific parts of the fusion zone despite their known brittleness [35].

4.3.2 GMAW with ER70S-6 fusion zone SEM analyses

A fine needle-like microstructure of Upper Bainite appears in the GMAW fusion zone according to the SEM image shown in Figure 5(b). The Upper Bainite displays thin needle-like structures surrounded by ferrite layers containing retained austenite and martensite areas. Ferrite exists along the prior austenite grain boundaries with a white or lighter appearance. The dark areas found in the image could represent M/A islands because the E70S wire used in the welding process contains small alloy additives that support the retention of austenite during rapid cooling. The fusion zone exists as completely solid material with needle-like features showing evidence of columnar solidification grains, although they remain undetectable in this area. Acicular features aligned in a highly directed manner indicate the direction (perpendicular to the weld pool boundary) in which heat extraction occurred rapidly. The welded structure shows rapid phase transformation and solidification due to the high cooling rate found in GMAW processes, which results in acicular appearance and absent equiaxed grains.

Table 5. SMAW (E8018) and GMAW (E70S) fusion zone SEM microstructure characteristics comparison

The combination of GMAW with E70S wire typically runs at moderate to high currents to produce a penetration depth middle range while also creating fast drying times when welding in flat positions with minimal protection. The fast solidification rate promotes bainite formation instead of pearlite or ferrite-pearlite through the aid of alloying elements found in the filler material, particularly C, Mn, and Si. The E70S filler wires consist of electrodes containing carbon steel and copper with low hydrogen content, while also including Mn (1–1.5%) and Si (0.5–1%) components. The combination of manganese and silicon with steel increases its hardness, which transforms its structure from ferrite and pearlite to bainite and martensite/austenite (M/A). The addition of silica to steel aids in stabilizing the residual austenite between laths through carbide inhibition. The complete melting of the fusion zone results in δ-ferrite formation before the material transforms into austenite while it cools. Austenite transforms into bainite when fast cooling occurs, while the alloy is present. The portions of high manganese or carbon content create isolated spots where austenite or martensite remains untransformed [36]. A comprehensive Table 5 presents below an evaluation of SMAW (E8018) and GMAW (E70S) fusion zone microstructure characteristics in A516 grade G through the integration of SEM data and established metallurgical knowledge.

4.4 Microhardness profile analyses

A516 grade G joining methods, SMAW and GMAW, exhibit distinct characteristics regarding heat input and cooling rates, together with dissimilar electrode behaviors. SMAW (with E8018 electrode) yields high heat input from its reduced speed and restricted heat distributions through the weld zone. Because of the fast-cooling rate, the weld metal develops martensitic microstructures while achieving higher hardness near the weld center. Using SMAW produces a wide HAZ together with elevated residual stresses, which raises the risk of weld cracking. Because of its continuous wire supply and shielding gas system, GMAW produces heat at a lower rate than SMAW when generating welds. The GMAW weld tends to form ductile ferritic or bainitic microstructural components. GMAW enables the manufacturing of welds with reduced HAZ width while attaining similar hardness throughout the entire weld area. Figure 6 below illustrates the microhardness results of both welding techniques.

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Figure 6. SMAW and GMAW microhardness test results charts

According to this hardness profile chart (Figure 6), the two processes display divergent results. The hardness measurement of SMAW welds at the center shows increased values, which point to martensite formation resulting from fast cooling rates. The material exhibits such behavior within the fragile section of the weld zone. The welding procedure employing GMAW produces lower and more uniform hardness levels, which indicates that it results in a weld material that is less brittle and tougher at a slower cooling rate.

When welding occurs, the material achieves a full melting state while directly experiencing high welding process heat before solidifying after cooling down. The fusion zone generally displays enhanced hardness levels, which result primarily from martensitic formation, particularly in SMAW processes. A sharp peak identifies microhardness at its peak in the FZ hardness profile, which appears close to the weld's center in the presented chart. The depth of hardness attains its maximum value in SMAW steel because of its fast-cooling rate and resulting rigid microstructure formation. GMAW produces slightly less hardened steel at this location compared to both HAZ and BM, but the metallurgical changes lead to higher values than the base metals. The fusion zone in SMAW tends to have a slightly wider dimension than in SMAW because of its increased heat application. The weld bead size with GMAW remains narrow because of the shielding gas system. The Fusion Line acts as the border between melted fusion zone material and thermally changed heat-affected zone material. The materials found in this zone transition from being totally melted to partially melted, making it a vital operational area. The fusion line Hardness Profile shows an intense decrease in resistance values. The microhardness experiences a significant decrease within the area preceding the HAZ, where this location is situated. The weld center typically shows a thin and indistinct band, which constitutes this narrow area.

