09-17

Welding procedures-fillet welds

2022-09-17

Fillet weld as steel structure welding process in the most common form of weld splicing, so many welders will encounter different styles of fillet welds, but also have different standards, so how can a good fillet weld, and get to meet the requirements of the fillet weld: 1 Selection of base material Below we take the base material as Q345, the size of 200x 125x 12mm test plate as an example for a brief introduction. 2 Selection of welding materials Welding material: select E5015(J507) electrode, the drying temperature is 350~400 ℃, heat preservation for 2 hours, the welding wire adopts the national standard ER50-6 (American standard ER70S-6). 3 Cleaning before welding For fillet welds, clean within 15~20mm on both sides of the weld. The oxide film, rust, oil and water on the surface of the plate are removed by grinding with grinding wheel or chemical treatment. 4 Assembly point fixing Generally, fillet welds require the weld gap to be as small as possible. Point solid is generally carried out on the back of the plate, and the point solid length is about 10mm. General point solid two to three points can be: 5 Welding parameters 6 Welding operation Base welding 1.1 the angle of welding rod and the method of driving rod The angle of the welding rod for root fillet welding is shown in the figure: use the right welding method, start the arc on the left side of the test plate, and weld the short arc. Weld to the right in a straight line, align the arc with the apex angle of the root, lower the arc, and ensure the fusion of the apex angle and the plates on both sides. When backing welding, the swing method adopts straight line transportation and fast welding without swing, so that the required penetration can be achieved. At the initial welding end and the final welding end of the weld, the phenomenon of magnetic bias blowing is easy to occur, which affects the quality of the weld. At this time, the angle of the welding rod should be adjusted appropriately, and the arc should be directed to the molten pool to control the magnetic bias blowing. Figure. 1.2 weld bead joint The joint shall start the arc 10mm in front of the crater. When the elongated arc moves to the crater quickly, the crater shall be filled along the crater shape and then welded normally. cover welding Before welding the cover surface, remove the root pass weld slag and spatter to prevent slag inclusion defects. The cover is welded in two passes, first welding the lower pass, and then welding the upper pass. When welding the lower weld bead, the arc should be aligned with the lower edge of the root weld bead, and the rod should be moved in a straight line, and the angle of the welding rod should be greater than 45; When welding the upper weld bead, the arc should be aligned with the edge of the root weld bead, and the straight wire can also swing horizontally, and the angle of the welding rod should be less than 45. Theoretical weld throat: the vertical distance from the root of the joint to the hypotenuse in the largest inscribed right triangle drawn in the fillet weld profile. Effective weld throat: the minimum distance from the fillet weld surface to the weld root, minus the raised portion. Actual weld throat: shortest distance between fillet weld surface and weld root. For concave fillet welds, the effective weld throat is equal to the actual weld throat because there is no convex part. Leg size: the distance from the root of the joint to the fillet weld toe. Weld toe: The intersection of the weld surface and the base metal. When determining the size of the fillet weld, it must be clear whether the fillet weld is convex or concave. Convexity refers to the slight convex on the surface of the weld, which is related to the height of the convex. The raised height of a fillet weld is the same as the reinforced height of a groove weld. If a weld is dimpled, it indicates that the weld surface is dimpled. For these two shapes, the weld size of the fillet weld with the same weld leg height is expressed as "the straight edge of the largest isosceles right triangle obtained in the fillet weld profile (the two weld legs are the same length)." Therefore, for a convex fillet weld, the weld leg is equal to the weld size, but for a concave fillet weld the weld size is slightly smaller than its leg length.

09-16

Is welding undercut a welding appearance defect? How to prevent the occurrence of undercut.

2022-09-16

Undercut refers to the depression or groove formed in the base metal part along the weld toe. It is the gap left by the arc melting the base metal at the edge of the weld without sufficient replenishment of the deposited metal. The main reason for the generation of undercut is that the arc heat is too high, that is, the current is too large and the speed of the strip is too small. Incorrect angle between electrode and workpiece, unreasonable swing, too long arc, unreasonable welding sequence, etc. will cause undercut. The magnetic deflection of the arc during DC welding is also a cause of undercut. Some welding positions (vertical, horizontal, upturned) will aggravate undercut. The undercut reduces the effective cross-sectional area of the base metal, reduces the carrying capacity of the structure, and also causes stress concentration, which develops into a crack source. Prevention of undercut: correcting the operating posture, selecting reasonable specifications, and adopting good transportation methods will help eliminate undercut. When welding fillet welds, AC welding instead of DC welding can also effectively prevent undercut.

