Selection of welding process

Posted by chase Liu on December 15th, 2022

At present, in addition to manual welding, the welding process of welded steel tubes and seamless steel pipes mostly uses submerged arc automatic welding process and gas shielded welding process.

Submerged arc automatic welding process

Submerged arc automatic welding has the characteristics of good weld shape, high welding efficiency and low welding cost. For pipeline construction, submerged arc automatic welding can be used for double-pipe joint welding, referred to as "two-to-one", that is, the welding torch is fixed at a certain position and the pipe rotates. Obviously, it is impossible to rotate the pipe during long-distance pipe welding, so "two-to-one" can only be used in the prefabrication of pipes.

If the submerged arc welding process is adopted for the all-position automatic welding of pipelines, the welding device must be equipped with a flux feeding, supporting and recovering mechanism, which makes the structure of the welding device more complicated and brings inconvenience to operation and assembly. Moreover, the load of the walking trolley is increased, which affects the stability of the trolley walking. Submerged arc welding generally adopts the welding method of thick wire and high current. It is used for all-position automatic welding, which may cause welding defects such as droplet sagging and flow due to the high deposition rate, which will affect the shape and quality of the weld. Therefore, it is very difficult to apply submerged arc welding to all-position automatic welding of pipelines.

Gas shielded welding process

If the welding process of flux-cored welding wire and gas shielding is used, if it is formed in multiple times, it will be labor-intensive and time-consuming to clean the slag on the surface of the weld each time. If it is forced forming, a forming copper slider that moves with the welding torch must be added, and circulating cooling water can be introduced to greatly improve the welding efficiency. In this way, not only the structure of the welding device is complicated, but also the weight increases.

Because the price of flux-cored wire is higher, and the gas source of shielding gas must be solved at the same time, the welding cost is higher. The single use of self-shielded welding wire saves the shielding gas, but there is a problem of difficult slag removal.

Welding process using solid wire plus gas shielding. If it is multi-pass molding, the welding process can be simply divided into three stages: priming, filling, and covering. There is no need to clean the surface of the weld and proceed directly to the next process, but the welding speed is slower than that of forced molding.

The protective gas is generally a mixture of pure carbon dioxide gas, carbon dioxide and argon or carbon dioxide and oxygen. The mixed gas of carbon dioxide and argon can make the arc burn stably and have less spatter during welding, but the source of argon gas is difficult to find and the price is high in field construction. From an economic point of view, it is best to use pure carbon dioxide as a shielding gas when welding oil pipelines. For construction in areas where conditions permit, it is ideal to use carbon dioxide and argon as protective gases.