• OCCLUSION

    The chemical property of some metals to absorb gases and retain them, usually resulting in porosity in welds. Aluminum, iron and many other metals absorb hydrogen, oxygen and other gases in varying volumes, particularly when the metals are in molten or powder form. The real nature of metallic occlusions is unknown. 

  • OFF TIME, Resistance Welding

    The time during which the electrodes are off the workpieces. The term is generally used when the welding cycle is repetitive. 

  • OHM

    A unit of electrical resistance. It is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere. 

  • OHM’S LAW

    The rule that gives the relation between current, voltage and the resistance of an electrical circuit. The voltage (E) is equal to the current (I) in amperes times resistance (R) in ohms, i.e. E = I X R; or R = E/I. 

  • ONE SIDED ARC WELDING

    A term generally applied to welding applications in which all of the filler metal is deposited from one side. The resulting welds are usually free of imperfections on the backside so that it should not be necessary to do any welding on the backside. Submerged arc welding (SAW) has been considered a “one-sided” method. For the most part, welding can be completed from one side, but sometimes there may be imperfections that necessitate back-chipping or gouging and welding on the underside. 

  • OPEN BUTT JOINT

    A nonstandard term for a butt joint with an open root. 

  • OPEN CIRCUIT VOLTAGE

    The voltage between the output terminals of the power source when no current is flowing to the torch or gun. 

  • OPEN GROOVE

    A nonstandard term for OPEN ROOT JOINT. 

  • OPEN HEARTH STEEL

    Steel which has been manufactured by the open hearth process. In this process steel is smelted in a gas fired, regenerative furnace consisting of a shallow trough or hearth. 

  • OPEN JOINT

    A nonstandard term for OPEN ROOT JOINT. 

  • OPEN ROOT JOINT

    An unwelded joint without backing or consumable insert. 

  • ORBITAL WELDING

    Orbital welding is a mechanized version of the gas tungsten arc welding (GTAW) process. In manual GTAW, the welder moves the welding torch and controls the welding current. In orbital GTAW, the tungsten is installed in a weld head that clamps on the tube or pipe. The tube remains in place while the weld head rotor revolves or orbits around the weld joint circumference to complete the weld.

    The process may be used to produce autogenous welds (without the addition of filler materials), or filler may be added that becomes part of the finished weldment. The welding is done in an inert gas atmosphere to protect the metal from oxidation as it is heated to melting temperature. Gas metal arc welding (GMAW) may also be used for orbital welding.

    Power Supplies- Orbital tube welding power supplies control weld parameters that typically include welding currents, background and pulse amperes (which determine the amount of heat input into the weld), travel speed (RPM), timers that control the amount of time at a particular setting, delay of rotation at the start of the weld, and a current downslope at the end of the weld.

    A timed prepurge and postpurge are usually used to time the flow of inert gas into the weld head before arc initiation and to continue the purge for a timed period after the arc has been extinguished. This allows the weld to cool sufficiently to prevent oxidation before the weld head is opened to remove the welded tube.

    Orbital tube welding is generally done autogenously, so additional controls used for wire feed are not required. Power supplies used for orbital tube welding generally supply 100to 150 amperes of welding current, direct current, electrode negative.

    Modern orbital tube welding power supplies are microprocessor-based. This permits the storage of weld programs or schedules for a large number of tube sizes. The programs can be written, entered into the power supply, and modified by the operator based on welding results, and programs can be changed without loss of other programs. The power supply may be able to print out the weld schedule or to interface with a computer for documentation of operational weld parameters.

    Weld Heads- Weld heads for orbital tube welding typically span a range of sizes. For example, a weld head for tubing up to 38.1 mm (1-1/2 in.) outside diameter (OD) may also be able to weld tube measuring 6.4 mm (1/4 in.), 9.5 mm (3/8 in.), 12.7 mm (1/2 in.), 19.1 mm (3/4 in.), and 25.4 mm (1 in.).

    Autogenous (fusion) tube welds are practical in diameters from 3.2 rnm (0.125 in.) up to about 152 mm (6 in.) with wall thicknesses up to 4 rnm (0.154 in.). Standard orbital weld heads have tube clamp inserts on both sides of the weld to hold the tubes during welding, and the tungsten electrode is located in the rotor in the centerline of the head. To weld two tubes or fittings in a particular size weld head, the length of tubing or the straight section of the fitting must reach from the outside of the head to the electrode location in the weld head center.