09-15

How to realize the safety of welding operation? Please remember "ten no welding!

2022-09-15

Electric welding is a special operation. If the personnel engaged in electric welding operation do not understand or ignore the safety protection work during the operation, once the operation is not standardized or the open fire operation is illegal, it is very easy to cause explosion and fire accidents! We should learn and reflect while regretting. How can welding safety work be realized? "Ten not welding" please remember 1. No special operation certificate, no welding and cutting 2. No reliable safety measures for rainy and open-air operation, no welding and cutting 3. Containers that have been filled with flammable, explosive and harmful articles have not been thoroughly cleaned, combustible concentration detection has not been carried out, and welding and cutting are not carried out. 4. There is no 12V low voltage lighting and poor ventilation when working in the container, and no welding and cutting 5. There is no power failure in the equipment, the equipment is not uncompressed, and the equipment is not welded and cut. 6. Inflammable and explosive articles around the operation area have not been eliminated, and welding and cutting are not carried out. 7. The nature of the welding body is unclear, the Mars flying direction is unknown, and no welding and cutting is carried out. 8. Equipment safety accessories incomplete or failure, not welding

09-14

Causes and Control Methods of Welding Deformation

2022-09-14

Causes and Control Methods of Welding Deformation The welding deformation is harmful, and the deformation control equipment; 1. Causes of deformation To put it simply, it should be caused by the unbalanced shrinkage in the process of cooling and shrinkage of the steel due to thermal expansion and contraction after the steel is heated and melted during the welding process. 2. Influencing factors of welding deformation There are many factors that affect welding deformation, which can be summarized as material, structure and process. 01 1) Material factors Mainly due to the physical characteristics of the material itself, especially the thermal expansion coefficient of the material and the yield limit and the elastic modulus of the material, the greater the expansion coefficient of the material, the greater the welding deformation, the elastic modulus increases, the welding deformation decreases, and the yield limit will cause higher residual stress caused by increased deformation. The expansion coefficient of stainless steel is greater than that of carbon steel, so the welding deformation tendency of stainless steel of two materials of the same thickness is greater than that of carbon steel. 02 2) Structural factors The design of welding structure has the most critical influence on welding deformation. The general principle is that with the increase of restraint degree, the welding residual stress increases and the welding deformation decreases accordingly. 03 3) Process factors The main influencing factors are welding method, welding heat input (current and voltage), positioning or fixing method of components, welding sequence, and the use of welding fixtures. The biggest impact is the welding sequence. 3. Welding deformation control 1) Design measures 1 reasonable choice of welding size and form Under the condition of ensuring the bearing capacity of the structure, the weld size should be as small as possible to reduce the influence of welding heat input on the material performance. 2 Reasonable selection of weld length and quantity As long as allowed, the use of profiles, stamping parts; weld more and dense place can be cast-welded joint structure, can reduce the number of welds. In addition, the appropriate increase in the thickness of the wall plate to reduce the number of ribs, or the use of pressure structure instead of rib structure, can prevent the structural deformation of the thin plate. 3 reasonable arrangement of weld position Arrange the weld as far as possible to use symmetrical to the neutral axis of the section, or make the weld close to the neutral axis, which has a good effect on reducing the flexural deformation of the beam and column. 2) Process measures 1 Anti-deformation method The use of anti-deformation to control welding deformation is the most commonly used welding method. 2 Allowance method During blanking, the actual length or width dimension of the part is appropriately increased compared with the design dimension to compensate for the shrinkage of the weldment. This method is suitable for preventing the shrinkage and deformation of the weldment. 3 Rigid fixation The use of clamps or rigid fixtures to fix the welded parts as much as possible can effectively control the angular deformation and bending deformation of the components to be welded. ① Fix the weldment on a rigid platform (suitable for rigid fixation when splicing thin plates) ② Combine the weldments into a more rigid or symmetrical structure (suitable for the control of T-beams and other structures) The use of welding fixture to increase the rigidity and restraint of the structure ④ Use temporary support to increase the restraint of the structure. 4 Choose a reasonable assembly welding sequence The assembly sequence has a great influence on the deformation of the welded structure. ① Large and complex welding structure, as long as the conditions allow, he is divided into a number of simple structure components, separate welding, and then assembly. ② The weld being welded shall be close to the neutral axis of the structure section. ③ For the structure with asymmetrical arrangement of welds, the side with less welds shall be welded first during assembly and welding. ④ The structure with symmetrical arrangement of welds shall be welded symmetrically by even number of welders. ⑤ When welding a long weld (more than 1m), the direction and sequence shown in Figure 12 can be used for welding to reduce the shrinkage deformation after welding. In view of welding deformation, we should pay attention to the selection of welding methods and welding process parameters, try to choose the method and process parameters with small welding heat input, and avoid the increase of welding deformation caused by large welding parameters and welding methods. We should still experience and summarize more in practice.

