• Abnormal Grain Growth

    The formation of unusually large polycrystalline grains in a metal. This condition frequently occurs when a critical amount of strain (in the range of 2%) is present during heating to elevated temperatures.

  • Abrasion

    A grinding action caused by abrasive solids sliding, rolling or rubbing against a surface; a scraped, ground, or worn area.


    An instrument for measuring absorption of gases by liquids.

  • AC

    Abbreviation for: Alternating Current.

  • AC Arc Welding

    An arc welding process, using a power source that supplies an alternating current to the welding arc

  • Acetone

    (C3H60) A compound of carbon, hydrogen and oxygen; it is a volatile, flammable, liquid ketone (an organic compound containing a carbon atom connected to an oxygen atom by a double bond and to two carbon atoms) used mainly as a solvent for such materials as resins, gums, oils, and cellulose.

    Acetone is odorless and colorless; it evaporates rapidly. Acetone boils at 56°C (133°F). One liter of acetone weighs about 1 kg.

    An important use for acetone is to stabilize acetylene gas. The safe, practical use of acetylene gas for welding and other applications would not be possible without acetone.  Compressed acetylene itself is highly explosive; however, it can be safely compressed and stored in high-pressure cylinders if the cylinders are lined with absorbent material soaked with acetone. As a solvent agent for acetylene gas, acetone has an absorptive capacity of 25 volumes of acetylene per volume of acetone per atmosphere of pressure, or about 420 volumes of acetylene at 1724 kPa (250 psi) pressure.

    Another important feature of the acetone-acetylene solution is that the exothermic properties of the acetone counteract the endothermic properties of the acetylene; consequently, the acetone-acetylene solution is, to a certain extent, immune from a complete dissociation in case an ignition or explosion is introduced into it.


    Acetylene, a hydrocarbon (C2H2), is a colorless, flammable gas shipped dissolved in a solvent. It has a garlic-like odor. Users are cautioned not to discharge acetylene at pressures exceeding 103 kPa (15 psig), as noted by the red line on acetylene pressure gauges. Other specifications of acetylene are:

    Molecular weight: 26.038

    Specific Gravity (Air = 1): 0.91 at 0°C (32°F)

    Specific Volume: 0.09 m3 kg at 156°C (14.5 ft3/lb at

    Critical Temperature: 35.2"C (95.3"F)

    Critical Pressure: 6139.3 kPa (890.4 psia)

    Acetylene is said to have an endothermic quality because it absorbs heat in formation and liberates it during combustion. In this respect, acetylene differs from most hydrocarbons: they are exothermic and give off heat during formation. As a fuel gas, acetylene generates 1433 Btu per cuft; 277 are derived from hydrogen combustion, 928 Btu result from the combustion of carbon into carbon dioxide, and 228 Btu result from its endothermic quality.


    Chemical Characteristics

    The chemical structure of acetylene is given in the formula C2H2, showing that two atoms of carbon (atomic weight 12) are combined with two atoms of hydrogen (atomic weight 1.008), which can be expressed as 92.3% carbon and 7.7% hydrogen. The nearest gaseous hydrocarbon is ethylene (C2H4), which consists of 85% carbon and 15% hydrogen.

    Acetylene contains the highest percentage of carbon of all the gaseous hydrocarbons and is the only one of the unsaturated hydrocarbons with endothermic properties (absorbs heat during its production, and liberates heat when it is decomposed). Because of these characteristics, the oxyacetylene flame creates intense heat. The theoretical maximum for the oxyacetylene flame is 4359°C (7878"F), although the working temperature is about 3316°C (6000°F). The temperature of the oxyacetylene flame cannot be approached by any other gas, and is only exceeded by the heat produced in the electric arc or electron beam and laser processes.


    Metalworking with Acetylene

    Acetylene is usually combined with oxygen to intensify the heat of the acetylene flame for welding. It can also be combined with air, but with a much lower flame temperature. The principal application for the air-acetylene mixture is in soldering operations.

    Mixed in equal amounts and burned at the tip of a welding torch, oxygen and acetylene create the so called neutral flame. This flame can be identified by the luminous, well-defined white cone at the torch tip, and by a fairly long, almost colorless outer envelope that is blue or orange at its leading edge. See Figure A-1. The neutral flame is the correct flame with which to weld many metals. See OXYACETYLENE .

