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Research on joint cracking and quality control of welded thick-walled copper pipe fittings for chemical industry

DataTime:[2023-2-7]

Based on the analysis of the welding defects and causes of thick-wall copper pipe fittings (teeelbowreducer, 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.

Research on joint cracking and quality control of welded thick-walled copper pipe fittings for chemical industry

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.

welded thick-walled copper pipe fittings for chemical industry

1. Copper material weldability and thick-walled pipe fittings welding technology difficulties analysis

1.1 Construction environment and its conditions selection

  • (1) First, a brief analysis of the basic characteristics of copper materials. Industrial pure copper materials are mainly T1, T2, T3 and T4 and other common grades (according to the number of impurities to be divided, the more forward the higher the purity). Due to the color and chemical composition purity, also known as purple copper. T2 for example (the focus of this article), can be seen in the chemical composition of such materials (mass fraction) and its performance characteristics: 99.90% copper, and 0.002% bismuth, 0.005% lead, 0.005% sulfur, 0.06% oxygen and other elements of the total impurity content of 0.10%; melting point 1083 ℃, density 8.94g/cm3, tensile strength 196 - 235MPa (soft). 235MPa (soft state), elongation of 50%. In general, the welding properties of such materials are poor, and it is difficult to obtain high-quality joints. Joint properties such as mechanical properties, electrical conductivity and corrosion resistance are reduced, low melting point alloy elements evaporate when welding, high porosity sensitivity, weld and heat-affected zone grain coarseness, easy to produce cracks, not weld through, not fused and other defects. At present, the main technical measures must be carried out before welding moderate preheating, and good protection of the weld and other related optimization work.
  • (2) Industrial pure copper T2 thermal conductivity is much greater than iron, welding heat is rapidly conducted from the heating zone, so that the base material and filler metal difficult to fuse, and its liquid flow coefficient of carbon steel more than one times the surface tension equivalent to carbon steel 1/3, welding and base material is easy to fuse, shallow depth of fusion and poor surface formation, resulting in local cracking and other defects, so welding must be used when the high-powered heat source welder and large line Energy welding and take a sufficient amount of preheating to ensure that the molten pool fusion.
  • (3) On the other hand, due to the strong thermal conductivity of pure copper T2, welding requires a large supply of heat, resulting in serious grain growth in welding, a variety of low melting point eutectic aggregation of grain boundaries, weld metal plasticity is severely reduced is very likely to cause cracks in the weak zone. In order to prevent the emergence of lead-rich or bismuth-rich low melting point eutectic, after welding should be made to quickly cool, hammering to reduce the internal stress of the weld. In order to ensure smooth welding, and to ensure that the root of the weld forming and closing the arc at the joint smooth not to crack, reduce heat loss, welding should use asbestos and thermal graphite baffle.

