1 Introduction

Welding processes play a fundamental role in the manufacturing industry, significantly contributing to the fabrication of a wide range of products across diverse sectors. These sectors include automotive, aerospace, construction, shipbuilding, consumer goods, industrial machinery, infrastructure, and various applications, spanning from small-scale components to large structures [1, 2].

Welding is a primary technique for permanently join materials by applying an appropriate amount of energy. Its primary objective is to create strong and durable connections between metal parts, often achieving a continuity of material at the microstructural level. This capability enables the fabrication of assemblies that require structural integrity. Furthermore, welded structures can be engineered to withstand different environmental conditions, making them suitable for use in a variety of settings [3, 4]. This adaptability is particularly crucial in industries where products are exposed to fluctuating temperatures, pressures and corrosive elements [5].

Ongoing research and innovation in welding techniques contribute to the development of new materials and processes. This innovation propels advancements in manufacturing technologies, resulting in improved product quality and performance. While welding is primarily used for connecting metallic materials, the numerous process variations also allow for the welding of polymeric, ceramic and composite materials [6,7,8,9,10,11,12].

This study specifically focuses on a particular arc welding variant, namely the submerged arc welding process (SAW) [8]. SAW is commonly employed for joining large-sized components and thick material sections, particularly in industries such as shipbuilding, oil and gas and structural fabrication. It finds application in various contexts, including the construction of tube and pipeline joints, pressure vessels, nuclear containers, wind tower structures and the fabrication of oil, water or liquefied natural gas tanks [13,14,15]. SAW offers several key advantages, including (i) the high deposition rate and productivity; (ii) deep penetration, made possible by the high current intensities that can be utilized; (iii) the capability to weld thick and large pieces in a single pass and (iv) the ability to achieve high-quality welds [16, 17].

To perform the SAW process, it is essential to establish an arc between a continuously fed electrode (a wire) and the workpiece. The arc’s parameters must be set to reach the melting temperature of both the base material and the filler material, allowing the creation of the weld bead. Temperature control plays a fundamental role in the joint formation, as a thermal cycle occurs during the welding process. This cycle induces changes in the metallurgical structure, affecting the material’s microstructure and altering its physical and mechanical properties in the final joint [13].

The uniqueness of SAW lies in submerging the arc beneath a layer of granular flux, stored in a hopper. This granular flux acts as a protective coating over the molten pool, shielding it from the surrounding atmosphere. Moreover, it aids in reducing splatter and fumes, ultimately improving working conditions compared to other arc welding variants. It is important that the flux has a lower melting temperature and density than the base metal. During the process, a portion of the flux melts and solidifies over the weld bead, while the remaining granular flux is removed through an aspiration system at the end of each pass and then can be reused. The operation of the electric motor is controlled by a unit that ensures a continuous delivery of the wire from the coil through feeding rolls. This controlled wire speed ensures that the electrode wire’s feed rate matches its melting rate, resulting in a stable and controlled arc [16, 17]. The operating principle of the SAW is depicted in Fig. 1.

Fig. 1
figure 1

Operating principle of the SAW [8]

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Due to the inherent nature of the welding process, the materials involved are subjected to thermal cycles. A high specific heat input results in elevated temperatures and reduced cooling rates, which in turn promote the growth of crystalline grains. The cooling rate is also influenced by the thickness of the plates. In this context, thermal energy is primarily dissipated through conduction across the thickness of the two welded pieces, while the portion dissipated to the external environment through radiation and convection is negligible. The power losses to the external environment are further reduced due to the insulating behaviour of the granular flux which shields the molten pool during the welding process. Therefore, for the same specific heat input, the greater the thickness of the joint components, the faster their cooling occurs, leading to a more severe thermal cycle.

In this study, the focus was on a specific joint configuration, and on the analysis of the heat, dissipation within the material at the microscopic level was conducted. This analysis involved observing the heat-affected zone (HAZ), which represents the portion of the base material adjacent to the melted zone undergoing solid-state transformations. These transformations cause microstructural variations compared to the original base material. The grain morphology and the extent of the HAZ primarily depend on the base material used, especially regarding its chemical composition and its initial metallurgical structure, which is influenced by the manufacturing process adopted for sheet metal production, and on the welding process conditions. These latter determine the thermal cycle of the material; in fact, the main parameters which play a crucial role in these terms are the welding current, the arc voltage and the travel speed. Predicting the evolution of the microstructure within the HAZ can be challenging, especially in the case of multi-pass welding, as subsequent passes generate heating effects on the underlying microstructure obtained from previous passes [18].

