• V-BLOCK

    A jig made of a casting with a V-shaped notch used to hold shafts or rods in alignment while they are welded. Small jobs are facilitated by using V-blocks. 

  • V-GROOVE WELD

    A type of groove weld. 

  • VACUUM BRAZING

    A nonstandard term for various brazing processes that take place in a chamber or retort below atmospheric pressure.

  • VACUUM CHAMBER

    A container voided of air and other matter to provide a controlled atmosphere in which highly reactive metals such as titanium, zirconium and tantalum are welded. Various welding processes, such as electron beam welding, diffusion bonding and welding, and furnace brazing are performed in a vacuum.

    A vacuum chamber, sometimes called a “dry box,” is evacuated by a pumping system. The chamber provides a completely sealed enclosure which allows a wide visual range so that all stages of the welding operation can be observed. It may also provide access ports for placing or removing parts without contaminating the atmosphere, and remote control devices to manipulate the weldment. Refinements of the system can allow placement of parts in all positions, multiple glove openings, automatic gas controls, and other accessories and equipment specific to the particular manufacturing requirement.

    Applications- The nuclear power and the aerospace industries have applications for controlled-atmosphere welding systems. Extreme corrosion problems encountered in the nuclear industry and the need for materials with low neutron absorption characteristics create numerous applications in which such materials as zirconium and zircaloy-2 must be welded under ideal conditions.

    A demand for materials with high strength-to-weight ratios and the capability to retain strength at high temperatures, such as titanium and beryllium, has similarly directed aircraft manufacturers to controlled-atmosphere welding.

    A basic controlled atmosphere system is comprised of a vacuum chamber and pumping console, purification trains, power supplies, travel and rotational fixtures, air locks, fully automatic operational controls, and other accessories.

    Highly reactive metals readily absorb oxygen, nitrogen and hydrogen when heated to temperatures above 316°C (600”F), which is detrimental to their mechanical and corrosion resistant properties.

    For gas tungsten arc welding, the vacuum chamber is a pressure-tight vessel in which the work, GTAW torch, fixtures and power leads can be installed and which can be evacuated to a range of 0.1 to 5 microns, then refilled with helium or argon at atmospheric pressure.

    The vacuum chamber atmosphere can be further purified of residual air (due to insufficient pump-down or leakage during refilling) by holding an arc for several minutes on a scrap piece of the alloy to be welded. The operation is halted when weld beads free of discoloration are observed.

    Helium, argon, or a mixture of the two may be used as a shielding gas. The most important consideration in the choice between argon and helium as an inert gas for welding is their individual arc characteristics. The normal GTAW welding voltage in an argon atmosphere is from 10 to 12 volts direct current electrode negative (DCEN), and 16 to 20 volts in helium. These values are for similar arc lengths.

    For this reason, when all other variables are held constant and equal, power input is greater with helium at the same welding current. It is difficult to strike an arc in helium at less than 30 amperes, whereas an arc can be initiated in argon at 10 amperes. For this rea son, it is generally necessary to use argon on thin materials to prevent excessive penetration or burnthrough.

    The most common vacuum chamber is a cylindrical vessel with removable plates for loading, and for access to the operator’s protective gloves extended inside. Specifications for construction of the vessel require that it have the capability to withstand vacuums in the order of lo4 mm of mercury. As an alternate to a steel vessel, rigid or flexible plastic containers have been used successfully.

    While normal design procedures for GTAW can be followed, preference should be given those avoiding the use of filler wire to reduce possible contamination from wire surface impurities. 

  • VACUUM PLASMA SPRAYING (VPSP)

    A thermal spraying process variation using a plasma spraying gun confined to a stable enclosure that is partially evacuated.

  • VACUUM TUBE

    A predecessor of solid state electronics. An electron tube evacuated sufficiently high to allow electrons to move with low interaction with remaining molecules of air or gas.

    Although they have been largely replaced by solid state electronics, vacuum tubes of interest to the welding industry are the thyratron, which changes alternating current into direct current and regulates the flow, and the ignitron, which also changes high-voltage alternating current into direct current. The ignitron depends on the presence of liquid mercury inside the tube. Some tubes, such as the ignitron, are housed in large tanks which have running water to cool parts of the tube because of the high heats that are generated. See ELECTRONIC TUBE. 

  • VALVE

    A device with a movable part which starts, stops, or regulates the flow of liquids or gases. 

