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Selection of shielding gas for flux cored wire gas shielded arc welding

Edit£º   Browse£º688  Date£º2021-07-20
Flux cored wire gas welding (fcaw-g) is a widely used welding process. It is widely used in heavy manufacturing, construction, shipbuilding, marine facilities and other industries of low carbon steel, low alloy steel and other alloy materials welding. The fcaw-g welding process often uses 100% pure CO2 or 75% - 80% AR and 20% - 25% CO2 mixture as the protection gas. So what kind of gas, CO2 or ar/co2 mixture should be selected when the flux cored wire gas welding is implemented? Each type of protective gas has its own advantages and disadvantages. When choosing welding gas, the cost, quality, productivity and other factors should be considered. Sometimes the choice of gas protection is contradictory to these factors. This paper mainly describes the advantages and disadvantages of fcaw-g in the selection of two basic protective gas in welded steel.
Before discussing the advantages and disadvantages of gas protection, it is better to review some basic knowledge. It is necessary to note that this paper only discusses a few kinds of protective gas. For a more comprehensive introduction, please refer to ansi/awsac5.32/ a5.32m. The technical requirements of the gas protection are specified in the welding gas specification, including test, packaging, qualification, acceptance, etc. In addition, it also includes some useful information such as ventilation and ventilation during welding, and takes safety requirements into consideration.
Working principle of protective gas
One of the main functions of all protective gases is to isolate oxygen, nitrogen and water vapor in the air, and to protect welding pool and electrode. The protective gas enters through the welding gun, and ejects from the welding nozzle, surrounds the electrode, replaces the air around the electrode, and forms a temporary protective gas cover around the molten pool and arc. This can be achieved by CO2 gas and ar/co2 mixture.
These gas protection promote the formation of arc plasma zone, which is the current channel of welding arc. The type of protective gas also affects the conduction of arc heat and the magnitude of arc force applied to the molten pool. On these issues, the performance of CO2 and ar/co2 mixtures is not the same.
Characteristics of gas protection
The reaction of CO2 and AR in arc heat is different. Analyzing these differences helps to understand how the characteristics of each gas affect the welding process and weld deposit.
Ionization potential. Ionization potential is the amount of energy needed for gas ionization (for example, converting a gas into an charged ion state) so that the gas can conduct electricity. The lower the ionization potential, the easier the arc ignites and keeps stable. The ionization potential of CO2 is 14.4ev and AR is 15.7ev. Therefore, CO2 gas is more easily ignited than Ar gas.
Heat conduction. The heat conduction of gas refers to the capacity of gas to conduct heat energy. Its quality will affect the way of droplet transition (such as jet transition and big drop transition), arc shape, weld depth and arc temperature distribution. The CO2 gas has higher heat conduction capacity than Ar gas and ar/co2 mixture.
Reactivity. The reactivity of gas refers to whether the gas reacts with the molten welding pool. Gases can be divided into two categories: inert gas and active gas. Inert gas, in the welding pool, does not react with other elements. AR is inert gas. Active gas, in the welding pool, will be combined with other elements or react to form new compounds. At room temperature, CO2 belongs to inert gas, but in the arc plasma region, CO2 will be decomposed to form carbon monoxide (CO), oxygen (O2) and some independent oxygen atoms (o). Therefore, CO2 becomes an active gas under the arc, and can oxidize with other metals. Ar/co2 mixture is also active gas, but it is lower than CO2.
When other welding specifications are consistent, the welding dust produced by different shielding gas is different. Specifically, ar/ CO2 gas produces less welding dust than CO2 protection gas, which is due to the oxidation of CO2. In addition, due to the specific welding situation and welding sequence, the number of welding dust is different.
Introduction to inert gas
Although inert gas can protect welding pool, they are not suitable for welding of flux cored wire of iron base metals (such as low carbon steel, low alloy steel, stainless steel, etc.). For example, if AR is used as the shielding gas to weld stainless steel, the weld performance will become very poor. This is because the use of inert gas protection will cause the arc length to lengthen and the outer steel sheet of the electrode melts prematurely. The arc range is larger and difficult to control, which leads to weld stacking. Therefore, when the flux cored wire is used to weld the base metal, the mixture gas protection of inert gas and active gas is usually used.