Saleem and Lakshmanaswamy [19] focused on optimizing input setting parameters to enhance weld bead characteristics. They developed a mathematical model, and their experimental results closely matched the predicted values. Küçüköner et al. [20] used SAW to join two types of steel and assessed the quality and mechanical properties of the welded samples. Microstructure examinations revealed dendritic formation in the welded metal, with larger grains near the melting boundary on the primary material side.

Renwick [21] evaluated flux composition and its impact on the weld bead. Sharma and Chhibber [13] designed various submerged arc welding flux compositions using an extreme vertices design approach and studied their effects on the weld bead and mechanical properties. The flux, in combination with welding parameters, significantly influences arc stability and the final joint. Datta et al. [22] developed mathematical models to understand the influence of process parameters on the final geometry of the weld bead. Prasad and Dwivedi [23] investigated the effect on the microstructure and on the heat-affected zone. Rathi [24] highlighted that electrode diameter, trolley speed and wire feed rate are key factors for controlling weld bead dimensions. Vishwakarma and Dwivedi [25] applied the Taguchi method to optimize five welding parameters to achieve minimal hardness and maximum tensile strength.

The literature review underscores that the dimensions of the weld bead and all mechanical properties of the welded joint are significantly affected by several parameters of the welding process investigated. Understanding how to control the process is crucial for ensuring the mechanical performance of the welding joint and meeting quality standards required by the normative of the industrial sector considered as well as by the customers.

In this study, starting from an industrial benchmark configuration, a novel approach was investigated. Tests were performed by setting different combinations of process parameters, taking into account the voltage and current ranges imposed by the machine and the fixed travel speed values, but keeping constant the heat input, in order to optimize the process productivity and to preserve, at the same time, the mechanical properties of the specific joint configuration.

2 Materials and methods

2.1 Materials

ASTM A516 steel, also known as ASME SA516, was employed for the experimental tests of this work. It is a carbon-manganese steel, representing the standard specification for the fabrication of pressure vessels [26]. This material is typically chosen for applications with moderate to low operating temperatures due to its excellent notch toughness properties [27].

The ASTM A516 steel plates used in the experiments were obtained through the hot-rolling process. Specifically, the dimensions of the plates involved in the experimental tests were 100 mm in width, 400 mm in length and thicknesses of 10 mm and 25 mm.

ASTM A516 steel comes in different grades (55, 60, 65, and 70), each offering varying levels of mechanical strength. Grade 70 is the one used to the aim of this work. It has higher yield strength and tensile strength values than other grades, providing higher tensile strength at low temperatures. These characteristics make it suitable for products operating under high-pressure conditions. Key mechanical properties of the material are summarized in Table 1.

Table 1 Main mechanical properties of ASTM A516 Grade 70
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Table 2 shows the chemical composition of the weld material. The main alloying elements in ASTM A516 Gr. 70 steel are carbon (C), manganese (Mn) and silicon (Si). The amount of these elements affects the mechanical properties of the material and its microstructure in a different manner. Indeed, C enhances heat treatment hardening and wear resistance in steels, but if it is present in excess, it results in the formation of martensite or bainite. These are undesirable structural constituents because they can lead to hydrogen cracking. Mn makes possible to increase the steel’s toughness and contrasts the damaging effects of sulfur, but it causes a deterioration of the material’s resilience property. In addition, from the microstructural point of view, it was found that the presence of Mn (1.37 wt. %) in ASTM A516 Gr. 70 steel promoted the acicular ferrite formation in the joint molten zone and reduced the grain size of polygonal ferrite in heat-affected regions in multi-pass welds [27]. Finally, the presence of Si increases the strength and hardenability of steels and acts as a deoxidizer; however, it negatively affects the toughness and weldability of the material.

Table 2 Chemical composition of the base material (ASTM A516 Gr.70)
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In addition to the mentioned elements, ASTM A516 carbon steel also contains other alloying elements, including molybdenum, chromium, and nickel, albeit in smaller quantities. These elements enhance the hardenability of the material, with nickel contributing to increased toughness. It is worth noting that an excess of molybdenum can lead to the formation of phases that can reduce notch toughness at low temperatures.

