Discussion on welding technology and quality control of pressure pipeline engineering
In the process of pressure pipeline installation, welding defects are easy to form due to various reasons. The common welding defects include incomplete fusion, undercut, slag inclusion, welding porosity, etc. Welding defects will not only seriously affect the quality of the pipeline, but also form the pipeline transportation liquid leakage, and then produce a series of safety risks, causing serious economic losses, thus increasing the investment cost of the pipeline, and then inhibiting the development of the enterprise. In view of the pressure pipeline welding needs comprehensive monitoring, to ensure the welding quality, to ensure the safety and stability of the pipeline.
Overview of pressure pipeline welding
- 1) Pipe positioning and welding assembly. When carrying out work tasks, the staff should select the most suitable joints for welding joints. According to the process regulations, the fundamental problem in the welding gap range is that the form of the interface should be matched with the seam between the welding edges to improve the pipeline in all aspects. The quality of the welding interface ensures that there will be no major quality problems such as indentation and incomplete penetration of the interface after welding.
- 2) Weld bottoming work. Before the first and bottom welds are welded, determine whether to perform heat treatment according to the material. If heat treatment is required, first prepare the place to be welded by pre-heat treatment, and it needs to be depressed when beaded iron beads appear Arc and rotating welding rods are used to extinguish the arc, and finally a molten pool is formed. In the process of the secondary arc starting, the inner fillet welding electrode is moved closer to ensure the full contact of the arc tube wall to ensure that the high quality standards of welding are achieved.
- 3) Supplement of welding port. The necessary preparations must be made before the formal welding of the pipeline weld. It is necessary to ensure that the welding coating and welding slag are completely cleaned up and there will be no excess waste affecting the work. In the specific work process, the entire pressure pipeline is filled and welded. It is necessary to ensure that the width of the welding banner to be welded meets the corresponding standard, and the length of the arc must be fully grasped. Only in this way can the temperature of the molten pool be quickly raised to the corresponding level, and the residue and the residue on the pipe interface The dissolution of stomata is the basis for the scale and quality of the next work.
- 4) Welding of the cover layer. To some extent, the bonding and welding process of the pressure pipeline is similar to the welding of the cover layer, so the same operation method can be used for construction. However, it is important to pay special attention to the fact that the staff, during the process of swinging the electrode left and right, correctly ensure the uniformity of the action, thereby improving the appearance of the weld. At the same time, the weld height should be offset by about 0.2cm in advance, which means that the two ends of the weld bead at the cover layer should not be less than the 0.2cm length of the groove position.
- 5) Seal welding. The welding process operation of the plugging must be carried out after the exchange of assembly, positioning, and filling layer. First of all, the staff must remelt each weld bead in the pressure pipeline, and then start the back cover welding operation to ensure that the size, width, and height of the weld are consistent with each other, and further improve the smoothness and appearance of the welding result , While reducing the adverse effects on the pipeline flow function. Finally, the staff must deal with the problems that arise during the gradual welding process, such as depressions, weld bumps and shrinkage holes, so as to improve the stability and reliability of the pressure pipeline.
- 6) Pipeline inspection. Inspection is the last construction method of pressure piping engineering. It requires the staff to carry out meticulous inspection of the welding of the pipeline in accordance with scientific and reasonable technical standards. First of all, the staff measured the pressure of the pipeline to check whether its tightness was qualified. Subsequently, X-ray internal flaw detection was performed to check whether all welded parts of the pressure pipeline had excessive overlap. Finally, the anti-corrosion performance test of the welded pressure pipeline is to ensure that the pipeline will not be affected by external factors and cause corrosion.
Analysis of welding defects of pressure pipeline
Welding unfused pressure pipelines
During the welding process, unfused welding is one of the main reasons for the current welding defects. Because the welding material and the base material are not fused together during high temperature fusion, the two are in a separated or semi-separated state. As a result, the unfused part of the pipe installation is formed, and the position will generally appear at 11 o’clock and 1 o’clock, and it may also appear at 6 o’clock. These three positions are basically high-prone areas where welding is not fused. The main manifestations of non-fusion defects are incomplete fusion of the welding material and the base material. The specific location at the groove of the workpiece is called the groove not fusion, and the specific location at the root is called the root not fusion. Once there is no fusion, it is easy to cause the concentration of stress at the weld bead, and if this happens, it is easy to cause a series of safety accidents.
Pressure pipe cracks refer to cracks in the weld and heat-affected areas during welding operations or after pipe welding .
