This is the 14732nd article published by Metalworking (mw1950PUB).
Editor's Note
This paper mainly introduces the domestic RMD welding equipment in the Qianjiang -Shaoguan gas pipeline project five-bid section application process root joints unfused and dense pore defects, through the RMD welder built-in parameters thermal arcing, arc closing, external environmental factors and welder operation skills and other factors, put forward the equipment built-in parameters accurate setting, eliminate external factors affecting gas protection and other improvement measures, and achieved satisfactory results.
1 Preamble
Imported RMD welding equipment has been used in China for many years, its welding control is simple, the performance is stable, and it is widely used in the construction of large-diameter long-distance pipelines. With the introduction of domestic RMD welding equipment and the increasing improvement of technology, it is gradually promoted and used in the domestic market. Since 2018, the qianjiang-Shaoguan gas pipeline engineering line of the fifth bid section of the Qianjiang-Shaoguan pipeline construction by our company has a total length of 151.307km and a design pressure of 10MPa. The second type of the whole line adopts φ1016mm× 17.5mm spiral seam submerged arc welded pipe (SAWH), and the third type of area adopts φ1016mm×21.0mm straight seam submerged arc welded pipe (SAWL), and the steel pipe material L485M. The main line welding process selection is RMD root welding + semi-automatic self-protection cover, of which RMD root welding selects domestic welding equipment, the material adopts metal powder core welding wire ER70C-6MH4, the welding process regulations are WPS-QJSG-X02 (wall thickness 17.5mm) and WPS-QJSG-X06 (wall thickness 21mm), nondestructive testing using X-beam + phased array ultrasonic double detection.
2 Welding process
(1) Joint design
The joint form is a butt, V-shaped groove (see Figure 1). Groove angle a=22°±2.5°; blunt edge p=1.6mm±0.8mm; counterpoint gap b=2.5~3.5mm; when the wall thickness is 17.5mm, the wrong variable ≤ 2.2mm, when the wall thickness is 21mm, the wrong variable ≤ 2.5mm.
(2) Process requirements
The shielding gas is a mixture of gas (80% Ar+20%CO2); the purity of the protective gas: Ar≥99.99%, CO2≥99.9%; the gas flow rate is 15~20L/min; the preheating temperature is 80~150°C; the welding ambient temperature is ≥5°C; the humidity ≤ 90RH; the wind speed is ≤2m/s; the thickness of the root welding layer is ≤3.0mm.
(3) RMD root welding parameters
The specific values are shown in Table 1.
(4) Welding power supply
Select the DC welding power supply with RMD characteristics of domestic welding equipment with the corresponding wire feeder.
3 RMD root welding technical features
1) It can accurately control the size of the droplet transition and arc blowing force, the welding process is stable, there is less splash, the internal forming of the weld is uniform and beautiful, and the back of the weld is formed as shown in Figure 2.
2) During the welding process, the arc is soft and the heat is concentrated, which produces a high-quality depth of penetration at the root, which can effectively solve the welding deficiencies such as the root is not fused caused by the wrong edge of the nozzle and the uneven gap (see Figure 3).
3) Semi-automatic gas shielded welding metal powder core welding wire welding, welding surface without welding slag, high welding efficiency [1].
4 RMD root welding defect generation and solution measures
RMD root welding process belongs to the melting pole metal powder core welding wire semi-automatic gas shielded welding, in the construction process due to the domestic RMD welding equipment parameter setting is not accurate, welder operation technology is not skilled and external environmental factors and other reasons, resulting in radiographic and phased array ultrasonic detection found unqualified defects are mainly dense pores and interlayer unmelted [2]. Through the search of multiple weld defects, it was found that 90% of the defects were present in the RMD root weld layer, and the type and location of the defects were shown in Table 2.