    If excess oxygen is fed into the torch, an oxidizing flame results. This flame is characterized by a short inner cone and a short outer envelope. The flame is hotter than a neutral flame, burning acetylene at the same rate. When this situation is reversed and an excess of acetylene is used, the resulting flame is termed carburizing. This flame appears as a greenish feather-shaped form between the inner cone and outer envelope. There are white-hot carbon particles in this feather which are dissolved to some extent in molten metal during welding.



    Because of its intense heat, and because it can be accurately controlled, the oxyacetylene flame can be applied to literally hundreds of welding and cutting operations, including hardfacing, brazing, beveling, gouging, and scarfing. The heating capability of acetylene is utilized extensively in bending, straightening, forming, hardening, softening, and strengthening many types of metals.


    Historical Background

    Acetylene gas was discovered by Edmund Davy in 1836, but it was not until 1862 when Woehler's discovery that acetylene gas could be produced from calcium carbide that the gas became well known. These developments were of little consequence, however, until 1892, when Thomas L. Wilson, of Spray, N. C., invented a process for producing calcium carbide and established facilities to produce it. He and James Morehead devised an economical commercial production method, and by 1895 acetylene gas was becoming recognized as a valuable gas for lighting.

    However, acetylene producers’ hopes for widespread use of acetylene for illumination of streets and buildings were dashed by the growing use of incandescent lamps. Acetylene’s potential in metalworking became apparent in World War I, when welding was adopted as the most effective and expedient method of constructing and repairing war ships and merchant vessels.

    (A) Carburizing

    (B) Neutral

    (C) Oxidizing

    Figure A-1- Types of Oxacetylene Flames


    Producing Acetylene 

    Acetylene is produced either in generators, by the reaction of calcium carbide and water, or by the cracking of hydrocarbons in a chemical plant.

    In the generator method, water is allowed to react with calcium carbide (CaC,), a chemical compound produced by fusing lime and coke in an electric furnace. The reaction between water and carbide is instantaneous, and as a result, the carbon in the carbide combines with the hydrogen in the water, forming slaked lime, or calcium hydrate.

    There are two methods of generating acetylene: (1) carbide-to-water, and (2) water-to-carbide. The carbide-to-water method is generally used in the United States, while the water-to-carbide method is favored to a large extent in Europe. A carbide-to-water generator operates on a “batch” basis, with a ratio of one gallon (8.3 lb) of water to one pound of carbide. This mixture is designed, in some generator models, to produce one cubic foot of acetylene per hour per pound of carbide hopper capacity. Some stationary generators are “double-rates’ for capacities of 2 ft3/hr per pound of carbide hopper capacity.

    There are two further classifications for acetylene generators: low pressure and medium pressure. The low-pressure generator carries out the calcium carbide-to-water reaction process at pressures below 7 kPa (1 psi). A medium-pressure generator produces acetylene at between 7 and 103 kPa (1 and 15 psi).

    Calcium carbide used for acetylene generation in the U. S. normally produces gas containing less than 0.4% impurities other than water vapor. Because of this favorable factor, there is no need for further purification of acetylene used for welding and cutting.

    The welding supply distributor receives and resells acetylene in its most common form: dissolved in acetone and compressed in cylinders. These rugged acetylene cylinders have nominal capacities of 0.28, 1.1, 2.8, 6.4, or 8.5 m3 (10, 40, 100, 225 or 300 cu ft) and hold the gas at a pressure of 1724 kPa (250 psi). 




    Because of the characteristics of acetylene gas, acetylene cylinders are constructed in an entirely different manner from those made to contain other gases.

    Historical Background

    Until 1904, no suitable acetylene container had been developed. The gas was used mainly for illumination and was generally piped directly from generators to the area to be served. In that year in Indiana, P.C. Avery displayed to two of his home state’s most famous promoters, James Allison and Carl Fisher, a portable cylinder containing acetylene gas designed to power auto headlights. Then engaged in auto sales, Allison and Fisher were immediately interested, and with Avery, set up a small factory in Indianapolis to fabricate this “tank.”

    The shop was known as Concentrated Acetylene Company, until Avery withdrew in 1906. The company then became the Rest-0-Lite Company, the forerunner of the Linde Division of Union Carbide Corporation.

