The proper use of welding electrodes directly affects the chemical composition and service performance of the weld, which is a crucial part of the welding preparation for large - diameter seamless steel pipes. To select the appropriate welding electrode, multiple factors need to be considered comprehensively. Sometimes, tests are required for verification before a final determination can be made. These factors include the service performance requirements of the weld metal, the shape, rigidity of the welding electrode and the weld position, the crack resistance of the weld metal, the operability of the welding process, the equipment and construction conditions, as well as economic rationality.
Heat treatment of large forgings for large - diameter seamless steel pipes: Large forgings are mainly forged directly from steel ingots. Therefore, when conducting heat treatment, the impacts of processes such as smelting, ingot casting, and forging on the internal quality of the forgings must be taken into account. The main influencing factors are: uneven chemical composition due to the existence of various smelting defects; coarse and uneven grains; a relatively large amount of gases and impurities; significant forging stress and heat - treatment stress.
The purpose of heat treatment after forging large forgings is to prevent white spots and hydrogen embrittlement, improve the internal structure of the forgings, eliminate forging stress, reduce hardness and improve machinability, so as to obtain good mechanical properties or prepare the microstructure for subsequent heat treatment. The forging of large - diameter seamless pipes belongs to the forging of large forgings.
The main difference between large - diameter seamless steel pipes and welded pipes lies in the forming process. Ordinary steel pipes, such as tap - water pipes, are generally formed by bending flat plates and then welding. You can find a weld on them. For relatively thick - diameter pipes, there are usually spiral welds. Seamless steel pipes are generally formed by extruding molten steel through an annular slit and then processed by stretching and other processes, so there is no weld. In terms of performance, especially in terms of pressure - bearing capacity, seamless steel pipes have a significant improvement compared to ordinary steel pipes, so they are often used in high - pressure equipment, such as in the pipeline connections of hydraulic equipment. The welded part of ordinary steel pipes is their weak point, and the weld quality is the main factor affecting their overall performance. People who have lived in the north generally have the experience of tap - water pipes or heating pipes bursting in winter, and the burst parts are usually at the welds. Welded pipes are not seamless steel pipes.
The production process of seamless - like steel pipes: The seamless - like treatment of steel pipes is mainly achieved through stretch - reducing. The stretch - reducing process is a continuous rolling process of hollow base metal without a mandrel. On the premise of ensuring the welding quality of the base pipe, the stretch - reducing process of welded pipes is to heat the entire welded pipe to above 950 degrees Celsius, and then roll it into a finished pipe with a certain outer diameter and wall thickness through a stretch - reducing mill (the stretch - reducing mill has 24 passes). The hot - rolled pipes produced by this process are essentially different from ordinary high - frequency welded pipes. After heating in a heating furnace, the metallographic structure and mechanical properties of the weld and the base metal can be completely consistent. In addition, through multi - pass rolling by the stretch - reducing mill and automatic control, the dimensional accuracy of large - diameter seamless steel pipes (especially the roundness of the pipe body and the wall - thickness accuracy) is superior to that of similar seamless pipes.
