Annealing a stack of several sheets of metal instead of a single sheet. Pack annealing minimizes oxidation and scale formation on the surfaces of the sheets.


    This term is no longer in general use; the term buildup is generally used when surfacing material deposited has essentially the same chemistry as the base metal. See BUILDUP. 


    An electrical circuit in which the current divides at a connection and flows through two or more devices connected to it. 


    The connection of two or more arc welding machines so that higher welding currents are provided than are available from one machine separately.

    For parallel operation, the welding machines must be similar and the recommendations of the manufacturers must be closely followed to adjust control settings correctly and to use the equalizer connections. All machines in parallel must be set for the same polarity and open circuit voltage, and current settings should be kept as nearly equal as possible on all the machines. 


    A nonstandard term for series welding with closely spaced electrodes. 


    An electrical circuit in which a number of devices are connected in series with one another, forming a group. Several groups of series circuits can be connected in parallel with one another to form a parallel. 


    A resistance welding secondary circuit variation in which the secondary current is divided and conducted through the workpieces and electrodes in parallel electrical paths to simultaneously form multiple resistance spot, seam or projection welds. See Figure P-1. 


    A substance which displays a small but positive susceptibility to a magnetic field, varying little with field strength. Examples are aluminum and platinum. 


    A nonstandard term for BASE METAL or SUBSTRATE. 


    A joint root condition in a groove weld in which incomplete joint penetration exists. See Figure I-3. See also COMPLETE JOINT PENETRATION, COMPLETE JOINT PENETRATION WELD, INCOMPLETE JOINT PENETRATION, and JOINT PENETRATION. 


  • PASS

    See also THERMAL SPRAY PASS and WELD PASS. A pass is a single progression of welding along a joint, resulting in a weld bead or layer. 


    A mixture of finely divided brazing filler metal with a flux or neutral carrier. 


    A mixture of finely divided solder with a flux or neutral carrier.


    A sheet of material used to place a patch in a flat, round or warped plate where cracking has occurred or is expected to occur during or after welding, or while in service. See Figure P-2. The cross section of the diagram shows that the patch is slightly dished to allow for contraction in the weld. The preferred circular patch shape equalizes stresses around the weld. However, if a circular patch is not practical, a patch shape as nearly circular as possible, such as oval or elliptical, should be used. If the opening is rectangular, corners of the patch and opening should be rounded.

    Where equipment for forming a dished patch is not available, the patch can be dished by hammering, which should be done when the plate is hot. The diagram shows a simple method for holding the patch in place. Holes are drilled through at the joint, and bolts are used to clamp lugs on both surfaces of the patch. See also CORRUGATED PATCH. 


    An archaic term for annealing. In wire production, it refers to an annealing treatment applied to medium- or high-carbon steel before drawing the wire, or between drafts. The process consists of heating to a temperature above the transformation range, then cooling to a temperature below the transformation range in air, molten lead or salt. See ANNEALING. 


    A microstructural aggregate or a mechanical mixture of ferrite and cementite (iron carbide) platelets which normally occurs in steel and cast iron. This lamellar structure can be observed only through a metallographic microscope because the platelets are very thin, on the order of 0.001 mm (0.00004 in.).

    Pearlite was given its name by H. M. Howe because its lamellar appearance resembles mother-of-pearl. See METALLOGRAPHY. 


    A destructive method of testing that mechanically separates a lap joint by peeling. 


    The mechanical working of metals using impact blows.  Peening is accomplished by repeated hammer blows to the surface of the metal. The blows may be administered manually, as with a hammer, or with pneumatic tools. Peening tends to stretch the surface of the cold metal, thus reducing contraction stresses. 


    A penetrameter, or image quality indicator (IQI), is a device used to measure the quality of radiographic images. Penetrameters consist of a piece of metal of simple geometric shape, with similar absorption characteristics as the weld to be tested. The thickness is generally 2% of the weld thickness. A penetrameter usually has three holes, the diameters of which are 1, 2, and 4 times the thickness of the penetrameter. The penetrameters are placed on a test piece during setup and are radiographed at the same time as the test piece. Sharpness of the penetrameter features in the developed image is a measure of image quality. See RADIOGRAPHIC EXAMINATION. 


    Penetrant inspection is a non-destructive test method for revealing fine surface discontinuities such as cracks, pores or seams in weld metal or base metal. It is useful for detecting discontinuities in magnetic and nonmagnetic materials where magnetic particle inspection cannot be used.

