The Influence of Water Flow Characteristics on the Physical and Mechanical Qualities of Underwater Wet Welded A36 Marine Steel Plate

Abstract

Underwater welding has proven to be a successful method of joining two similar or dissimilar metals and takes place underwater. This technique is frequently used for maintenance purposes, such as repairing piping systems, ships, and other marine structures. This study investigates the effect of different water flow types on an underwater weld’s physical and mechanical properties of welded bead on the A36 steel plate. The SMAW method with an E7018 electrode is used for welding A36 steel in saltwater. In this simulation, underwater welding is performed using three types of flow (without flow, non-uniform flow with a baffle plate, and non-uniform flow without a baffle plate) to compare metallography, hardness, tensile, impact, and bending testing results. The findings revealed that the saltwater flow caused more porosity defects. Moreover, the highest penetration depth was observed in specimens with the non-uniform flow with a baffle plate. The heat energy is concentrated due to droplets accumulating in the weld area. The microstructure of welding metals such as acicular ferrite and ferrite with the second phase grows as the water flow becomes non-uniform. Furthermore, as the rate and variability of the water flow increased, the value of the mechanical properties of the specimens increased relatively.

1. Introduction

In recent decades, there has been an increase in the demand for sophisticated ships and other marine structures. These structures are generally made of various materials such as steel and alloy steel. This material is intended to withstand harsh and corrosive environments and the increasing need to repair and maintain underwater marine structures. As offshore environments are remote, hostile, and characterized by extreme temperatures and windy surroundings [1], tapping resources from such environments for global energy market stability requires stringent implementation of effective safety measures and selecting a suitable selection of modern welding technologies. However, to combat failures in offshore structures installed underwater, modern welding technologies must be used for underwater welding of offshore structures.

Although there is always a potential for more research and investigation in underwater welding of marine and offshore structures, several studies have been conducted in this area. Underwater welding can be divided into wet and dry underwater welding [2,3]. Wet underwater welding is manually performed at ambient water pressure by divers/welding at shallow depths. In contrast, dry underwater welding is performed in a watertight closed environment surrounding the welded structure. During the welding process, weldments formed using the dry welding technique have no direct contact with water. The dry atmosphere assures welding process stability, removes or reduces the presence of hydrogen and oxygen in the weld, ensures a reasonable cooling rate, and significantly improves weld strength and ductility. In contrast to wet underwater welding, which uses a water-proof stick electrode and is operated directly without additional equipment, the closed chamber in underwater dry welding is filled with a mixture of helium and oxygen gas. Thus, it is economically viable due to its versatility compared to other underwater welding methods [4]. Wet underwater welding can generally use shielded metal arc welding, flux core, submerged arc welding, gas-shielded metal arc welding, laser, friction, and explosive welding [5].

However, the significant disadvantages of underwater wet welding have sparked widespread concern and the need for further research and development. Due to increased porosity, lower ductility, and hydrogen cracks, welds' quality using the wet welding process has not been satisfactory [6]. These problems are caused due to rapid cooling of the weld metal by the surrounding water. It can cause an increase in tensile strength in the weld area but decrease ductility and impact strength [5]. In addition, rapid cooling can increase the occurrence of porosity and improve the hardness of the welded joint [7].

Moreover, determinants of the water environment such as water temperature and water flow rate can significantly impact underwater welding. Both of these factors affect the welding qualities. The heat energy from welding causes the water temperature to increase significantly, so the water around the welding arc immediately evaporates. This problem causes the size of the protective welding bubble to be larger [8,9]. Increasing the water temperature also causes the protective bubble's maximum size to decrease gradually. The dissociation of the electrolyte by the welding arc causes the electrolyte to split into ions which will appear in the molten weld metal. The increase in the inclusion of these ions results in the emergence of porosity and cracks which will change the microstructure of the weld area [10]. Water types in underwater wet welding can affect the results in the weld area. The microstructure and mechanical properties, such as tensile strength and hardness in the weld area, can be affected by the mineral content in the water because each type of water contains different mineral levels. Seawater generally contains mineral salts of approximately 3.5%, which is dominated by NaCl (sodium chloride). However, not all places in the ocean include the same levels of mineral salts [11]. Due to temperature fluctuations in an area, each location has different levels. Seas with greater temperatures have higher quantities of mineral salts in general. This type of sea can be found around the equator, such as in Indonesia.

