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How to choose the appropriate welding materials in the process of shipbuilding

In the shipbuilding mode, welding technology is one of the key technologies of shipbuilding, and how to effectively shorten the shipbuilding cycle is an eternal topic in the field of shipbuilding in China. There is no doubt that the improvement of production efficiency is fundamental to shortening the manufacturing cycle. Since ancient times, people have understood that the work must be done first. In shipbuilding, the choice of welding materials will directly affect the shipbuilding efficiency, casting cost, and construction quality. Therefore, its role must attract people’s attention. Selecting appropriate welding materials in the shipbuilding production process is important in improving the economic benefits of shipbuilding enterprises.

1. Commonly used welding materials

The welding materials used in the shipbuilding process mainly include carbon steel, low alloy high strength steel, stainless steel (including duplex stainless steel and super duplex stainless steel), low temperature steel (316L, 9Ni), and nickel base alloy (Inconel 625). Carbon steel and low alloy steel welding are mainly FCAW, and the ultra-high strength steel is still mainly manual. The duplex and super duplex stainless steel are mainly tungsten argon arc welding. The 9Ni and nickel-based alloys are welded by tungsten argon arc welding and manual welding. With the promotion of high-efficiency welding technology, the use of welding rods is decreasing yearly, and the proportion of automatic and semi-automatic welding is gradually increasing. In recent years, with the change in project requirements, tight schedules, and cost control, more and more problems are faced in the selection of welding methods and welding materials.

2. Main problems and solutions

(1) 9Ni steel welding project, 9Ni steel welding not only to meet the high strength, -196 ℃ low-temperature impact performance, but also to meet the requirements of the SSC (antihydrogen sulfide corrosion test), and 9Ni steel is magnetic, difficult to demagnetize, welding cracks easily, which put forward a higher demand on our choice of welding materials and the development of welding technology.
Firstly, regarding the magnetic problem of 9Ni steel, we must undergo demagnetization treatment before welding. Positioning welding is carried out above the groove, and the magnetic field on both sides of the groove is demagnetized to ensure smooth welding. It was found through analysis that the selection of welding materials is very important for welding cracks. The NiCrMo-3 type welding material selected by our company has a linear expansion coefficient similar to that of 9Ni steel, effectively reducing welding thermal stress. The carbon content is similar to that of 9Ni steel, while wNi=55%~60%, ensuring the content of the austenite structure and avoiding the occurrence of hard brittle martensite bands on the fusion line. wC ≤ 0.03% and the brittle temperature range is small. Using NiCrMo-3 type welding material to weld 9Ni steel effectively ensures the low-temperature toughness of the steel. This is because the high Ni content effectively prevents carbon migration, and the weld seam is of austenitic structure, effectively ensuring the low-temperature toughness of the weld seam.
Welding parameters to ensure the welding quality of 9 nickel steel is also extremely important; this project, involving the welding of this material for the thick-walled pipe, the use of GTAW bottoming, SMAW filler, cover the surface of the method of welding, to ensure that the quality of the welding material to save, the bevel design as shown in Figure 1. To ensure that the root of the weld channel molding and filler rod arc welding, as well as to avoid the phenomenon of burn-through, bottoming weld at least two layers of welding, weld thickness of at least 6.4mm, and then filler rod arc welding. Welding channel layout is shown in Figure 2.
For 9Ni steel welding process made the following provisions: heat input ≤ 2.2 kJ/mm; welding interlayer temperature ≤ 100 ℃; due to the 9% of the steel depth of fusion being shallow, poor penetration, the bevel surface is easy to produce unfused, so when the welding arc swing to the bevel surface, do a short stay to obtain a good fusion; in the multi-channel weld cover weld channel, bevel on both sides of the cover weld channel requires the extension of 1-2mm to ensure good welding fusion. These are strong to ensure the quality of 9Ni steel welding.
(2) Module project Inconel 625 composite pipe welding project Inconel 625 composite pipe is made of pipe (carbon steel – API 5L B) + liner (Inconel 625 material, thickness 3mm) composite. In addition to solving the Inconel 625 material itself is difficult to weld, the two materials combined with the welding of the layer are extremely complex. The most important thing to solve is the composite layer of the Fe element of the dilution of the problem; the choice of what material welding and welding process has become a difficult problem.

Through the specification, specification reading, and test, the whole weld used NiCrMo-3 type welding materials. To avoid the dilution of the iron element, we will be in the bevel with NiCrMo-3 type welding materials pre-stacking a layer (see Fig. 3), and then use GTAW priming, SMAW filler, cover the surface of the way to weld. We used foreign welding materials (UTPA 6222Mo and UTP 6222Mo) to ensure welding stability. The welding parameters and processes are also strictly controlled to ensure the smooth progress of the project.

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Figure.1 Bevel design

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Figure.2 Welding channel layout

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Figure.3 Pre surfacing of NiCrMo-3 type welding material inside the groove

3. Some ideas for the future development of welding materials

First of all, the current market competition is fierce, product manufacturing requirements are increasingly stringent, and they often have to face the problem of tight deadlines; the current project involves a lot of special welding materials (such as 9Ni steel, duplex stainless steel, and super duplex stainless steel welding materials), these welding materials need to be imported from abroad, the cycle is particularly long, and the price is very expensive, I hope that the domestic welding materials manufacturers to strengthen the research and development, to provide high-quality and high stability of the product, which is extremely important for the cost control and schedule. Cost control and schedule are extremely favorable.
Secondly, ultra-high-strength alloy steel welding materials, such as EQ70, for the 6G position, are the current basic use of welding rods and submerged arc welding wire. Try a lot of domestic and foreign FCAW flux-cored wire; low-temperature toughness is difficult to guarantee. I hope that the domestic and foreign welding materials enterprises, given this problem, the development of suitable for production of welding materials used to improve productivity and safeguard the stability of welding is very meaningful.
Finally, for green welding – health, environmental protection, and safety production, I think the future of welding materials research and development to improve its process, reduce dust, pollution, spatter, and noise, etc., to reduce the harm to the environment and the welder, at the same time, with the promotion of high-efficiency welding, I hope that the development of more and better welding materials, and I hope that welding robots can be faster to promote the marine industry, improve productivity. We also hope that welding robots can be promoted in the shipbuilding industry to improve productivity and liberate the labor force.
Author: Yu Caixia



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