    Allison and Fisher devoted much of their time relocating their plant into progressively larger quarters.  Not until 1910 did they build one of sufficient size in what was then suburban Indianapolis, across the street from the site of the famed motor speedway they later constructed.

    Carbide production continued to increase, and in 1913, a much improved acetylene cylinder similar to that used today was introduced. With these two major achievements, gas welding began replacing other metal joining methods.


    Cylinder Stabilizing Fillers

    The need for a porous substance in a cylinder to stabilize compressed acetylene was realized by the French scientist Fouche, one of the men responsible for the oxyacetylene mixture. The size of the filler, however, left very little room for gas in the cylinder. One filler was a magnesium oxychloride cement type; another was made of asbestos discs. The charcoal-cement filler was not developed until 1919, and in 1950 a sand-lime material became popular.

    In 1897 a French team, Claude and Hess, demonstrated the value of acetone. This colorless, flammable liquid, when added to the porous material, is capable of absorbing 25 times its own volume of acetylene for each atmosphere 101 kPa (14.7 psi) of pressure applied. Thus, at full cylinder pressure of 1724 kPa (250 psi at 70”F), it can absorb over 400 times its own volume of acetylene.

    In 1958, cylinder manufacturers announced a lightweight calcium-silicate filler with 92% porosity. This new filler lessened cylinder weight by 30%, increased cylinder capacity, and improved charging and discharging characteristics. Although only 8% solid, this filler had extraordinary strength, longer life, no deterioration, and could be charged and discharged much faster.

    The calcium silicate filler, composed of sand, lime and asbestos, lined the cylinder and conformed to its shape. Its crushing strength, an indication of cylinder life, is 6205 kPa (900 psi).

    When medical research indicated that asbestos fibers are carcinogenic due to the size of the fibers (less than 3.5 microns in diameter and 10 microns in length, which is small enough to allow the fibers to penetrate the respiratory tract of the lungs), cylinder manufacturers set about to produce an asbestos-free filler. A non-asbestos alkaline-resistant glass fiber filler was developed by the Linde Division of Union Carbide Corporation and patented in 1982.

    A cut-away view of a modern acetylene cylinder is shown in Figure A-2.

    How Acetylene Cylinders are Manufactured

    Cylinder production and testing is a step-by-step procedure which insures ultimate quality and safety. Seamless shells are cold drawn in hydraulic presses with capacities up to 454 000 kg (500 tons). Center seams and foot ring attachments are welded using the submerged arc process. Cylinders are then normalized (stress relieved) to increase cylinder life and corrosion resistance.


    Measure and Weight

    In the filling area, cylinders are measured and weighed to determine exact volume. At another location, filler is mixed to correct proportions in hoppers, weighed, and mixed with water in agitators. Before each new batch of filler is used, a sample containing one cubic foot is weighed and examined to ensure correct mixture.

    Cylinders are then filled automatically and weighed again. Factoring in the weight and volume of the cylinder confirms that it is accurately filled to specification. The cylinders are then oven-baked at 315°C (600°F) to eliminate the water. Baking time ranges from 40 to 120 hours, depending on cylinder size. After baking, another weight check is made to determine if any water remains. Since 1% moisture in the filler will affect ultimate performance, cylinders are baked again if only a slight moisture content is detected.

    Fuse plugs and valves are installed, and cylinders are shot-blasted and painted. (Fuse plugs are small steel machine bolts with holes filled with a low melting alloy designed to release gas in case of fire, and to lessen the acetylene pressure to reduce the possibility of an explosion).

    Finally, strength proof tests at 4140 kPa (600 psi) are run. Pressure is then reduced to 2070 kPa (300 psi), and the cylinders are immersed in water to check for leaks. Drawn to a vacuum, they are charged with acetone and weighed again to determine if they are fully charged.

    Cylinders are checked after each procedure during the manufacturing process. Those not meeting the rigid requirements of federal law and company rules are rejected regardless of the stage of manufacture. For example, a number of cylinders are selected from each completed lot, charged with acetylene, and tested to ensure proper discharge. If the cylinders do not meet specifications, the entire lot is rejected.