As for the filler material for the welded joint, a low-carbon steel coded as AWS 5.17 of EH14 grade was used.

This high-Mn, copper-coated solid wire is typically employed in single-pass and multi-pass SAW procedures.

The chemical composition of the electrode used is provided in Table 3 while Table 4 lists its key mechanical properties.

Table 3 Chemical composition of AWS 5.17 EH 14
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Table 4 Main mechanical properties of AWS 5.17 EH14
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The filler wire was selected to achieve a chemical composition and mechanical strength of the joint metal similar to that of the base material. Two different filler wire diameters, specifically 2.5 mm and 4 mm, were used during welding tests, based on the pass type.

Finally, the covering flux used for the experiments was the ESAB OK 10.61, a highly basic agglomerated flux commonly used for SAW. Its nominal density is 1.1 kg/dm3 and nominal basicity index is 2.6.

2.2 Methods

As mentioned previously, the SAW process is a widely utilized electric arc welding method in heavy industries for joining medium and thick metals. It relies on an electric current passing between the filler wire and the workpiece to generate the thermal energy needed for material melting. Due to its operational characteristics and the equipment involved, the SAW can only be conducted in fully automatic or semi-automatic mode. In this study, the semi-automatic mode was employed.

Tests were conducted using the LINCOLN ELECTRIC’s Place Power Wave® AC/DC 1000 SD CE machine, designed and suitable only for SAW. The machine carries an output rating of 1000 Amps and 44 Volts at 100% duty cycle.

The plates were butt-welded in a flat position, and a total of five passes were performed. Figure 2 illustrates the procedure used for creating the joints. The test execution method involved initially coupling plates of varying thicknesses to create the first root weld pass (referred to as pass 1 in Fig. 2) at room temperature. A 2.5-mm-diameter electrode was used for this purpose, providing support for the plates during the subsequent weld bead formation.

Fig. 2
figure 2

Schematization of the numbering of the passes sequence during the experimental tests

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A milling operation was then conducted on the opposite side of the previously welded area to create a groove, as indicated in Fig. 2. This groove was then filled by performing four additional passes along the milled channel, using a 4-mm diameter filler wire. It is important to note that no plate preheating or post-weld heat treatment was performed. The process parameters set for each pass are the arc voltage (V), the welding current (I) and the travel speed (v). The selected values for these parameters are provided in the next paragraph. It is worth highlighting that the machine ranges (specifically the maximum values) in terms of current and voltage have been considered, while the travel speed could be set only considering preset values (i.e. 320, 384, 448, 512, 544, 576, 608, 672, 704, 768 mm/min).

These parameters play a crucial role in the welding process and determines the amount of heat input introduced in the workpiece. Higher current values result in increased arc intensity and penetration depth, leading to a larger resistant section of the joint under external loads. The travel speed, on the other hand, is inversely proportional to the thermal input which the material is exposed to. Both too low and too high travel speeds can result in significant defects within the weld bead.

The influence of current, voltage and travel speed on the weld bead and on the microstructure has been assessed. To this aim, the transformation phases that occur usually in steels during welding process have been analysed. These phases depend on the specific heat input, which is a key process variable encompassing all three individual investigated parameters [16], as well as on the cooling rate of the workpiece. The specific heat input is calculated using the following equation:

$$Q [J/\text{mm}]= \frac{\eta *V*I*60}{ws}$$
(1)

where η represents the arc efficiency factor, and in case of the SAW, it is equal approximately to 0.95 [28], V, I and ws are, as already specified, the arc voltage, the welding current and the welding speed, respectively.

The dimensions of the molten and the heat-affected zone, as well as of their grain size, depend on the amount of heat (Q) transferred to the material during the welding. Besides, the transient thermal cycle plays a crucial role in controlling the material’s cooling rate, which, in turn, is a key factor in determining phase changes and the formation of microstructural constituents.