The reasons for cracks in pressure pipelines are as follows:
- (1) Pipeline welds are too concentrated or unreasonably distributed.
- (2) There is a problem with the welding sequence, which will cause excessive contraction of the pressure pipeline during the welding process, which will hinder the free expansion and contraction of the weldment.
- (3) Preheating before welding of pressure pipes and unreasonable post-heat treatment measures can easily lead to cracks in pressure pipes during application, which will have a negative impact on the later applications of the pipes. Cracks are the most dangerous defect in pressure pipelines. On the one hand, the existence of crack defects in pressure pipelines will cause the strength of the material to decrease during application, and on the other hand, there will be a high degree of stress concentration. If the pipe is subjected to load, this will cause The cracks in the pipeline during application further expand, which will damage the performance of the pipe fitting and affect its application.
Welding slag inclusions
During the welding process, welding workers often do not have superb technical level, so that slag remains in the weld metal and forms welding slag inclusions. Welding slag inclusions have no clear location and have a high probability of occurrence. , It will seriously affect the quality of welding metal, and even affect the setting of welding current and the cleaning of welding position.
Welding pores are prone to occur during pipeline installation, mainly due to the residual gas inside the welding material, and the cavities formed by the undischarged gas are welding pores. The formation of welding pores is very closely related to the specific location of welding, the shape of welding, the process used by the welding personnel, and the material selected during welding. Therefore, during pipeline installation, the welding materials should be carefully selected to ensure that the welding materials meet the construction standards. At the same time, the welding position should be carefully selected to ensure that the pipeline is evenly stressed and reduce the welding pores caused by insufficient welding technology, thereby improving the overall pipeline installation Level, so that the pipeline can be safely put into use, extend the service life of the pipeline, and reduce the cost of pipeline investment.
The undercuts are caused by the arc heat being too high and the current being too large, but the speed of the strip is relatively low. Generally, corresponding depressions or grooves are formed in the base material, and the gaps left are not supplemented by welding. Metal, conduction causes the base material at the edge of the weld to melt and form an undercut. There are many reasons for the formation of undercuts. Incorrect selection of the welding rod and workpiece angle or excessive arc length is one of the reasons for the formation of undercuts. In addition, deviations in the welding position will expand the undercut range.
Application of pressure pipeline welding technology
Technical points before welding pressure pipeline
Quality requirements of welders
The installation work of pressure pipelines requires high professional quality and professional technical level of welders, and requires delicate welding operations. Therefore, welders performing pressure pipeline installation and welding need to have a more professional skill level and a solid theory Basic, and with a careful and serious work attitude, it is also necessary to conduct theoretical and practical examinations in accordance with relevant national regulations to ensure that they are certified.
Requirements for welding equipment
In the pressure pipeline welding process, commonly used equipment includes argon arc welding machine, manual arc welding machine, automatic welding machine and welding seam heat treatment equipment. For welding equipment, it must have a certain degree of stability and reliability, and to ensure that the measurement results of the ammeter and voltmeter of the welding equipment are accurate. The ammeter and voltmeter need to be checked regularly to avoid parameter measurement errors .
Weldment processing requirements
Pressure pipeline installation groove processing, large diameter pipelines generally use oxygen-acetylene flame cutting or plasma cutting, small diameter pipelines mostly use groove machining, the type of groove and the processing angle of the groove must be Meet the specification requirements. After the groove processing is completed, it is necessary to clean the slag, oil stain and oxide scale on the groove surface to ensure that the groove surface is clean and avoid improper groove processing that affects the welding quality. For alloy steel pipes, after the thermal cutting process is completed, the hardened layer on the surface needs to be removed. Before aligning the pipe, the surface of the base material on both sides of the groove needs to be cleaned within 20mm, and the metal luster should be exposed. Pipeline aligning must ensure a uniform pairing gap. The aligning gap is generally controlled at 2 to 3mm to ensure that the weld can be welded through without causing unfusion or incomplete penetration defects. During the positioning process, ensure that the connecting pipe is flush, and the amount of staggered side of the inner wall should be within 10% of the wall thickness and not more than 2mm.