Welding quality is influenced by the welding arc transfer mode and the metal transfer method. Several factors in the water environment influence these variables, including flow velocity, temperature, water type, and pressure. There are two types of fluid flow rates: uniform and non-uniform. In non-uniform flows like seas, rivers, and lakes, the fluid’s shape, direction, and velocity vector change according to the fluid flow. Several remarkable findings in investigating the physical and mechanical properties of underwater wet welding have been discussed. The influence of porosity variation on weld metal properties was examined by Pessoa et al. [12]. Mechanical studies revealed that samples removed from the weld end had better characteristics. Chen et al. [9] investigated the influence of flow velocity on underwater wet welding arc stability and metal transfer. The increased flow rate promotes heat dissipation during welding, resulting in lower arc temperature, arc shrinkage, higher current density, and deeper penetration. The mechanical performance and morphology of marine structure underwater welding were investigated by Çolak et al. [13]. According to the findings, defects considerably impact the mechanical characteristics and width of the weld zone. Di et al. [14] investigated the effect of cooling rate on the microstructure, inclusions, and mechanical properties of weld metal during local dry underwater welding. High strength and toughness values are associated with the existence of acicular ferrite in weld metal with a rapid cooling rate. Moreover, Omajene et al. [15] discussed the effects of welding parameters on the weld bead shape of underwater welds. Water has a higher convective heat transfer coefficient than air, resulting in rapid cooling during underwater welding.

The effect of water temperature and different types of water flow on mechanical and microstructural properties of A36 marine steel was studied by Surojo et al. [16]. The tensile and impact strength values increased as the cooling rate increased. Surojo et al. [17], investigated the effect of water flow speed and water depth on the crack growth rate of underwater wet welded low carbon steel SS400. The fatigue crack growth rate in underwater wet welded joints decreases as water depth and flow rate increase. Surojo et al. [18], investigated the effect of water flow rate on an underwater welded bead’s physical and mechanical properties on a freshwater A36 steel plate. The control variables used a freshwater environment. The findings revealed that changes in water flow impact underwater weld defects, microstructure, and mechanical properties [19].

The physical and mechanical properties of flow underwater wet welding, particularly in saltwater, are poorly understood. There has previously been researched into the physical and mechanical aspects of still underwater welding, but more research into flowing saltwater was required. Wet underwater welding with saltwater is more commonly used in this application. This study was carried out to observe the physical and mechanical properties of the wet underwater welding in saltwater with different water flow types. In this simulation, underwater welding is performed using three types of flow, including without flow, non-uniform flow with a baffle plate, and non-uniform flow without a baffle plate, to compare physical and mechanical testing results.

2. Materials

2.1. Low Carbon Steel A36

Low carbon steels have less than 0.3% carbon content by weight. As a result, they have excellent welding properties and are suitable for grinding, punching, tapping, drilling, and machining processes, making them a special type of general-purpose steel. The material used in this research was A36 low carbon steel with dimensions in width × length × thickness of 300 mm × 400 mm × 4 mm. The chemical composition based on the spectrometer test is shown in

Table 1. Chemical composition (wt %) of ASTM A36.

wt %	ASTM Steel A36
Carbon (C)	0.190
Silicon (Si)	0.128
Manganese (Mn)	0.472
Phosphorus (P)	0.044
Sulfur (S)	0.053
Chromium (Cr)	0.025
Nickel (Ni)	0.014
Copper (Cu)	0.015
Niobium (Nb)	0.0082

, while the microstructure of low carbon steel is shown in Figure 1. It can be seen in Table 1 that carbon, silicon, and manganese are the three highest compositions of low carbon steel A36.

2.2. E7018 Electrode

In wet underwater welding with salt water, the E7018 electrode was used. The E7018 electrodes were classified according to ASME Section IIC, 5.1 Specification for carbon steel electrodes for shielded metal arc welding. E7018 rod was one of the most frequently used SMAW/ stick welding low hydrogen electrodes/ rod for mild steel and carbon steel welding. E7018 electrode was made of carbon steel wire with a Titanium coating. The E7018 electrode was particularly suitable for low-carbon steel structures.

Table 2. Chemical composition (wt %) of the E7018 electrode [20].

wt %	Electrode E7018
Carbon (C)	0.15
Silicon (Si)	0.75
Manganese (Mn)	1.6
Phosphorus (P)	0.035
Sulfur (S)	0.035
Chromium (Cr)	0.2
Nickel (Ni)	0.3
Molybdenum (Mo)	0.3
Vanadium (V)	0.08

shows the chemical composition of the E7018 electrode.

2.3. Salt Water Environment

Wet welding in the water is called underwater welding. Under these conditions, the surrounding environment affects the welding rod, arc, and weldment, affecting the metallurgical welding process and the welded joint structure. It is critical to investigate the aqueous environment’s role in welding and take safety measures in response to this. In this case, underwater wet welding was carried out in seawater. Seawater has a salinity level of 35 ppt (parts per thousand) where 35 g of dissolved salt for every 1 kg of water. Figure 2 shows the salinity level of the brine used in this wet-salt underwater welding.

