Based on the analysis of the welding defects and causes of thick-wall copper pipe fittings (tee, elbow, reducer, etc.), through the welding test research, the welding process of thick-wall copper pipe fittings for chemical industry (manual argon tungsten arc welding) is explored and summarized, and the corresponding welding process evaluation and special technical measures are formulated, which has accumulated experience for the on-site high-quality welding of copper pipe fittings and the improvement of one-time repair qualification rate, It also plays a positive role in guiding the installation and welding of devices with similar materials and structures.
Because copper and copper alloys have excellent resistance to atmospheric, seawater and some chemical media corrosion resistance, can maintain safe and long-lasting work in the harsher corrosive media environment, so in recent years the application is increasing, more is used as a strong corrosion resistance of the chemical industry, such as pipe fittings and other equipment structure materials. Pipe fittings play an important role in the connection of pipes and equipment.
Data show that for the chemical industry with thick-walled pipe fittings installation welding, domestic tungsten arc welding method is relatively rare. Mainly welding energy seems to feel inadequate, but this method has the characteristics of flexibility, convenience and practicality, and in recent years there has been new progress in the development of this process. Practice has proved that manual tungsten arc welding is prone to and the most harmful problem is welding cracks; then is the porosity and not welded through, there is a higher preheating temperature, the material is easy to carburization and embrittlement and other deficiencies. Of course, the latter defects also have some intrinsic connection with the welding crack, are prompted or accelerate the formation and expansion of welding cracks. In a large chemical production plant, the use of manual tungsten arc welding installation welded part of the industrial pure copper thick-walled fittings (material T2). In full consideration of quality, environmental and cost factors, based on the development of field welding process. The results show that the copper pipe fittings after welding the appearance of the weld is well formed, to solve the prevalence of welding cracks, porosity and not weld through the defects, to meet the requirements of the design.
1.1 Construction environment and its conditions selection
1.2 Common welding cracks and causes of analysis of pipe fittings
2.1 Pre-welding preparation and the required conditions
Data show that the welding of thick-walled copper pipe fittings with manual tungsten arc welding method is relatively rare, mainly because of the material weldability and the special requirements of the welder's operating techniques and welding power is difficult to ensure. After research using a step preheating followed by welding (argon arc welding) can meet the construction requirements. The key issue in welding precious non-ferrous metals is the weld protection and the adaptability of the welding conditions, both on the front and back of the weld, which are equally important. Due to the presence of impurities such as oil, water and oxide film on the bevel area and the surface of the welding wire, it is easy to cause defects such as welding cracks in the welding process. Therefore, before welding, the bevel of copper pipe parts and both sides of the 30mm range, the surface of the wire for strict inspection and treatment. Construction site using chemical cleaning methods, cleaning solution formulations, as shown in Table 1.
Table.1 Copper pipe fittings bevel and joint parts cleaning solution formula
Table.2 Copper pipe welding with the chemical composition of the wire
|Grade||Main chemical composition (mass fraction%)||Melting point/°C|
(Note: welding wire HSCu201 can also be used SCu-2 said)
Welding materials need to be carefully selected. Copper T2 thick-walled fittings welding wire should be based on the principle of matching the composition of the parent material to choose, matching the wire selection and chemical composition as shown in Table 2 (selected wire grade HSCu201). The surface of the wire should not have cracks and oil sludge and other defects or debris. The quality of the wire should be confirmed before use. According to the above weldability analysis, the construction characteristics of copper pipe fittings and equipment conditions, the use of DC positive connection, the welding machine selected Miler400ST type.