The HAZ extends next to the fusion zone, yet the metal remains solid because welding heat exposure has altered its structure. The HAZ high temperature modifies the metal microstructure, while the cooling speed determines its hardness level. The central part of the heat-affected zone marks the position where grain coarsening develops, thereby reducing material hardness while possibly increasing brittleness [37]. The HAZ Hardness Profile demonstrates a softening effect that extends outward from the fusion zone, and this behavior appears equally in SMAW and GMAW. The SMAW HAZ shows a greater hardness inconsistency compared to the GMAW HAZ because of its swift cooling speed and risk of forming martensitic microstructures. GMAW produces a HAZ with steady and mellow properties through its gradual cooling process. Table 6 below illustrates the maximum hardness values recorded for the three distinct welding joint zones.

Table 6. Three welding joint zones' maximum hardness values

The standard deviation (or frequently denoted as SD) in the above table is a statistical measure to quantify how much the data values differ from the arithmetic mean (or average). It is a measure of dispersion or variation that exists within a set of data. A small value of SD means that there are small deviations from the mean, which indicates high consistency of observations. On the other hand, a large SD indicates observations are widely spread in a larger range, indicating a large variability of the data. In applied situations, in which results are presented in tabular form as "mean+- SD," the SD indicates the average magnitude of deviation of individual measurements from the mean. The use of the standard deviation is indispensable in evaluating the degree of reliability and precision of experimental findings. It allows one to test whether observed differences, for example, between two welding techniques, are specific differences or simply due to random variability among samples. Formally, the value of SD is found in the following procedure: (1) find the arithmetic mean of the values of the observations; (2) find the differences between each observation and this mean; (3) square each of the deviations; (4) average the squared deviations; and (5) take the square root of this average value.

The Heat Affected Zone stretches up to multiple millimeters beyond the fusion interface. The size of the heat-affected zone in SMAW extends wider with a more pronounced decrease in material hardness, but GMAW produces a narrower and better-controlled HAZ zone. The Fusion Zone's position can normally be assessed to exist between 5 mm on each side of the weld centerline because the hardness evaluation showed a swift increase followed by a decline. The hardness measurements demonstrate that the SMAW heat-affected zone spreads up to 12 mm beyond the fusion line. The larger HAZ occurs with SMAW welding because the process generates higher heat that leads to sizeable material structure modifications. The GMAW HAZ maintains a small width measuring 7 mm next to the fusion line because of heat control and the gradual cooling characteristic. Table 7 below summarizes these zones' dimensions and characterizes them.

Table 7. Welding zones with approximately dimensions according to the microhardness results

4.5 Tensile and Charpy test results analysis

Figure 7 presents the tensile strengths of the base metal and the welded joints. The yield strength of the base metal was 446 MPa, while its tensile strength was 617 MPa. Nonetheless, the SMAW joint has a yield strength of 468 MPa and a tensile strength of 581 MPa, which is 5.2% inferior to that of the parent metal. The yield and tensile strengths of the GMAW joint are 491 MPa and 588 MPa, respectively, reflecting a 5.1% reduction from the parent metal's values. Joints fabricated using the GMAW technology exhibited superior strength compared to other types of welded connections. The elongation rate of the parent metal is 27%, but the GMAW joint exhibits a rate of 22%. Overall, the GMAW procedure appears to diminish ductility by 23%. In comparison to the parent metal, the SMAW joints exhibit an elongation percentage that is 33% lower, measuring at 17%. In the comparison of the two welded joint types, the ductility values of GMAW joints exceeded those of SMAW joints. Figure 8 illustrates the stress-displacement curves of the base metal and the welded joints. Table 8 presents the results of the Charpy impact toughness test. The parent metal exhibits an impact hardness of 81 J at ambient temperature. The weld metal's 72 J after GMAW welding is 36% inferior to that of the parent metal. Joints fabricated using the SMAW technique had the lowest impact strength (65 J) compared to those produced using the GMAW method (72 J).

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Figure 7. Base metal and welded joints: Load vs displacement curves

Table 8. Parent metal and welded joints' mechanical properties

4.6 Tensile test sample fracture SEM analyses

Examining fractures with SEM constitutes an essential method to understand the mechanical properties that emerge in welded joints. The method presents the step-by-step failure processes along with ductile-brittle examination, followed by microvoid coalescence and crack growth procedures. The features that stress SEM analysis are vital because they find fundamental aspects of residual stresses, together with phase transitions and material flaws that decrease welded joint strength and structural integrity. Examining broken materials enables improvement of welding parameters while advancing material production methods, which subsequently ensures dependable performance for critical uses. Figure 8 (a) and (b) below illustrate the tensile test samples' fracture surface SEM.

Figure 8 (a) demonstrates flexible breaking through a tiny hole unification on the surface of SMAW. Ductile fracture surface characterizes itself through its large, irregular voids combined with fibrous textures that appear due to enormous plastic deformation preceding failure. The fracture occurs within the HAZ of the material because the welding heat affected the surrounding material area. Stress residuals and hardness modifications in this region cause it to become weaker than the base material, thus becoming a vital structural point. Small holes within the SEM image represent a typical sign of ductile fracture due to material voids producing larger cracks, leading to material failure. The SEM image displays a textured appearance, which probably combines fibrous patterns with round-shaped voids that are frequently observed in ductile HAZ fracture locations. The plastic deformation process generates a special texture whose appearance signals stretching of the material before voids start forming and eventually grow until breaking happens. The surrounding material demonstrates signs of necking through the reduction of its cross-sectional area shortly before the crack reaches a complete length [38].