    Penetrant inspection is accomplished by applying a liquid with high wetting capabilities to the surface which can be drawn into surface cavities or openings. The excess penetrant is then removed from the surface, and a liquid-propelled or dry powder developer is applied. If there is a significant discontinuity, the penetrant will be held in the cavity. Blotter action draws the penetrant from the discontinuity to provide a contrasting indication on the surface. This is a relatively reliable and inexpensive method for obtaining information on questionable welds.

    The following sequence is normally used in the application of a typical penetrant test. When the order is changed or short cuts are taken, the validity of the test is suspect.

    (1) Clean the test surface.

    (2) Apply the penetrant.

    (3) Wait for the prescribed dwell time.

    (4) Remove the excess penetrant.

    (5) Apply the developer.

    (6) Examine the surface for indications, and record the results.

    (7) Clean to remove the residue.

    Dye Penetrant

    The dye penetrant method uses a bright red dye with high wetting capabilities. To begin this method, the part is cleaned with a cleaning solution to prepare the surface. The dye penetrant is applied by brush, spray, or dipping, and allowed to remain for at least five minutes. (Detection of very small cracks may require two or three such applications). After applying the dye penetrant, excess penetrant is removed with a cleaning solution, and the developer is sprayed on or applied with a brush. As the developer dries, the penetrant is drawn to the surface, and the discontinuities are revealed.

    Fluorescent Penetrant

    In fluorescent penetrant examination, a highly fluorescent liquid with good wetting or penetrating properties is applied to the surface of the part to be inspected.

    The liquid is drawn into very small surface openings by capillary action. Excess liquid is removed from the surface and a developer in the form of a fine powder, or water suspension of a fine powder, is applied to the surface. The developer draws the penetrant from the pores and cracks and makes them more visible under ultraviolet light.

    Before the fluorescent penetrant test is started, the part must be thoroughly clean, because any dirt, grease, or paint could close the discontinuities to the penetrant. The penetrant may be applied by spraying, dipping or brushing. The time the penetrant must remain on the surface will vary from a few minutes to several hours, depending on the thickness of the work-piece, but 5 to 15 minutes is usually required.

    After the penetrating time, the excess penetrant must be removed carefully to avoid removing more than the surface penetrant. Water-wash penetrants can be washed with a low-pressure water spray. Some commercial penetrants require a solvent wash or an emulsifier. The emulsifier is applied to the surface and allowed to remain for one to four minutes before washing with the water spray. The parts are then dried by wiping, air blower, or hot air oven.

    The dry developer is applied to the dried parts with a powder gun, spray bulb, or by dipping the part into the developer powder. The penetrant is drawn from the discontinuities, making the discontinuities visible in ultraviolet light. If a wet developer is used, drying after washing is not necessary. The wet developer in the form of a colloidal suspension is applied by spraying or dipping. The developer should remain on the surface for at least half the penetrating time. After this, the part is dried by hot air.

    When viewed under ultraviolet light, the indications of discontinuities are brilliantly fluorescent, revealing the depth and length of discontinuities by the amount of penetrant which bleeds out. Contrast is enhanced when viewed in a darkened location, which allows the finer indications to be observed.

    Historical Background- The old “oil and whiting” method might be considered the forerunner of penetrant inspection. A light oil was applied to a surface, wiped off, then the surface was coated with chalk. The oil showing through the chalk pointed up the location of cracks. 


    A nonstandard term when used for DEPTH OF FUSION, JOINT PENETRATION, or ROOT PENETRATION. 


    A nonstandard term when used for FERRITE NUMBER. 


    A welding process that produces coalescence with an arc resulting from a rapid discharge of electrical energy. Pressure is applied percussively during or immediately following the electrical discharge.

    The electrical energy is stored in a capacitor or group of capacitors at a relatively high voltage and discharged directly, or through a transformer, to the part to be welded. Discharge is initiated by closing a mechanical or electronic switch.  Percussion welding is the process used in the electronics industry for joining wires, contacts, leads, and similar items to a flat surface. However, if the item is a metal stud that is welded to a structure for attachment purposes, it is called capacitor discharge stud welding.

    In applying the process, the two parts are initially separated by a small projection on one part, or one part is moved toward the other. At the proper time, an arc is initiated between them. This arc heats the faying surfaces of both parts to welding temperature. Then, an impact force drives the parts together to produce a welded joint. There are basically two variations of the percussion process: capacitor discharge and magnetic force.

