Common welding methods for steel structure members

Welding is the process of combining atoms (molecules) of two separated objects to form a whole with the help of energy. The welding method can not only connect metal materials, such as steel, aluminum, copper, titanium, etc; It can also connect nonmetals, such as plastics and ceramics; It can even solve the connection between metal and nonmetal, which is collectively referred to as engineering welding. The structure made by welding method is called welding structure, also called engineering welding structure.
According to the object and purpose, it can be roughly divided into four types: building welding structure, tank and container welding structure, pipeline welding structure, and conductive welding structure. The so-called building steel structure includes these four types of welding structures. The selected structural material is steel, and most of them are ordinary carbon steel and low alloy structural steel. The main welding methods include manual arc welding, gas shielded welding, self-shielded arc welding, submerged arc welding, stud welding, spot welding, etc.
(1) Manual arc welding
The method of welding relying on the heat of the arc is called electric arc welding. Manual electric arc welding is a kind of electric arc welding with manual operating electrode, which is the most commonly used method in the welding of building steel structures.
The principle of manual arc welding is shown in Figure 7-1. The welding rod and weldment are two electrodes that produce an arc. The arc generates a lot of heat to melt the welding rod and weldment. The end of the electrode is melted to form a droplet, which is transferred to the base metal of the melted weldment and fused to form a molten pool and undergo a series of complex physical-metallurgical reactions. As the arc moves, the liquid molten pool gradually cools and crystallizes to form a weld.
Under the action of high temperature, the coating on the steel core of the welding rod is melted into molten slag, which is covered on the surface of the molten pool metal. It can not only protect the high-temperature molten pool metal from chemical reaction with harmful oxygen and nitrogen in the air, but also participate in the chemical reaction of the molten pool and penetrate into the alloy, and form a protective slag shell on the cooled and solidified metal surface.
(2) Gas shielded arc welding
It is also called gas arc welding. The welding wire and the weldment are used as the two poles. Arc heat is generated between the two poles to melt the welding wire and the base metal of the weldment. At the same time, the shielding gas is sent to the welding area to separate the arc, the molten welding wire, the molten pool and the nearby base metal from the surrounding air. The welding wire is automatically sent in. It is continuously melted under the action of the arc and fused with the molten base metal to form the weld metal. Its principle is shown in Figure 7-2. This welding method, called GMAW (Gas Metal Arc Welding) for short, can also be divided into CO2 gas shielded arc welding due to the different shielding gases. It is the most widely used welding method at present, characterized by the use of large current and fine welding wire, the welding speed is fast, the penetration is large, and the operation efficiency is high.
(3) Self-shielded arc welding
Self-shielded arc welding was once called gas-free arc welding. Compared with gas shielded arc welding, it has good wind resistance. When the wind speed reaches 10 m/s, it can still obtain a weld with no pores and excellent mechanical properties. Due to automatic welding, the welding efficiency is extremely high. The welding gun is light and does not use gas cylinders, so the operation is very convenient, but the price of welding wire is higher than that of CO2 shielded welding.
(4) Submerged arc welding
Submerged arc welding is a kind of arc welding in which the arc burns under the cover of fusible granular flux. The bare welding wire continuously fed into the molten pool is both a metal electrode and a filler material. The arc burns under the flux layer, melting the welding wire and base metal to form a molten pool. The molten flux becomes molten slag, which covers the surface of the molten metal pool and separates the high-temperature molten pool metal from the air. In addition to protecting the molten slag formed by the flux, it also participates in the metallurgical reaction with the molten metal, thus affecting the chemical composition of the weld metal.
(5) Narrow gap welding
This method uses the existing special technology of gas shielded welding. The groove section area of welded joints is smaller than that of manual arc welding or gas shielded welding, which is the characteristic of this method.
Narrow gap welding can be carried out at the position of flat welding, horizontal welding and vertical welding. The horizontal welding is suitable for the column joints on the project site. The flat welding and vertical welding are respectively suitable for the corner joints of box columns and the welding of columns and beams in the factory.
(6) Stud welding
Stud welding is a method of applying welding current between the stud and the base metal to make the local contact heating and jointing. It is mainly used for the welding of shear connectors and concrete anchor bolts. In addition, it is also widely used for the installation of connectors with heat insulation and sound insulation materials.
(7) Spot welding
First of all, the spot welding here is different from the spot welding in the assembly of building steel structure components. It is a method of resistance welding, which directly energizes the welding area, uses its resistance heating to locally increase the temperature of the welded part, and joins under pressure. Spot welding is commonly used in automobile industry and household appliances, and it is also used for complex joints in steel structures.