1.2 Common welding cracks and causes of analysis of pipe fittings

  • (1) Experimental evidence, industrial pure copper T2 welding process, porosity is one of the most easily produced defects, but also an important source of cracking. Porosity is mostly concentrated in the fusion line, near the center line of the weld. Hydrogen in pure copper liquid solubility is much greater than in the solid state, while the metallurgical reaction of oxygen to form reaction pores, reduction reaction of water vapor diffusion pores, due to the rapid cooling of copper welds, high temperature molten pool of hydrogen absorbed in the cooling process is not easy to precipitate hydrogen content to supersaturated state, resulting in gas is difficult to escape, it is easy to form pores in the weld, when the pores are arranged more dense, but also the formation of A seemingly broken non-broken source of small cracks, may be connected at any time through the macro crack, so the source of impurities such as hydrogen and oxygen should be strictly controlled when welding, surface cleaning measures must be taken, and welding arc as low as possible.
  • (2) In addition, pure copper T2 Bi, Pb, S, P and other impurities and Cu can generate a variety of low melting point eutectic, low melting point eutectic at the weld to form a liquid film, and its heating process without isomeric transformation, the surface crystallization tendency is serious, coupled with a large expansion coefficient and shrinkage caused by the joint stress, resulting in the formation of welded joints prone to thermal cracking.
  • (3) Pure copper T2 coefficient of linear expansion is 15% greater than iron, in order to avoid cracks in the near seam area, must ensure sufficient group gap; at the same time its shrinkage rate is more than one times greater than iron, in order to prevent the melt pool from the liquid phase into a solid state, the temperature difference between the layers to form cracks, the application of welding should ensure that the temperature between the layers in a certain range, usually (300 - 600) ℃. Due to the offset of the pipe joint and take the strong group of stresses generated by the pair can not be neglected.
  • (4) In addition, the high temperature fluctuations in welding is prone to arc cracking (in recent years, our scholars have discovered a new form of thermal dynamic cracking), from the essence of the weld area in addition to the role of thermal stress cycle, there are changes in the inherent quality of the material itself, such as the gradual precipitation of compounds, accumulation and extension, coupled with the irreversible return of material weakness and deterioration and has a non-uniformity characteristics, first in the weld area most The weakest edge of the weld cracking, and finally lead to sudden arc fracture and cause leakage, fire and explosion of the pipe joint and other malignant accidents (from the chemical device online safety use, the hidden danger of its pre-cracking can not be ignored). In fact, it also contains elements of a variety of other forms of cracking. The main is to control the number of welding arc wave do not approach the critical number of arc wave cracking (determined by the welding test).

2. Thick-walled copper pipe fittings welding test and process evaluation

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

H20/ml H2SO4/ml HNO3/ml NaCl/g
1000 1500 2250 30

Table.2 Copper pipe welding with the chemical composition of the wire

Grade Main chemical composition (mass fraction%) Melting point/°C
Cu Zn Sn Si Mn Fe P
HSCu201 Allowance - -1.1 -0.4 -0.4 - - 1050

(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
12 360-420 20-24 Double-sided forming

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
10 T2 174.6 123.2 18.3 180 No
12 178 124.5 20

3. Thick-walled copper pipe fittings field welding and optimization measures

(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:

  • 1) Welding environment temperature shall not be lower than 5 ℃, relative humidity within 80%.
  • 2) Strengthen the welding area protection devices, so that it is not affected by wind, rain, snow and other weather.
  • 3) The construction site should be kept clean and dry, if necessary, set up a dust shed, lay the floor, equipped with a de-humidifier.
  • 4) Forbid arbitrary arcing on the surface of the pipe, welding should be piloted on the arcing plate before welding, determine and confirm the process parameters and related data are in line with the process requirements.
  • 5) By the construction worker with the materials clerk to the incoming materials for strict management, if necessary, the incoming materials for re-inspection.
  • 6) During the construction process, try to weld a weld at once and then proceed to the next link of welding to reduce the number of welded joints and the possibility of welding cracks.
  • 7) The beginning of the welding speed should be slightly slower, after the appropriate speed, the welding process should pay attention to control the weld formation, such as the discovery of porosity or micro-grain should be promptly back to the arc burn melt to eliminate.
  • 8) In order to refine the grain size of the joint, improve the mechanical properties of the joint, the stresses or more important pipe joints must be used after welding hammering and post-heat treatment and other technological measures.
  • 9) For the welding of dissimilar copper, should minimize the amount of melting of the base material, can also be used to form a joint in the low melting point of the working side of the melt welding process, and in the high melting point of the side of the formation of a special process of brazing.
  • 10) Welding wire can not be used to stir the molten pool, in order to ensure the normal welding conditions, should be used as short as possible, the arc length of narrow fluctuations in welding. Field welding, according to the actual pipe material and wall thickness, the appropriate wire diameter and appropriate welding speed to ensure adequate penetration, weld formation and weld denseness.

4. Conclusion

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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