  • VAN STONE JOINT

    This is a type of bolted flange pipe joint in which the ends of the pipe are heated and flanged outward to form circular contacting flanges. A gasket is placed between the flanged pipe ends and the bolted flanges are slipped over the flanged pipe ends and tightened to draw the pipe ends tightly together. 

  • VANADIUM

    (Chemical symbol: V). A rare bright white ductile metallic element usually found in nature as a compound of lead or lead and copper. It is used in the production of steel to promote control of grain size and provide corrosion resistance and hardenability. The addition of vanadium tends to produce fine grain structure during the heat treating process. Because of this property, vanadium often eliminates the harmful effects of overheating. Once used in armor plate, its principal application is in high-speed steels. 

  • VAPOR FLUX

    A flux that is brought to the oxyfuel gas torch by passing acetylene through a liquid flux held in a dispenser. The dispenser is connected in the acetylene line between the regulator and the torch so that all of the acetylene passes through it. The flux in a vapor form is picked up by the acetylene and carried through the hose and torch to the point of welding. Vapor flux provides automatic fluxing and accurately regulates the amount of flux used; it permits continuous welding without stopping to reflux the rod. 

  • VARIABLE RESISTOR

    A resistor that can be changed or adjusted to different values. 

  • VENTILATION

    In welding, brazing, cutting, or bonding operations, a system of removing fumes, vapors, or gases from the workplace and replacing them with fresh air. Refer to ANSIJASC 2-49.1 Safety in Welding, Cutting and Allied Processes. 

  • VERTICAL POSITION, Pipe Welding

    A nonstandard term for the 2G position in pipe welding. 

  • VERTICAL WELD

    A butt or fillet weld with its linear direction vertical or inclined at an angle less than 45" to the vertical; made by fusion welding. 

  • VERTICAL WELDING POSITION

    The welding position in which the weld axis, at the point of welding, is approximately vertical, and the weld face lies in an approximately vertical plane. 

  • VERTICAL-DOWN

    A nonstandard term for DOWNHILL. 

  • VERTICAL-UP

    A nonstandard term for UPHILL.

  • VICKERS HARDNESS TEST (HV)

    The Vickers hardness test is an indentation test that measures a metal’s resistance to deformation using a diamond pyramid indenter. A standard method for using this method is available in ASTM E92, Vickers Hardness of Metallic Materials. See HARDNESS TESTING. 

  • VISUAL INSPECTION

    The most extensively used nondestructive examination of weldments. It is a primary method of determining important information about conformity to specifications. Visual inspection does not normally require special equipment; however, the requirements are that the inspector is knowledgeable and has good vision.

    Advantages- Visual inspection should be the primary evaluation method of any quality control program. It can, in addition to flaw detection, discover signs of possible fabrication problems in subsequent operations, and can be incorporated in process control programs. Prompt detection and correction of flaws can result in significant cost savings. Conscientious visual inspection before, during, and after welding can detect many of the discontinuities that would be found later by more expensive nondestructive examination methods.

    Equipment- Auxiliary lighting equipment may be needed to assure good visibility. If the area to be inspected is not readily visible, the inspector may use mirrors, borescopes, flashlights or other aids.

    Inspection of welds usually includes quantitative as well as qualitative assessment of the joint. Numerous standard measuring tools are available to make various measurements, such as joint geometry and fit-up, weld size, weld reinforcement height, misalignment, and depth of undercut. Contact pyrometers and crayons should be used to verify that the preheat and interpass temperatures called for in the welding procedure are being used.

    Prior to Welding- Before welding, the base metal should be examined for conditions that tend to cause weld defects. Scabs, seams, scale, or other harmful surface conditions may be found by visual examination. Plate laminations may be observed on cut edges.

    Dimensions should be confirmed by measurement. Base metal should be identified by type and grade. Corrections should be made before work proceeds.