3 Experimental investigation

3.1 The experimental plan

For the experimental analysis, specific working conditions were established. As previously mentioned, a constant number of passes for each sample was considered, and it was set at five owing to the depth of the welding gap. Table 5 outlines the designed experimental plan and the values assigned to voltage, current and welding speed. It is essential to note that test 0 represents the working condition necessary to achieve a sound joint in accordance with UNI EN ISO 3834 quality standards; therefore, it was considered as a benchmark. Consequently, the experimental plan was constructed by maintaining a constant specific heat input for each pass, based on test 0. The first pass was consistent across all tests, as it was performed on the bottom side of the plates to create a foundational layer that supports subsequent welding steps. The second passes (labelled pass 2.1 and pass 2.2) had varying parameters in each test, but the nominal specific heat input for all of them was approximately 2145 J/mm. The third passes (labelled pass 3.1 and pass 3.2) were conducted by employing the same process parameters in all tests, aligning with the specific heat input of test 0, which was approximately 1790 J/mm.

Table 5 Standard condition and experimental plan
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3.2 The experimental procedure

As mentioned, the experimental investigation aimed to assess how process parameters affect joint quality and process efficiency by reducing the time required for welding. To evaluate the process accuracy, a metallurgical analysis of the final joints was conducted, focusing on the thermal effects caused by the process parameters. Specifically, the HAZ, the microstructure and the mechanical properties of the joints have been examined and discussed below.

It should be pointed out that geometric effects and the resulting induced potential stresses were disregarded in the following discussion since they were assumed unchanged in the several tests. Indeed, it was possible to assume identical distortions for all the tests due to the method of performing the experimental trials. In fact, in each test the sequence and number of weld passes remained unchanged as well as the mechanical constraint conditions, interpass temperatures and the overall heat input value for each welding pass.

3.2.1 Sample preparation

To conduct the required studies, samples have been extracted from the joined plates by cutting them at low temperatures. This was done to avoid the alteration of the microstructure already affected by the welding process. From each weld bead, three samples have been extracted to ensure result repeatability and account for the non-uniformity of the weld bead along the entire joined plates.

Before conducting macro and micro analysis, all specimens underwent a specific preparation process to eliminate cutting-induced scratches. First, the samples were cold-mounted using acrylic resin to prevent thermal alterations. The drying time for this process was approximately 45 min. Once the samples were ready, they were secured in the holder of the Struers Tegramin-25 machine (as shown in Fig. 3a), and their surfaces were prepared for metallographic inspection. The preparation involved the use of abrasive papers with different granularities to remove residual resin and achieve a flat surface. Subsequently, specific cloths were used for mirror polishing (Fig. 3b). The samples were prepared by chemically treating their polished surfaces with a 3% nitric acid-based solution, named Nital, for 25 s. This treatment made the macrostructure of the weld beads clearly visible to the naked eye (Fig. 3c), allowing each pass to be distinctly recognized, as well as under the microscope (Fig. 3d).

Fig. 3
figure 3

a Mounted samples, b polished samples, c macrograph of the weld bead, d micrograph of the HAZ

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It is important to note that the immersion time in the solution was determined through experimental observations. Several tests were conducted to establish the right immersion time required to reveal the joint microstructure. Low time (underetching) was insufficient for visualizing the grains, while prolonged immersion (overetching) resulted in a surface burning effect from a microstructure perspective.

3.2.2 Metallurgical observations

Metallographic investigations were conducted using the LEICA DM 4000M microscope. The applied method, as outlined in Fig. 4, enabled the analysis of the heat-affected zone (HAZ). For this purpose, at least three pictures were captured close to each welding pass to provide a comprehensive description of the entire joint microstructure.

Fig. 4
figure 4

Metallographic investigation of the welded joints: a identification of the HAZ produced by pass 2.1, b micrograph at 5x of the HAZ

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To determine the amount of microstructural constituents at room temperature, specifically ferrite and pearlite, micrographs were analysed using a specific image processing technique that allowed distinguishing and estimating the magnitudes of those two components.

The micrograph displayed in Fig. 4b shows the distinction between base material (not affected by heat) and region 1 and region 2, which belong to the HAZ and are characterized by differences in microstructure, morphology and grain size. More in detail, region 1 refers to the part of the HAZ bordering the base material, while region 2 indicates the area adjacent to the weld bead.

