In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.


A single sheet of A4 paper is 0.088mm thick, while the Invar steel is only 0.7mm thick. This kind of steel sheet is equivalent to the thickness of 8 sheets of A4 paper. Only China and France can produce it in the world. Invar steel is the most important material for the production of LNG ships. LNG turbo locomotives are liquefied natural gas carriers. The construction of such a ship is very difficult. South Korea, known as the "crown jewel" of the shipbuilding industry.

In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.

Whoever comes in contact with the Invar should wear moisture-wicking gloves

The main core technology of invar steel production is in the hands of the French company GTT, and at present, only my country can independently develop invar steel. The patented Invar steel technology mastered by GTT has reached the level of monopoly. The United States, China, South Korea, Japan and other major countries in the world cannot do without the technology approval granted by GTT. Even if GTT does not produce Invar steel, but only provides authorized technical services, still remains a huge profit.

French company GTT

LNG ships are used to transport liquefied natural gas. To facilitate transportation, natural gas must be liquefied. The main component of natural gas is methane. 162°C for methane liquefaction. The problem is that at such a low temperature, ordinary metals will be subject to cold cracking, that is, cracks that form when the internal stress of the metal exceeds the tensile strength of the alloy in a low-temperature environment.

In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.

LNG ship

The strength of the metal will greatly decrease at low temperatures. An aluminum plate 1 meter long will become shorter by 2.5 millimeters if the temperature drops by 50 degrees. This is a devastating disaster for LNG ships. The best material for building LNG ships is Invar steel, discovered by a French physicist back in 1896. The main components are 63.8% iron, 36% nickel, 0.2% carbon and a very low coefficient of thermal expansion. to put it bluntly, it does not expand when heated and does not contract when cooled.

In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.

Cold cracking occurs in metals at low temperatures

Invar alloy has low expansion coefficient, low strength and hardness, low thermal conductivity, high ductility and toughness, and is mainly used in aerospace remote sensors, low expansion thermal bimetal layers, sealing materials for electronic components, microscopes and astronomical telescopes. Zhongda lens support system, precision instruments, structural parts in the waveguide, special transmission cables, increase the stability of ion lasers, etc. There is also a layer of insulating tank lining between the LNG body and the cabin, all of which are made of Invar steel, which does not expand and withstands low temperatures.

In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.

Internal wall of the cargo compartment of a gas carrier of a membrane type

Although Invar steel can be used to "freeze" natural gas, it is very brittle. The thinnest steel plate is only 0.7mm. If there is sweat on your hands, if you touch it with your bare hands, it will rust in less than a day. Thus, the entire steel sheet is useless, and it will affect the neighboring steel sheets. Therefore, during the welding process, workers should wear special moisture-wicking lambswool gloves without sweat.

In the world, only China and France can produce Invar steel, which is the most difficult material to weld, and France has monopolized it for 40 years.

Interior of an LNG ship's cargo tank

Invar steel material thickness is divided into three specifications: 0.7mm, 1.0mm and 1.5mm. Making this special steel as thin as paper is not an easy task, especially in the manufacturing process. complex and must go through many processes, such as electric arc furnace melting, vacuum induction melting, low carbon steel refining, vacuum oxygen decarburization, vacuum arc remelting, electroslag remelting, continuous casting, hot rolling, cold rolling, etc. And Germany cannot produce Invar Steel. Currently, only China and France have such technology.

Invar steel connection

South Korea is the country that produces the most LNG-powered ships in the world. Last year, three shipyards in South Korea, Hyundai Heavy Industries, Samsung Heavy Industries and Daewoo Shipbuilding, built 100 LNG-powered ships with a design size of 137.4 billion yuan for Qatar. However, South Korea wants to import French company GTT's Invar steel and uses GTT's No. 96 Liquid Cargo Containment System, so for every LNG produced by South Korea, it has to pay about US$10 million in patent fees to France.

Inside the tanker

The cost of Invar steel is very high, it is 20 times higher than the price of ordinary steel. It is sold not in tons, but in kilograms. 1 kg costs at least 100 yuan. A standard 147,000 cubic meter LNG ship needs about 450 tons of invar steel, the cost of invar steel alone is about 48 million yuan.

