Why is CO2 used in welding?
MIG (GMAW) welding with shielding gas and a solid wire electrode produces a clean, slag-free weld without the need to continually stop welding from replacing the electrode, as in Stick welding. Increased productivity and reduced clean up are just two of the benefits possible with this process.
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Shielding gas can play a significant role in improving or impeding welding performance.
To achieve these results in your specific application, however, it helps to understand the role of shielding gas, the different shielding gases available and their unique properties.
The primary purpose of shielding gas is to prevent exposure of the molten weld pool to oxygen, nitrogen and hydrogen contained in the air atmosphere. The reaction of these elements with the weld pool can create a variety of problems, including porosity (holes within the weld bead) and excessive spatter.
Different shielding gases also play an important role in determining weld penetration profiles, arc stability, mechanical properties of the finished weld, the transfer process you use and more.
Choosing MIG gun consumables that provide consistent and smooth shielding gas delivery are also important to making successful MIG welds.
Why is it important to choose the right gas?
Many MIG welding applications lend themselves to a variety of shielding gas choices, and you need to evaluate your welding goals in order to choose the correct one for your specific application. The cost of the gas, finished weld properties, preparation and post-weld clean up, the base material, weld transfer process and your productivity goals all need to be taken into account when selecting a shielding gas.
Argon, Helium, Carbon Dioxide and Oxygen are the four most common shielding gases used in MIG welding, with each providing unique benefits and drawbacks in any given application.
Porosity, as can be seen on the face and interior of the weld bead, can be caused by inadequate shielding gas and can dramatically weaken the weld.
Carbon Dioxide (CO2) is the most common of the reactive gases used in MIG welding and the only one that can be used in its pure form without the addition of inert gas. CO2 is also the least expensive of the common shielding gases, making an attractive choice when material costs are the main priority. Pure CO2 provides very deep weld penetration, which is useful for welding thick material; however, it also produces a less stable arc and more spatters than when it is mixed with other gases. It is also limited to only the short circuit process.
For many companies, including those that place emphasis on weld quality, appearance and reducing post-weld clean up, a mixture of between 75 – 95 percent Argon and 5 – 25 percent CO2 will provide a more desirable combination of arc stability, puddle control and reduced spatter than pure CO2. This mixture also allows the use of a spray transfer process, which can produce higher productivity rates and more visually appealing welds. Argon also produces a narrower penetration profile, which is useful for fillet and butt welds. If you’re welding a non-ferrous metal — aluminium, magnesium or titanium — you’ll need to use 100 percent Argon.
Oxygen, also a reactive gas, is typically used in rations of nine percent or less to improve weld pool fluidity, penetration and arc stability in mild carbon, low alloy and stainless steel. It does cause oxidation of the weld metal, however, so it is not recommended for use with aluminium, magnesium, copper or other exotic metals.
Helium, like pure Argon, is generally used with non-ferrous metals, but also with stainless steels. Because it produces a wide, deep penetration profile, Helium works well with thick materials, and is usually used in ratios between 25 — 75 percent Helium to 75 — 25 percent Argon. Adjusting these ratios will change the penetration, bead profile and travel speed. Helium creates a ‘hotter’ arc, which allows for faster travel speeds and higher productivity rates. However, it is more expensive and requires a higher flow rate than Argon, so you’ll need to calculate the value of the productivity increase against the increased cost of the gas. With stainless steels, Helium is typically used in a tri-mix formula of Argon and CO2.
This graphic shows the difference that consumables can make in shielding gas coverage. The photo on the left shows good coverage, while the coverage in the photo on the right allows the air environment to contaminate the shielding gas.
Is it ok to place the gas in the weld pool?
All of your efforts selecting the right shielding gas will be wasted, however, if your equipment isn’t getting the gas to the weld. The MIG gun consumables, consisting of a diffuser, contact tip and nozzle, play a crucial role in ensuring that the weld pool is properly protected from the air atmosphere.
If you choose a nozzle that is too narrow for the application or if the diffuser becomes clogged with spatter, for example, there might be too little shielding gas getting to the weld pool. Likewise, a poorly designed diffuser might not channel the shielding gas properly, resulting in turbulent, unbalanced gas flow. Both scenarios can allow pockets of air into the shielding gas and lead to excessive spatter porosity and weld contamination.
This cutaway shows a consumable system in which the contact tip is seated in the diffuser and held in place by the spatter guard inside the nozzle. When selecting MIG gun consumables, choose ones that resist spattering build-up and provide a wide enough nozzle bore to ensure adequate shielding gas coverage. Some companies offer nozzles with a built-in spatter guard that also adds a second phase of shielding gas diffusion, resulting in even smoother, more consistent shielding gas flow.
Choosing the right shielding gas for your specific application will require a careful analysis of the type of welding you are doing as well as your operational priorities. Using the guidelines above should provide a good start to the learning process, but be sure to consult your local welding supply distributor prior to making a final decision.
