According to China's long-distance pipeline construction of the development process, sum up the whole fiber, hybrid, complex three kinds of manual welding techniques and to the activity under the gas, flux cored wire from under the protection of two kinds of semi-automatic and automatic welding technology activity to flux-cored arc Welding Wire and automatic welding technology to the next process characteristics and long-distance pipeline construction in China in the application state that the automatic and fully automatic active gas shielded flux-cored welding to weld will be our next long-distance pipeline under the direction of the welding technology.
With oil and gas and petrochemical industries to natural gas projects as a symbol of China's long-distance pipeline construction peak has arrived. More to the long-distance oil and gas pipelines of large diameter, high-pressure transmission direction. Under the long-distance pipeline welding technology since the 1960s since the introduction of our country, after decades of development, now under the hand of a mature technology to welding, are popular semi-automatic gas shielded welding technology, welding technology and automatic gas under the combination of the welding technology as a long-distance pipeline welding technology trends, will be in the national long-distance pipeline construction to promote.
Flux-Cored Gas Shielded Welding (referred to as FCAW-G) is a widely used welding process, shown in Figure 1. It is widely used in heavy manufacturing, construction, shipbuilding, offshore and other industries in low-carbon steel, low alloy steel and various other alloys welding. FCAW-G welding is often used 100% pure CO2 or 75% to 80% Ar and 20% to 25% CO2 gas mixture as a protective gas.
So in the implementation of gas-welding flux cored wire, whether the choice of a protective gas, CO2 or Ar/CO2 mixture it? Each type of protective gas has its own advantages and shortcomings. Choose gas welding protection, when important consideration of cost, quality, productivity and other factors. Sometimes the choice of protective gas and these factors are contradictory. This paper describes the FCAW-G weld steel in a protective gas in the choice of two basic advantages and disadvantages.
Choose the specific pros and cons on the protection of air, the best review some basic knowledge. Be noted that this paper only discussed a few protective gas. More comprehensive introduction, refer to ANSI/AWSA5.32 / A5.32M, welding shielding gas specification details the technical requirements to protect air, including testing, packaging, identification, inspection and so on. It also includes ventilation during welding and other useful information, take full account of safety requirements.
Protection of gas works
All to protect a gas main role is to cut off oxygen in the air, nitrogen and water vapor, to protect the weld pool and electrode. Protective gas through the torch to enter, ejected from the nozzle, surrounded by the electrode around displaces the air around the electrode in the arc around the pool and form a temporary protective hood. CO2 gas mixture and Ar/CO2 can achieve this goal.
The protective gas for the arc plasma formation, plasma arc welding arc is the current channel. Also affects the protective gas arc type heat conduction and the pool size of the force applied to the arc. On these issues, CO2 and Ar/CO2 mixture performance is not the same.
Characteristics of protective gas
CO2 and Ar arc enthusiastic response in the different. Analysis of these differences can help to understand the characteristics of each gas welding process and how it affects the weld deposition.
Ionization potential. Ionization potential is the size of the gas ionization energy required (for example, the gas charged into the plasma state), the gas can conduct electricity. Ionization potential is lower, the more easily ignite and maintain arc stability. Ionization potential of CO2 is 14.4eV, Ar ionization potential is 15.7eV. Therefore, CO2 shielding gas Ar protection than more easy to ignite the gas arc.
Thermal conductivity. Gas thermal conductivity is the heat capacity of the gas transmission size, it's good or bad will affect the metal transfer method (such as jet transition and large drops of the transition), the arc shape, weld penetration and arc temperature distribution. CO2 gas is heavier than air and Ar/CO2 Ar gas mixture has a higher thermal conductivity.
Reactivity. The reactive gas is gas with the molten weld pool is a chemical reaction. Gas can be roughly divided into two categories: inert gas and reactive gas. Inert gas in the weld pool does not react with other elements. Ar on the part of inert gas. Reactive gases in the weld pool will be combined 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 broken down to form carbon monoxide (CO), oxygen (O2) and some independent oxygen atoms (O). Therefore, CO2 under the arc becomes active gases can occur with other metal oxide. Ar/CO2 mixed gases are reactive gases, but the activity is lower than CO2.