Dear Invar

The inner tank of a conventional LNG vessel is welded from at least 3,600 pieces of conventional large Invar steel sheets and tens of thousands of irregular small Invar steel sheets. The total length of these welds can reach 140 kilometers. Most welds can be done with welding robots, but many small and irregular corners must be done by hand, and 10% of welds require manual labor. A hole the size of a grain of rice can be fatal, causing the natural gas to vaporize and explode.

Invar steel welding

Until 2005, there were 16 best welders in my country who could weld Invar steel. Invar steel is one of the most difficult materials to weld due to its special physical properties, low hardness and thinness, much higher alloy content than regular steel, and poor weldability. If you want to learn Invar steel welding technology, you will need at least 200,000 RMB for tuition fees, and you must pass the rigorous assessment of professional institutions before you can get the highest level welder certification from GTT. the certificate has the right to weld invar steel and review every month.

Invar Steel Welder Qualification Certificate - Highest Level

It's liquidth cargo tank inside an LNG tanker. Its inner wall looks shiny and is welded from pieces of Invar steel sheets. The thinnest part is only 0.7 mm, and the length of one weld is about 100 kilometers. For normal operation at a low temperature of minus 163°C, the LNG vessel cargo hold maintenance system consists of two layers of heat-insulating boxes and two layers of film, which is also the most technically meaningful and most complex part of the design. the whole body.

Inside the tanker

The external thermal insulation material of the LNG vessel is perlite-filled laminate, and the internal main and secondary barriers are made of invar steel. There are usually 4 huge liquefaction tanks loaded with natural gas in the hull, and the welds of hundreds of kilometers can not have a small leak . If the invar steel is inadvertently exposed to fingerprints or sweat, the corroded invar steel will also corrode the surrounding invar steel, and they all need to be replaced, which will take a long time, and the shipyard will delay the construction period and be unable to bear it.

The welding process is very complex and long

The dryness of the air in the workshop should be above 60%. To ensure the sealing of the welding workshop, dehumidification and air conditioning are necessary. Wipe with anhydrous alcohol above 99.7% first to remove moisture, dust, oil, etc., and remove burrs with a file. Do not polish quickly back and forth, but lightly polish from top to bottom, and clean immediately after polishing. During the welding process, senior welders should have good protection, wear sheep's wool gloves, and there should be no sweat or fingerprints on the parts to be welded. Welders are required not only to be careful in their hands, but also to be calm in their hearts. Emotional fluctuations of welders directly affect the quality of welding.

Machine welding can only be done at 90%

A tungsten argon arc welding machine is used, which must be shielded with argon with a purity of 99.999%. When positioning, the beginning and end of the arc must be completed as soon as possible. Just click once to start the arc. .Extinguish the arc immediately. When welding, the tungsten electrode should be close to, but not touching, the edge corners of the two overlapping Invar steel plates. It should be welded back and forth in a Z-shape from right to left. welding torch oscillates 3 times in 2 seconds pause. The welds between the steel plates must be fired just right so that they cannot be welded, and the insulation box at the back cannot catch fire due to the high temperature.

Many places can only be soldered by hand

The above requirements are easy, right? But for a weld less than 4 meters long, it takes 5 hours for the senior welder to complete the weld, and there is no pause in the middle, it needs to be formed once, and it is uniform, like fish scales. It takes 3 hours to stop and rest. Basically, it's more likeembroidery, the stitches are embroidered evenly, and finally, as in tailoring, pieces of invar steel are joined together. At present, there are less than 200 workers in China with GTT welder qualification certificates.

Great cargo tank

The thickness of the Invar steel sheet is consistent with the application of laser welding, but the energy of laser welding is concentrated, and the edge part has a large temperature difference, which increases the welding stress and affects the strength. Therefore, laser welding also has great disadvantages. The solution is to use the spectroscopic method to obtain double beams, one preheating beam and one welding beam, which can be carried out at the same time, which can effectively reduce the welding temperature gradient and reduce the welding of invar steel is similar for embroidery.

Senior welder welding invar steel

Because this kind has become too difficult to weld, so only the Daewoo shipyard in South Korea used to be able to build LNG ships in the world, but the current situation has changed. My country can produce not only Invar steel, but also attaches great importance to the qualifications of senior welders. The training mainly involves traveling to Europe to learn welding technology, which also makes China one of the few countries in the world that is proficient in both invar steel production and welding technology, and can produce LNG ships.