3. Methods: Welding Apparatus and Testing Procedures

3.1. Welding Equipment

The SMAW KOBEWEL KA-602 welding machine (obtained from PT Bumi Teknik Utama, Jakarta, Indonesia) at Solo Technopark (Surakarta, Indonesia) has been used in the welding process of testing specimens. For developing a water flow simulation glass box, welding was carried out in a glass case that was made in such a way to get the water flow with a particular water velocity. The water pump used in this simulation was a 1.5 HP Yamamax with a flow rate of 450 L/min and an input/output line of 2 inches. The water flow was pumped from the saltwater in the Solo Technopark Surakarta. The exhaust pipe was equipped with a valve that functions as a regulator of the speed of water flow to the spray hose with a nozzle tip size of 1 ¼ inch, while two perforated plates were used to regulate the type of water flow. The velocity of the water flow used in this simulation was 0.6 m/s with a depth of 0.5 m from the pool water's surface. The welding process was carried out with a travel speed of 0.4 cm/s. The water flow simulation glass box is shown in Figure 3, while the dimensions of the water flow simulation glass box are shown in Figure 4.

3.2. Variables and Procedures

Research variables consisted of independent, dependent, and controlled variables. As an independent variable, water flow types consisted of three flows: without flow, non-uniform flow with a baffle plate, and non-uniform flow without a baffle plate. The dependent variable in this study is in the form of tests such as tensile strength, hardness, macrostructure, microstructure, impact strength, and bending strength. Moreover, controlled variables were welding speed at 0.4 mm/s, current 120 A, water temperature at 27 °C, E7018 electrode with a diameter of 4 mm, welding depth of 0.5 m, and saltwater. The variables used in the study are shown in

Table 3. Research variables on flow types.

No.	Variation	Flow Type
1	A	Land-based welding
2	B	Non-flow
3	C	Non-uniform flow with a baffle plate
4	D	Non-uniform flow without a baffle plate

.

3.3. Welding Process and Water Flow Set-Up

The first step was conducted by specimen cutting of A36 steel with a size of 300 mm in width, 400 mm × 4 mm, as shown in Figure 5, and then leveling and cleaning the surface of the specimen. The second step was assembling the water flow simulation glass case, turning on the pump until it gets the type of flow determined, and installing the workpiece. A schematic of the welding and the position of the welding arc just before starting welding can be seen in Figure 6.

The wet underwater welding process was carried out in the water with a non-uniform flow. A baffle plate was intended to distinguish the flow of water formed in a simulated water glass box. The use of non-uniform water flows with a baffle plate aimed to create more uniform flow results, while non-uniform water flows without a baffle plate were used to obtain water flow results that tend to be more random. The use of non-uniform flow was enforced because the laminar flow was difficult to form in operation.

3.4. Specimen Manufacturing and Testing Procedures

The welded specimens were subjected to morphological and mechanical testing in this investigation. Morphological testing consisted of evaluating the macro and microstructure. Mechanical testing was performed on the welded sample by conducting hardness, tensile, impact, and bending tests.

In this case, micro-observations and hardness testing used a similar specimen. The testing procedure and specimens were developed based on the ASTM E92 standard, as indicated in Figure 7. Observations of macro and microstructures were conducted at the Mechanical Engineering Materials Laboratory, Sebelas Maret University, Surakarta. An Olympus SZ2 ILST microscope (by Labexchange, Germany) was used for macro-photo testing, and a Euromex Holland microscope (by Euromax, Arnhem, The Netherland) was used for micro-photo testing. This test aimed to determine the weld metal, HAZ, and base metal area in the weld results. Observation of macro and microstructures began with sanding the surface of the cross-section. The sanding process started with a roughness grade of 150 to 2000. Then proceeded to the polishing process using a polishing paste, which was carried out until smooth. In testing the macro and microstructures, the etching process was carried out first with an etching solution (2 mL HNO₃ and 98 mL alcohol) for less than 5 s, according to the ASTM E407 standard. Macro images of the welded specimens were examined with an Olympus microscope type SZ2 ILST, while micro-photos were acquired with an optical microscope of the Euromex Holland brand. The macrostructure was reviewed to ascertain the weld metal, HAZ, and base metal area. In contrast, the microstructure was examined to determine the phase changes in the area of weld metal, HAZ, and base metal in each specimen variation.

The hardness test was performed at the Mechanical Engineering Materials Laboratory at Sebelas Maret University in Surakarta. The Vickers Highwood engine type HWMMT-XX7 was used for hardness testing. The indenter used was a pyramid-shaped diamond. This test was performed to determine the hardness value of the weld metal, HAZ, and base metal from each welding variation. The hardness test was carried out using 200 g for 10 seconds according to the ASTM E92 standard. The number of points in the hardness test was 15 points. There were five points in the weld metal area, five in the HAZ area, and five in the base metal area. The distance of each point in the weld metal and HAZ was 0.25 mm, while for the base metal, it was 1 mm. Moreover, tensile testing was performed using a SANS type SHT 4106 Universal Testing Machine (UTM) (by SANS Testing Machine Co., Ltd., Shenzhen, China). The tensile test started with the specimen cut by the JIS Z 2201 standard [21]. Tensile test specimens were established by the JIS Z 2201 standard, as seen in Figure 8.