4.1 The roots are not fused
Through ray negatives viewing and phased array data analysis, most of the root welding head unmelted defects are generated above the ring weld vertical welding, and at the same time, the angle grinder is used on the spot to repair the defect location, and the unmelted defect is that the root joint is not fused between the layers or the root unilateral side.
(1) Causes
1) When welding, the root welding starting point is from 12 points, and the welder generally adopts a small wire feeding speed in order to control the height of the back side of the molten pool, which can effectively ensure the forming height of the back of the weld. Although this small weld parameter can control weld forming, the probability of unfused defects is also very high.
2) When the RMD root welding arc welding, due to the low temperature of the melt pool during the new arc, the welding speed is too slow, so the molten pool will form a pile, resulting in the root weld seam is not completely fused.
3) When the root welding head, the welder will grind the arc into a slope in order to ensure the quality of the joint, and when the arc joint is raised, the welding torch runs to the slope-like gap of the grinding, which belongs to the surfacing welding, and the slope-like gap of the grinding generally has the characteristics of narrowness, depth and front width and narrowness, then the welding parameters of the re-arc position cannot effectively melt the front weld bead, and the molten pool here is small through the mask. The wedge-shaped ramp has not yet fully opened the molten pool, and the liquid metal formed by the molten weld wire flows onto the weld bead on the rear side, forming a joint that is not fused.
(2) Solution measures
Through communication with the R&D engineers of domestic welding machine manufacturers, and with the cooperation of experienced welders, it is proposed to adjust the arcing current in the built-in parameters of the welding machine and increase the precise control of the arc receiving current parameters, which can reduce or eliminate the defect of non-fusion of the joint.
The specific operation is implemented as follows:
1) Arcing parameter adjustment: Through the field multiple test adjustment, after the arc increase a period of constant pressure control, that is, the use of gas shielded welding mode to start the arc, during this period of time to increase the thermal arcing current, is conducive to quickly open the melt pool, enhance the fusion ability, thereby effectively reducing the probability of unfused deficiencies. The arcing phase waveform is shown in Figure 4, where the yellow is the arc voltage and the green is the welding current.
2) Arc receiving parameter setting: the special four-step function is used when the arc is closed, and when the downward welding head is about to close the arc, the welding torch switch can be pressed to increase the arc energy of the waveform control, increase the arc receiving current at this stage, ensure the effective melting of the base material, and achieve the purpose of reducing the lack of arc closing without fusion. The arc closing control scheme is shown in Figure 5, where the yellow is the arc voltage and the green is the welding current.
4.2 Dense stomatals
Through the ray inspection negative, the dense pore defect mainly appears in the upper part of the ring weld, and for the top 6 point position of the most difficult welding, due to the vertical angle of the welding torch and the shortening of the length of the shaft, there are basically no dense pores.
1) Through the on-site welding operation observation, the welder in the 12 point position after ignition arc continuous downward welding, in order to facilitate the observation of the shape of the molten pool and the root fusion, the angle of the welding torch, dry elongation has undergone great changes, resulting in the lack of gas protection on the back of the root welding pool, the occurrence of dense pores.
2) Because RMD root welding uses 80% Ar+20% CO2 as a protective gas, the gas stiffness is poor, the protective area is easily disturbed by the outside world and the air flow inside the tube, and the molten pool is not effectively protected during welding to form dense pores.
3) During the use of the welding torch, the nozzle is easily blocked by the splash, resulting in the weld due to the lack of protective gas flow and the formation of pores.
4) In the process of use, the pressure in the cylinder is too low, the flow rate is too small, the air supply system is damaged and the link is loose, etc., which is also easy to cause dense pore defects during the welding process.
1) Before the welding begins, external windproof measures should be done, especially the combination of the windbreaker and the ground and the pipe should be carefully inspected, and the wind inside the pipeline should also be prevented. Depending on the length of the connected pipe and the outdoor temperature, choose to block or open the nozzle at both ends to balance the pressure.