3.2.3 Hardness measurement

Vickers microhardness measurements were performed in order to assess the mechanical strength of the welds obtained. The mechanical characterization of the weldments was executed using the Micro Combi Tester hardness machine (CSM Instruments SA, Switzerland) on mounted, polished and chemically etched specimens. The test load was increased linearly to a maximum value of 3 N at a loading rate of 6 N/min for a time of 15 s. Microhardness measurements were performed focusing on welding passes 2.1, which, as described later, are of great interest in assessing the effects of different process parameters on welded joint quality. Figure 5 provides a schematic pattern of the indentation grid for the microhardness measurement relative to welding pass 2.1 of test specimen T8. Specifically, indentations were performed along four lines orthogonal to the weld bead running through pass 2.1 in order to assess hardness values in the welded zone, in the HAZ regions (i.e. region 1 and region 2), as well as in the base material. For this purpose, a 50-micron distance was set between two consecutive measurements.

Fig. 5
figure 5

Representation of the indentation grid for hardness measurement of welded joints for pass 2.1 relative to T8 specimen

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4 Discussion of the results

The welded joints were observed through an optical microscope in order to investigate their microstructure and welding-induced phase transformations. Since the welding passes were performed with the same heat input value for passes 2.1 and 2.2 in which the process parameters were changed, the thermal cycle experienced by the material was the same for each test. This resulted in a similar microstructure for each test specimen. Figure 6 shows the distinctive microstructures of the base material (a), HAZ region 1 (b), and HAZ region 2 (c), at 50x magnification, respectively.

Fig. 6
figure 6

Microstructure of the a base material, b HAZ region 1, and c HAZ region 2 at 50x magnification. P pearlite, F ferrite, PF pro-eutectoid ferrite, WF Widmanstatten ferrite

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The base material is a hypoeutectoid C-Mn steel (according to ASTM A516 Gr. 70 specifications) with a carbon content of 0.31% by weight. Its microstructure (Fig. 6a) is composed of coarse ferrite grains with an equiaxial morphology, i.e. approximately the same dimension in all directions, and thin pearlite bands aligned in the rolling direction. The average ferritic grain area in the base material is approximately 183 µm2. This value was obtained as the average of several measurements taken using an image processing software on micrographs with 50x magnification.

Region 1 of the HAZ is the region of the parent material that reached such a temperature during welding that resulted in recrystallization, i.e. a grain refinement. Indeed, the microstructure in region 1 presents equiaxial ferrite grains with a considerably lower average size than those in the base material (Fig. 6b). Specifically, the average grain area in region 1 is approximately 41 µm2. Furthermore, the perlite bands were clearly distorted.

Region 2 of the HAZ reached a higher maximum temperature during the welding thermal cycle and therefore experienced different phase transformations, as well as a slower cooling rate than region 1. This resulted in a different microstructure in both grain size and morphology (Fig. 6c). Specifically, the distinctive microstructure of region 2 consists of pro-eutectoid ferrite and Widmanstatten ferrite. Following the complete austenitization of the grains at high temperatures, pro-eutectoid ferrite with a polygonal shape initially formed from the austenitic grain boundaries during the cooling of the steel. Phase transformation from austenite to pro-eutectoid ferrite occurred until the eutectoid temperature was reached. Below this temperature, a second phase transformation occurred, i.e. the austenite that was not transformed into ferrite is transformed into ferrite plates separated by thin cementite regions starting from the austenite grain boundaries. This grain morphology is referred to as Widmanstatten ferrite.

Since region 2 of the HAZ reached a higher maximum temperature and was characterized by a lower cooling rate than region 1, grain growth occurred in this area. As can be observed from the comparison of Fig. 6b, c, the grain size is significantly larger in region 2 even though the grain morphology is different. Due to the evidence of this phenomenon and the significant difference between the microstructures of regions 1 and 2, a grain size comparison between these two HAZ regions was not feasible.

Being ferrite very low in carbon content, its high level within the investigated region can mainly be explained by two physical phenomena occurring during the steel welding. The first one is the decarbonisation, i.e. the loss of C from the base material due to its reaction with the oxygen in the air, while the second one is related to the migration of C atoms from a higher to a lower content area. In fact, the material of the HAZ is the ASTM A516 Gr.70 steel, and it has a C content of 0.31% by weight, which is higher than that in the melted zone. Indeed, the latter is a mixture of base and filler metal, with a carbon content of about 0.10%, as already specified in Table 3.

