Introduction

Nuclear power equipment always runs under the condition of high radioactivity, high pressure, and high temperature. Thus, it is prone to abrasion, aging, and even failure. To ensure safety and have a stable operation in nuclear power plant, it is necessary to check and maintain the nuclear power equipment and components regularly (Ref 1-3). As both coolant and moderator, water plays an important role in the nuclear power plant. Generally, during the maintenance of nuclear power equipment, water cannot be drained to prevent nuclear radiation pollution. Therefore, underwater welding, which is an important technology in modern marine engineering, is also extensively demanded in the field of nuclear equipment repair (Ref 4, 5). In addition, because of boron’s strong ability to absorb neutrons (Ref 6), a mass of boric acid is dissolved into water in nuclear power plant reactor to control the reaction rate and make the nuclear reactor run stably and safely. Therefore, compared with normal underwater welding, the boric acid environment is a specific condition for underwater welding technology used in nuclear power equipment repair. Many studies show that boric acid has an obvious effect on the materials’ corrosion behaviors and mechanical properties (Ref 7, 8). Due to the potential for high deposition rates and adaptation to automated equipment, flux-cored wire arc welding (FWAC) has been widely used in the underwater welding. It is notable that a thick slag layer plays an important role in protecting the weld metal from the water in flux-cored wire arc underwater welding (Ref 9). Therefore, the performance of the slag directly affects the underwater welding process and the properties of the joints. However, the effect of the boric acid concentration on the slag properties has rarely been reported. Viscosity, which refers to the internal friction of the relative motion between the layers, is one of the vital physical properties of the welding slag. If the slag viscosity is too high, then its protection effect and metallurgical function can deteriorate. However, if the viscosity is too low, then the protection effect is also poor and is detrimental to the appearance of the weld. Thus, the study of the influence of boric acid on the viscosity of the underwater welding slag is vitally crucial for underwater welding in the maintenance of nuclear equipment.

Oxide-based slag has very complex structures, and their properties, particularly the viscosity, can vary over a wide range. Because of these considerations, many studies have focused on oxide-based slag. These many studies show that the slag viscosity is closely linked with the short-range network structures of silicates, alumino-silicates, and aluminate, which are produced in metallurgical processing (Ref 10, 11). Therefore, the composition of welding slag is a decisive influence on the viscosity. According to Kim and Sohn, an increase in the MnO and TiO2/SiO2 ratio in the TiO2-MnO-SiO2 slag system lowers the viscosity slightly (Ref 12). Furthermore, the relationship between the TiO2 content and the slag viscosity has been researched by Morizane, Handfiel, and Charette (Ref 13, 14). TiO2 is a type of amphoteric oxide. Thus, whether TiO2 is the acidic oxide or basic oxide is determined by the relative ratio of the other components in the slag system. If the TiO2 content is increased, then more O2− is supplied to depolymerize the complex silicate network structures and lower the slag viscosity. Although many studies have focused on the influence of the composition on the slag viscosity, research about the effect of the boric acid underwater environment on underwater FWAC welding slag viscosity has rarely been performed.

In terms of the properties of underwater welding joints, the majority of existing research pays close attention only to the unfavorable influence of the water environment. Due to the rapid cooling of the molten pool, the micro-pores can appear easily in the welding joint. Thus, the strength of the welding joint will decline (Ref 15). Furthermore, the rich hydrogen environment tends to increase the diffusion hydrogen content and then result in cold cracks (Ref 9, 16). Although many studies have focused on the influence of the water environment on weld metal, few of those studies are related to the effect of boric acid.

In this study, the effects of the underwater environment and boric acid solution of different concentration on the viscosity of an underwater welding slag and the properties of the welding joint are investigated.

Experimental

The base metal prepared for this experiment is Q235 mild steel. Every mild steel plate with dimensions of 200 mm × 60 mm × 6 mm should be polished before welding. In this study, self-developed underwater welding dedicated to flux-cored wire with a 1.6 mm diameter is used.

To obtain a good appearance of the weld, an automatic control platform for underwater welding, an SAF welder, and a set of matched automatic wire feeding systems are used in this experiment. At the same time, the dimensions of the experimental tank are 800 mm × 600 mm × 500 mm. DCEP (Direct Current Electrode Positive) is used during the experimental welding. The welding voltage is 30 V and the current is 210A with a 1.5 mm/s welding speed. The influence of the hydrostatic pressure on the experiment can be negligible because of the shallow water.