09-13

Prevention and treatment of welding bead, trachoma, burn-through, welding leakage and other phenomena in welded copper pipe

2022-09-13

Selection of 1. Solder Copper pipe is an important raw material for refrigeration equipment, which is mainly used to make heat exchangers and connecting pipes and fittings. Refrigeration pipe fittings are mostly copper (purple) pipes, and commonly used solder types are copper-phosphorus solder, silver-copper solder, copper-zinc solder, etc. When welding, according to the characteristics of the pipe material, the correct choice of solder and skilled operation, to ensure the quality of welding. Copper and copper soldering: phosphorus copper solder or phosphorus copper solder with low silver content, such as 2% or 5% silver-based solder. This kind of solder price is relatively cheap, and has a good solution, uses the filling and wetting process, does not need the flux. Welding of copper and iron: phosphor copper solder or brass bar solder can be used, but corresponding fluxes, such as borax, boric acid or boric acid mixed flux, are also required. 2. welding operation Use sandpaper to remove grease, oxides, stains and dust from the surface of the copper pipe inserted into the joint. The copper pipe is inserted into the welded part. Pipeline welding requirements of the length of the neutral flame is generally 20~30cm, gas output is greater than the oxygen output will produce not suitable for welding operation of the carbonized flame, the use of welding torch on the copper pipe interface heating. Then evenly heat the pipe to be welded until the welding joint is dark red, put in the welding rod and make the melted welding rod wrap around the welding joint for more than 0.8cm, then remove the welding rod, and stop heating when the brazing filler metal is completely melted. Wipe the connection with a damp cloth after several minutes after welding. In this way, the welding part can be stabilized, and the slag on the welding surface can be removed to prevent corrosion. Normal welds after welding shall be free of porosity, cracks and incomplete fusion. Analysis of Common Defects in 3. Welding 1. Welding bead Features: The brazing filler metal flows out of the brazing seam and forms an accumulation. Cause of formation: 1. The welding thermal efficiency is too large; 2, the amount of solder applied too much or improper application; 3. The welding seam is not at the same level during welding; Precautions: 1. Swing the welding gun back and forth during heating to prevent local overheating; 2. Add solder to slide along the weld from the back of the heating position; 3. Stable positioning of weldment. 2. Undercut Features: The edge of the weld is burned into a rotten shape by the flame, but it is not completely burned through, and the pipe wall itself is burned. Cause of formation: 1. The welding thermal efficiency is too large 2. The heating method is wrong. Precautions: 1. The flame is neutral welding. 2. The flame swings the welding torch back and forth to avoid centralized heating. 3. Over-burning or corrosion Features: The oxide on the surface of the joint naturally falls off after welding. After the joint is wiped clean, black pitting is inlaid on the surface of the joint. Cause of formation: 1, welding thermal efficiency is too large (temperature is too high); 2. The welding time is too long. 3, the use of oxidation flame welding. 4. Repeat welding (repair welding) Precautions: 1. Neutral flame welding; 2, do not repeat welding. 4. trachoma Features: The weld has impurities, resulting in deep holes on the surface of the weld. Cause of formation: 1. The brazing filler metal or the base metal itself has oil pollution, impurities or oxide film. 2. The weld temperature is too high, causing the phosphorus in the solder to volatilize. 3. The heating speed is too fast. Precautions: 1. Ensure the cleanliness of the solder and base metal; 2. The welding efficiency is appropriate, so that the gas dissolved in the high-temperature molten pool can be effectively discharged when the weld solidifies. 5. Air hole Features: Insufficient heating time, the gas in the liquid seam metal fails to overflow, forming holes on the surface or inside. Cause of formation: 1. The fiber material or base material itself has oil pollution or oxidation; 2. The welding speed is too fast, and the gas in the welding is effectively discharged; 3. The flame is a reducing flame, producing reducing hydrogen holes; Precautions: 1, to ensure the cleanliness of the base material fiber; 2. Select neutral flame welding with moderate speed. 6. Burn through Features: The weldment is burned and pierced near the seam. Cause of formation: 1. The operation is not skilled and the welding torch is not swung; 2. Improper flame adjustment and uneven temperature control; Precautions: 1. Swing the welding torch back and forth during welding to prevent overheating; 2. Select neutral flame connection. 7. Cracks Features: cracks in brazed joints include cracks in parent metal and brazed joints. The mechanical properties of the base metal and brazing seam decrease, and cracks occur under the action of residual stress. Cause of formation: 1. The phosphorus content of the solder is too high, and the toughness of the material is weakened; 2. The cold shrinkage rate of copper is obvious, and the residual thermal stress causes cracks; Precautions: 1, switch to better welding performance; 2. Tempering treatment to eliminate residual stress. 8. Welding leakage Features: The weld is incomplete, part of the weld is not completely fused to form a whole weld, and there is a leakage channel in the weld. Cause of formation: 1, flame brazing welding heat input is too small; 2. The temperature is uneven when applying the solder; 3. There are oxide skin, oil and other impurities on the surface of the weld. Precautions: 1. Uniform heating; 2. Use flame to heat the workpiece and add it when the workpiece is dark red. 9. Preventive measures In welding: 1. In the process of brazing heating, the welding torch swings back and forth at the part to be heated, and fixed-point heating is not allowed; 2. When adding solder, solder must be added from the back of the flame heating direction; 3. When welding, the nozzle should not be too close to the workpiece, otherwise it is easy to temper; 4. When brazing copper, generally preheat the inserted tube first to make the tube fit tightly, and then focus on heating the outer piping. Welding completed: The thermal expansion and contraction ratios between the mating workpieces are different, and the internal stress between the mating workpieces is generated during the welding process. After welding, tempering is required to eliminate the internal stress between the welding workpieces. The operation method of tempering: After the workpiece is welded, the temperature is reduced to 200 ℃ ~ 300 ℃, and the welding edge is tempered within 30mm. During tempering, the outer flame is used to quickly sweep the welding torch at the joint back and forth, and fixed-point heating is not allowed. Each workpiece lasts for about (3-5) seconds.