    Basic Tests

    A bonfire test is designed to check cylinder performance under conditions similar to a fire in a building. A fully charged cylinder is placed horizontally on racks, and specified sizes and amounts of wood strips are ignited around it. The cylinder passes the test if there is no appreciable shell bulge, no penetration of filler by decomposition, and no breakup of the filler.

    The flashback test simulates torch flashback entering the cylinder, assumed to be at full pressure when the operator closes the valve immediately afterward. If the flash is immediately quenched in the cylinder with only a minimum of decomposition and without release of fusible plugs, the cylinder passes the test.

    A hot spot test simulates negligent impinging of a torch flame against the cylinder. Flame is directed at the cylinder sidewall until a 3 to 20 mm (1/8 to 3/4 in.) bulge develops. If filler decomposition is limited to the area closely adjacent to the resulting cavity, performance is satisfactory.

    The bump test determines the filler’s resistance to mechanical shock received during normal service. The cylinder is mounted on a foundry mold-bumper and subjected to minimum 200 000 bumping cycles. At the conclusion of the test, satisfactory performance is indicated when there is no attrition, sagging, or cracking of the filler.




    At ambient conditions, increased pressure and decreased temperature can liquefy acetylene. At extremely low temperatures, acetylene can solidify. The danger at the point of liquefaction or solidification (and the major reason why acetylene cannot be distributed in this form) is that the necessarily high pressures create a very unstable product. At the slightest provocation, compressed acetylene will dissociate into its chemical components, carbon and hydrogen. This dissociation is accompanied by drastic increases in both temperature and pressure, and results in an explosion.  The acetylene distributor, as well as the user, must observe important precautions:

    (1) Slings, hooks or magnets cannot be used to move cylinders. Cylinders of acetylene must be kept in an upright position. Cylinders cannot be dragged, and can never be used or stored in a horizontal position.

    (2) A hand truck should be used when an acetylene cylinder must be moved, or the cylinder should be tilted slightly and rolled it on its bottom edge.

    (3) A cylinder storage area should be chosen that is well removed from any heat sources, and the area should be posted with conspicuous signs forbidding smoking or the use of open flames or lights.

    (4) If cylinders are stored outdoors, dirt, snow or ice should not be allowed to accumulate on valves or safety devices.

    (5) The cylinders should be secured with chains or heavy rope so that they cannot be accidentally tipped over.

    (6) A leaking cylinder must be handled with extreme care; it should be removed immediately from the storage area after checking to be sure that no sources of ignition are brought near it. The supplier should be notified immediately.

    (7) One cylinder should not be recharged from another, or other gases mixed in an acetylene cylinder.

    (8) Copper tubing should never be used to convey acetylene. Acetylene will react with the copper to form copper acetylide, an unstable compound which can explode spontaneously.


    The intense white, feathery-edged portion adjacent to the cone of a carburizing oxyacetylene flame. See STANDARD WELDING TERMS.


    In the United States, common practice has established a preference for the carbide-to-water machines, and they are almost universally used. There is another type of generator using calcium carbide molded into cakes, in which the water drops into the calcium carbide. This type of generator, while common in Europe, is almost unknown in the United States.

    Insurance Regulations

    The Underwriters’ Laboratories is an organization maintained by the insurance companies of the United States which provides for the inspection and testing of all types of equipment which may be considered a fire or accident hazard, including welding and cutting equipment and acetylene generators. There are established sets of rules governing the design, construction, and installation of acetylene generators, including acetylene pipe lines.

    Another insurance authority which publishes rules for acetylene generators is the Factory Mutual Engineering Organization, Norwood, Mass. Regulations of the American Insurance Service Group, New York, N.Y. and the National Fire Protection Association, Quincy, Mass. are also followed. See GAS SYSTEMS.




    Brittleness induced in steel, especially wire or sheet, by pickling in dilute acid for the purpose of removing scale. This brittleness is commonly attributed to the absorption of hydrogen.


    A solder wire or bar containing acid flux as a core.


    A rosin base flux containing an additive that increases wetting by the solder.

  • ACTIVE FLUX, Submerged Arc Welding

    A flux from which the amount of elements deposited in the weld metal is dependent on the welding conditions, primarily on the arc voltage.


    The shortest distance between the weld root and the face of a fillet weld.