Carbon dioxide is often used as shielding gas for GMA welding of carbon steels. In the case of other metals, it may provoke weldment oxidation, impairing the metallurgical attributes. Still, in carbon steels, oxygen content assists in achieving some useful weld characteristics rather than vitiating your weld. Using carbon dioxide shielding in carbon steels, may not produce elegant welds. Still, the use of some other gases (like Argon) in conjunction with carbon dioxide, renders improvement of some other factors like arc stability, weld pool fluidity etc. to enhance the soundness and quality of welds.
While traditional stick welders knew very little about gases with their welding, the rise of the MIG and TIG welding machines over the last 70 – 80 years has brought in the need for gas as a common commodity in most workshops.
As we jump into the leading gases and mixtures used in the welding world, it’s fascinating to learn how much we have progressed over the short time since they were first implemented. The progression is enormous, and what’s in store for new gases, or new ways to use these gases, is exciting.
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What is the purpose of gas in welding?
Gas is used in a range of different ways. These include shielding the arc from impurities like air, dust, and other gases; keeping welds clean on the underside of the seam opposite the arc (or purging); and heating metal. Blanketing gases are also used to protect metal after the welding process.
What are the different types of gases used in welding?
Inert and reactive gases
Gases come in two categories: inert or reactive. Inert gases do not change or create change when in contact with other substances or temperatures. Reactive gases do the opposite. They react in different circumstances, creating a change of state in the other substances and/or themselves.
Inert gases are useful, as they allow welds to be achieved naturally without unwanted occurrences weakening or distorting the weld. Reactive gases provide a positive change during the process of the weld, which enhances the way the material is fused.
When the air gets into the arc while you’re welding, it causes air bubbles to form within the molten metal, creating a weak and very ugly weld. You cannot MIG or TIG weld without a shielding gas unless the filler material being used is flux-cored or flux coated. This serves the same purpose as a shielding gas, keeping impurities out, but in a different way.
Most shielding gases are inert, which makes them ideal for shielding a welding process as they remain stable under welding’s extreme conditions. They also nurture the weld in different ways, depending on the gas being used, including more penetration, more fluidity when molten, and a smoother surface on the bead.
Purging gases are used to cover the underside of the material you’re welding in the same way a shielding gas does, and only it’s done separately from the natural process of the weld.
While you weld the top of a joint, the bottom of the joint is sealed off and has a flow of gas purging it. It’s frequently used with stainless steel items, and it can be the same type of gas or a different gas than what’s used on the top of the joint.
Certain welding, like gas welding and brazing, requires gas to heat the metal or the filler rods to achieve the welding. This replaces the need for an arc.
Specific types of welding require the metal to be preheated before welding, which this gas is used for. The gas is simply a fuel mixed with air or oxygen, which is lit by a flame to warm or melt the metal.
Blanketing is a process where tanks and confined spaces are filled with gas after they’re completed to keep air and other contaminants from damaging or staining the finished product.
Sometimes it’s used to fill the completed projects entirely. Other times, the gas is added to the air-filled tank, creating a mixture to keep the tank pure against other gases or reactions.
What are the reasons for using carbon dioxide in welding?
Here are some main reasons for using carbon dioxide shielding in gas metal arc welding of carbon steels.
Carbon dioxide shielding provides better joint penetration as it promotes high arc voltage during welding. In this way, you can achieve good results for sidewall and root penetration.
The advantage of low cost enhances its worth among other shielding gases. Using carbon dioxide shielding instead of oxygen, will not allow oxidation in the weld metal, as oxygen does. Being heavier, it provides better shielding characteristics. Though it is cheaper than Argon and Helium but comparatively fewer quality welds are obtained.
Due to the high-temperature arc, carbon dioxide dissociates into carbon monoxide and oxygen that encourages oxidation. In this case, a little bit oxidizing may prove an accompaniment to GMA welding of carbon steels, by reducing the polar spots during the process as the polar spots formation may cause unstable arc and spatter during welding. During spray transfer mode electrodes are connected to positive terminals (anode) of the power source and workpiece with negative (cathode), this type of settings constitutes to reverse polarity.
Oxidation may reduce the deoxidizers for consumables, for example, silicon content can be lessened, and as a result, glassy slag (black in colour) is developed in the weldment. So controlled oxidation is a key to achieve good penetration, weld bead definition. On the other hand, carbon dioxide may help in providing fluxing and prevent porosity by eliminating any impurity present on a joint.
Combination with other gases
In the spray transfer mode, Carbon dioxide singly not gives better results and may cause severe spatter. By developing an association with other gases, mutual benefits can be achieved. For example, in combination with inert gases (like Argon), smooth spray transfer is achieved with low voltage settings, eliminating the problem of spattering and arc instability.