When the other parameters the same welding specifications, different protective gas welding fumes generated by different size. Specifically, compared with CO2 shielding gas, Ar / CO2 shielding gas welding fumes produced less because CO2 has oxidation. In addition, due to the specific welding and welding sequence of different occasions, the amount of welding fumes are not the same.
Inert gas introduced
Although the inert gas to provide protection for the weld pool, but they themselves are not suitable for iron metals (such as low carbon steel, low alloy steel, stainless steel, etc.) gas-welding Flux Cored Welding Wire. 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 longer rod outer steel skin and premature melting. Increased range and difficult to control the arc, leading to accumulation of the weld. Therefore, using gas shielded flux cored wire welding ferrous base metals, usually using an inert gas and reactive gas combination of mixed gas.
CO2/Ar gas mixture introduced
In North America, gas-welding stainless steel flux-cored welding is often used as a protective gas Ar/CO2 mixed gas, which accounted for 75% and Ar 25% CO2. Sometimes also used 80% of Ar and 20% CO2 mixture, but this mix is â€‹â€‹not common. Some gas shielded flux cored wire requires the use of 90% Ar and 10% CO2 gas mixture protection. However, if the protective gas mixture of Ar content of less than 75%, it will damage the arc performance, it is necessary to ensure the protection of the percentage of Ar gas. Furthermore, Ar / CO2 percentage of non-standard configuration mixing tank is usually a percentage than the standard configuration mixing tank (such as 75% Ar/25% CO2 or 80% Ar/20% CO2) more difficult to obtain. Retention and mechanical properties of alloys
Because the nature of the activity of CO2, when using Ar / CO2 mixed gas for flux cored wire welding, the use of pure CO2 than gas, electrode alloy in the weld metal deposited 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, also used as a deoxidation agent. Part of these alloys and CO2 obtained free of ionizing oxygen react to produce oxides remain in the slag rather than remain in the weld metal. Therefore, using Ar/CO2 mixed gas than CO2 gas shielded welding using deposited metal in the Mn and Si content higher. Shown in Figure 2.
Deposited weld metal in the higher Mn and Si content, the higher weld strength, weld the lower the elongation, while Sharp V-notch impact toughness will change. Simply replace the protective gas from the CO2 gas mixture Ar/CO2, tensile and yield strength will increase 7 ~ 10ksi, elongation decreased by 2%, 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 welding procedures to ensure that the weld provides a series of specific performance 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 cap. Choice of welding shielding gas welding results must ensure that these provisions do not exceed the strength limit, which also depends on the welding rod and welding design. Welding process for the pre-amendment standard, D1.1: 2008 requirements of the specific filler to support and protect the gas composition test data.
D1.1: 2008 of the 3.7.3 entry provides two forms of support: one is to protect the gas used for welding purposes of classification; one is filler metal manufacturer's data consistent with the AWS A5 requirements, and with the provisions of the WPS protect the gas line. If these two conditions are not met, D1.1: 2008 requirements to protect the gas mixture to the qualification test.
According to the gas type of filler metal classification
Since 2005, American Welding Society filler metal-cored gas-type classification will be incorporated into the electrode to protect the classification symbols. Mild steel FCAW-G electrodes AWS number EXXT-XX, the last symbol refers to the protection of gas type. If the last one for the C, on behalf of the protective gas as CO2; if M, on behalf of Ar/CO2 gas mixture (for example, E71T-1C or E71T-1M). For low-alloy steel welding rod, gas sign with the provisions of protection of the last symbols of the same deposited metal components symbol (for example E81T1-Ni1C). On the contrary, self-protection 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. Some electrodes can also use CO2 gas or Ar/CO2 mixed gas, in this case, the electrode must meet the requirements of the two classifications.
FCAW-G protective gas choice
For flux cored wire welding, is to select or choose Ar/CO2 CO2 gas mixture of gas need to consider the following three aspects.