    A device for connecting two parts (i.e., of different diameters) of an apparatus, or for adapting apparatus for uses not originally intended.


    Welding with a process control system that automatically determines changes in welding conditions and directs the equipment to take appropriate action. Variations of this term are adaptive control brazing, adaptive control soldering, adaptive control thermal cutting, and adaptive control thermal spraying. See STANDARD WELDING TERMS.

    Adaptive (feedback) control systems are automatic welding systems which make corrections to welding variables based on information gathered during welding. The objective is to maintain weld quality at a constant level in the presence of changing welding conditions. Automatic adjustment of individual weld variables, such as arc current or arc length, is made by monitoring a weld characteristic, such as pool width. Other feedback control systems are available to provide electrode guidance and constant joint fill. See also AUTOMATIC WELDING, MANUAL WELDING, MECHANIZED WELDING, ROBOTIC WELDING, and  SEMIAUTOMATIC WELDING.


    Adhesive bonding is a materials joining process in which a nonmetallic adhesive material is placed between the faying surfaces of the parts or bodies, called adherends. The adhesive then solidifies or hardens by physical or chemical property changes to produce a bonded joint with useful strength between the adherends.

    Adhesive is a general term that includes such materials as cement, glue, mucilage, and paste. Although natural organic and inorganic adhesivesare available, synthetic organic polymers are usually used to join metal assemblies. Various descriptive adjectives are applied to the term adhesive to indicate certain characteristics, as follows:

    (1) Physical form: liquid adhesive, tape adhesive

    (2) Chemical type: silicate adhesive, epoxy adhesive, phenolic adhesive

    (3) Materials bonded: paper adhesive, metal-plastic adhesive, can labeling adhesive

    (4) Application method: hot-setting adhesive, sprayable adhesive.

    Although adhesive bonding is used to join many nonmetallic materials, the following paragraphs refer only to the bonding of metals to themselves or to nonmetallic structural materials.

    Adhesive bonding is similar to soldering and brazing of metals in some respects, but a metallurgical bond does not take place. The surfaces being joined are not melted, although they may be heated. An adhesive in the form of a liquid, paste, or tacky solid is placed between the faying surfaces of the joint. After the faying surfaces are mated with the adhesive in between, heat or pressure, or both, are applied to accomplish the bond.

    An adhesive system must have the following characteristics:

    (1) At the time the bond is formed, the adhesive must become fluid so that it wets and comes into close contact with the surface of the metal adherends.

    (2) In general, the adhesive cures, cools, dries, or otherwise hardens during the time the bond is formed or soon thereafter.

    (3) The adhesive must have good mutual attraction with the metal surfaces, and have adequate strength and toughness to resist failure along the adhesive-to-metal interface under service conditions.

    (4) As the adhesive cures, cools, or dries, it must not shrink excessively. Otherwise, undesirable internal stresses may develop in the joint.

    (5) To develop a strong bond, the metal surfaces must be clean and free of dust, loose oxides, oil,

    grease, or other foreign materials.

    (6) Air, moisture, solvents, and other gases which may tend to be trapped at the interface between the adhesive and metal must have a way of escaping from the joint.

    (7) The joint design and cured adhesive must be suitable to withstand the intended service.

    A variety of adhesives can be used. Thermoplastic adhesives develop a bond through the evaporation of a solvent or the application of heat. The pressure-sensitive adhesives produce a bond when pressure is applied to the joint. Other adhesives, usually used for metals, react chemically with curing agents or catalysts. Some epoxy-based adhesives can produce joint strengths up to 70 MPa (10 000 psi) when cured at 175°C (350°F) for a few hours under pressures of about 1030 kPa (150 psi). The types of polymeric adhesives used to bond metal are listed in Table A- 1.

    Advantages and Applications

    Adhesive bonding has several advantages for joining metals when compared to resistance spot welding, brazing, soldering, or mechanical fasteners such as rivets or screws. Adhesive bonding is also capable of joining dissimilar materials, for example, metals to plastics; bonding very thin sections without distortion and very thin sections to thick sections; joining heat sensitive alloys; and producing bonds with unbroken surface contours.