Prevention of Undercut
As it is stated that carbon dioxide is a denser gas and capable of sound shielding. Having the ability to prevent severe weld imperfections like undercut and as a result, good profile weld beads are made.
Well, safety is another concern with carbon dioxide shielding. Less hazardous in the workplace, however, the threatening behaviour of released carbon monoxide may prove dangerous. Proper ventilation at the workplace is the recommended step in order to make procedures safer.
Removal of Rust
This gas assists in the removal of rust present on the joint. It eliminates rusting by reacting with rust oxides, and in this way, other impurities are also removed. Apart from atmospheric protection, it also helps to prevent weld defects like porosity, lack of fusion, lack of penetration in the weld metal.
Improvement in Toughness
In a welding procedure, the suitable composition of gases and suitable consumables are the primary concerns, to produce required toughness in weld metals. Carbon dioxide, in combination with other gases, also helps to enhance the toughness of weldment.
Reduction of Surface tension
Surface tension is another issue in carbon steels that causes less penetration. The molten weld gains high surface tension which cannot be lessened by the use of inert gases like Helium, Argon etc. Only in that case carbon dioxide is the only shielding gas that reduces the intensity of surface tension and provides better penetration results. This makes carbon dioxide more exceptional in carbon steels.
Gas welding involves the use of a gas-fed flame torch to heat the metal workpiece and the filler material to create a weld. The gas is generally a mixture of fuel gas and oxygen to create a clean, hot flame. Many different gases can be used as fuel for gas welding, and electricity is not needed to power the welding system, resulting in a flexible and portable fabrication method. All gas welding techniques require proper safety equipment for the welder and storage of the welding gases.
Oxy-acetylene welding uses a mixture of acetylene gas and oxygen gas to feed the welding torch. Oxy-acetylene welding is the most commonly used gas welding technique. This gas mixture also provides the highest flame temperature of available fuel gases. However, acetylene is generally the most expensive of all fuel gases. Acetylene is an unstable gas and requires specific handling and storage procedures.
Pressurized gasoline is used as a welding fuel where fabrication costs are an issue, particularly in locations where acetylene canisters are not available. Gasoline torches can be more effective than acetylene for torch-cutting thick steel plates. The gasoline can be hand-pumped from a pressure cylinder, a common practice by jewellery makers in impoverished areas.
MAPP Gas Welding
Methylacetylene-propadiene-petroleum (MAPP) is a gas mixture that is much more inert than other gas mixtures, making it safer for hobbyists and recreational welders to use and store. MAPP can also be used at very high pressures, allowing it to be used in high-volume cutting operations.
Butane and propane are similar gases that can be used alone as fuel gases or mixed together. Butane and propane have a lower flame temperature than acetylene but are less expensive and easier to transport. Propane torches are more frequently used for soldering, bending and heating. Propane requires a different type of torch tip to be used than an injector tip because it is a heavier gas.
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Hydrogen can be used at higher pressures than other fuel gases, making it especially useful for underwater welding processes. Some hydrogen welding equipment works off electrolysis by splitting water into hydrogen and oxygen to be used in the welding process. This type of electrolysis is often used for small torches, such as those used in jewellery making processes.
How to weld with Mapp gas?
MAPP is a gas mixture created by the Dow Chemical Company that is a combination of liquefied petroleum gas (LPG) mixed with methylacetylene-propadiene. MAPP gas can be highly pressurized and stored in the same way as LPG, and it is a favourite of hobby welders. However, MAPP torches provide a very hot flame, almost as hot as oxy-acetylene, and the gas can be used for industrial metal-cutting operations. MAPP should not be used for welding steel because the hydrogen in the gas mixture can result in brittle welds.
Fit the parts to be welded together and check for alignment. Light the welding torch and adjust the flame. Some MAPP torches use a separate oxygen cylinder; others rely on air to provide oxygen to the flame. Touch the flame to the workpieces and move in a small circle to melt the material at the weld zone.
Move the torch to move the pool of molten metal forward and add filler material to the weld with the filler rod as needed. The base metal should be hot enough for the filler rod to melt like solder when touched to the workpiece.
Continue to move the weld forward until it is complete. As the workpiece heats up, adjust the welding speed to avoid burning through the metal. Allow the weld to cool when complete.
Is it safe using gas?
All the gases used in welding have unique dangers according to their characteristics. While most are not flammable, any flammable gas used in a welding shop must be treated with extreme caution, especially acetylene.
Keep flammable gases well away from your welding area unless you’re in the process of using them. When using them, have a class B fire extinguisher nearby. If your extinguisher has no class labelled on it, a class B extinguisher will be filled with either C02 or some sort of dry chemical.
While inert gases pose little threat due to lack of flammability and they are not reacting with anything, they can cause asphyxiation if you’re welding in an enclosed space for too long. If you have to weld in a confined environment, make sure to have the right precautions in place. Gas detectors, extractor fans, a welding spotter, and regular breaks are great ways to minimize the danger.