What are the welding characteristics of carbon steel?

- Mar 18, 2020-

         (1) Low carbon steel: low carbon content, no tendency to harden, good plasticity, no special process is required during welding, no heat treatment is required after welding, and it is suitable for various welding methods.      

         However, it should be noted that the welding joint should be of equal strength, and there should be no defects such as pores and slag inclusions in the welding. These are related to the operation. Generally, the carbon content of the weld metal is lower than that of the base metal, which can be appropriately increased. Si, Mn to ensure the strength of the weld. When the carbon content is too high, above 0.24%, care must be taken to prevent hot cracks during welding and cold cracks after welding. The carbon content of some foreign steel grades is as high as about 0.30%, which can not help but be noticed. When welding these steels, it should be appropriately preheated. During welding of large-thickness structures, especially the first primer welding, cracks may occur if the steel is not preheated. In the winter (0-10 ℃) welding of 12-32mm low carbon steel plates, especially when different types of electrodes are used for electrode arc welding or T-corner welding of different plate thicknesses, micro-cracks of different degrees will occur. Such cracks will occur when the iron-iron ore plastic electrode is used to fillet welds with a thickness of more than 12mm. When using a low hydrogen electrode to weld a 500MPa strength steel fillet weld with a thickness of more than 32mm, micro-cracks are also generated in the weld. This will reduce the notch toughness and elongation of the weld metal, but has no effect on the tensile strength.

          In recent years, many boiler plants have found that the cold-bend test after welding cannot pass when submerged arc welding of medium-thickness No. 20 plates. but. If steel is selected according to the new standard, the phenomenon of high carbon, sulfur, and phosphorus can be overcome. Submerged arc welding of No. 20 steel is expected to pass the cold bending test. In the manufacture of large-scale power station boilers, large thickness steel plates are widely used. This requires electroslag welding to complete the welding work. Due to the obvious casting structure and coarse columnar grains in the welds of electroslag welding, normalizing is often performed after welding Tempering. Therefore, although we say that the weldability of low carbon steel is excellent, in actual work, it is necessary to analyze and deal with it on a case-by-case basis.

         (2) Medium carbon steel: Generally, medium carbon steel is used as forgings or hot-pressed parts to manufacture various gears, fasteners or shafts, and some are supplied in the form of steel plates and profiles. The common ones are Q275, 35 No. 45 and No. 45 steel. Due to the increased carbon content of medium carbon steel, there is a tendency to harden, so under the heat cycle of welding, defects such as overheating, hardening, porosity and cold cracks may occur in the joint. Therefore, pre-heating is required when welding medium carbon steel, and the pre-heating temperature is determined according to the carbon content, workpiece size and structural constraints. Generally, the temperature is 150-250 ° C. The electrode should be a low-hydrogen covered electrode with equivalent strength. Pay attention to drying; it should be slowly cooled after welding and high temperature tempering heat treatment is recommended. Generally, the tempering temperature is recommended to be 600-650 ℃. Special attention must be paid to the first layer when welding, because at the root of the groove, the fusion ratio of the weld is high, which increases the carbon content of the weld and has a large tendency to crack. If possible, U-shaped grooves should be opened and welded with small diameter electrodes and small process parameters. 

         (3) High-carbon steel: Generally high-carbon steel is not used to manufacture welded structures, and its welding is mostly repair welding or surfacing welding. The choice of welding electrode is based on the requirements of the welded part. E7015 G or E6015 can be used for high requirements, E5015 and E5016 for low requirements, or E309 and E309Mo welding electrodes can be used. Preheating should generally be above 250-350 ℃, and it should be slowly cooled and tempered at high temperature (650 ℃) after welding. No preheating is required when using austenitic electrodes. Welding should use small process parameters as much as possible, and hammering the weld should be used if necessary to reduce stress and prevent cracks.

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