1) the cost of gas protection
Typically, the soldering of the total cost, 80% of labor and management expenses, 20% of material costs, which protect the gas costs of materials cost 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 replaced by protective gas with CO2 shielding gas mixture Ar/CO2 way can greatly reduce welding costs. 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. World wide and rich resources of CO2. CO2 can usually get by by-product of other processes. The welding industry, on one hand by natural gas processing or isolated CO2, on the other hand you can get through the air CO2. Because the content of Ar in the atmosphere less than 1%, need a lot of processing and handling of air in order to extract a certain amount of Ar, and the need for specialized air separation unit to deal with air. Air separation unit to spend a lot of power, also needs to be placed in a special area.
2) welder's preferences and productivity of
When the same type and size of wire for welding, shielding gas used Ar/CO2 protection than simply using CO2 gas arc welding is more stable when the gain, weaker, less spatter, so loved by the welder. CO2 shielding gas when welding the welding arc prone to large metal transfer (droplet is usually greater than the wire diameter), resulting in the arc of instability, discontinuity, a larger splash. Ar/CO2 mixed gas transition of droplet splashing 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 extent of its love, protection and use of CO2 gas welding, compared to its low thermal conductivity, so it can maintain a pool of heat and liquid level. This enables 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 a lack of skills of the arc welder can be well controlled to improve welding productivity.
Using Ar/CO2 mixed gas Welding Wire. due to higher levels of Ar, the gas than CO2 protection welding more heat radiation to the welder. This means that the welder welding feel hotter. In addition, the torch will be hotter (Ar/CO2 duty cycle of the gun under the protection of gas than CO2 shielding gas of low duty cycle), which requires a larger gun or ask for the same model gun and vulnerable components more frequent replacement.
3) welding quality
As previously discussed, the use of protective gas mixed with Ar/CO2 protective gas welding using CO2 than it can heat and maintain the pool of liquid level, making the pool a more thorough response, weld toe part of the more easily melted fully. Therefore, it greatly increased the ability to weld and weld quality.
In addition, Ar/CO2 mixture spatter when welding protection, welding quality greatly improved, while reducing post-weld clean-up time and costs. Lower the amount of spatter also improved ultrasonic weld inspection costs, because splash too much, to ensure the accuracy of ultrasonic testing, must be clean prior 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 worms crawl chicken scratch marks or defects, some are sometimes located in a small groove weld surface. They are dissolved by the weld metal in the gas caused by these forward out of gas in the coagulation bath, was stranded in the solidification of the slag below.
Ar/CO2 mixed gas than pure CO2 gas has a higher sensitivity to gas mark. Ar/CO2 transition splash of gas protection features lead to a large number of small droplets, which increases the droplet surface area, resulting in a large number of gases dissolved in the weld metal. In addition to shielding gas type will affect the sensitivity of gas marks, there are other factors, but they do not belong within the scope of this article. Some of the major applications of commonly used shielding 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 welding welding high deposition welding applications, typically using CO2 gas protection, because in these welding position, Ar/CO2 mixed gas does not have much advantage.
Shipbuilding industry is also usually like to use CO2 gas as CO2 gas shielded arc better burn characteristics of the base metal primer. North American marine construction, welding next to the T-, Y-and K-groove weld to connect when they need to shape and smooth weld spatter less, so a Ar/CO2 mixed gas is more suitable. If used in the construction of the workshop is more than one gas-welding process, such as GMAW and FCAW-G, the two processes are usually protective gas standardization. Sometimes, in order to get a better rate and pulse spatter arc transition, and many manufacturers also choose Ar/CO2 mixed gas for GMAW welding.
When the application of choice for the FCAW-G protective gas, we should not only consider the cost of gas, but should consider the three aspects discussed in this article. Each gas type can affect the overall welding costs? Which gas can reduce the cost per meter weld? Some vendors find Ar/CO2 mixed gas can improve weld quality and productivity. Other companies that Ar/CO2 mixed gas arc welding is not satisfactory or not the advantages and CO2 gas shielded welding of low cost. However, for some firms, CO2 cost, used in some of their welding applications is very appropriate. FCAW-G process for users, how to choose the protective gas is that gas should be based on how it affects the cost of welding operations, quality and productivity to determine. Once selected the protective gas, FCAW-G electrode should be suitable for the gas welding.