Four sets of experiments are designed to explore the influence of an underwater boric acid environment on welding slag and welding joints. During the experiments, boric acid aqueous solution is obtained by adding solid boric acid of different amounts to a water tank. Except for the 1# on-land welding experiment, experiments 2#, 3#, and 4# are all underwater welded in the boric acid solution. The corresponding concentration of boric acid is 0, 2300, and 35000 ppm, respectively.

After the welding experiments, slag is removed and collected. The slag should be stove dried before the viscosity measurements, and the weld joints should be polished before performing property tests.

The viscosity is measured using a RTW-08 melt property testing device. Figure 1 shows its structure. A controlled torque range between 10 and 20% at a rotation speed of 100 rpm is targeted to ensure reliable and reproducible viscosity measurements. The high temperature furnace is calibrated with a reference B-type thermocouple, which has an error of within ± 3 K using a proportional integral derivative controller.

Fig. 1
figure 1

Structure of the melt property testing device

Full size image

100 g of welding slag is placed in a graphite crucible and heated to 1773 K (1500 °C) according to temperature-controlled parameters, which are automatically controlled by computer. Then, it is important to hold this temperature for 30 min under N2 gas to reach thermal equilibrium and obtain a fully liquid phase. The viscosity is measured under the cooling speed of 2 K/min. The measurement should be finished when the viscosity is more than 2 Pa s.

The composition of slag is measured by x-ray fluorescence (XRF) and x-ray diffraction (XRD). In fact, the focus of this study is to explore the effects of boric acid on slag; thus, it is worthwhile noting that the boron element, and boron compounds must receive special attention. It is very difficult to measure the boron element by applying common test analysis equipment such as an energy dispersive spectrometer (EDS) or XRF because of their light atomic weight and small sensitivity factor. Thus, secondary ion mass spectroscopy (SIMS) is applied to analyze the boron element in the slag and the welding joints in this study.

In this study, hardness, ductility, and tensile strength are adopted to evaluate the performance of the welding joint. Therefore, a micro-hardness tester (HV-1000DT) is used to obtain the hardness of the welding joint. The diameter of its pressure head is 10 µm, and the experimental force is 300 g (dwell time: 10 s). In addition, an electronic universal testing machine (INSTRON MODEL 1186) is used to perform tensile testing for the welding joint at room temperature. The shape and size of the tensile samples are according to the international standard (ISO 6892: 1998). The tensile rate is 3 mm/min. A total of 5 tensile samples are taken from one experimental welding joint. The average test result is considered to be the tensile strength.

Result and Discussion

Composition of the Welding Slag

Table 1 shows the mass percentage of the main elements in slag. This result is obtained from the original data of the XRF testing. According to Table 1, the main elements are Fe, Ti, Ca, Mn, Mg, Al, Si, and O. The basicity of the welding slag can be calculated using the following formula:

$$B = \frac{{0.018{\text{CaO}} + 0.015{\text{MgO}} + 0.006{\text{CaF}}_{2} + 0.014\left( {{\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}}} \right) + 0.007\left( {{\text{MnO}} + {\text{FeO}}} \right)}}{{0.017{\text{SiO}}_{2} + 0.005\left( {{\text{Al}}_{2} {\text{O}}_{3} + {\text{TiO}}_{2} + {\text{ZrO}}} \right)}}$$

The basicity of 1# welding slag is 2.96. The basicity of 2#, 3#, and 4# is 3.34, 3.18, and 3.64, respectively. Thus, the welding slag obtained from these experiments is alkaline because its basicity is greater than 1. To analyze the content of the boron element in different experimental slag, the secondary ion spectrum intensity value of each group of underwater welding slag (2#-4#) is described in Fig. 2. It is obvious that the content of the B element is related to the concentration of boric acid in the solution.

Table 1 Mass percentage of main elements oxides in slag
Full size table
Fig. 2
figure 2

SIMS average intensity of the B element in slag

Full size image

Effect of Boric Acid on the Viscosity of the Welding Slag

Figure 3 shows a comparison of the viscosity-temperature curves for slag from welding on-land and welding in boric acid solution, at different concentrations. The diagram clearly shows that at lower temperatures, such as 1441 °C, there is an obvious difference between the viscosities of the slag obtained from different welding environments. The viscosity measurements of the welding slag from the 2# experiment (0 ppm) is significantly higher than that of the 4# (35,000 ppm). At the same time, the 3# experiment (2300 ppm) corresponds to the lowest slag viscosity. However, when the temperature is above 1480 °C, the differences between the viscosity measurements become less pronounced, and the viscosity tends to a constant value.