2.2 Bevel form and its processing
Thick-walled copper pipe welding although there is a low melting point, high thermal conductivity and easy to fall characteristics, but can not use too much line energy to optimize the welding effect. Bevel form should be selected with a smaller bevel angle, for thick-walled copper pipe butt weld, generally take the bevel angle (50 ± 5)°, blunt edge (1 - 1.5) mm, gap (3 - 3.5) mm is appropriate. In the prefabricated workshop or temporary shed, pipe bevel processing using mechanical processing is appropriate, the bevel surface is smooth, group size control in place, while easy to clean up and adjust. Installation site of the pipe fittings weld mouth, can use special grinding machine processing. Weld mouth group welding before the use of stainless steel wire brush or stainless steel file, etc., the weld bevel and not less than 30mm on both sides of the range of cutting materials, oxide film, burrs, etc. to clean up. Cleaning tools should be dedicated and kept clean to prevent cross-contamination, affecting the quality of the weld. Grease, dirt and other pollutants are cleaned with acetone or water and wiped with a clean cotton cloth, and the degree of dryness is confirmed before welding. Preheating method using homemade two-headed oxy-acetylene flame grill step heating, this grill not only shortens the preheating time, and the heating point is slightly far from the operator, improving the labor conditions when welding.
2.3 Copper pipe fittings welding seam grouping and processing
Good alignment of the weld is an important prerequisite for high-quality pipe joint welding. Weld seam assembly should strictly control the alignment gap and the amount of misalignment, for two pipe joints of different thicknesses, the need to use the form of processing the internal bevel to ensure that the amount of internal misalignment meets the requirements. Pipe interface welds shall not be strongly grouped. The spot welding of the weld seam should not be carried out arbitrarily. Copper pipe fittings weld joint grouping spot welding spacing is generally (40 - 60) mm, the length of the spot welding to (6 - 8) mm is appropriate. Spot welding welding process conditions should be the same as the formal welding.
After the completion of the welding mouth grouping, the oxide layer and debris should be removed clean, revealing the metal luster in time for welding. Welding process, according to the weld and heat-affected zone after welding the surface color to initially determine the quality of its effect. But in the multi-layer, multi-channel welding, the final should be qualified for each layer of the weld as the standard, or need to take the appropriate technical measures to deal with.
2.4 Argon gas requirements and its replacement
After the completion of the weld port assembly, the gas inside and outside the weld port should be replaced. In order to ensure the quality of gas replacement, save argon, improve efficiency, according to the condition of the pipe weld mouth design and production of the right size of the combination of bracket cover, and give full consideration to its sealing, removability and post-protection. Can also be used to fill the pipe inside the argon method. For the site weld mouth or not easy to fill argon isolation inside the weld mouth, weld mouth grouping before, set up in the weld on both sides of the easy to dissolve paper, grouping after the appropriate location from the weld grouping gap filled with argon for replacement. Welding test, the use of gas shielding shield, it is appropriate to vary with the specifications of the pipe.
2.5 Welding process parameters and process evaluation
For pure copper material with high thermal conductivity, easy to overheat, welding grain growth and easy to crack the tendency, strict control of welding process parameters is very important. In particular, the welding line energy, simply by increasing or decreasing the welding current method, practice has not proved to be effective, it is appropriate to ensure that the welding seam fusion good conditions as far as possible using the method of increasing the welding speed to control the amount of heat input. At the same time, the interlayer temperature should be strictly controlled. The following are several commonly used thick-walled copper pipe fittings manual tungsten arc welding welding process parameters, as shown in Table 3.
Table.3 Several commonly used thick-walled copper pipe fittings manual tungsten arc welding process parameters
|Material||Wall thickness/mm||Tungsten electrode diameter/mm||Welding wire/mm||Welding current/A||Ar gas flow/(L/min)||Preheating temperature/°C||Note|
|Pure copper T2||10||5-6||4-5||340-400||20-22||500-600||One-sided welding|
It should be noted that the quality and quantity of argon gas is an important parameter that affects the protection effect of the weld. The flow rate is appropriate to obtain the key to quality welds, argon gas flow is too large, but easy to produce welding porosity slag cracks and other defects. According to experience argon flow should be controlled in (20 - 24) L / min better. In addition, welding techniques need to be appropriate. It should be coordinated with the characteristics of the material, melting and condensation speed. Welding operation, it is appropriate to use a short arc welding, arc length is generally controlled between (3.0 - 4.0) mm, each section of the weld joint, can be back to weld a small section, in order to facilitate the exclusion of gas or inclusions. Each time the arc should be filled when the arc hole to prevent arc hole cracking. After the end of the welding test, the production of welding process evaluation. Evaluation test specimens using the welding method, tools and other welding test process is identical. After completion of the test piece welding, through visual inspection and flaw detection, its internal and external weld and heat-affected zone is well formed, are pure purple and no cracks or other defects beyond the standard, indicating that the high temperature characteristics of industrial pure copper T2 material measures used is feasible. After the mechanical properties of the welded specimens tested to see whether the mechanical properties of the standard indicators, until the satisfactory application of welding results. As shown in Table 4, the welding test results of several thick-walled pure copper specimens.
