Comparative Study of Conventional Underwater Electrodes and Electrodes with Additional Modification of Mg

The influence of water depth, microstructure, chemical composition, welding defects, and mechanical properties affect underwater welding. As well as, the determination of electrodes for underwater welding requires unique properties, including being able to cause arc flame bursts and slag growing on the surface of metal deposits capable of protecting from the effects of oxides and low hydrogen solubility. The electrodes must meet the AWS D3.6M underwater welding specification standard. In this study, the steel plate material used in AH-36 and implemented for underwater wet welding by comparing two electrodes, namely E6013 with additional Mg modification and Broco E70XX electrode specifically for underwater wet welding, other Mg on the E6013 rutile electrode with modification 1 (3% Mg) and Modification 2 (5% Mg). This welding method with a heat input of 1.5 kJ/mm with 130 A and 2.5 kJ/mm with 140 A is carried out at 5m depts. The radiographic test results showed that the specimens welded at a depth of 5m showed incomplete penetration defects. Perhaps due to the influence of a significant enough pressure and a higher cooling rate, the molten Weld cannot penetrate completely into the parent material. Tensile test results also showed an insignificant increase in underwater welding strength and considerable elongation. It occurs at the E6013 electrode with modification 1 (3% Mg) and Modification 2 (5% Mg), not too large an increase, with incomplete penetration. Due to the influence of a significant enough pressure and a higher cooling rate, the molten Weld cannot penetrate completely into the parent material. However, for testing the Broco E70XX electrode, there is no change in the microstructure, such as a modified electrode with magnesium. It was only grain growth.


INTRODUCTION
Underwater wet welding (UWW) is gaining popularity in construction and maintenance. This manufacturing technology is starting to develop and is needed in archipelagic areas such as Indonesia; hence it is starting to be known by the public not only for welding done on land but also now being known for welding under the sea; about 62% of Indonesia's area is sea and water. As well as many offshore oil and gas drilling processes and many ships used to transport the results  Purnama) of oil and gas drilling for further processing. Efforts to maintain subsea pipelines, ships, and underwater constructions by conducting regular repairs for corrosion and leakage protection facilities require activities that are conducted underwater.
The main purpose of underwater welding technology has been developed for repair, with SMAW (Shielded Metal Arc Welding) being the most straightforward technology. The welding method is divided into two ways: wet welding (underwater wet hyperbaric), which is welding on materials in direct contact with air. Also, dry welding of the hyperbaric process is not in direct contact with water and is conducted in a hyperbaric tank [1].
The method used for air-wet welding is based on the American Welding Society (AWS) standard D3.6M stipulating five basic air welding methods: air-pressure welding, welding at ambient pressure in a large dry room, welding at ambient pressure in an open dry room. Bottom welding at ambient pressure in a transparent enclosure and welding at ambient pressure. There are several class codes in underwater welding, such as type A, type B, and type O welding. Type A welding defines stand-alone requirements. For underwater welding to be suitable for application and design stresses and more accessible and applicable, type B is defined by a set of mechanical and inspection requirements. For the intended less critical application, the decreased ductility, and increased porosity, can be tolerated on the type C welds, which meet lower requirements than other types. In addition, type O meets the requirements of governing "in the air" codes, specifications, or other mandatory documents, as well as the additional requirements specified in the specifications for Underwater Welding [2]. However, to evaluate several diverse types of commercial electrodes for wet welding.
The base electrode produces an unstable arc and an irregular bead appearance.
The rutile electrode is the most used type for underwater wet welding because of its excellent arc stability and good bead appearance. However, the rutile electrode produces a weld metal with a highly diffusible hydrogen content of up to 80 ml/100g, making it susceptible to cold cracking. Thus, due to the low diffusing hydrogen content resulting from the development of the oxy-rutile electrode, which can give a yield as low as 20 ml/100g, these electrodes have low arc stability and lower weld bead appearance and detachability. Bad slag. From the results of the JMN, Vol. 6, No. 1, Juni 2023, Hal. 1-11 development and study of oxy-rutile electrodes that try to combine the advantages of rutile and oxidizing electrodes [3]. Therefore, the research purpose is to study the comparison between the electrodes commonly used for underwater wet welding, such as the Broco E70XX electrode with the E6013 rutile electrode by the modification with the Mg. The resulting mechanical properties of the welded material, namely the AH-36 plate, would be observed as the high-strength lowcarbon steel that functions for ship hull materials.