2) Carefully check the pressure in the cylinder before use, when the pressure < 1.0MPa, it will cause insufficient air supply to the welding torch nozzle, insufficient shielding gas stiffness, easy to produce porosity, so the use of replacement cylinders should be stopped. At the same time, it is necessary to be equipped with gas ratio detection instruments to strictly control the mixing ratio in the cylinder.
3) The gas flow selection should be adjusted in strict accordance with the requirements in the welding process regulations, and the flow size should be adjusted according to the size of the nozzle type selected, and the flow size should be adjusted within the scope of the process regulations.
4) Before welding every day, carefully check along the gas walking route from the beginning of the gas supply system to prevent air leakage due to damage to the air belt or loose joint.
5) According to the welding time, clean up the splash in the nozzle in time to prevent the welding splash from blocking the nozzle, causing the weld seam to produce porosity due to lack of gas protection.
6) The welding torch angle directly affects the welding quality during the welding process, and in the ring weld welding process, the welding torch angle should be adjusted at any time with the change of the welding position. Generally, the angle of the flat welding position and vertical welding position welding gun is 75 ° ~ 85 °, but it is very easy to appear dense pores at 9 ~ 11 points and 13 ~ 15 points of the ring weld, so the angle of the welding torch should be accurately controlled within 80 ° ±2 °, so that the welding shielding gas can effectively protect the front and back liquid molten pools from external air erosion and avoid the formation of dense pores. The angle of the welding torch in the upward welding position is generally controlled at about 90 °, and the angle of the welding torch should be slightly adjusted at any time according to the size of the gap and blunt edge during the welding process, so as to effectively ensure that the internal and external welds are formed uniformly.
7) When the dry elongation is too long, the distance between the nozzle and the workpiece is too large, and the protective range of the molten pool is reduced due to insufficient shielding gas stiffness, which is also one of the main reasons for the dense pores during the welding process. Therefore, the welder in the welding process to ensure a clear view of the melt pool at the same time, to choose the shortest dry elongation for welding operations.
8) In the welding process, due to the uneven speed of the welder driving the molten pool, the root welding thickness is too thick, and the shape of the molten pool section is too large, which is easy to cause part of the molten pool to leave the protective gas range and produce dense pores. Therefore, in the process of operation, the welder needs to appropriately increase the welding speed while ensuring the quality of welding.
5 Conclusion
After the increase of the built-in parameter thermal induction arc of the domestic RMD welding equipment and the fine control of the welding current during the arc collection process, the unfused defects generated by the joint are effectively controlled. At the same time, after continuous summarization and induction of on-site practice, the refinement and analysis of external interference factors, the improvement of process parameters in the welding process and the improvement of the operation skills of root welders, the welding efficiency and root welding quality have been significantly improved, and the defects such as unfused joints and dense pores of roots have been basically overcome. After the improvement, in the 1568 weld joints constructed by a welding unit, the pass rate of root welding ray inspection and phased array ultrasonic inspection reached 99.87%, and satisfactory results were achieved.
bibliography:
Jin Haicheng,Wang Junhong,Li Guangmin. RMD root welding technology for the second line of west-to-east gas transmission[J].Electric Welding Machine,2009,39(5):87-89.
Li Guoqing,Zhang Jingzhou,Feng Yandong. Analysis of RMD semi-automatic root welding operation difficulties[J]. MetalWorking (Hot Working), 2009(2): 44-45.
This article was published in Metal Processing (Thermal Processing), No. 12, 2021, pages 31-33, by Tang Haidong, Yue Yuanming, Liu Guangyu of Sinopec Shengli Oil Construction Engineering Co., Ltd., and Wu Guofeng, Thermal Branch of Sinopec Shengli Oilfield. Original title: "Analysis of Common Welding Defects in Root Welding Operation Technology of Domestic RMD Equipment".
Recommend Video
Pay attention to the metalworking video number, pay attention to the industrial media with attitude
-End-
Notice for Submission