08-31

Selection of FCAW-G protective gas

2022-08-31

FCAW-G shielding gas selection. When welding flux-cored wire, whether to choose CO2 gas protection or Ar/CO2 mixed gas protection needs to consider the following three aspects. 1) The cost of protection. In general, 80% of welding assembly belongs to labor and management costs, 20% belongs to material costs, of which the cost of shielding gas accounts for about 1/4, or 5% of the material cost. Assuming that the cost of the shielding gas is the only determining factor, replacing the Ar/CO2 mixed gas with CO2 shielding gas can greatly reduce the welding cost. However, often other costs will also affect the welding assembly cost, which will be discussed later. CO2 is cheaper than Ar/CO2 because it is available at a lower cost. The world's carbon dioxide resources are extensive and rich. CO2 is often available as a by-product of other processes. For the welding industry, on the one hand, CO2 can be obtained through the processing or separation of natural gas, on the other hand, CO2 can also be obtained through air. Since the content of Ar in the atmosphere is less than 1%, a large amount of air needs to be treated and processed to extract a certain amount of Ar, and special air separation equipment is required to process the air. The air separation equipment consumes a lot of power and needs to be placed in a dedicated area. 2) Effect of welder preference and productivity. When welding with the same type and size of welding wire, the arc obtained by Ar/CO2 shielding gas welding is more stable, weaker and less spatter than that obtained by CO2 gas shielding gas welding, so it is deeply loved by welders. In the process of CO2 gas shielded arc welding, it is easy to produce large droplet transition (the droplet is usually larger than the diameter of the welding wire), resulting in unstable arc, discontinuous arc and large spatter. The Ar/CO2 mixed gas protects the small droplets from splashing (the droplets are usually smaller than the wire diameter), resulting in a more stable and continuous arc, and the splash is small, as shown in Figure 3. Another feature of Ar/CO2 mixed gas protection also increases the welder's love for it. Compared with CO2 gas welding, its heat transfer capacity is low, so it can maintain the heat and liquid degree of the molten pool. This makes the reaction of the molten pool more thorough and the weld toe portion more easily completely melted. When welding in special positions (such as uphill or inverted welding), the use of Ar/CO2 is more attractive because less skilled welders can also control the arc and increase welding productivity. When using Ar/CO2 mixed gas shielded welding, due to the high content of Ar, more heat is emitted to the welding machine than CO2 shielded gas welding. This means that the welder feels warm while welding. In addition, the torch will be hotter (the duty cycle of the torch under Ar/CO2 shielding gas is lower than that of CO2 shielding gas), which requires the use of a larger torch or more frequent replacement of the same type of torch and its vulnerable parts. 3) Welding quality. As mentioned earlier, the welding using Ar/CO2 shielding gas mixture can maintain the heat and liquid degree of the molten pool, make the molten pool react more thoroughly, and the weld toe part is easier to melt completely, compared with the welding using carbon dioxide shielding gas. Therefore, the weld forming ability and weld quality are greatly improved. In addition, the Ar/CO2 mixed gas splash is small, the weld quality is greatly improved, and the time and cost of post-weld cleaning are reduced. The lower amount of spatter also increases the cost of ultrasonic weld inspection, because if there is too much spatter, the spatter must be cleaned in advance to ensure the accuracy of ultrasonic inspection. Another quality issue that affects the appearance of the weld is the sensitivity of the shielding