Flux-cored arc welding (referred to as FCAW-G) is a widely used stainless steel welding wires process, shown in Figure 1. It is widely used in heavy manufacturing, construction, shipbuilding, offshore installations and other industries in the low carbon steel, low alloy steel and various other alloys welding. FCAW-G welding process is often a 100% pure CO2 or 75% to 80% of Ar and 20% to 25% CO2 gas mixture as a protective gas.
Then when the implementation of flux-cored arc welding, whether the choice of which protective gas, CO2 or mixed gas Ar/CO2 it? Each type of shielding gas has its own advantages and disadvantages. Select shielding gas when welding, to focus on considering cost, quality, productivity and other factors. Sometimes the choice of shielding gas, and these factors are contradictory. This paper describes the FCAW-G in the welding of steel in shielding gas selected two basic advantages and disadvantages.
Select the specific pros and cons of the discussion before the protective gas, the best review some basic knowledge. Should be noted that this paper discussed only a few protective gas. A more comprehensive introduction, refer to the ANSI/AWSA5.32 / A5.32M, welding shielding gas specification details the technical requirements for protection of gas, including testing, packaging, identification, acceptance and so on. In addition, the welding process also includes useful information such as ventilation, full account of safety requirements.
Protection of gas works
All the main role of a protective gas is cut off oxygen in the air, nitrogen and water vapor, protect the weld pool and electrode. Protection of gas through the torch to enter ejected from the tip, surrounded around the electrode, the electrode displacement off the air around the arc around the pool and the formation of a temporary protective hood. Ar/CO2 CO2 gas and mixed gas can achieve this goal.
The shielding gas for the formation of plasma arc, plasma arc welding arc is the current channel. Type also affects the protective gas arc heat conduction and the arc force exerted on the molten pool size. On these issues, CO2 and Ar/CO2 mixture performance is not the same.
Characteristics of protective gas
CO2 and Ar enthusiastic response in the different arc. Analysis of these differences can help to understand how the characteristics of each gas welding process and welding of the cladding.
Ionization potential. Ionization potential is the size of the gas ionization energy required (for example, the charged ions into the gas state), the gas can conduct electricity. The lower ionization potential, the arc ignition more easily and remain stable. CO2 ionization potential for the 14.4eV, Ar ionization potential is 15.7eV. Therefore, CO2 shielding gas Ar protective gas more easily than the arc ignition.
Heat transfer. Gas thermal conductivity is the size of the gas transfer capacity of heat, it's good or bad will affect the metal transfer means (such as jet transition and large drops of the transition), the arc shape, weld penetration and arc temperature distribution. Ar gas than CO2 and Ar/CO2 gas mixture has a higher heat capacity.
Reactivity. Reactive gas is gas and the molten weld pool is a chemical reaction. Gas can be roughly divided into two categories: inert gases and reactive gases. Inert gas, in the weld pool does not react with other elements. Belong to the inert gas Ar. Reactive gases, in the weld pool in combination with other elements or reaction to form new compounds. At room temperature, CO2 is an inert gas, but in the arc plasma, CO2 will be decomposed to form carbon monoxide (CO), oxygen (O2) and some independent oxygen atoms (O). Therefore, CO2 under the arc becomes the active gas, can occur with other metal oxide. Ar/CO2 also belongs to the active gas mixture, but lower than the activity of CO2.
Consistent when other welding parameters, different shielding gas welding fumes generated size are also different. Specifically, compared with CO2 shielding gas, Ar / CO2 shielding gas welding fumes produced less because CO2 has oxidation. In addition, the specific sequence of welding and welding of different occasions, the number of welding fumes are not the same.
Inert gas introduced
Although the inert gas to provide protection for the weld pool, but they themselves were not suitable for iron based metal (such as low carbon steel, low alloy steel, stainless steel, etc.) flux-cored gas shielded arc welding. For example, if only Ar as protective gas welding stainless steel, weld performance becomes very poor. This is because the use of inert gas will cause the arc length of the extension and the rod outer steel skin premature melting. Increase the scope and difficult to control the arc, resulting in weld deposits. Therefore, the use of flux-cored arc welding welding ferrous base metal, usually an inert gas and reactive gases using a combination of mixed gas.