The Charpy Tatonas impact test type TG-820 (by TUV Rheiland, China) was used for impact or notch testing. An impact test utilizing the Charpy method was performed to measure the material toughness using an infinity machine branded RS-8217. This machine was used to find the strength of the material in receiving shock loads. The ASTM E23 standard was used as a reference in creating specimens and testing criteria for this impact test. The impact test specimen was cut to ASTM E23 standard using the Charpy method in Figure 9.

Bending tests were performed using the SANS Universal Testing Machine (UTM) model SHT41006 at the Mechanical Engineering Materials Laboratory, Sebelas Maret University, Surakarta. The bending test was performed according to the AWS D1.1 standard to establish the bending strength and geometry of the fractures and visual defects in the specimen. The bending specimen dimension is shown in Figure 10.

4. Results and Discussion

4.1. Observation of the Welding Defect

4.1.1. Defect Identification

Underwater wet welding is vulnerable to defects, mainly when performed in a saltwater environment. This phenomenon is caused by more mineral content that can be dissociated and more particles in the area around the weld metal. As a result, the results for underwater wet welding need visual observations to see and investigate the occurrence of visible defects. Figure 11 compares welding results between land-based welding, underwater welding without flow, underwater welding with the non-uniform flow and baffle plate, and underwater welding with the non-uniform flow without a baffle plate.

Based on Figure 11, the welding results on land and welding in saltwater with wet conditions resulted in various types of defects.

Table 4. Data observations of visual defects in specimens.

No.	Specimen	Water Type	Type of Flow	Defect	Total
1.	A	Land-based	-	Porosity (P)	0
				Spatter (S)	79
				Irregular Surface (I)	3
2.	B	Saltwater	Non Flow	Porosity (P)	14
				Spatter (S)	48
				Irregular Surface (I)	11
3.	C	Saltwater	Non-uniform flow with a baffle plate	Porosity (P)	38
				Spatter (S)	43
				Irregular Surface (I)	8
4.	D	Saltwater	Non-uniform flow without a baffle plate	Porosity (P)	62
				Spatter (S)	86
				Irregular Surface (I)	12

demonstrates the number of various defects in welded specimens, especially the highest defect in wet-salt underwater welding conditions with the non-uniform flow and no baffle plate. Spatter (S) and irregular surfaces (I) were found in land-based welding specimens. In contrast, underwater welding showed that there were three types of visual defects of the baffle plate such as spatter (S), porosity (P), and irregular surface (I).

Spatter defects (S) are shown in land-based and wet-salt underwater welding results. Spatter defects are visible defects in welded specimens induced by the presence of molten welding metal splashes or overheated welding electrodes. This significant jump happens due to increased electric current during welding and the distance between electrodes and base metal being too far [22]. The transfer of molten weld metal occurs globularly or in large sizes due to the jump in electric current and the excessive distance of the electrode from the base metal during the welding process. As a result, controlling the transfer of molten weld metal becomes difficult, leading to molten weld metal splashing into the region around the weld [23]. Figure 12 illustrates a schematic of molten weld metal globular transfer. Because spatter defects are categorized as visual defects, they do not affect the mechanical properties of the welded material. The welding arc creates the splash of molten welding metal that causes this spatter defect when the welding processes are too long [24].

The land-welded specimens have 79 points of spatter defects, as shown in Table 4. However, in the variation of underwater welding, specimens from underwater welding without flow have a spatter defect of 48 points, while underwater welding with the non-uniform flow with a baffle plate has a spatter defect of 43 points, and underwater welding without a baffle plate has a spatter defect of 86 issues. The decrease in spatter from changes in welding on land to wet underwater welding occurs because the movement of saltwater in the region around the weld prevents the molten weld metal splash from expanding from the weld metal area. Consequently, specimens from wet saltwater welding with non-uniform flow through a baffle plate had fewer spatter defects than specimens from wet saltwater welding without flow. On the other, the samples from wet salted underwater welding with the non-uniform flow and no baffle plate display an anomaly where the spatter defects are more numerous. This anomaly can arise due to the flow’s turbulence, which is quite unpredictable. The droplets from the welding are spread following the random and opposite flow of water from the welding direction. As shown in Figure 13, the high flow rate and irregular flow direction lead the droplet transfer to split into spatter, which is dispersed across the surface of the welding specimen [24].

Furthermore, the welded specimens revealed irregular surface defects or uneven welded surfaces, as seen in Figure 11a–d. Poor transfer of molten weld metal might result in an irregular surface. Because of the increased water flow, the welding protection barrier is less stable [25]. However, as shown on land specimens, irregular surface defects might occur due to changes in Amperes and Volts during the welding process. The number of irregular surface defects in land-welded specimens is less than in underwater welding specimens.