In order to ensure that the welding quality of the pressure pipeline meets the requirements of the specified standards, the welding personnel need to carefully follow the welding process requirements during actual work. Special attention is needed to strictly control the welding current, voltage and preheating temperature. And the temperature between layers to ensure that the welding process meets the requirements of the process regulations. For pressure pipes with different wall thicknesses, welders should reasonably control the number of welding layers in accordance with the requirements of the process regulations, and appropriately increase vertical fill welding. For pressure pipeline on-site butt welding, welders should pay attention to the following issues:
- First, when the nozzles are paired, the welds of adjacent pipes are opened incorrectly at the pairing, and the distance is not less than 100mm.
- Second, when using the internal pairing device, the root welding can only be evacuated after the root welding is completed. When the roots are so hard to bear the laying stress higher than normal, and cracks may occur, the pairing device should be removed after the heat welding is completed. When using an external pairing device, the root welding must be at least 50% of the total length of the circumferential weld completed symmetrically before removing the pairing device.
- Third, the welds that cannot be completed on the same day should be completed at least 50% of the pipe wall thickness and no less than three layers of welds.
- Fourth, after welding, slag, spatter and other dirt on the weld surface should be removed.
- Fifth, when the welding environment conditions do not meet the requirements of welding procedures, effective protection should be taken, otherwise welding is strictly prohibited.
Technical Points after Welding
After the pressure pipeline is welded, the welder needs to clean up the debris on the surface of the welded pipeline, and use a marker to mark the weld number at the position near the weld, and 100% check the appearance quality of the weld to prevent Excessive defects such as cracks, non-fusion on the surface, pores, etc., will improve the welding quality of pressure pipelines.
Technical points of rework welding
After non-destructive testing, all welds with cracks except for crater cracks should be removed from the pipeline, and crater cracks and other non-crack defects can be repaired. Rework should be carried out according to the welding procedure specification, each repair should not exceed 2 times, and the length of each repair should meet the standard requirements. Non-destructive testing shall be performed again after repairing.
Quality control strategy of pressure pipeline welding technology
Quality control of pressure piping materials
The selection of pressure piping materials must first consider the type of pressure piping, the use environment, the conveying medium, and the requirements for strength, as well as the welding performance, economic and practical requirements. Choose seamless steel pipes with small diameters. The performance of pipes of the same grade and different standards are different. Spiral welded seam steel pipes with large pipe diameters need to be supervised and inspected by the local inspection agency. The selected pipes must be re-inspected in accordance with regulations when entering the factory. All pressure pipelines must have a quality certificate, a certificate of conformity, and the pipe must have a “TS” mark. The material must be re-inspected for chemical composition and mechanical properties. , Visual inspection, etc.
Strengthen the welding inspection of pressure piping
The welding inspection of pressure piping includes pre-weld inspection, in-welding inspection and post-welding inspection. Summary of experience in the inspection process: The welding wire and flux used in welding should meet the design requirements; the connection between butt welds and spiral welds should be smoothly transitioned to avoid stress concentration; the welding site should be protected from wind and rain, and winter welding should be done in winter Construction plan; during the welding process, the quality of the arc starting and ending points should be ensured. The arc pit should be filled when the arc is closed; the welding rod should be dried and placed in the insulation cylinder; after the weld is welded, the spatter should be polished and cleaned in time , Coating or welding slag, the weld surface should be visually inspected; after the visual inspection is qualified, radiographic inspection should be carried out, and the welding seam should be repaired for defects that are not allowed by the standard. The pipeline with heat treatment requirements should be heat treated after the repair [ 3].
Establish a complete management system
Welding work units must strictly follow the welding standards, improve the supervision and management system, analyze the specific conditions, put forward a management and monitoring system in a targeted manner, strictly require staff, and standardize work methods. Improve the welding management capabilities of the relevant units, one-to-one correspondence with the welding processing work and the corresponding person in charge, and at the same time ensure that there are substitutes to fill the gaps in time when a group of staff have problems. The distinction between rewards and punishments is also an important standard for the welding process. It is necessary to increase the rewards and punishments to ensure that merits are rewarded, and that there are penalties for passing, and promote healthy competition among staff to achieve the effect of improving work efficiency. In addition, it is necessary to strengthen pressure pipeline welding monitoring, establish a standardized, reasonable and efficient supervision system, fully implement the high-quality requirements of pipeline welding technology, and ensure high-level and high-efficiency welding.
To sum up, due to the many factors that affect the welding quality of pressure pipeline installation, in order to ensure the welding quality, the welder must clarify the technical points before, during and after welding, and take reasonable measures to achieve the welding quality. Effective control, thereby improving the quality of pressure pipes, and meeting application requirements.
Author: Wang Hongdan, Hong Yao
Source: China Pressure Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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