Is the domestic underwater welding technology mature?

Domestic underwater welding technology is immature. Underwater welding is considered as an economical and effective method for the maintenance of large ships. In addition, underwater welding provides immediate wound repair and rescue capabilities to improve the utilization and durability of ships. The formal service of underwater wet repair welding technology makes it unnecessary for ships to enter the dry dock, and the maintenance cost of ships is greatly reduced. Another alternative repair method of 1 wet welding is to adopt non-mechanical waterproof wet welding technology. The cost of manual wet welding with solder joints is low, but in the past, the cost saved is often difficult to make up for the poor welding effect. Indeed, if special protective measures are not taken, the arc extinguishing effect of surrounding water and the existence of hydrogen in the welding environment will often make the weld brittle and hard, and the weld in HAZ (heat affected zone) will easily burst. In addition, the high cooling rate also prevents gas from being discharged from the molten pool, thus promoting the appearance of pores and bubbles. Therefore, underwater wet welding has always been considered as a temporary repair method before the ship enters the dry dock. 2 test plan recently, an in-depth test plan was carried out at HYDREX's headquarters in Antwerp, which deepened the understanding of the change of metallurgical properties caused by the situation that HAZ and base metal were heated first and then cooled by surrounding water, and also enhanced the understanding of the physical properties of the final weld. In addition, after several months of further training and using suitable covered electrode specially designed to deal with possible problems in underwater welding, divers of Hydrex Company can now weld wet welds meeting the requirements of AWS D3.6-93, which is the most widely adopted and accepted specification for underwater welding. 3 Quality Assurance This paper points out that the quality of underwater wet welding can be comparable to that of dry welding, as long as the following three main conditions are met:-Engineers must ensure that welders have a deep understanding of the phenomenon, and the equipment used should be designed for underwater use to ensure the construction quality and the safety of welders. Only through constant practice can we get good results. Many people think that welding only uses covered electrode to fuse two identical or different metals together, but it is actually much more complicated. Heating and cooling during covered electrode welding, especially for alloy materials and steel, will cause phase transformation (mainly depending on cooling rate and covered electrode metal composition), hydrogen-induced cracking (or cold cracking), cracking after reheating, stress-induced cracking, crystal phase cracking and so on. , and may lead to almost no similarity between the weld and the base metal or completely different physical properties. Typical inhomogeneities (porosity, lack of penetration, undercut, flash, slag inclusion) between welding materials and base metal shown in AIS07 will adversely affect the physical properties of welded joints. Four main problems that may occur in cold cracking underwater wet welding: 1. Hydrogen induced cracking. 2. Welding metal and HAZ (heat affected zone) are very good. 3. The existence of pores and slag inclusion. As the name implies, hydrogen-induced cracking (cold cracking) occurs at low temperature (generally below 300℃). It needs sensitive welding microstructure, which is related to the hardened zone and the existence of hydrogen. High stress intensity (more than twice the yield strength) near the welding zone may also increase the possibility of cracking. Hydrogen plays an important role in cold cracking, because its solubility in steel decreases at lower temperature (FCC of austenite becomes BCC of ferrite). Therefore, when the solution cools and solidifies, hydrogen will escape from it. However, the existence of hydrogen escaping from grain boundaries promotes the cold cracking of hardened structures. The hardness of the joint is related to the chemical composition and cooling rate. Low carbon steel is usually not easy to be hardened, so it is generally not easy to be exploded by hydrogen. However, the existence of manganese in carbon manganese steel will increase the hardening ability. Obviously, high cooling rate and increased hardening ability will lead to increased hardness. The lower cooling rate makes hydrogen escape from the grain boundary and leave the wrong position. According to the welding situation on the water surface, it is considered that steel is prone to hydrogen-induced explosion, so it is necessary to preheat the base metal containing steel before welding to reduce the cooling rate. Obviously, although preheating in wet welding is not impossible, it is actually unrealistic. On the contrary, if we don't pay special attention to this influence and don't consider this influence when designing the welding process, the quenching effect (extremely high cooling rate) caused by the surrounding water will greatly increase the final stiffness of the welding place. Therefore, wet welding machine needs other methods to deal with abnormal cold cracking. 5 reducing toughness as mentioned above, wet welding is easy to form too high hardness of welding metal and HAZ hardness due to the quenching effect of surrounding water. Very high cooling rate will produce fine grain structure, which will reduce the toughness of weld in most cases. This reduction in toughness is extremely unfavorable to all dynamic structures (most ships obviously belong to this type of structure). Another negative effect caused by high cooling rate may be porosity (bubbles) and slag inclusion in the weld. If the cooling of the molten pool is very slow, bubbles will be discharged from the weld and spread to the atmosphere. At the same time, welding slag (covered electrode slag) will float on the surface of the molten pool to protect the molten pool from the negative influence of the surrounding atmosphere. However, if the welding metal solidifies rapidly, there will be no time for bubbles to escape. Engineers who fully understand these mechanisms and defects will consider these factors in welding design, which is the first step to achieve high-quality underwater wet welding. Lack of understanding, or worse, no engineers at all, may have serious consequences, resulting in welding that once brought bad reputation to wet welding. Recently, people have specially made covered electrode for use in water. Covered electrode of these coatings always depends heavily on the action of nickel mentioned above, but they also have other characteristics. Their coating is a very complex mixture of components, all of which are considered to have a positive impact on welding quality. Although it is expensive, the product can be used to obtain underwater wet welding with the same quality as that on water. The third and perhaps the most important human factor is the welder himself. The best ASME (American Society of Mechanical Engineers) 1X Gb water welder should be invited to perform fairly simple G3 wet welding. The result will be amazing, although it is a pity: it doesn't look like a welding operation. Compared with underwater welding, underwater welding is a skill that needs to be practiced for several years. An excellent underwater welder is an expert. He can judge whether the current, voltage and moving speed are appropriate by looking, touching, listening and smelling. The divers of Hydrex Company study in the company's own welding school, and apply what they have learned to the real world-the ship, and constantly improve their skills. The test results show that the above three factors have produced the expected effect in HYDREX. Recently, according to the requirements of AWSD 3.6-93 B welding specification, a covered electrode called WPS was developed and tested. It is found that both WPS and welders meet all the technical requirements of the above specifications, and formal or temporary underwater wet welding repair is usually allowed in the presence of the surveyor of classification society. The indoor fracture test even revealed that the hardness of Sharpy V-notch varied from 35 to 70J, and the welders of Hydrex burned the weld that met the most stringent bending test conditions (2t bending radius) for many times. These two tests have higher requirements for AWSD3.6—93A welding than for B welding. It is now used as the specification of the Underwater Hull Repair Act. Surveyors of major classification societies have witnessed the qualification test of Hydrex welding procedure specification and are surprised by the welding quality. It seems to be a big step towards formal underwater wet repair.