gas to the gas mark. Air marks, similar to earthworm marks or chicken claw marks, are small grooves, sometimes distributed on the surface of the weld. They are caused by dissolved gases in the weld metal that move out before the bath solidifies, but remain under the solidified slag. Ar/CO2 mixed gas protection has higher gas trace sensitivity than CO2 gas protection alone. The splash transition characteristic of Ar/CO2 shielding gas leads to the generation of a large number of fine droplets, which increases the surface area of the droplets, resulting in the dissolution of a large amount of gas in the weld metal. In addition to the type of shielding gas, there are other factors that affect the sensitivity of gas traces, but they are beyond the scope of this article. Common shielding gases for some major applications. Over the years, FCAW-G shielding gases have gradually become the standard in some major applications. For example, in high deposition welding applications such as flat welding and cross welding, CO2 gas shielding is usually used, because in these welding positions, Ar/CO2 mixed gas shielding does not have much advantage. The shipbuilding industry is also generally inclined to adopt CO2 gas protection, because the arc characteristics of the CO2 gas shield allow better burning of the underlying metal primer. In the North American marine industry, downward welding T, Y, K connection groove welds require smooth weld shape and small welding spatter, so Ar/CO2 mixed gas protection is more appropriate. If the construction shop uses more than one GMAW and FCAW-G process, the shielding gas for both processes is usually standardized. Sometimes, in order to obtain better spatter rate and pulse arc transition, many manufacturers also choose argon/CO2 mixed gas shielded GMAW welding.

08-30

Classification of filler metals according to gas type, self-shielded flux cored electrodes, no need for any shielding gas

2022-08-30

Filler metals are classified by gas type. Since 2005, the American Welding Society's core filler metal classification has included the shielding gas type into the electrode classification symbol. Mild steel FCAW-G electrodes have an AWS number of EXXT-XX and the last symbol indicates the type of shielding gas. If the last digit is C, the shielding gas is CO2. If M, it represents an Ar/CO2 gas mixture (e. g., E71T-1C or E71-T1M). For low alloy steel electrodes, the shielding gas symbol corresponds to the final deposited metal composition symbol of the specified symbol (e. g. E81T1-Ni1C). In contrast, self-protected drug-cored electrodes do not require any shielding gas, and there is no shielding gas code (such as E71T-8) in their classification number. Some electrodes can only be protected with CO2. Some electrodes can only be protected with an Ar/CO2 gas mixture. Some electrodes can be protected by CO2 gas or Ar/CO2 mixed gas at the same time. In this case, the electrode must meet both classification requirements. When choosing a shielding gas in a FCAW-G application, one should not only consider the cost of the gas, but should consider the three aspects discussed in this article. How each gas type affects the total cost of welding. Which gas can reduce the cost per meter of weld? Some manufacturers have found that Ar/CO2 mixed gas shielding can improve weld quality and productivity. Other manufacturers believe that the advantages of Ar/CO2 mixed gas welding are not ideal, or the cost is not as low as CO2 gas welding. However, for other manufacturers, CO2 is cheap and suitable for some of their welding applications. For users of the FCAW-G process, the shielding gas should be selected according to the impact of the shielding gas on the cost, quality and productivity of the welding operation. Once the shielding gas has been selected, the FCAW-G electrode shall be suitable for welding with that gas.

08-29

Welding processing escort, inert gas is introduced.