Introduction of co2/ar mixture
In North America, ar/co2 mixture is often used as the protection gas in the welding of stainless steel flux cored wire gas welding, of which AR accounts for 75% and CO2 accounts for 25%. Sometimes 80% AR and 20% CO2 are also used, but this mixing ratio is not commonly used. Some gas shielded flux cored wires need to be protected by 90% AR and 10% CO2 mixture. However, if the AR content in the mixed gas is less than 75%, the arc performance will be destroyed, so the percentage of AR in the gas must be ensured. In addition, the non-standard percentage ar /co2 tanks are generally more difficult to obtain than those with standard percentage configurations (such as 75% AR / 25% CO2 or 80% AR / 20% CO2).
Because of the active nature of CO2, when ar/ CO2 mixed gas protection is used for flux cored wire protection welding, the welding rod alloy has a higher degree of deposition in weld metal than the CO2 gas protection. This is because CO2 reacts with the alloy to form oxides, which, together with oxides in the flux, form slag. The core of electrode must include some active elements, such as manganese (MN) and silicon (SI), which can be used as deoxidizer in addition to other purposes. Some of these alloys react with free oxygen obtained by CO2 ionization, resulting in oxides remaining in slag rather than in weld metal. Therefore, the content of Mn and Si in the ar/co2 mixture is higher than that of the welding deposited metal under CO2 gas protection.
The higher Mn and Si content in the deposited metal, the higher the weld strength, the lower the weld elongation, and the change of impact toughness of sharp V-notch. Simply change the protective gas from CO2 to ar/co2 mixture, the tensile and yield strength will be increased by 7-10ksi, and the elongation will decrease by 2%. It is important to understand this. With the increase of AR content in the gas, the weld strength will increase and the toughness will decrease.
As the shielding gas will affect the final performance of the weld, AWS d1.1/d1.1m:2008, the welding code of steel structure specifies a series of specific requirements to ensure the performance of the weld. For all welding, the selection of shielding gas must be in accordance with AWS a5.32/a5.32m. The AWS classification of welding consumables (a5.20/a5.20 m and a5.29/a5.29m) of fcaw-g specifies the upper limit of weld deposited metal strength. The welding shielding gas selected must ensure that the welding results do not exceed the specified upper limit of strength, which also depends on the design of welding rod and welding process. For the welding procedure specification before modification, d1.1:2008 requires specific filler and protective gas composition to support test data.
The article 3.7.3 of d1.1:2008 specifies two supporting forms: one is the use of protective gas for electrode classification purposes; One is that the filler metal manufacturer's data is consistent with AWS A5 requirements and with the WPS specified protection gas. If both conditions are not met, d1.1:2008 requires mixed protective gas to be evaluated.
Classification of filler metals by gas type
Since 2005, the core filler metal classification of the American Welding Association has incorporated the type of protective gas into the classification symbol of the electrode. The AWS number of fcaw-g electrode of low carbon steel is exxt-xx, and the last symbol refers to the type of protective gas. If the last one is C, it means that the protective gas is CO2; If m, it represents ar/co2 mixture (e.g., e71t-1c or e71t-1m). For low alloy steel electrodes, the gas protection symbol is consistent with the last deposited metal composition symbol of the specified symbol (e.g. e81t1-ni1c). Instead, self protective core electrodes do not need any protective gas, and there is no code for the protective gas in its classification number (e.g. e71t-8).
Some electrodes can only be protected with CO2. Some electrodes can only be protected by ar/co2 mixture gas. Some electrodes can be protected by CO2 gas or ar/co2 mixture gas at the same time. In this case, the electrodes must meet two classification requirements.
Selection of fcaw-g gas protection
When welding flux cored wire, the following three aspects should be considered in the selection of CO2 gas protection or ar/co2 mixed gas protection.
1) Cost of gas protection
Generally, 80% of the total welding costs are labor and governance costs, 20% are material costs, of which the cost of gas protection accounts for about 1/4 of the material cost, or 5% of the total welding cost. Assuming that the cost of the gas is the only determinant, the welding cost can be greatly reduced by replacing ar/co2 with CO2. However, other costs generally also affect the total cost of welding, which will be discussed later.
CO2 is cheaper than ar/co2 because it can be obtained at a low cost. The world has a wide range of CO2 resources. CO2 can usually be obtained by-products of other processes. For welding industry, CO2 can be obtained by processing or separating natural gas, on the other hand, CO2 can be obtained by air. Because ar content in the atmosphere is less than 1%, a large amount of air needs to be processed and processed to extract a certain amount of AR, and special air separation device is needed to process air. Air separation devices consume a lot of power and need to be placed in special areas.