To examine the effects of the different working parameters on the microstructural characteristics of welded plates, the focus of the study was placed on the HAZ. Both regions constituting the HAZ were analysed in order to estimate the ferrite content, which could be representative of the weakest portion of the joint.

An image processing software was employed to this aim. Micrographs at 50x magnification (Fig. 7a–c) were converted into black-and-white images (Fig. 7b–d). Ferrite was clearly identified because of the white colouring, while the pearlite was dark. Therefore, the percentage of ferrite was determined by calculating the ratio of white pixels to the total ones. It is important to note that the method of calculating these values relates to ferrite as a phase, thus, to the amount of ferrite in both the pro-eutectoid and Widmanstatten ferrite structural constituent.

Fig. 7
figure 7

a Region 1 micrograph at 50x and b its ferritic (69%) and pearlitic (31%) content; c region 2 micrograph at 50x and d its ferritic (75%) and pearlitic content (25%)

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Quantitative analyses were conducted to determine the average extension of the overall HAZ and the size of regions 1 and 2 for each welding pass. In Fig. 8a, for instance, pass 2.1 is indicated by a red arrow, denoting the length of the entire HAZ produced at the end of the process, whereas Fig. 8b shows a 5x magnification micrograph of the HAZ produced by pass 2.1 for the same specimen. On this latter, the white arrows indicate the extension of region 1 and region 2 resulting from the welding. To determine the lengths of both the HAZ and the two regions, multiple equally spaced lengths were measured, considering potential variations along the edge of each welding pass. For each pass, the lengths were obtained by means of segments orthogonal to the weld boundary line. Ten length measurements were taken for each welding pass to ensure consistency and repeatability of results.

Fig. 8
figure 8

a Example of an analysed macrograph and indication of the passes and of the HAZ extension, b 5x micrograph and indication of region 1 and region 2 length

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Additionally, the percentage of ferrite within region 1 and region 2 was estimated following the above-mentioned procedure. Data collected from the regions under investigation were then analysed by comparing the different working conditions set in the experimental study (Table 5). First of all, the data related to pass 2.1 were considered, since, looking for instance at Fig. 8a, pass 2.2 was covered by pass 3.1 and pass 3.2 making an assessment of the corresponding outputs impossible. Just for test 2, test 5 and test 7, it was possible to compare the effect caused by pass 2.2, as it was not covered by subsequent pass 3.

Histograms in Fig. 9a display the extension of the HAZ produced by pass 2.1 for each considered test, while Fig. 9b reports the extension of region 2. From the histogram observation, it is easy to deduce that region 1 area is wider than that of the region 2. Figure 9c and d show, respectively, the percentage of ferrite within region 1 and region 2.

Fig. 9
figure 9

a Average HAZ extension resulting from pass 2.1, b average extension of region 2 resulting from pass 2.1, c ferrite content percentage in region 1, d ferrite content percentage in region 2

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The data analysis related to pass 2.1 revealed that both the average size of the whole HAZ (Fig. 9a) and the percentage of ferrite within both regions remained relatively consistent for all the investigated process conditions, and therefore in all the tests. In fact, the corresponding values were uniformly distributed as the process conditions changed. Specifically, the average length of the HAZ ranged from approximately 1.84 mm in test 8 to 2.93 in test 7, with an average value of 2.33 mm. Whereas the ferrite percentage in region 1 ranged from 64% in test 5 to 73% in test 3 with an average value of 69.22%, in region 2, it ranged from a minimum of 64% in test 5 to a maximum of 77% in tests 0 and 3, with an average value of 73.66%.

Considering the number of tests investigated and the outcomes data distribution under different working conditions, no correlation was identified with the process parameters employed for performing the joint.

As previously mentioned, test 2, test 5 and test 7 were compared for evaluating the effect caused by pass 2.2. The comparison, both from a qualitative and a quantitative point of view, is reported in Fig. 10, where the macrographs and the measurement of the average HAZ extension and the ferrite content just in region 2, for sake of simplicity, are depicted. The analysis revealed higher values for both the HAZ extension and ferrite percentage related to pass 2.2 for all the tests, likely due to the different thicknesses of the welded plates. pass 2.2, in fact, was consistently performed on the 10-mm-thick side, resulting in a lower heat dissipation rate.