  • ORIFICE GAS

    The gas that is directed into the plasma arc torch or thermal spraying gun to surround the electrode. It becomes ionized in the arc to form the arc plasma, and issues from the constricting orifice of the nozzle as a plasma jet. 

  • ORIFICE THROAT LENGTH

    The length of the constricting orifice in the plasma arc torch or thermal spraying gun. 

  • OSCILLATOR, ELECTRICAL

    Any of various electronic devices that produce alternating electrical current, commonly employing tuned circuits and amplifying components. 

  • OSCILLATOR, MECHANICAL

    A mechanical device used to impart oscillatory motion to electrode holders, within limitations of stroke and amplitude in arc welding. It is used to meet a wide range of welding conditions, particularly in gas metal arc welding (GMAW).

    Commercial units are available to linearly oscillate loads at frequencies of 30 to 240 cycles per minute, with infinite adjustments within this range and with running amplitude adjustment of from 0 to 18 mm (0 to 3/4 in.). Other oscillators are available in which the motion is that of a pendulum, with strokes up to 62 mm (2-1/2 in.) wide.

    The types of motions imparted by oscillators are harmonic or uniform, or a combination, with dwell as required. The motions are produced by linkages or cams. The units are driven by electric motors and are controlled mechanically or by an electronic governor. 

  • OSHA

    The Occupational Safety and Health Act of 1970, known as Public Law 91-596, is the most far-reaching safety and health regulation ever enacted by the federal government. It became effective April 28, 1971. Most states also have an OSHA regulatory board that enforces safety and health regulations.

    The Occupational Safety and Health Act provides the federal government with an instrument to support, encourage and carry forward into new areas the safety and health activities that American industry pioneered on a voluntary basis. Initially, the Act offered no new standards but has relied on accepted industry-developed standards. The responsibility is to build on what management, labor and government (state and national) have accomplished in job safety since the early years of this century.

    The provisions of OSHA have had a great impact on employers and industry in observing specific safety and health standards. The following is required of employers:

    (a) The employer must furnish to each employee, employment and a place of employment which is free from recognized hazards that are causing or likely to cause death or serious harm to these employees.

    (b) Comply with the Occupational Safety and Health standards promulgated by this Act. While the Act covers all industries and most employees, there are many areas that are of specific interest to welding. Much of the healthy and safety information available to the fabricator and consumer has originated with manufacturers of welding equipment and consumables, metals and materials, as well as the professional associations that support these groups. 

  • OSMIUM

    (Chemical Symbol: Os) A bluish-white, hard, crystalline metallic element belonging to the platinum family of elements. Discovered in 1803 by Tennant, it is used as a hardening alloy in platinum. Osmium is used for fine machine bearings, for pen points and instrument pivots. With iridium, it forms an alloy, osmiridium, which is used for making filaments in incandescent lamps. Atomic number: 76; atomic weight: 190.; melting point: 3000°C (5432°F); specific gravity: 22.48 at 20°C (68°F). 

  • OVEN SOLDERING

    A nonstandard term for FURNACE SOLDERING. 

  • OVERHANG

    A nonstandard term when used for EXTENSION. See OVER-HEAD WELDING POSITION. 

  • OVERHEAD WELD

    A butt or fillet weld made by a fusion welding process with its linear direction horizontal or inclined to an angle less than 45" to the horizontal, the weld being made from the lower or under side of the parts joined. See WELDING POSITION. 

  • OVERHEAD WELDING POSITION

    The welding position in which welding is performed from the underside of the joint. 

  • OVERHEATING

    A term applied to metals which have been heated to such high temperatures that grain growth is caused to occur, yet not heated sufficiently high to cause partial melting. In the case of steels, to which the term is usually applied, the overheated grain structure can be removed by normalizing. 

  • OVERLAP

    A nonstandard term when used for INCOMPLETE FUSION. 

  • OVERLAP, Fusion Welding

    The protrusion of weld metal beyond the weld toe or weld root.