Acetylene is a colorless, combustible gas with a distinctive garlic type odor. When acetylene is liquefied, compressed, heated, or mixed with air, it becomes highly explosive. Acetylene is used as the fuel component in oxy-acetylene welding and metal cutting.
Acetylene is a hydrocarbon consisting of two carbon atoms and two hydrogen atoms. Its chemical symbol is C2 H2 . For commercial purposes, acetylene can be made from several different raw materials depending on the process used.
These processes use high temperature to convert the raw materials into a wide variety of gases, including hydrogen, carbon monoxide, carbon dioxide, acetylene, and others. The other gases are the products of combustion with oxygen. In order to separate the acetylene, it is dissolved in a solvent such as water, anhydrous ammonia, chilled methanol, or acetone, or several other solvents depending on the process.
Because acetylene is highly explosive, it must be stored and handled with great care.
When acetylene is pressurized and stored for use in oxy-acetylene welding and metal cutting operations, cylinders are used. The cylinders are filled with an absorbent material, like diatomaceous earth, and a small amount of acetone. The acetylene is pumped into the cylinders at a pressure of about 300 psi, where it is dissolved in the acetone. Once dissolved, it loses its explosive capability, making it safe to transport. When the cylinder valve is opened, the pressure drop causes some of the acetylene to vaporize into gas again and flow through the connecting hose to the welding or cutting torch.
Argon is a noble gas which under normal circumstances doesn’t react with any other element. This is why it’s used as a shield gas during welding. Its use protects the metal that’s being worked on from the oxygen in the air.
Argon is a food preservative and is also used to fill the “dry” type of scuba diving suits.
Though other inert gases can fulfill the functions of argon, argon is especially attractive because it’s inexpensive and plentiful. It makes up nearly one percent of the atmosphere and can be obtained through the production of liquid oxygen and liquid nitrogen.
Because 100% argon can be used to TIG weld all metals and thicknesses you only need one type of gas in your shop to handle all of your welding projects. MIG welding aluminum is different than welding steel. For aluminum, 100% argon is the gas of choice.
Hydrogen is the lightest gas known. It is a colorless, odorless, tasteless and nontoxic gas at atmospheric temperatures and pressures, but can be an asphyxiant. Hydrogen burns in air with a pale blue, almost invisible flame. Its ignition temperature will not vary greatly from 1050° F in mixtures with air or oxygen at atmospheric pressure. The flammable limits are four to 94 percent hydrogen by volume.
Used as a fuel, the oxy-hydrogen flame has a 4000° F (2200° C) temperature, good for low-temperature brazing and welding of aluminum, magnesium and lead, and for underwater cutting at pressures that would liquefy other fuels.
The most common metalworking use for hydrogen is as an “oxygen getter”, as in some shielding-gas mixtures. In these cases, it combines readily with the oxygen in metal oxides to form water, leaving reduced metal behind.
Many metals are sensitive to hydrogen embrittlement, and caution must be exercised in its use and in the piping equipment handling it.
Helium is one of the basic chemical elements. In its natural state, helium is a colorless gas known for its low density and low chemical reactivity. It is probably best known as a non-flammable substitute for hydrogen to provide the lift in blimps and balloons. Because it is chemically inert, it is also used as a gas shield in robotic arc welding and as a non-reactive atmosphere for growing silicon and germanium crystals used to make electronic semiconductor devices. Liquid helium is often used to provide the extremely low temperatures required in certain medical and scientific applications, including superconduction research.
Gaseous helium is distributed in forged steel or aluminum alloy cylinders at pressures in the range of 900-6,000 psi.
The Compressed Gas Association establishes grading standards for helium based on the amount and type of impurities present. Commercial helium grades start with grade M, which is 99.995% pure and contains limited quantities of water, methane, oxygen, nitrogen, argon, neon, and hydrogen. Other higher grades include grade N, grade P, and grade G. Grade G is 99.9999% pure.
Nitrogen constitutes 78 percent of the atmosphere and is a constituent of all living tissues. It has two characteristics that make it the world’s most widely used gas. Liquid nitrogen is intensely cold. This makes it a highly effective (and totally non-polluting) agent for freezing and chilling. Nitrogen is also inert; under normal conditions it is not chemically active. An atmosphere of nitrogen is like a blanket that prevents oxidation and combustion from taking place. Gaseous nitrogen at atmospheric pressure has no taste, color, or odor. It’s a poor conductor of heat and electricity, and has low solubility in most common liquids. Nitrogen has no toxic properties at atmospheric pressure, other than the possible hazard of suffocation due to displacement of air.
At -320° F (-196° C) and standard atmospheric pressure gaseous nitrogen condenses into a water-white liquid, with 696.5 volumes of gaseous nitrogen becoming one volume of the liquid. The liquid is non-magnetic, stable against mechanical shock and does not produce toxic or irritating vapors. The only caution required in handling liquid nitrogen is due to its low temperature.
Liquid nitrogen is classified as Type II by the Compressed Gas Association (CGA). CGA-10.1, Type II Grade L or The National Formulary Specs (similar to USP) are considered “commercial” liquid nitrogen standards.
Oxygen is used in many industrial, commercial, medical, and scientific applications. It is used in blast furnaces to make steel, and is a component in the production of many synthetic chemicals, including ammonia, alcohols and various plastics. Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting. When oxygen is cooled below -297° F, it becomes a pale blue liquid that is used as a rocket fuel.
The Compressed Gas Association establishes grading standards for both gaseous oxygen and liquid oxygen based on the amount and type of impurities present. Gas grades are called Type I range from A, which is 99.0% pure, to F, which is 99.995% pure. Liquid grades are called Type II and also range from A to F, although the types and amounts of allowable impurities in liquid grades are different than in gas grades. Type I Grade B and Grade C and Type II Grade C are 99.5% pure and are the most commonly produced grades of oxygen. They are used in steel making and in the manufacture of synthetic chemicals.
Periodic sampling and analysis of the final product ensures that the standards of purity are being met.
Carbon dioxide is used by the food, oil industry, chemical and industrial industry for welding applications. Co2 is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor.
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 an 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 spatter than when it is mixed with other gases. It is also limited to only the short circuit process.
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