    Although the steps may differ in certain applications because of process variations, the essential sequence of events in making a percussion weld is as follows:

    (1) Load and clamp the parts into the machine.

    (2) Apply a low force on the parts or release the driving system.

    (3) Establish an arc between the faying surfaces

    (a) with high voltage to ionize the gas between the parts or

    (b) with high current to melt and vaporize a projection on one part.

    (4) Move the parts together percussively with an applied force to extinguish the arc and complete the weld.

    (5) Turn off the current.

    (6) Release the force.

    (7) Unclamp the welded assembly.

     (8) Unload the machine.


    Operation- Welding heat is generated by an arc between the two parts to be joined. The current density is very high, and this melts a thin layer of metal on the faying surfaces in a few milliseconds. Then the molten surfaces are brought together in a percussive manner to complete the weld.

    Capacitor Discharge

    With the capacitor discharge method, power is furnished by a capacitor storage bank. The arc is initiated by the voltage across the terminals of the capacitor bank (charging voltage) or a superimposed high-voltage pulse. Motion may be imparted to the movable part by mechanical or pneumatic means.

    Magnetic Force

    For magnetic force welding, power is supplied by a welding transformer. The arc is initiated by vaporizing a small projection on one part with high current from the transformer. The vaporized metal provides an arc path. The percussive force is applied to the joint by an electromagnet that is synchronized with the welding current. Magnetic force percussion welds are made in less than one-half cycle of 60 Hz. Consequently, the timing between the initiation of the arc and the application of magnetic force is critical.

    Advantages of Percussion Welding

    The extreme brevity of the arc in both versions of percussion welding limits melting to a very thin layer on the faying surfaces. Consequently, there is very little upset or flash on the periphery of the welded joint, only enough to remove impurities from the joint.

    Heat-treated or cold-worked metals can be welded without annealing them. Filler metal is not used and there is no cast metal at the weld interface. A percussion welded joint usually has higher strength and electrical conductivity than a brazed joint. Unlike brazing, no special flux or atmosphere is required.

    A particular advantage of the capacitor discharge method is that the capacitor charging rate is easily controlled and low compared to the discharge rate. The line power factor is better than with a single-phase a-c machine. Both these factors contribute to good operating efficiency and low power line demand.  Percussion welding can tolerate a slight amount of contamination on the faying surfaces because expulsion of the thin molten layer tends to carry any contaminants out of the joint.


    The percussion welding process is limited to butt joints between two like sections, and to flat pads or contacts joined to flat surfaces. In addition, the total area that can be joined is limited, since control of an arc path between two large surfaces is difficult.  Joints between two like sections can usually be accomplished more economically by other processes. Percussion welding is usually confined to the joining of dissimilar metals not normally considered weldable by other processes, and to the production of joints where avoidance of upset is imperative. Another limitation of this process is that two separate pieces must be. joined. It cannot be used to weld a ring from one piece.


    Mechanical- The welding machine should be equipped with appropriate safety devices to prevent injury to the operator’s hand or other parts of the body. Initiating devices, such as push buttons or foot switches, should be arranged and guarded to prevent them from being actuated inadvertently. Machine guards, fixtures, or operating controls should prevent the hands of the operator from entering between the work-holding clamps or the parts to be welded. Dual hand controls, latches, presence-sensing devices, or any similar device may be employed to prevent operation in an unsafe manner.

    Electrical- All doors and access panels on machines and controls should be kept locked or interlocked to prevent access by unauthorized personnel. When the equipment utilizes capacitors for energy storage, the interlocks should interrupt the power and discharge all the capacitors through a suitable resistive load when the panel door is open. A manually operated switch or other positive device should also be provided in addition to the mechanical interlock or contacts. Use of this device will assure complete discharge of the capacitors.  A lock-out procedure should be followed prior to working with the electrical or hydraulic systems.

    Personal Safety Equipment- Eye protection with suitable shaded lenses should be worn by the operator.  When the welding operations produce high noise levels, operating personnel should be provided with ear protection. Metal fumes produced during welding operations should be removed by local ventilating systems. 


    A requirement of electrical service that demands operation for alternate periods of loads and rest in which the load conditions are well defined, with recurrent magnitude, duration and character. 


    A ferromagnetic material which can be magnetized permanently by applying a magnetic field to the material. A permanent magnet retains its magnetization and magnetic poles for a long period of time after the magnetizing field is removed.