The underwater welding process also creates porosity defects, as shown in Figure 11b–d, with the numbers and sizes listed in Table 4. Including hydrogen gas and other mineral components in the brine causes this porosity defect. Because the saltwater is in direct contact with the welding arc, hydrogen gas and other mineral components are present. Water particles dissociate into hydrogen and oxygen gases due to the high temperatures around the weld area [8]. The hydrogen gas is trapped in the weld metal and is known as a hydrogen inclusion in the weld metal. Hydrogen gas becomes trapped in the weld metal because of the rapid cooling and solidification rates [26]. Figure 14 shows a schematic of the occurrence of porosity.

4.1.2. Data Observation of Macrography

Evaluating data from macro pictures is carried out by studying specimens cut transversely by land-based and underwater welding, as illustrated in Figure 5. The macro photo data of welding specimens on land and underwater welding reveals changes in weld penetration depth between the welded specimens, with variations in each welding specimen. The material and water flow type may theoretically produce forced convective heat transfer during the specimen’s welding process. As a result of the increased cooling rate around the weld, gas inclusions increase [14]. Underwater welding with the non-uniform flow and no baffle plate warms up faster than wet-salt underwater welding with the non-uniform flow and a baffle plate. This phenomenon is caused by the Reynolds number in non-uniform flow welding without a baffle plate is higher than the Reynolds number in non-uniform flow welding with a baffle plate. The high Reynolds number also indicates that the heat transfer value is significant, and the heat transfer is related directly to the cooling rate. As a result, the higher the Reynolds number of a flow, the higher the heat transfer and cooling rate. The cooling rate of the welding processes is also affected by the medium of sea and land water. Welding in saltwater has a rapid cooling rate in wet circumstances, affecting the specimen's welding penetration rate.

According to Figure 15, welding specimens performed on land have a penetration of 1.48 mm, but underwater welding without flow has the lowest penetration of 1.38 mm. The deepest penetration depth in specimens with the non-uniform flow with a baffle plate is 2.23 mm. Underwater wet welding with a non-uniform flow and no baffle plate had a lower penetration depth of 1.45 mm. Figure 16 shows the comparison of the penetration depths of the four specimens.

According to the welding penetration depth graph, the wet-salt underwater welding specimens with the non-uniform flow with a baffle plate have the deepest penetration depth. As shown in Figure 17, this is caused by droplets that form when the molten weld metal flows and is pushed by the opposing flow of water. Water will flow from the molten weld metal towards the rear of the welding electrode, where it gathers, allowing more heat energy to be absorbed by the base metal [8]. The effect of the molten weld metal being driven by the water flow is greater than the rise in cooling rate, which leads the weld metal to penetrate deeper.

4.1.3. Data Observation of Micrography

The micro photo observation aims to detect the phase composition and microstructure, or phase grains, present in the weld metal region, heat-affected zone (HAZ), and base metal. The cooling rate experienced by the welded specimens influences changes in the form of the microstructure. Changes in the microstructure of the welded specimen cause changes in the specimen’s mechanical properties.

Based on findings from microstructure photographs, A36 carbon steel base metal comprises ferrite and pearlite. The ferrite structure is ductile but has low tensile strength and hardness. On the other hand, the pearlite structure has higher tensile strength and hardness and is more brittle than ferrite [26]. Three types of mixes were discovered in the weld metal area, namely grain boundary ferrite (GBF), ferrite with the second phase (FSP), and polygonal ferrite (PF) [27]. The weld metal region in the wet underwater welding specimen, on the other hand, exhibits an acicular ferrite (AF) structure. The rapid cooling rate caused by the water medium and flow types during the welding process resulted in a rise in the number of microstructures of the FSP and AF types, whereas the grain size and microstructures of the GBF and PF types dropped. Compared to PF and GBF, the AF microstructure has higher tensile strength, hardness, and a higher impact value, but FSP has a lower impact and elongation value [28].

After creating austenite at 1400–850 °C, the PF structure is formed at 1000–600 °C. The GBF structure is formed at temperatures of roughly 1000–650 °C along the austenite grain boundaries and will be the location of FSP core production when the cooling temperature reaches 800–500 °C. As a result, the FSP’s position is always next to the GBF.

Table 5. Comparison of micro-test results under different welding types.

Specimen Variation	Base Metal	HAZ	Weld Metal
Land-based welding
Underwater welding without flow
Underwater welding with the non-uniform flow and baffle plate
Underwater welding with the non-uniform flow and baffle plate without a baffle plate

reveals that the weld metal in the onshore welding specimens is greater than the PF and GBF structures in the wet-salt underwater welding specimens. The comparatively substantial heat input received accounts for the lack of the AF structure in the weld metal region of the welded specimen on land. This increases the grain size of GBF and FSP while decreasing the quantity of FSP and AF [27,28].