    The adhesive that bonds the component may serve as a sealant or protective coating. Adhesives can provide thermal or electrical insulating layers between the two surfaces being joined, and different formulations of the adhesive can make the bonding agent electrically conductive. These properties are highly adaptable to mass-produced printed circuit boards, and to the electrical and electronic components industry.

    Smooth, unbroken surfaces without protrusions, gaps, or holes can be achieved with adhesive bonding. Typical examples of applications are the vinyl-to-metal laminate used in the production of television cabinets and housings for electronic equipment. Other examples are automotive trim, hood and door panels, and roof stiffeners.

    The ability of flexible adhesives to absorb shock and vibration gives the joint good fatigue life and sound-dampening properties. A specific example is the improved fatigue life of adhesive-bonded helicopter rotor blades.

    A combination of adhesives and rivets for joints in very large aircraft structures has increased the fatigue life of joints from 2 x 10^5 cycles for rivets alone to 1.5 x l0^6 cycles for bonded and riveted joints. The large bonded area also dampens vibration and sound.

    Adhesive bonding may be combined with resistance welding or mechanical fasteners to improve the load carrying capacity of the joint. The adhesive is applied to the adherents first. Then the components are joined together with spot welds or mechanical fasteners to hold the joints rigid while the adhesive cures. Figure A-3 illustrates typical design combinations. These techniques significantly reduce or eliminate fixturing requirements and decrease assembly time when compared to conventional adhesive bonding methods.

    Adhesive bonding may permit significant weight savings in the finished product by utilizing lightweight fabrications. Honeycomb panel assemblies, used extensively in the aircraft industry and the construction field are excellent examples of lightweight fabrications. Although weight reduction can be important in the function of the product, adhesive bonding of products may also provide considerable labor and cost savings in packing, shipping, and installation.


    Adhesive bonding has certain limitations which should be considered in its application. Joints made by adhesive bonding may not support shear or impact 1oads.These joints must have an adhesive layer less than 0.13 mm (0.005 in.) thick, and must be designed to develop a uniform load distribution in pure shear or tension. The joints cannot sustain operational temperatures exceeding 260°C (500°F).

    Capital investment for autoclaves, presses, and other tooling is essential to achieve adequate bond strengths. Process control costs may be higher than those for other joining processes. In critical structural bonding applications, surface preparation can range from a simple solvent wipe to multi-step cleaning, etching, anodizing, rinsing and drying procedures; and joints must be fixtured and cured at temperature for some time to achieve full bond strength. Some adhesives must be used quickly after mixing. Nondestructive testing methods normally used for other joining methods are not generally applicable to evaluation of adhesive bonds. Both destructive and nondestructive testing must be used with process controls to establish the quality and reliability of bonded joints.

    Service conditions may be restrictive. Many adhesive systems degrade rapidly when the joint is both highly stressed and exposed to a hot, humid environment.


    Safe Practices

    Corrosive materials, flammable liquids, and toxic substances are commonly used in adhesive bonding. Manufacturing operations should be carefully supervised to ensure that proper safety procedures, protective devices, and protective clothing are being used. All federal, state and local regulations should be complied with, including OSHA Regulation 29CRF 1900.1000, Air Contaminants. The material safety data sheet of the adhesive should be carefully examined before the adhesive is handled to ensure that the appropriate safety precautions are being followed.

    References: American Welding Society. Welding Handbook, 8th Edition, Vol. 1. Miami, Florida: American Welding Society, 1987; and American Welding Society. Welding Handbook, 8th Edition, Vol. 2. Miami, Florida: American Welding Society, 199 1.


    An alloy which is 70% copper, 29% zinc and 1% tin, commonly used for condenser and heat exchanger tubing. See COPPER ALLOY WELDING.


    A term applied to a property exhibited by some of the light alloys, such as aluminum or magnesium, of hardening at ordinary temperatures after solution treatment or cold work. The controlling factors in age hardening are the composition of the material, degree of dispersion of the soluble phase, solution time and temperature, and aging time and temperature.

  • AGGLOMERATED FLUX, Submerged Arc Welding

    A type of flux produced with a ceramic binding agent requiring a higher drying temperature that limits the addition of deoxidizers and alloying elements. This is followed by processing to produce the desired particle size.


    A term applied to metals and particular alloys which show changes in physical properties on exposure to ordinary or elevated temperatures.