Fig. 3
figure 3

Comparison of viscosity-temperature curves for slags under different welding environments

Full size image

Figure 4 shows the slag viscosity at high temperature more clearly. The viscosity measurement of the 2# slag is almost equal to that of 3#, which is approximately 0.28 Pa s. Comparing the viscosity at high temperatures between on-land welding slag and underwater welding slag, the former reveals a significant decrease, to 0.2 Pa s.

Fig. 4
figure 4

Comparison of viscosity-temperature curves at high temperatures

Full size image

Although the slag viscosity can be affected by many factors, the main two factors are the composition and temperature. At a low temperature, the viscosity is determined mainly by the composition of the slag. The more complicated slag structure composition and the larger anion size will result in the slag particles having a higher difficulty of movement. Therefore, the slag viscosity could be increased. Otherwise, the viscosity would decrease. Because the arc temperature of underwater welding can reach 3500 °C, the boric acid, which is in the welding area and the surrounding water environment, will follow the decomposition reaction 2H3BO3 = 3H2O + B2O3. The compound B2O3 is produced from this reaction and goes into slag quickly. Slag’s quick cooling makes B2O3 gather in the slag. Therefore, the amount of B2O3 in welding slag will be more significant as the boric acid concentration in the water is increased. As shown in Fig. 3, at a low temperature (1440 °C), the viscosity of the 3# and 4# slag is clearly lower than that of 2#. Therefore, the boron oxide generated from the boric acid reaction in the welding process can significantly lower the slag viscosity. This relationship is identical to the findings obtained in many existing research studies. As a type of acidic oxides, B2O3 can easily form a eutectic mixture (such as \({\text{MgO}} \cdot {\text{B}}_{2} {\text{O}}_{3}\)), which would greatly improve the superheating degree of the slag. This property will promote complex anionic bonding group to disintegrate because of the drastic thermal vibrations. Thus, the slag viscosity will be decreased. In addition, the oxygen ions in the slag are distinguished within three typical structure types. The bridged oxygen (O°) is connected to two cations. Compared with the bridged oxygen, then on-bridged oxygen (O−) is connected to only a single cation. Moreover, free oxygen (O2−) is connected to two metal cations. Among them, the bridge oxygen can form a complex anion network. Therefore, reducing the amount of bridged oxygen in the slag is the main way to decrease the slag viscosity. In the welding slag, the Si-O bond tends to form a complex network, while the B-O bond energy in B2O3 is much higher than that of the other oxides, which can reach 561-561 kJ/mol. Thus, the strong affinity ability of the boron element makes it easy to seize bridge oxygen from other types of bonds (e.g., Si-O bond) to form a boron oxygen ionic group. This process can lead to the simplification of anionic polymerization and lower the slag viscosity. Therefore, the viscosity of slag that is obtained from experimental welding in boric acid aqueous solution will be decreased.

It is necessary to pay close attention to Fig. 3. This figure shows that the viscosity of 4# slag is higher than 3# even if the content of B2O3 in the 4# welding slag is more than 3#. The main reason is that during the 4# experiment, the concentration of boric acid solution is 35,000 ppm, which is close to saturation already. Thus, the acidity of the water environment of the 4# experiment is far higher than that for 3#. As a type of amphoteric oxide, TiO2 is an alkaline oxide under acidic conditions. Thus, the basicity of slag can be improved. Existing studies show that an increase in the slag basicity can significantly enhance the slag viscosity (Ref 17). Under an acidic water environment, the Ti element from TiO2 is more likely to form four ligand ions, which is a typical body to form a network. Therefore, more complex network structures appear in the slag, which increases the slag viscosity. The above analysis shows that boric acid solution with a high concentration could lower the slag viscosity when at a low temperature.

As the temperature is increased, the slag viscosity becomes markedly lower. The reason is that sufficient heat energy can prompt complex networks in slag to depolymerize and become destroyed. Moreover, a higher temperature can still cause the decomposition of TiO2 to provide free oxygen O2−, which will conduct the following reaction: \({\text{O}}^{2 - } + {\text{O}}^{0} = 2{\text{O}}^{ - }\). It is obvious that this reaction consumes bridged oxygen and decomposes the complex network that is composed of bridge oxygen. Thus, the slag viscosity could be reduced significantly at a higher temperature. When the temperature reaches a certain value, the complex network structure largely disappears. As a result, the viscosity will not continue to decline and arrive at a stable value, as shown in Fig. 4.

