Low carbon steel is a carbon steel with a carbon content of less than 0.25%. It is also called mild steel because of its low strength and hardness. It includes most of the ordinary carbon structural steel and a part of high-quality carbon steel, which are used for engineering structural parts without heat treatment, and some are used for wear-resistant mechanical parts after heat treatment such as carburizing and nitriding. The low carbon steel annealed structure is ferrite and a small amount of pearlite, which has low strength and hardness, and good plasticity and toughness. Therefore, it is cold-formed well, and can be subjected to cold working such as crimping, bending, punching, stretching, etc.; it is easy to forge, weld, and the like. Due to the low carbon content of low carbon steel, the content of alloying elements Mn and Si is not high. In general, it has good weldability and does not cause hardening due to rapid cooling of the welding thermal cycle but rather tissue embrittlement. Therefore, the weldment having a thickness of less than 70 mm generally does not need to be preheated before welding, and it is not necessary to maintain the interlayer temperature. However, for important welding structures such as boilers and pressure vessels, it is necessary to consider preheating and maintaining the interlayer temperature.
Low carbon steel can be welded by almost all welding methods. Binding can guarantee welding quality, such as: electrode arc welding, submerged arc welding, argon arc welding, secondary welding, flux cored wire gas shielded welding, electroslag welding, plasma arc welding , laser welding, electric resistance welding, friction welding, brazing, gas welding, etc. Low-carbon steel welding is the most widely used for electrode arc welding. The principle of using low-carbon steel for welding rods is mainly the principle of equal strength. Generally, the welding rods of E43×× series can be used, which can be selected according to the specific base material, load and environment; For more important structures or structures with complex loading conditions, low-hydrogen welding rods should be used as much as possible. For the selection of low-carbon steel welding consumables, refer to JB/T4709-2007. The commonly used low-carbon steel welding consumables are shown in Figure 1.
The preparation of low carbon steel groove can be carried out by cold working method or hot working method. The welding groove should be kept flat, without cracks, delamination, slag inclusion, etc., oil stain and rust in the range of 20~30mm on both sides of the groove. Clean impurities such as slag and water are cleaned up. The electrode is dried as specified. If it is cold weather in winter, it should be preheated 50~100°C when welding thick parts and rigid structures. The positioning welding of low carbon steel is the same as that of normal welding. The positioning weld can not be cracked, otherwise it must be re-welded; the two ends of the positioning weld welded into the permanent weld should be made to facilitate the arc. Generally, the arc should be led in the arc runner or the groove. The arc should not be arced at the non-welded part, and the arc pit should be filled when the arc is extinguished. Each weld is welded as soon as possible to avoid welding interruption. For important components, the interlayer temperature should not exceed the specified range during the welding process; when the weldment is preheated, the interlayer temperature should be controlled not lower than the preheating temperature.
Galvanized steel sheets are one of the harder to weld in low carbon steel. The main reason is that the galvanized layer brings welding cracks and stomata sensitivity to welding, zinc evaporation and soot, oxide slag, plating The zinc layer melts and destroys. Low carbon steel galvanized steel sheets can be welded by manual arc welding, gas metal arc welding, argon arc welding and electric resistance welding. For manual arc welding, the groove should be properly opened before welding, and the galvanized layer near the groove should be removed. The removal method can be flame baking or sandblasting. The electrode is selected to be as close as possible to the base metal. J421/J422/J423 welding rods are generally preferred; for galvanized steel sheets with a strength grade of 500 MPa or more, E5001/E5003 welding rods may be used; for galvanized steel sheets with a strength of 600 MPa or more, E6013/E5503/E5513 welding rods shall be used. When welding, use short arc as much as possible, do not swing, to prevent the melting area of the galvanized layer from expanding, to ensure the corrosion resistance of the weldment and reduce the amount of smoke.