• ACETYLENE CUTTING

    See OXYFUEL GAS CUTTING

  • ACETYLENE CYLINDERS

    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.

     

     

  • ACETYLENE CYLINDERS, Safe Handling

    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.

  • ACETYLENE FEATHER

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

  • ACETYLENE GENERATOR

    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.

  • ACETYLENE WELDING

    See OXYACETYLENE WELDING and OXYFUEL GAS.

  • ACID BRITTLENESS

    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.

  • ACID CORE SOLDER

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

  • ACTIVATED ROSIN FLUX

    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.

  • ACTUAL THROAT

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

  • ADAPTER

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

  • ADAPTIVE CONTROL WELDING

    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

    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.

    Limitations

    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.

  • ADMIRALTY BRASS

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

  • AGE HARDENING

    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.

  • AGING

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

  • AIR ACETYLENE WELDING (AAW)

    An oxygen gas welding process that uses air and acetylene. The process is used without the application of pressure. This is an obsolete or seldom used process. See STANDARD WELDING TERMS.

  • AIR CAP

    A nonstandard term for the nozzle of a flame spraying gun for wire or ceramic rod.

  • AIR CARBON ARC CUTTING (CAC-A)

    A carbon arc cutting process variation that removes molten metal with a jet of ail: See STANDARD WELDING TERMS.

    The air carbon arc cutting process uses an arc to melt metal which is blown away by a high-velocity jet of compressed air. The electrodes are rods made from a mixture of graphite and carbon, and most are coated with a layer of copper to increase their current-carrying capacity. Standard welding power sources are used to provide the current. Air is supplied by conventional shop compressors, and most applications require about 550 kPa (80 psi) at between 560 to 840 liters/min (20 to 30 cubic feet per minute), Manual rod holders are very similar in appearance to shielded metal arc welding electrode holders, and supply both compressed air and current.

    In gouging operations, the depth and contour of the groove are controlled by the electrode angle, travel speed, and current. Grooves up to 16 mm (5/8 in.) deep can be made in a single pass. In severing operations, the electrode is held at a steeper angle, and is directed at a point that will permit the tip of the electrode to pierce the metal being severed.

    In manual work, the geometry of grooves is dependent on the cutting operator’s skill. To provide uniform groove geometry, semiautomatic or fully automatic torches are used to cut “U” grooves in joints for welding, When removing weld defects or severing excess metal from castings, manual techniques are most suitable.

    Voltage controlled automatic torches and control units are used for very precise gouging, with tolerances of less than 0.8 mm (1/32 in.), and are generally mounted on standard travel carriages.

    Reference: American Welding Society. Welding Handbook, Vol. 2, 8th Edition. Miami, Florida: American Welding Society, 1991.

  • AIR CARBON ARC CUTTING TORCH

    A device used to transfer current to a fixed cutting electrode, position the electrode, and direct the flow of air: See STANDARD WELDING TERMS.

  • AIR FEED

    A thermal spraying process variation in which an air stream carries the powdered sugacing material through the gun and into the heat source. See STANDARD WELDING TERMS.

  • AIR-ACETYLENE TORCH

    A torch which produces a flame by burning a mixture of acetylene and air. The flame is as easily controlled and manipulated as the oxyacetylene flame, but has a lower temperature.

    The air-acetylene torch operates on the same principle as the Bunsen burner, that is, the acetylene flowing under pressure through a Bunsen jet draws in the appropriate amount of air from the atmosphere to provide combustion. The flame is adjusted by controlling the amount of air admitted to the Bunsen jet. The mixer on the torch must be carefully adjusted to draw the correct volume of air to produce an efficient, clean flame. The air-acetylene flame ignites at 480°C (896°F) and produces a maximum temperature of 1875°C (3407°F).

    The air-acetylene torch is used for brazing, soldering, and heating applications, but the flame temperature is not sufficient for welding, except for joining materials with a low melting point, like lead. It is widely used for soldering copper plumbing fittings up to 25 mm (10 in.) in diameter.

  • ALIGNED DISCONTINUITIES

    Three or more discontinuities aligned approximately parallel to the weld axis, spaced sufficiently close together to be considered a single intermittent discontinuity.