2022-08-29

Inert gas is introduced. Although inert gas can provide protection to the weld pool, it is not suitable for flux-cored welding of iron-based metals (such as low carbon steel, low alloy steel, stainless steel, etc.). For example, if stainless steel is welded using only Ar as a shielding gas, the weldability becomes poor. This is because inert gas protection will lead to longer arc length and premature melting of the outer steel skin of the electrode. The arc range is large and difficult to control, resulting in weld accumulation. Therefore, in the case of welding an iron-based metal using a flux-cored wire GMAW, a mixed gas of an inert gas and an active gas is generally used. Introduction of CO2/Ar gas mixtures. In North America, the welding of stainless steel flux-cored wire gas shielded arc welding often uses Ar/CO2 mixed gas as the shielding gas, of which Ar accounts for 75% and CO2 accounts for 25%. Sometimes 80% Ar and 20% CO2 are mixed, but this mixing ratio is not commonly used. Some gas-shielded flux-cored wires require the use of a mixture of 90% Ar and 10% CO2 for shielding. However, if the Ar content in the mixed protective gas is less than 75%, the arc performance will be affected, so the proportion of Ar in the mixed protective gas must be ensured. In addition, tanks containing non-standard percentage Ar/CO2 mixtures tend to be more difficult to obtain than tanks containing standard percentage mixtures (e. g., 75% Ar and 25% CO2 or 80%). Ars and 20% CO2). Alloy retention and mechanical properties. Due to the active nature of CO2, when using Ar/CO2 mixed gas shielding for flux-cored wire welding, the degree of electrode alloy deposition in the weld metal is higher than when using pure CO2 gas shielding. This is because CO2 reacts with the alloy to form oxides, which together with the oxides in the flux form slag. The flux core of the electrode must contain some active elements, such as manganese (Mn) and silicon (Si), and can be used as a deoxidizer, among other uses. Some of these alloys react with free oxygen obtained by CO2 ionization to form oxides, which remain in the slag rather than in the weld metal. Therefore, the content of Mn and Si in the deposited metal using Ar/CO2 mixed gas is higher than that using CO2 gas. The higher the content of Mn and Si in the weld deposited metal, the greater the weld strength, the lower the weld elongation, and the impact toughness of the sharp V-shaped notch will also change. Simply changing the shielding gas from CO2 to Ar/CO2 mixture will increase the tensile strength and yield strength by 7 to 10 ksi, and reduce the elongation by 2%, as shown in Figure 2. It is important to understand that as the Ar content in the shielding gas increases, the weld strength increases and the toughness decreases. Since the shielding gas will affect the final performance of the weld, AWS D1.1/D1.1 M:2008 "Steel Structure Welding Specification" stipulates a series of specific requirements to ensure the performance of the weld. For all welds, the selection of shielding gas must be consistent with the AWS A5.32/A5.32M Standard AWS FCAW-G Welding Consumables Classification (A5.20/A 5.2 M and A5.29M) which specifies the maximum strength of the weld deposit. The welding shielding gas selected must ensure that the welding result does not exceed these specified strength limits, which also depends on the electrode design and welding process. For unmodified welding procedures, D1.1:2008 requires specific filling and shielding gas compositions to support the test data. D1.1:2008 Item 3.7.3 specifies two forms of support: one is the use of shielding gas for electrode classification. One is that the data of the filler metal manufacturer meets the requirements of AWS A5 and meets the shielding gas specified by WPS. If neither of these conditions is met, D1.1:2008 requires an evaluation of the protective gas mixture.

08-28

Selection of shielding gas for co2 flux core gas shielded welding, introduction of inert gas