The presence of an AF structure is shown by the microstructure of the weld metal region on the wet-salt underwater welding specimen. The AF structure is generated at a cooling temperature of 600–400 °C with a rapid cooling rate. The rapid cooling rate allows for the creation of more AF structures. As a result, flat GBF and PF structures with smaller grain sizes are produced instead of offshore welding structures [29]. Underwater welding with the non-uniform flow with and without baffle plates exhibits the lowest GBF and PF compared to other welding outcomes. This is owing to the enhanced cooling rate caused by the brine flow. The microstructure will increase the mechanical strength of the welded specimen with tiny and fine grain sizes [30]. The following equation calculates the cooling rate t_8/5 in wet underwater welding.

$$H=UI/\left(v\right)$$

(1)

$$Rc=6.25\times 105{\left(10000H\right)}^{-0.95}{\times t}^{0.17}$$

(2)

$$\Delta t_{8/5}{=455Rc}^{-1.09}$$

(3)

H is welding heat input (kJ/mm), U is arc voltage (V), I is welding current (A), v is for welding speed (mm/s), Rc is cooling speed (°C/s), and t is plate thickness (mm).

Table 6. Parameter cooling rate of underwater welding.

H (J/mm)	I (A)	U (V)	v (mm/s)	Rc (°C/s)	∆t8/5 (s)
7500	120	25	0.4	2.74	0.00042

displays the results of the computations based on these equations. The observed results are consistent with the ASM Handbook, which states that t_8/5 in SMAW underwater welding is worth 1–6 s depending on the heat input for values less than 3.6 kJ/mm [30].

Mechanical qualities vary depending on the microstructure phase in the weld metal area. The tensile strength and hardness values of the AF structure are greater than those of the GBF and PF structures. The FSP structure has a higher tensile strength than the AF structure but poorer flexibility [26]. The distribution of GBF, PF, and AF structures from each variant of the welded specimen is summarized in Table 5. There is a thermal cycle in the heat-affected area (HAZ) of heating to rapid recrystallization and cooling temperature. This alters the microstructure of the HAZ area. The HAZ area shares the base metal region’s ferrite and pearlite microstructure. However, the grain size and quantity vary depending on the welding specimen variance. Onshore welding specimens include less ferrite than pearlite, whereas wet-salt underwater specimens contain more pearlite than ferrite. As a result, the hardness value of the wet-salt underwater welding specimen is more significant than that of the onshore welding specimen.

Table 6 shows that the microstructure of wet-salt underwater welding specimens tends to generate smaller grain structures when the water flow velocity is higher, notably in the HAZ area. Welding under wet seawater with the non-uniform flow without a baffle plate produces the smallest grains compared to welding onshore, welding under wet saltwater without flow, and wet-salt underwater welding with the non-uniform flow with a baffle plate. Changes in the kind of water flow affect the increase in heat transfer, resulting in an increase in the cooling rate but a decrease in heat input in the welding region. Compared to the grain size in the ground-welded specimens, the very rapid cooling rate interferes with grain growth and causes the grain size to be reduced [28]. The high cooling rate in the HAZ area of the wet underwater welding specimen promotes the recrystallization process, resulting in a microstructure with smaller and denser grain sizes [29]. The mechanical characteristics of A36 steel can be affected by the microstructure phase. The smaller the grain size of the microstructure, the greater the tensile strength and hardness. Based on the discovered microstructure, wet-salt underwater welding in non-uniform flow without a baffle plate has an enormous influence on the changes in the microstructure, leading its mechanical characteristics to alter significantly compared to ground welding specimens.

4.2. Mechanical Properties

4.2.1. Hardness Testing

The hardness of the resulting specimen is examined to identify the distribution of hardness in the areas around the welding, such as the weld metal, heat-affected zone (HAZ), and base metal. The tests were performed on specimens of each variation previously sliced crosswise. A Micro Vickers hardness testing equipment was used to measure the hardness of the welded samples (Micro Hardness Vickers). The test was performed using a conventional pyramid indenter diamond at 136° with a load of 200 gf, pressed for 10 s. The distance of emphasis for each point in this hardness test is determined by the region surrounding the weld. A distance of 0.2 mm is supplied from one place to another in the weld metal and base metal sections, whereas a distance of 0.1 mm is given in the heat-affected zone (HAZ). Because each welding area has a distinct area, this is done. The HAZ area is thinner than other locations, such as weld metal and base metal. The amount of hardness of each point in the welding region for all changes is shown in Figure 18. The graph allows us to detect and classify the welded specimens for each variation, including ductile or brittle materials. If a material has a low hardness value, it is considered ductile, and if it has a high hardness value, it is said to be brittle [31].

