Hydrogen is an element that seriously harms the performance of welds. The presence of hydrogen may cause defects such as hydrogen embrittlement, white spots, pores, and cracks in welds, which may lead to failure of welded structures in severe cases. Therefore, the hydrogen content in some important welds must be controlled. Due to the high operating pressure of large-diameter oil and gas pipelines, the welds are subjected to greater tensile stress. During production, submerged arc automatic welding technology is used, and low-hydrogen welding materials are selected to ensure that the hydrogen content in the welds is low. However, in the production process of straight seam submerged arc welded pipes, welds occasionally have a small amount of hydrogen-related defects. Finding and eliminating the source of hydrogen in the weld is the main task of preventing hydrogen-induced defects in straight seam submerged arc welded pipes.
Cause analysis
Hydrogen-induced defects in welds are related to the amount of hydrogen content. When the hydrogen content in the weld is low, hydrogen embrittlement occurs; when the hydrogen content in the weld is high, white spots often appear on the tensile or bending section; when the molten pool absorbs a large amount of hydrogen, pores will appear in the weld; when the diffused hydrogen content in the weld is high, the tensile stress is large and cold cracks are generated.
The measures to control hydrogen in the weld are:
First, limit the hydrogen content in the welding material;
Second, remove the rust, oil, adsorbed water, etc. on the surface of the welding wire and the weld groove before welding;
Third, reduce the hydrogen content in the weld through metallurgical treatment; Fourth, control the welding process parameters; Fifth, dehydrogenation treatment after welding. In the production process of straight seam submerged arc welded pipes, the hydrogen content in the weld is mainly controlled by the first two measures.
① To ensure the ultrasonic flaw detection effect of steel plates, the amount of water used as coupling water is large. When the production cycle is fast, the residual water on the surface of the steel plate does not dry when the steel plate reaches the forming stage. After forming, it remains in the steel pipe. Since the internal welding is performed at the 6 o’clock position below after the steel pipe groove rotates to the bottom, the water in the steel pipe flows into the groove, and pores are generated after welding.
②Due to the high temperature in summer, the hydraulic station of the forming machine dissipates heat slowly, the oil temperature rises, the hydraulic cylinder leaks oil, and some oil drops into or near the inner weld groove, resulting in pores in the inner weld during welding; since the location of the pores is close to the root of the inner weld, the root of the pores is easily penetrated by the molten pool metal during external welding, and the metal flows into the pores to produce beads; when the pores in the inner weld reach the weld surface, the root of the pores is penetrated and the liquid metal flows to the weld surface to form weld nodules.
③The inner welding flux of the straight seam submerged arc welded pipe has a long conveying distance and is conveyed by compressed air. Since the compressed air contains moisture, to prevent the water in the compressed air from affecting the welding, a heatless regeneration dryer is used to remove water with spherical aluminum chloride as a desiccant. After long-term use, the adsorption capacity of the desiccant is greatly reduced or even ineffective. In summer, the air temperature is high and the moisture content in the compressed air is high. The compressed air treated by the dryer still contains a large amount of moisture. Although the flux is strictly dried before use, it absorbs moisture during transportation. The moisture in the compressed air enters the flux and enters the welding pool through the flux, which eventually leads to hydrogen embrittlement in the weld.
④ Butter is the butter applied to the crane hook for the convenience of loading and unloading during the transportation of steel plates. It is not cleaned up during loading. The cleaning belt sweeps the oil to the edge of the steel plate when cleaning the surface before pre-bending. When the steel plate passes through the centering roller, the oil on the edge sticks to the centering roller. When the steel plate that has been milled off the line is put back on the line, the edge of the plate contacts the centering roller, and the oil on the centering roller sticks to the downslope, resulting in pores in the outer weld.
⑤ The amount of oil dripped by the crane is small, and hydrogen embrittlement occurs in the weld. When pores are found in the nearby welds and repaired, the local tensile stress increases, causing cold cracks in the weld. The above cases show that in the long-term production process, the abnormality of related equipment may cause water or oil to exist near the welding groove, causing hydrogen-induced defects in the weld. Only by timely eliminating equipment failures and eliminating the source of hydrogen can hydrogen-induced defects in the weld be prevented.
Hydrogen-induced defects in straight seam submerged arc welded pipes are related to oil stains and adsorbed water near the weld groove. Preventing water and oil from entering the welding molten pool is the main measure to prevent hydrogen-induced defects in the weld. The oil stains and adsorbed water near the weld groove can be controlled by the following points:
① The coupling water on the surface of the steel plate should be completely removed after flaw detection, and the steel plate surface should be prevented from freezing in winter;
② Prevent the molding machine oil from dripping near the steel plate groove;
③ Before welding, check whether there is oil, water, and rust near the steel pipe groove. If there is, it should be removed before welding; the desiccant in the compressed air dryer should be replaced regularly;
④ The steel plate should not come into contact with oil during transportation, and the oil stains on the steel plate surface should be removed when loading;
⑤ The overhead crane in the welding area should stay in the position after external welding as much as possible.
Post time: Nov-06-2024