Fig. 10
figure 10

Macrographs of welding test a T2, b T5, c T7; d average extension of the HAZ produced by pass 2.1 and 2.2, and e percentage of ferrite contained in region 2 related to pass 2.1 and 2.2

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In addition to that, it is worth pointing out that the difference in the cooling rate of the welded plates, resulting from variations in plate thickness, also had an impact on the microstructure. Specifically, in region 2, thicker Widmanstatten ferrite was observed on the thinner plate side (10-mm thickness). For instance, in test 7, a comparison of micrographs at 50x magnification revealed the characteristic microstructure of region 2 produced by pass 2.2 (Fig. 11a) and pass 2.1 (Fig. 11b).

Fig. 11
figure 11

Cooling rate effect on the Widmanstatten ferrite. Test 7, a pass 2.2 side 10 mm and b pass 2.1 side 25 mm

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The metallographic investigation highlighted that the percentage of ferrite in the two regions remained fairly consistent when modifying the main welding parameters while maintaining a constant specific heat input.

To further verify the influence of the process variables on the mechanical properties of the final joints, numerous microhardness measurements were performed across the sample, following the procedure illustrated in paragraph 3.2. To this aim, test 0, which represents the benchmark since it satisfies the required standard for the specific application field, was compared to test 8, which was performed by setting the highest value of the welding speed, maintaining always constant the specific heat input. The hardness profile aimed at showing how the hardness varied across the identified zones, which were the welding zone, region 1, region 2 and base material, for the two above-mentioned tests. As clearly seen from the histogram displayed below (Fig. 12), in both tests, the hardness values are comparable, demonstrating that the increase in the welding speed, but guaranteeing a specific heat input, does not affect the mechanical response of the joint.

Fig. 12
figure 12

Hardness profile of test 0 (benchmark) and test 8 (faster process condition)

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Following this observation, the efficiency increase, resulting from a higher welding speed, was quantified. More in detail, in test 0, passes 2 had a welding speed of 320 mm/min and passes 3 of 544 mm/min, while in test 8, passes 2 had a welding speed of 576 mm/min and passes 3 had a speed of 768 mm/min. The time necessary to perform pass 1 was not included in the calculation, as the same parameters were used for all tests, as well as the time between pass 1 and the next ones, which was also ignored since it was the same in all tests. Therefore, only the active time of the welding process was calculated by considering the four passes and the total length of the plates (400 mm). Table 6 reports the active welding times of the compared tests. Test 0 lasted approximately 4 min while test 8 approximately 2 min and half. The time saving provided by the new process conditions, which guarantee the same mechanical properties of the joint, was around 61%. This represents a significant result for such kind of industrial applications where the joints to perform are much larger than the ones herein examined; therefore, the competitive industrial advantage provided in these terms becomes significant.

Table 6 Comparison of T0 and T6 tests for active welding time calculation
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5 Conclusion

This study focused on the submerged arc welding technology employed on carbon steel plates, aiming at demonstrating the process optimization based on substantial experimental evidences. Plates of different thicknesses were joined using multi-pass welding according to an experimental plan. The joints were meticulously analysed from macroscopic and microscopic perspectives to understand the effects of the thermal cycle on the heat-affected zone. The key welding process parameters, and the resulting specific heat input, significantly impact on the microstructure of the weld bead first and, subsequently, on the heat-affected areas. The percentage of ferrite and perlite of the welded steel was measured, and the analysis of the results has shown how the process efficiency and, therefore, the productivity can be improved while ensuring the required welding quality. Test 0, in fact, represented the test that satisfies the UNI EN ISO 3834 quality standards and, therefore, it was considered as the reference point for both the microstructural analysis and the hardness measurements. All the other tests were carried out gradually increasing the welding speed, and the microstructure was analysed comparing the content of ferrite to the one obtained in test 0. Furthermore, for verifying the influence of the process parameters on the mechanical properties of the joint, microhardness was measured in different zones. It was demonstrated that for the same heat input set considering the output rating of current and voltage, the welding speed increase, within a range of preset values depending on the machine, does not affect the heat-affected zone, as well as the mechanical strength of the joint. Therefore, the productivity of the welding process under investigation may be increased, but the process parameters have to be set maintaining constant the specific heat input.

These findings can serve as a guide for future research and practical applications, ultimately leading to enhanced welding practices and the development of more robust and reliable welded joints.