Figure 18 shows specimens from wet-salted underwater welding with the non-uniform flow and no baffle plate had the maximum hardness value in the HAZ area, with 345.2 HVN. Wet-saltwater welding with the non-uniform flow with a baffle plate produced specimens with a hardness value of 317.7 HVN. In contrast, wet-saltwater welding without flow produced specimens with a hardness value of 327.9 HVN. The disparity in hardness ratings across the variants is due to the various cooling rates encountered by each variant. The variable cooling speeds are caused by the welding cooling media utilized and the flow pattern employed in the area around the weld. The HAZ region exhibited the fastest cooling rate compared to the weld metal and base metal sections. As a result, the hardest value was obtained in the HAZ area of each specimen variation.

According to the findings, each metal region of all wet-salt underwater welding specimens has a higher hardness value than the specimens of land-based welding. The heat-affected area generally had the highest hardness values for all specimens (HAZ). The HAZ area has the maximum hardness value compared to other places because the specimen undergoes heat transfer from the welding arc in the HAZ area and extremely quick cooling owing to ambient circumstances where the cooling medium is saltwater. As a result, heat transmission and cooling occur at a quick pace. This quick heat transmission and cooling, also known as forced convection, produces a high hardness value in the HAZ area [29]. The cooling rate increases the development of AF and FSP [28]. The hardness of the wet-salt underwater welding outcomes is affected by the rise in AF and FSP. The rapid cooling rate reduces the heat entering the weld region or heat input, resulting in a microstructure with fine and minute grains. This increases the hardness and strength of the HAZ area [30].

Furthermore, when HAZ comes into contact with the recrystallization temperature, it changes to create pearlite grains, which are tougher than ferrite grains [26]. Because of the absurdly short recrystallization time, new crystal grains were unable to form in the HAZ area. The new crystal grains that cannot develop are smooth, tiny, and dense, increasing the hardness and tensile strength in the HAZ area [26]. As a result, specimens from wet-salted underwater welding with non-uniform flow and no baffle plate have the highest HAZ hardness values. In contrast, specimens from ground welding have the lowest HAZ hardness values.

4.2.2. Tensile Testing

The tensile test was performed at Sebelas Maret University’s Materials Laboratory utilizing a Universal Testing Machine (UTM) labeled SANS type SHT4106 with an automated loading mechanism. Tensile testing is performed using the JIS Z 2201 standard and the material's thickness to determine the material's standard tensile speed. The tensile strength of the welded specimens was evaluated three times, with each variation evaluated three times. Figure 19 depicts a graph of the welded specimen’s tensile strength and elongation test results.

Figure 19 shows that the tensile strength of the specimens from underwater welding with non-uniform flow without a baffle plate is greater than the tensile strength values of land-based welding, underwater welding without flow, and underwater welding with non-uniform flow with a baffle plate, which is equal to 611, 809 MPa, and the elongation rate is 10.6%. In comparison, the specimens from land-based welding have a tensile strength value of 435 MPa and an elongation rate of 4%. Changes in tensile strength and elongation level are affected by changes in the quantity and size of the welded specimen’s microstructure. The increasing amount of accicular ferrite (AF) in the specimen’s microstructure can enhance its tensile strength [32]. Additionally, the grain size formed in the heat-affected zone (HAZ) affects the tensile strength of the welded specimen. Materials having a dense and fine grain microstructure have high mechanical strength. This tensile strength test revealed that the grain size found in wet underwater welding had a higher density than the grains found in land-welded specimens in wet underwater welding without flow, the non-uniform flow with a baffle plate, and the non-uniform flow without a baffle plate.

The tensile strength value for the wet underwater welding specimen without flow is 442.885 MPa, and the elongation rate is 6.2%. In contrast, the tensile strength value for the wet underwater welding specimen with the non-uniform flow with a baffle plate is 500.4 MPa, and the elongation rate is 8%. The cooling rate influences the difference in tensile strength and elongation values in the weld region and the microstructure of the welded specimen. The rapid cooling rate increases the number of acicular ferrite structures [33,34]. Furthermore, the rapid cooling rate prevents grain growth, resulting in a relatively small grain size generated during the welding process [35,36]. This microstructure’s small grain size will increase the mechanical strength of the welded specimen. Figure 20 shows images of the tensile test results that have been conducted.

Figure 21 shows that simulation the fracture caused by the tensile strength test on the welded specimen happened along the boundary between the heat affected area (HAZ) and the base metal region rather than in the center of the specimen. This is due to weld reinforcing that is not eliminated throughout the tensile testing procedure. Weld reinforcement is not eliminated since welding is performed on a single carbon steel plate or bead on the plate, resulting in a fracture that differs from the fracture generated by welding on two plates or joints. However, research also demonstrates that welding on a single carbon steel plate, also known as bead on plate welding, strengthens the carbon steel material, allowing this welding process to be employed as reinforcement in a carbon steel construction.