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Figure 8. Tensile test samples fracture morphology (A) SMAW sample, (B) GMAW sample

Several variables related to welding procedures generate micro-cracks inside the HAZ of SMAW fractures, as seen in Figure 8 (a). Material stress responses together with welding-induced structural modifications become visible through the formation of these vital indicators known as cracks. Material structure develops different types of cracks during welding when it experiences fast heating and cooling conditions. The welding process creates residual stresses that focus on the HAZ region alongside the weld bead because of thermal expansion and contraction. High thermal gradients lead to material shrinkage together with thermal expansion that produces micro-cracking at HAZ locations, primarily because this material section does not efficiently handle these stresses. Due to the welding heat application, the HAZ grain material structure changes substantially. Differentially treated A516 alloy steel may undergo martensitic transformation in the heat-affected zone and produce brittle phase zones. The brittle phases in this material become easier targets for tensile stress, which ultimately leads to multiple micro-cracks. The welding method with SMAW allows hydrogen materials to enter both the welding metal and the heat-affected zone material. The process of hydrogen embrittlement causes micro-cracks to occur within regions experiencing high levels of stress. The HAZ represents a zone where rapid cooling and microstructural modifications cause hydrogen retention that decreases Micro-cracks in Figure 8 (a) present through the SEM images display short and pointed discontinuities inside voids or along the borders between grains. Fractures normally originate from particular vulnerable areas, including martensite phases together with brittle substances and voids or sulfide and oxide particles. The SMAW fracture surface extends either through the grains as trans granular fractures or along the grain boundaries as intergranular cracks. The crack propagation occurs either within material grains through trans granular paths or it spreads along the intergranular spaces between grains. Intergranular cracks are significant indicators of material imperfection at grain boundaries because welding stresses or HAZ changes can cause this weakness between grains. Plastic deformation leads to micro-cracks and micro voids joining together. The combination of cracks spreads toward each other, creating greater fractured areas. The material shows small crack formation at initial weakening points that may lead to the development of the bigger voids discovered in tests of the SMAW sample. Sheared areas near material cracks increase overall damage to the point of material failure [39].

The morphology of the GMAW fracture in Figure 8 (b) exhibits slight distinctions compared to the other welds. The HAZ is the location where this fracture forms in both welding processes, yet the GMAW results in more uniform fractures than SMAW. Morphology in Figure 8 (b) shows smaller voids and a more uniform fracture surface, likely indicating less plastic deformation. The material showed reduced plastic behavior at this location, possibly because it had lower strength in the specific area. Due to a shift between ductile and brittle material behavior within this condition, smaller, distinctive void shapes appear in the fracture surface. The observed change in material strength possibly arises from excessive residual stress, together with pronounced welding procedure effects or unfavorable HAZ grain arrangement. The fracture from GMAW welding shows smaller grains than the SMAW weld. GMAW enables controlled heat input to the material, which potentially creates alterations in the material structure. The fracture surface of the GMAW sample features smaller void characteristics with uniform distribution, which indicates reduced localized strain before breakage occurs. The specific hardness, together with the microstructure of the HAZ area, will determine how many brittle fracture characteristics are present in the material. The fracture occurs through a trans granular fracture that runs along the grain boundaries.

From the macroscopic examination and scanning-electron-microscope (SEM) fractographic analysis of tensile test specimens, material failure basically occurred in the HAZ of both welding configurations. The fracture surface of the SMAW joints was found to consist of coarse facets illustrating cleavage cracking and irregular cracking, characterizing intergranular cracking, which is consistent with the appearance of the martensitic microconstituents and high residual stresses. This brittle fracture mode is in agreement with the low elongations and low impact toughness that are measured. In contrast, GMAW specimens showed more ductile fracture behavior with many fine deep dimples, which suggests a superior plastic deformation before failure. Such morphology is incorporated with an additional homogenous bainitic microstructure feature and achieves a balanced toughness, which adds support to the realizations of concurrently improved elongation and impact energy. Experimental results show that the GMAW welds exhibit a slower and prolonged strain-hardening phase beyond the yield point due to gradual growth of dislocation density and following interaction with similar fine bainitic microstructure (i.e., fine bainite microconstituent), and the progression and this hardening mechanism improves the ductility and toughness. In contrast, SMAW welded joints have a distinct yield point with fast strain hardening saturation, as a result of a dominant martensitic microstructure, which inhibits dislocations early in plastic deformation, reducing ductility and causing earlier necking. The microstructural and mechanical mechanisms of GMAW are superior, indicating the outstanding fabrication of welds with higher toughness and energy absorption capabilities, which are critical for applications where impact resistance and fatigue durability are required. Furthermore, the strain-hardening behavior of the GMAW fusion zone supports the greater stability and homogeneity of the GMAW fusion zone compared to the SMAW.