CO2/Ar mixed gas introduced
In North America, stainless steel flux-cored gas shielded arc welding is often used as a protective gas Ar/CO2 mixed gas, which accounted for 75% and Ar 25% CO2. Sometimes also used 80% Ar and 20% CO2 mixture, mixing ratio, but this is not commonly used. Some gas shielded flux cored wire need to adopt 90% CO2 and 10% Ar gas mixture protection. However, if mixed in the Ar shielding gas content is less than 75%, the performance will be produced on the arc damage, it is necessary to ensure the protection of the percentage of Ar gas. Furthermore, Ar / CO2 tank mixture of non-standard configuration is usually a percentage than the standard percentage of the configuration mixing tank (eg 75% Ar/25% CO2 or 80% Ar/20% CO2) more difficult to obtain. Retention and mechanical properties of alloy
Nature of the activity of CO2, when using the Ar / CO2 mixed gas for flux cored arc welding, the use of pure CO2 than gas, electrode alloy in the welding alloys deposition in a higher degree. This is because CO2 and alloy reacts oxide, together with the flux in the oxide to form slag. The cored electrode must include active elements, such as manganese (Mn) and silicon (Si), etc., in addition to other uses, but also can be used as oxygen scavenger. Part of these alloys and CO2 ionization of free oxygen to obtain the reaction of oxides rather than remain stuck in the slag in the weld metal. Therefore, using Ar/CO2 CO2 gas mixture protection than the use of deposited weld metal Mn and Si content in the higher. Figure 2.
Deposited weld metal Mn and Si content is higher, the higher weld strength, weld the lower elongation, while Sharp V-notch impact toughness will change accordingly. Simply replace the protective gas from the CO2 gas mixture Ar/CO2, tensile and yield strength will increase 7 ~ 10ksi, the elongation decreased by 2%, as shown in Figure 2. It is important to understand, with the protection of Ar gas content increases, the weld strength increases, toughness will be reduced.
As the weld shielding gas will affect the final performance, AWS D1.1/D1.1M: 2008, steel for the welding of weld performance requirements to ensure a range of specific requirements. For all the welding, the shielding gas selection standards must be consistent with the AWS A5.32/A5.32M. FCAW-G welding consumables (A5.20/A5.20M and A5.29/A5.29M) of the AWS classification provides the strength of the weld deposited metal ceiling. Choice of welding shielding gas welding results must ensure that these provisions do not exceed the strength limit, which depends on the design of electrode and welding process. Before the welding procedure specification for the unmodified, D1.1: 2008 requirements of the specific composition of filler and protection of air support for test data.
D1.1: 2008 in 3.7.3 provides two entry form of support: one is the use of shielding gas for welding purposes of classification; one is filler metal manufacturer's data consistent with the AWS A5 requirements, and provided with the WPS protect the gas line. If these two conditions are not met, D1.1: 2008 requirements of shielding gas mixture of tests to be assessed.
According to the type of filler metal classification of gas
Since 2005, American Welding Society classification cored filler metal shielding gas type will be incorporated into the classification of welding symbols. Mild steel FCAW-G electrode AWS number EXXT-XX, the last type of symbol refers to the protective gas. If the last one for the C, on behalf of the protective gas as CO2; if M, on behalf of Ar/CO2 mixed gas (for example, E71T-1C, or E71T-1M). For low-alloy steel electrodes, the provisions of protection of gas symbols and signs the final agreement signed deposited metal components (such as E81T1-Ni1C). On the contrary, since the protection of cored electrode, without any protective gas, in its classification number in the code there is no protective gas (for example, E71T-8).
Some electrodes can be protected with CO2. Some electrodes can only Ar/CO2 mixed gas. There are a number of electrodes can use CO2 gas or gas mixture Ar/CO2 protection, in this case, the electrode must meet the requirements of the two classifications.
FCAW-G the choice of shielding gas
The flux-cored wire welding, is to select or choose Ar/CO2 CO2 gas mixture protection needs to consider the following three aspects.
1) the cost of protective gas
Typically, the soldering, 80% of the total cost of labor and management expenses are 20% of material costs, including the cost of gas protection material costs account for about 1 / 4, or accounting for 5% of the total cost of welding. Assume that the cost of protective gas is the only determining factor, then protected through the use of CO2 gas shielding gas mixture Ar/CO2 alternative ways to greatly reduce the welding cost. Often, however, other costs also affect the total cost of welding, which will be discussed later.
Ar/CO2 cheaper than CO2, because it can get low cost. CO2 world wide and rich resources. CO2 can usually get by-product of other processes. The welding industry, the one hand, the processing of natural gas or by isolated CO2, on the other hand can also be obtained through the air CO2. Because the content of Ar in the atmosphere less than 1%, you need to process and handle large volumes of air to extract a certain amount of Ar, and the need for special air separation unit to handle the air. Air separation unit spend a lot of power, also needs to be placed in a special area.
