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Invar steel, I believe many people do not know this steel. This is a material with a very small application area, but its welding is notoriously difficult to weld. For a long time, the manufacturing process of Invar steel has been monopolized by France. After 4 years of research and development, my country has successfully broken the monopoly of French manufacturing and achieved independent production.
Invar steel is a nickel-iron alloy with 36% nickel, 63.8% iron and 0.2% carbon discovered by French physicists in 1896. Its expansion coefficient is almost zero at 230°C, so it is Named Invar, translated as "unchanged", and transliterated into Invar steel.
The application of Invar steel is mainly in the manufacture of LNG carriers. The LNG ship is a dedicated transport ship for liquefied natural gas. The storage of liquefied natural gas is very limited, that is, it needs to be stored and transported in an environment of minus 162°C. If the hull container uses general steel materials, the hull will burst due to the low temperature, and in severe cases, a huge explosion will occur. The characteristics of Invar steel are very suitable as storage containers for liquefied natural gas.
Invar steel is very different from ordinary steel. The alloy content of Invar steel is much higher than that of ordinary steel. It is easy to cause poor welding performance during the welding process. Therefore, the welding of Invar steel is also known as the most difficult in the world. Welding Technology. Invar steel currently only uses argon tungsten arc welding. The surface of the material needs to be cleaned before welding. The concentration of argon shielding gas must reach 99.999%, and the ambient air dryness must be higher than 60%. Welding personnel must be professionally trained and obtained Welders can only engage in welding with the highest grade certificate and G grade certificate.
Such demanding and difficult welding is bound to have its limitations. The TIG welding process has a large heat input, and it is difficult to control workpiece deformation and thermal cracks. TIG welding is definitely not a good welding technique. At present, the thickness of Invar steel is mainly 0.7mm, 1.0mm, and 1.5mm, which is very suitable for the application of laser welding. The laser welding energy is concentrated and the edge part has a large temperature difference, which will increase the welding stress and affect the welding. The strength of the site. Laser welding also has certain limitations. There are currently envisioned laser welding methods: using spectroscopy to obtain dual beams, one beam preheating, and one beam welding, which can be performed simultaneously, which can effectively reduce the temperature gradient during welding and reduce the stress. Improve the strength of the welded part. I believe that in the near future, laser welding will surely provide a perfect answer for the welding of Invar steel.
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