During the underwater welding, there could be some small metal particles that splash into the liquid welding slag. The strong cooling effect of the water makes slag solidify rapidly. Thus, the particles cannot be isolated from the solid slag. Due to the high melting point, metal particles usually cannot be melted under the high temperature in Fig. 4. So they will produce certain obstacles and prevent the flow of liquid slag at high temperatures. This circumstance can visibly increase the slag viscosity under a high temperature. Thus, in Fig. 4, the slag viscosity of underwater welding (2#, 3#, and 4#) is higher than on-land welding (1#).

Effect of Boric Acid on the Properties of Underwater Welding Joint

In terms of the molten pool reaction zone in underwater welding, it is inevitable for the boron element to enter into slag and weld joints through a metallurgical reaction. As shown in Fig. 5, when increasing the boric acid concentration, the content of the boron in the joint will be improved markedly. The boron can reduce the proeutectoid ferrite and increase the acicular ferrite to improve the hardness of the steel because of its strong ability to improve the hardenability (Ref 18). Thus, boric acid at a higher concentration in the water environment can enhance the hardness of the joints significantly, as shown in Fig. 6. However, in terms of the strengths of the joints, although the quantity of acicular ferrite will be increased with the improvement in the content of boron in the weld, the ferrite will be refined substantially; thus, the weld metal will have a higher strength when the content of boron has a smaller value. If the content of boron is increased to higher value, the boron tends to gather in the austenite grain boundary in the cooling process, to reduce the grain boundary energy. Therefore, the nucleation rate will be reduced as soon as the austenite transformation occurs. However, the boron element has almost no effects on the grains growing speed. Thus, the average size of the grain can become larger. Due to the heredity of the structure, the formative ferrite is also coarse, and the strength of the welding joints can thus be reduced (Ref 19). According to Fig. 7 and 8, with increasing boric acid concentration in the water environment, the strength of the weld metal will increase and reach a maximum at 2300 ppm and then decrease sharply for 35,000 ppm.

Fig. 5
figure 5

Secondary ion spectrum intensity of the boron element in weld

Full size image
Fig. 6
figure 6

Hardness of the 3# and 4# experimental welds

Full size image
Fig. 7
figure 7

Tensile strength of the 2#, 3#, and 4# experimental welds

Full size image
Fig. 8
figure 8

Stress-strain curves for the 2#, 3#, and 4# tested samples

Full size image

Considering the ductility of the welding joints, the quantity of acicular ferrite in weld metal can be larger as a result of the addition of the boron element. Because the crossing angle of the acicular ferrite grain boundary is large, the crack expansion requires more energy. The result is that the ductility of the welding joints is strengthened and the elongation can be improved, as shown in Fig. 9. It is important to note that although the boron content in the 4# weld is much larger than in 3#, their ductility are almost equal. A possible reason is as follows: although the increase in the boron content can improve the ductility of the welding joints to some extent, the boron can make the number of second-phase particles in the weld metal (such as the M-A constituent) be greater and then weaken the bonding strength if it is too large in the weld metal. Therefore, the ductility of the welding joints can be reduced by a certain amount. In general, even if the boron content in the 4# weld is much larger than in 3#, their ductility could be almost equal.

Fig. 9
figure 9

Elongation of the 2#, 3# and 4# experimental welds

Full size image

Conclusion

This paper outlines the effects of the boric acid concentration on the viscosity of underwater welding slag and the properties of the weld metal.

  1. (1)

    At temperatures that are lower than 1441 °C, B2O3 is generated from the boric acid reaction and reduces the slag viscosity observably. However, if the acid of the water environment becomes strong, then TiO2 will show the characteristics of alkaline oxide. Therefore, the slag basicity can be improved to increase the slag viscosity. To summarize, the viscosity of the welding slag decreases first and then increases with increasing boric acid concentration.

  2. (2)

    As the temperature is raised, the slag viscosity becomes markedly lower because of depolymerization and the destruction of complex networks in the slag. Moreover, when the temperature is above 1480 °C, the viscosity tends to a constant value.

  3. (3)

    The hardness of the joint increases with the concentration of boric acid solution in the process of underwater wet welding. The maximum tensile strength with an approximately 15% elongation is obtained at 2300 ppm.