4.2.3. Impact Testing

The welded specimens are subjected to impact testing to establish the toughness value. A Charpy impact tester with a swing load of 9.5 kg, arm length of 0.85 m, and an initial angle (α) of swinging of 90° were used for the impact testing process. Because the heat-affected zone (HAZ) was the weakest section of the welded specimens, the impact test was conducted by bead on the plate. The impact test findings are the absorbed energy (J) and the impact strength in units of J/mm². Figure 22 compares the toughness of specimens from onshore welding and wet-salt underwater welding. The highest amount of toughness against impact testing is obtained for specimens from underwater welding without flow, 37.152 Joules, and the impact strength is 1.61 J/mm². In contrast, the lowest value for specimens from land-based welding is 20.48 Joules and 1.37 J/mm². Toughness values decreased with changes in flow in the area around the welding, such as specimens from wet-salted underwater welding with non-uniform flow with a baffle plate, which had toughness values of 33.444 Joules and 1.69 J/mm², and specimens from wet-salted underwater welding with non-uniform flow without a baffle plate, which had toughness values of 32.187 Joules and 1.57 J/mm². This is owing to the extremely fast cooling rate of the cooling medium, saltwater. However, the impact strength of the non-uniform welding specimen with the flow, dropped slightly because the flow generated forced convection heat transfer, reducing the heat input or heat entering the region around the weld. The welded specimen will absorb less impact energy as the heat input is reduced [14].

4.2.4. Bending Testing

The bending test is performed on the resulting specimen to verify the welding results' quality, particularly on the weld seam. The bending test was performed at the Materials Laboratory, Faculty of Engineering, Sebelas Maret University, using the AWS D1.1 standard for the face bend test type. The face bend test results on the welded specimen are shown in

Table 7. Face bend test results of welding specimens on land and in wet-saltwater.

No.	Variation	Identification	Mandrel Diameter	Test Results Long Crack	Results
1	C0	Face bend 1	32	-	Accepted
2	C1	Face bend 2	32	-	Accepted
3	C2	Face bend 3	32	-	Accepted
4	C3	Face bend 4	32	-	Accepted

. The toughness of material after welding can assess its bending strength. Toughness is a combination of the material’s strength and ductility. Toughness may also be defined as a material’s ability to absorb energy until it fractures. The amount to which the specimen can be bent to the maximum angle also indicates the toughness of the welded specimen. If the bending angle during the bending test on the welded specimen is greater, the material is characterized as tough. The bending test results can be declared passed or accepted if they meet the acceptance requirements specified in the standard [38]. The typical approval requirements for the bending test, including a maximum allowable fracture of 3 mm, are measured from all directions on the bent, convex surface. The maximum number of faults in defects varying in size from 1 mm to 3 mm shall not exceed 10 mm. Surface cracks at the corners are limited to 6 mm unless caused by slag inclusions or other fusion defects. The maximum allowable size is 3 mm.

Figure 23 shows the visual results of the bending test on the specimens from onshore welding and wet-salt underwater welding. Based on this visual observation, a study of the faults generated by bending tests on the welded specimens can be performed by the AWS D1.1 standard.

Figure 23 shows the results of the welding bending test performed on land-based and underwater welding specimens using the face bend test method. Underwater welded specimens exhibited an extension of cracks from previous defects, such as porosity created by underwater welding in saltwater conditions (water environment). This is due to the flow and increased flow rate in the welding process, causing larger porosity defects than on land welding. The flaws discovered in Figure 23B–D are accepted or permitted since the faults detected in the welding specimens are caused by defects during the welding process, not defects induced by the welding process bending test. In terms of the bending angle created with a material thickness of 2 mm, this welding specimen may be classified as a material with outstanding toughness qualities.

5. Conclusions

In this study, the effect of flow and depth of saltwater on the physical properties and microstructure of the welded under wet-saltwater using the bead on plate method on A36 steel. The results are summarized as follows.

• The presence of flow and an increase in the flow rate in wet-salt underwater welding causes larger and more porosity defects to arise. Most porosity and irregular surface defects were found in wet-salt underwater welding with non-uniform flow variations without a baffle plate. The microstructure in the weld metal, such as acicular ferrite (AF) and ferrite with the second phase (FSP), increases with the non-uniformity of water flow and cooling rate. AF and FSP are most commonly found in non-uniform, non-uniform flow wet-salt underwater welding without a baffle plate. The heat affected zone has a smaller grain size with increasing flow and cooling rates.
• The type of water flow during the welding process, namely without flow, non-uniform flow and a baffle plate, and non-uniform flow and a baffle plate can affect the results of underwater welding. Based on the results of mechanical testing (micro-Vickers and tensile test), non-uniform flow with a baffle plate has a higher hardness and tensile strength than other types of flow. In the Vickers test, the hardness value increased by 1.7% compared to on land. In the tensile test, the UTS value increased by 1.5% compared to on land. The impact test increased by 1.6% compared to on land. However, the impact test on underwater welded specimens with non-uniform flow and a baffle plate decreased compared to other types of flow. This shows that the correlation between the value of hardness is inversely proportional to the value of toughness