2022-08-28

When selecting GMAW shielding gas, factors such as welding cost, welder preference and welding quality should be considered. Flux-cored wire gas shielded welding is a widely used welding technology. It is widely used in the welding of low carbon steel, low alloy steel and other alloy materials in heavy manufacturing, construction, shipbuilding, marine facilities and other industries. The FCAW-G welding process often uses 100% pure CO2 or 75% ~ 80% Ar and 20% ~ 25% CO2 mixed gas as the protective gas. So in the implementation of flux-cored wire gas welding, which shielding gas should be selected, CO2 or Ar/CO2 mixed gas? Each shielding gas has its own advantages and disadvantages. When selecting a welding shielding gas, it is important to consider factors such as cost, quality, and productivity. Sometimes the choice of protective gas and these factors are contradictory. The advantages and disadvantages of FCAW-G in the selection of two basic shielding gases for welding steel are introduced. Before discussing the advantages and disadvantages of protective gas in detail, it is best to review some basic knowledge. It should be noted that only a few shielding gases are discussed in this article. For a more comprehensive introduction, please refer to ANSI/AWSA5.32/A5.32M. The welding shielding gas specification specifies the technical requirements for shielding gas, including testing, packaging, marking, acceptance, etc. In addition, it also includes some useful information, such as ventilation during welding, and full consideration of safety requirements. Working principle of protective gas. One of the main functions of all shielding gases is to isolate oxygen, nitrogen and water vapor from the air, and to protect the weld pool and electrodes. The shielding gas enters through the welding torch, is sprayed from the nozzle and surrounds the electrode, displaces the air around the electrode, and forms a temporary shielding gas around the molten pool and the arc. Both CO2 gas and Ar/CO2 mixed gas can achieve this goal. These shielding gases promote the formation of the arc plasma region, which is the current path for the welding arc. The type of shielding gas also affects the heat transfer of the arc and the arc force exerted on the molten bath. CO2 and Ar/CO2 mixtures exhibit different behavior on these issues. Characteristics of the shielding gas. The reaction of CO2 and Ar in arc heat is different. Analyzing these differences can help understand how the properties of each gas affect the welding process and weld deposit. ionization potential. The ionization potential is the energy required to ionize a gas (e. g., convert the gas to a charged ionic state), making the gas conductive. The lower the ionization potential, the easier it is for the arc to ignite and remain stable. The ionization potential of CO2 is 14.4 eV The electron volt of argon is 15.7 eV. Therefore, CO2 shielding gas is easier to arc than Ar shielding gas. Heat conduction. The heat transfer of gas refers to the ability of gas to conduct heat energy, which will affect the transfer mode of molten droplets (such as jet transfer and large droplet transfer), arc shape, weld penetration and arc temperature distribution. The CO2 gas has a higher thermal conductivity than the Ar gas and the Ar/CO2 mixed gas. Reactive. The reactivity of the gas refers to whether the gas reacts with the molten bath. Gases can be broadly divided into two categories: inert gases and reactive gases. An inert gas that does not react with other elements in the bath. Ar is an inert gas. The reactive gas combines or reacts with other elements in the bath to form new compounds. At room temperature, CO2 is an inert gas, but in the arc plasma region CO2 is decomposed into carbon monoxide (CO), oxygen (O2) and some independent oxygen atoms (O). Therefore, CO2 becomes an active gas under the arc and can be oxidized with other metals. The Ar/CO2 mixture is also a reactive gas, but is less reactive than CO2. When the parameters of other welding specifications are consistent, the welding smoke produced by different shielding gases is also different. In particular, Ar/CO2 shielding gas produces less welding dust than CO2 shielding gas because CO2 is oxidizing. In addition, due to the different welding occasions and welding sequence, the amount of welding fume is also different.

08-27

CO2 gas shielded welding, flat butt welding, single-sided welding, double-sided forming operation process