2) The influence of welder preference and productivity
When welding with the same type and size of welding wire, ar/co2 protection gas is more stable, weaker and less splashing than that of CO2 shielded gas welding, so it is favored by welders. When CO2 shielding gas is applied, welding arc can easily produce large droplet transition (the droplet is usually larger than the diameter of welding wire), which leads to the arc instability, discontinuity and large spatter. The arc is more stable and continuous and the spatter is smaller (the droplet is usually smaller than the diameter of the wire) in the arc / CO2 mixture gas protection splash transition.
Another feature of ar/co2 gas mixture protection also increases the degree of preference of welders. Compared with the welding with CO2 shielding gas, it has low heat conduction capacity, so it can keep the heat and liquid level of the molten pool. This can make the reaction of the pool more thorough, weld toe part more easily melt full. When welding in special positions (such as uphill welding or inverted welding), ar/co2 is more attractive because the welding workers with poor technology can also control the arc well and improve the welding productivity.
When ar/co2 is used in the welding, because of the high ar content, it emits more heat to the welder than that of CO2 gas welding. This means that the welder feels hotter when welding. In addition, the welding gun will also be hotter (the duty cycle of welding gun under ar/co2 protection gas is lower than that of CO2 protection gas). This requires the use of larger welding gun or the replacement of the same type of welding gun and its vulnerable components more frequently.
3) Welding quality
As discussed earlier, ar/co2 hybrid shielding gas can keep the heat and liquid level of the pool, make the reaction of the pool more thorough and the weld toe part melt more easily and fully compared with the CO2 gas. Therefore, it greatly improves the welding seam forming ability and the weld quality.
In addition, ar/co2 mixture gas protection welding spatter is small, weld quality is greatly improved, and the cleaning time and cost after welding are reduced. The lower spatter also improves the cost of ultrasonic weld detection. If there are too many splashes, it is necessary to clean up the spatter in advance to ensure the accuracy of ultrasonic detection.
Another quality problem that affects the appearance of weld is the sensitivity of gas to gas trace. Gas marks, like earthworm crawling marks or chicken scratches, are some small grooves that sometimes distribute on the surface of the weld. They are caused by dissolved gases in weld metal, which are removed before the pool solidifies, but remain under the solidified slag.
Ar/co2 mixed gas protection has higher gas trace sensitivity than that of CO2 gas protection. The spatter transition characteristics of ar/co2 shielding gas lead to a large number of small droplets, which increases the surface area of the droplet, and results in the dissolution of a large number of gas in the weld metal. Besides the influence of gas protection type on the sensitivity of gas marks, there are other factors, but they are not covered by this paper. Some common protective gases in some major applications
Over the years, the protection gas of fcaw-g in some major occasions has gradually formed the standard. For example, CO2 gas protection is usually used in high deposition welding applications of flat and transverse welding, because ar/co2 mixed gas protection does not have too much advantage in these welding positions.
Shipbuilding also likes CO2 gas protection, because the arc characteristics of CO2 gas protection can better burn the base metal primer. In North America, the offshore construction industry requires smooth weld shape and small welding spatter when welding T-type, Y-type and K-joint groove weld, so ar/co2 mixed gas protection is more suitable. If more than one gas welding process is used in the construction workshop, such as GMAW and fcaw-g, the two processes are usually standardized. Sometimes, in order to obtain better spatter rate and pulse arc transition, many manufacturers also choose ar/co2 mixed gas protection for GMAW welding.
Concluding remarks
When choosing the protective gas for fcaw-g application, the cost of gas should not be considered, but the three aspects discussed in this paper should be considered. How does each gas type affect the total welding cost? Which gas can reduce the cost of each meter of weld? Some manufacturers have found that ar/co2 gas mixture protection can improve the quality and productivity of welding seam. Some other manufacturers think that the advantages of ar/co2 hybrid gas shielded welding are not ideal or lower than that of CO2 gas protection. However, for other manufacturers, CO2 is cheap and suitable for some welding occasions. For users of fcaw-g process, how to choose the protective gas should be determined according to how the gas affects the cost, quality and productivity of welding operation. Once the shielding gas is selected, the electrode of fcaw-g shall be suitable for welding of this gas.
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