Table.4 Test piece welding test results
|Wall thickness/mm||Material||σb/MPa||σs.MPa||δ(%)||Cold bending/(°)||Cracks and pores|
(1) According to the test qualified welding process evaluation, the preparation of targeted welding process guidance for specific guidance on site welding operations. Strictly control the welding line energy, control the thickness of each layer of welding, the temperature and cooling rate of the region is conducive to grasp, the temperature between the welding layers shall not be too high. Practice has proved that too many layers of welding is prone to arc cracking, the correct approach is to ensure that the grain size is small and stress concentration is not too large on the basis of reducing the number of layers of welding as much as possible. The welding process must confirm the "human, machine, material, method and ring" the scientific and suitability of various elements, and check the welding energy, conditions and stability of the weld formation, so that the welding work is carried out smoothly.
(2) Welding process, filler wire melting end must always be in the gas protection, rest arc should be waiting for the melting end of the cooling, before the protection can be removed from the wire head such as pollution or oxidation, the pollution or oxidation part must be removed or ground off. In addition, the welding process, should always pay attention to observe the changes in the color of the weld channel. If there are obvious welding oxidation defects in the welding process, should immediately stop the operation, to cut or polish off the oxidation area, identify the cause of oxidation and take corrective measures before continuing welding. During the construction process, the amount of grinding and surface temperature should be strictly controlled to prevent re-oxidation or deterioration of the bevel surface. When installing copper pipe fittings weld mouth site operations, for the actual situation of the atmosphere and dust, take the necessary anti-pollution measures. Site using the production of canvas canopy, in order to temporarily isolate the welding area from the external environment, to produce a moderate space conducive to welding operations, to effectively ensure the welding quality of copper pipe fittings weld.
(3) The weld surface exceeds the standard defects or non-destructive testing found in the unqualified parts, must be reworked. Before repair, should be based on the specific circumstances, analysis of the causes of defects, and the development of targeted rework program. Weld rework must be strictly in accordance with the program requirements for operation, welding with the same or more stringent process of formal welding. It is best to be applied by experienced technicians to weld difficult parts, the same part of the weld rework not more than twice, in order to prevent the generation of arc cracking and local material deterioration. When tungsten arc welding patch welding, welding area drying, preheating and purification work must also be done. Once found that the color after welding is not pure purple, should be promptly ground to re-welding. If there are pores, cracks, should also be ground to re-weld. But also to strictly control the number of rework and welding, to prevent the reduction of fatigue strength of the weld. If the same part of the weld is welded more than twice, the weld should be approved by the unit in charge of the technology before the patch, and should take special and reliable technical measures by the senior technician welding.
(4) Other relevant considerations. Chemical industry with pure copper thick-walled fittings using manual tungsten arc welding, should also pay attention to the following points:
Research and explore the welding characteristics of thick-walled copper pipe fittings in a specific field environment, the development of a rigorous and reasonable welding process, the welding process at all stages of quality control and protection of the weld, is to ensure the quality of thick-walled copper pipe fittings field welding key factors. In the welding process, strict operating procedures, in accordance with the process requirements of the steps to be implemented one by one, is to ensure the quality of industrial pure copper pipe fittings - the premise and basis of the field welding of the pipe interface. In a chemical engineering of the welded copper pipe fittings weld, 100% of the X-ray nondestructive flaw detection, by checking a pass rate of 96.3%, the defective part of the one-time rework also reached the specified requirements. Practice has proved the feasibility of the above welding process, for the domestic welding of similar materials devices to accumulate a certain amount of experience.
Source: China Copper Pipe Fittings Provider - Yaang Pipe Industry Co., Limited (www.yaang.com)
Tel No：+86-18267732328 / Email:[email protected]
Address：Longwan District, Wenzhou, Zhejiang Province, China.
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