2022-08-27

Features of semi-automatic CO2 welding. When the plate is welded, the molten pool is in a suspended state, and the metal liquid is easy to fall under the action of gravity. During the process, the angle, swing amplitude and welding speed of the welding torch must be adjusted in time according to the changes in the assembly gap and the temperature of the molten pool to control the size of the molten hole and ensure the uniform weld on the back of the sample. 2. the welding operation process. 1. Preparation before welding. (1) The test piece is processed by using the normal strength ship structural steel Class B test piece with a thickness of 10mm. The processing size is 300mm × 150mm × 10mm, the bevel processing angle is 300±10mm, and no blunt edge is left, as shown in Figure 3-1 below. Semi-automatic CO2 welding is very sensitive to rust and oil prevention, so it is necessary to clean the 20mm range around the groove before welding, and use an angle grinder to polish the metallic luster. (3) See Table 3-5 for the dimensions of the test piece group and the positioning of the welding blank. The two ends of the specimen are positioned welded, and the length of the positioning weld is 10~15. The welding wire and welding parameters used in the positioning welding are the same as those used in the formal welding. After the tack welding, grind both ends of the tack welding into inclined surfaces with an angle grinder to remove the spatter on the groove. (4) Welding parameter selection B plate butt flat welding semi-automatic CO2 welding welding parameter selection is shown in Table 3-6. Table 3-6 Selection of welding parameters. 2. Welding. Place the test piece flat on the workbench with enough space on the back, place the end with the smaller gap as the starting end on the right, and use the left welding method for welding. (1). Semi-automatic CO2 gas shielded welding, single-sided welding, double-sided forming, backing welding, using filaments, small current welding, and molten droplets are transferred in the form of short circuit. In order to avoid spatter caused by low short-circuit voltage, the end of the welding wire should be cut into a bevel shape with pointed nose pliers before starting the arc to reduce the short-circuit contact area and start the arc smoothly. At the same time, pay attention to the length of the wire. Generally, the arc is started at about 10mm. If the length is too long, it is easy to increase the resistance and heat, causing the wire to burst, and the arc cannot be smoothed. The arc position shall be at the top of the slope of the tack weld, see Figure 3-2. (2). Bottom welding. When the bottom layer is welded, the angle between the welding torch and the weldment is shown in Figure a, and the extension length of the welding wire is 20~25mm. After the arc is ignited, the welding torch runs along the root of the groove without pressing the arc. The welding torch swings slightly laterally at the root and pauses on both sides of the cut. During welding, pay attention to observe the size of the weld hole. The diameter of the weld hole is 0.5~1mm larger than the groove gap. Adjust the transverse swing range and welding speed of the welding torch according to the change of groove spacing and weld aperture. When the gap between the groove and the weld hole increases, the transverse swing of the welding torch should be widened to ensure uniform weld formation on the back. During the welding process, the end of the welding wire should always be burned in the front half of the molten pool, and the welding wire should not be separated from the molten pool. In addition, the nozzle height should be strictly controlled. That is, the operating line of sight cannot be blocked, and the gas protection effect is good. During welding, the arc burns at a distance of 2~3mm from the root of the groove to keep the thickness of the female weld at about 4mm. Semi-automatic CO2 gas shielded welding does not need joints. Generally, it can be completed continuously at one time. If it cannot be completed at one time, the arc should be polished and then arc. The welding method is the same as the above method. From the above analysis of the welding process, it can be seen that the operation points of CO2 semi-automatic welding single-sided welding double-sided welding are: when backing welding, attention should be paid to the swing mode of the welding torch, the position of the welding torch corner arc in the groove and the influence of the melting hole. The angle of welding torch should be controlled within a certain range. If the back angle of welding torch is too small, it will not only destroy the protective atmosphere, but also easily make the liquid metal flow forward, hinder the formation of molten hole, so that the back weld can not be formed. The back angle of the welding torch is too large, which not only affects the operation line of sight, but also makes the end of the welding wire out of the molten pool and extends to the back of the sample through the groove gap, causing the welding process to be interrupted. The welding torch swing method should be appropriate to effectively control the size of the molten pool and prevent the liquid metal from falling through the groove gap. During flat welding, the small amplitude transverse swing method should be adopted, and the arc should be suspended on both sides of the groove to control the temperature rise of the molten pool. During the welding process, attention should be paid to controlling the size of the weld hole so that it is always greater than the gap between 0.5~1mm on both sides of the groove to obtain uniform and smooth back weld formation. (3). Filling welding. Before welding the filling layer, the welding slag and spatter on the surface of the bottom weld shall be cleaned to prevent pollution and affect the quality. The angle and lateral swing mode of the welding gun are the same as that of the master welding, and the extension length of the welding wire can be slightly greater than 1~2mm. During welding, pay attention to the even swing of the welding torch in place, and pause on both sides of the groove to ensure smooth welding seam. At the same time, it is conducive to full melting of the edges on both sides of the groove without slag inclusion defects. In the welding process, attention should be paid to controlling the welding speed to ensure that the thickness of the weld is appropriate and the filling layer and the underlying metal are well fused. After the filling layer is welded, the weld surface shall be 1~1.5mm away from the specimen surface, and the groove edge shall not be damaged. (4) Cover welding. In front of the welding cover, the welding slag and metal spatter on the surface of the welding seam of the filling layer shall be removed first, the uneven part of the joint shall be leveled with an angle grinder, and the spatter around the conductive nozzle and nozzle shall be removed. The angle and swing mode of the welding gun are the same as that of filling welding, but the swing amplitude of the welding gun should not be too large, otherwise the welding wave thickness and undercut phenomenon are easy to occur. When the welding gun swings, both sides of the groove shall be evenly and slowly to prevent the concave. The welding process can be completed continuously. When the welding is interrupted, the gas should be delayed to avoid oxidation of the molten pool at high temperatures.