2) Welder's preferences and productivity of
When using the same type and size of the welding consumables , shielding gas used Ar/CO2 protection than simply using CO2 gas arc welding obtained more stable, more weak and less spatter, so loved by the welder. CO2 shielding gas when welding arc prone to large droplet (droplet is usually greater than the wire diameter), leading to arc instability, discontinuous, splash bigger. Ar/CO2 mixed gas splash droplet transition is small (typically less than wire droplet diameter), resulting in more stable and continuous arc, little spatter, shown in Figure 3.
Ar/CO2 Another feature of mixed-gas welders have also increased the level of its love, protection and use of CO2 gas welding compared to its ability to lower thermal conductivity, so it can maintain a pool of heat and liquid level. This allows a more thorough reaction pool, weld toe part of the more easily melted fully. When the special position of welding (eg, Yang uphill welding or soldering), the use of Ar/CO2 more attractive, because of poor technique can also be a good control of arc welders and welding productivity.
Mixed gas arc welding with Ar/CO2, due to higher levels of Ar, CO2 shielding gas welding than when more heat radiation to the welder. This means that the welder welding feel more heat. In addition, the torch will be more heat (Ar/CO2 protective gas than CO2 under the protection of the duty cycle of torch gas with low duty cycle), which requires a larger gun or require the same model gun and vulnerable components more frequent replacement.
3) the quality of welding
As previously discussed, as protective gas mixed with Ar/CO2 protection and use of CO2 gas welding compared to the heat and can maintain a pool of liquid level, so that a more thorough reaction pool, weld toe part of the more easily melted fully. Therefore, it greatly improves the ability to weld and weld quality.
In addition, Ar/CO2 mixture of small spatter when welding protection, welding quality greatly improved, while reducing the time and post-weld clean-up costs. Flying low volume also improved ultrasonic weld inspection costs, because if too much splash, to ensure the accuracy of ultrasonic testing that must be cleared in advance, splash.
Another factor affecting the appearance of the shape of the weld quality is to protect the sensitivity of gas on gas marks. Gas mark, similar to the earthworm crawling chicken scratch marks or defects are sometimes distributed in the weld surface of the small grooves. They are dissolved by the weld metal caused by the gas, these gases in the molten pool solidification forward out, but was stranded in the freezing of the slag below.
Ar/CO2 mixed gas CO2 gas than simply a higher sensitivity of the gas marks. Ar/CO2 transition splash protection features of the gas result in a large number of small droplets, which increases the droplet surface area, resulting in a large number of weld metal dissolved gases. In addition to shielding gas type will affect the sensitivity of gas marks, there are other factors, but they are not within the scope of this article. Some of the major applications of commonly used protective gas
Over the years, some of the major occasions of FCAW-G protection standards for gas has been gradually formed. For example, in flat and horizontal position welding of high deposition welding applications, typically using CO2 gas protection, because in these welding position, Ar/CO2 mixed gas does not have much advantage.
① consider the weld metal mechanical properties and chemical composition For normal aluminum electrode structural steel, weld metal and base often require materials such as strength, tensile strength of deposited metal should be used is equal to or slightly higher than the base metal electrodes. For alloy steel, sometimes also called the base metal alloy composition and the same or close. Structural rigidity in the welding, the joint stress and high, weld cracks easily under adverse conditions, should consider using low intensity than the base metal electrode. When the base material of carbon, sulfur, phosphorus and other elements of the content is too high, the weld is easy to crack, crack resistance should be a good choice of basic low hydrogen electrodes. ② consider the performance of welded components and working conditions Bear the load and impact load on the welds, in addition to meet the strength requirements, primarily to ensure that the weld metal has a high impact toughness and plasticity, the choice of plastic, high toughness, low hydrogen electrode. Contact with corrosive media weldment should be based on the nature of the media and the corrosion characteristics of type electrodes made of stainless steel or other corrosion-resistant electrodes. In the high-temperature, low temperature, wear, or other work under special conditions welding parts, should be used in the corresponding heat-resistant steel, low temperature steel, welding rods or other special purposes. ③ consider the structural characteristics of welding and stress conditions On the structure of complex shape, the Heavy rigid welded, the welding process with great stress, easy to weld crack, crack resistance should be a good choice of basic low mig welding electrode . On Shouli Bu large, difficult to clean up the welding parts welding parts, should be used for rust, scale, oil is not sensitive to the acidity of electrodes. Can not be overturned by the conditions of the welding parts, should be used for all position welding electrodes. ④ construction conditions and economic considerations Product performance requirements to meet the circumstances, should be used with good acidity electrode process. In small or poorly ventilated places, should be used in acidic or low dust and welding electrodes. Large amount of welding work on the structure and conditions should maximize the use of high-efficiency welding rod, such as iron rods, electrodes and other high-efficiency gravity, or use the bottom electrode vertical down welding electrode and the like dedicated to improve welding productivity.