EXPERIMENT METHOD
Steel plate used AH-36 as standard high-strength steel. The chemical composition is shown in Table 1. The electrode materials used are rutile electrodes E6013+Mg and Broco E70XX electrodes with a diameter of 3.2 mm with the addition of % magnesium as shown in Table 2, and the chemical composition of Broco electrodes can be seen in Table 3.  Mg) & modification 2 (5% Mg). The diameter of the electrode is 3.2 mm. In the first stages, welding was conducted using an E6013 electrode with modified Mg at a depth of 5 m using a heat input of 1.5 and 2.5 kJ/mm. Afterwards, the welding results with a modified Mg electrode were compared between the Broco E70XX electrode at the same depth and heat input as the modified E6013 electrodes.       There was no phase change in samples B1 and B2 with broco E70XX electrodes.

RESULTS AND DISCUSSIONS a. Non-Destructive Test (NDT) of Welding
Only cementite was formed where other micro constituents, such as pearlite, cementite, and martensite, could also occur. At faster cooling rates, the formation of bainite or ferrite with aligned carbides (AC) and martensite is possible [7]. is shown in Figure 5. One of the differences in heat input between 1.5 kJ/mm and 2.5 kJ/mm can be seen in the size of the austenite grains. The grain size of B2 is larger than the grain size of B1. The higher heat input causes the cooling process in the weld metal to be slower even though it is conditioned in water so that the grain size is still coarse. In addition, higher heat input can also increase grain size, which can contribute to reducing hardness. It is similar to the study reported by previous researchers [8].

b. Destructive Test (DT) of Welding
The value of tensile strength with a heat input of 1.5 kJ/mm (B1-UW) using electrode Broco E70XX has a value of 527 MPa. However, the tensile strength for heat input of 2.5 kJ/mm (B2-UW) using electrode Broco E70XX has a value of 533 MPa. The Broco E70XX electrode, specifically for underwater application, has different weld properties with the electrode E6013 + Mg. The ductility of the weld metal increases with increasing heat input [8].   The hardness value between the heat input of 1.5 kJ/mm and 2.5 kJ/mm has the Vickers hardness value, which tends to be the same. With a higher heat input, the cooling rate can be slower, allowing carbon atoms to diffuse so that martensite may reduce [9]. That the maximum hardness value occurs in HAZ followed by WM and BM. The hardness value in HAZ is highest compared to WM and BM because the HAZ microstructure has a large amount of martensite. When compared to atmospheric welding, underwater welding has a higher hardness value [10].
Another thing is that the low heat input will cause the cooling rate to be faster when compared to the high heat input, so it tends to form a more complex phase in the microstructure, but this is not following the data in almost all test samples show in  area, so the location of the fracture is in the base metal. The rupture in BM may result in a higher impact toughness value on the welded specimens because the area on the base metal (BM) is not exposed to heat, so BM does not undergo a grain structure transformation which can reduce the toughness of a material [9]. However, when compared to the E6013 modified magnesium electrode, it is better than the c. All the tensile properties on the Mg modification of the E6013 electrode and the Broco E70XX electrode have shown more ductile properties except for the welded steel of 2.5 kJ/mm heat input with the modified electrodes of 5% Mg.
The welded steel with an Mg-modified electrode with a heat input of 1.5 kJ/mm has a higher average tensile strength value than the welded steel with a heat input of 2.5 kJ/mm. In contrast to the Broco E70XX electrode, the average value with a heat input of 2.5 kJ/mm has a higher average tensile strength value than the welded steel with a heat input of 1.5 kJ/mm. d. The welded steel using an Mg modified E6013 electrode with a heat input of 1.5 kJ/mm is a higher impact toughness value when compared to a heat input of 2.5 kJ/mm. The optimum impact energy is 50.61 J, and it is occurred on the welded steel with the heat input of 1.5 kJ/mm using 5% Mg modified of the E6013 electrodes