    After the parts are assembled for welding, the inspector should check the weld joint for root opening, edge preparation and other features that might affect the quality of the weld. The inspector should check the following conditions for conformance to the applicable specifications:

    (1) Joint preparation, dimensions, and cleanliness

    (2) Clearance dimensions of backing strips, rings, or consumable inserts

    (3) Alignment and fit-up of the pieces being welded

    (4) Welding process and consumables

    (5)Welding procedures and machine settings

    (6) Specified preheat temperature

    Inspection During Welding- Visual inspection is the primary method for controlling quality during welding. Some of the aspects of fabrication that can be checked include the following:

    (1) Treatment of tack welds

    (2) Quality of the root pass and the succeeding weld layers

    (3) Proper preheat and interpass temperatures

    (4) Sequence of weld passes

    (5) Interpass cleaning

    (6) Root condition prior to welding a second side

    (7) Distortion

    (8) Conformance with the applicable procedure

    The most critical part of any weld is the root pass because many weld discontinuities are associated with the root area. Competent visual inspection of the root 

    pass may detect a condition that would result in a discontinuity in the completed weld. Another critical root condition exists when second-side treatment is required of a double welded joint. This includes removal of slag and other irregularities by chipping, arc gouging, or grinding down to sound metal.

    The root opening should be monitored as welding of the root pass progresses. Special emphasis should be placed on the adequacy of tack welds, clamps, or braces designed to maintain the specified root opening to assure proper joint penetration and alignment.

    Inspection of successive layers of weld metal usually concentrates on bead shape and interpass cleaning, sometimes with the assistance of workmanship standards. Standards show examples of joints similar to those in manufacture in which portions of successive weld layers are shown. Each layer of the production weld may be compared with the corresponding layer of the workmanship standard. Each weld layer should be visually checked by the welder for surface irregularities and adequate interpass cleaning to avoid subsequent slag inclusions or porosity.

    After Welding- The following are among the items that can be checked by visual inspection:

    (1) Final weld appearance

    (2) Final weld size

    (3) Extent of welding

    (4) Dimensional accuracy

    (5) Amount of distortion

    (6) Postweld heat treatment.

    Most codes and specifications describe the type and size of discontinuities which can be accepted. Many of the following discontinuities on the surface of a completed weld can be found by visual inspection:

    (1) Cracks

    (2) Undercut

    (3) Overlap

    (4) Exposed porosity and slag inclusions

    (5) Unacceptable weld profile.

    The weld surface should be thoroughly cleaned of oxide and slag in order to accurately detect and evaluate discontinuities.

    When a postweld heat treatment is specified, the operation should be monitored and documented by an inspector. Items of importance in heat treatment may include the following:

    (1) Area to be heated

    (2) Heating and cooling rates

    (3) Holding temperature and time

    (4) Temperature measurement and distribution

    (5) Equipment calibration.

    Discretion should be used when judging the quality of a weld from the visible appearance alone. Acceptable surface appearance does not prove careful workmanship or subsurface weld integrity; however, proper visual inspection procedures before and during fabrication can increase product reliability over that based only on final inspection. 

  • VOLATILE

    Capable of vaporizing at a relative low temperature. 

  • VOLT

    A measurement of electrical potential and electromotive force calculated between two points on a conducting wire carrying a constant current of one ampere, when the power dissipated between the points is one watt. See ELECTRICAL UNITS. 

  • VOLT-AMPERE

    The unit of apparent power; it is the product of volts times amperes in a given electrical circuit. 

  • VOLT-AMPERE CURVE

    A plot of output-voltage values versus output-current values usually used to describe the static characteristic of a welding power source.

    Static volt-ampere characteristics are generally published by the power supply manufacturer. There is no universally recognized method by which dynamic characteristics are specified. The user should obtain assurance from the manufacturer that both the static and dynamic characteristics of the power supply are acceptable for the intended application.

    Constant Current

    Typical volt-ampere (V-A) output curves for a conventional constant-current power source are shown in Figure V- 1. It is sometimes called a drooper because of the substantial downward (negative) slope of the curves. The power source might have open circuit voltage adjustment in addition to output current control. A change in either control will change the slope of the volt-ampere curve.

    The effect of the slope of the V-A curve on power output is shown in Figure V- 1.With curve A, which has an 80 V open circuit, a steady increase in arc voltage from 20 to 25 V (25%) would result in a decrease in current from 123 to 115 A (6.5%). The change in current is relatively small. Therefore, with a consumable electrode welding process, electrode melting rate would remain fairly constant with a slight change in arc length.

    Setting the power source for 50 V open circuit and more shallow slope intercepting the same 20 V, 123 A position will give volt-ampere curve B. In this case, the same increase in arc voltage from 20 to 25 V would decrease the current from 123 to 100 A (19%), a significantly greater change. In shielded metal arc welding, the flatter V-A curve would give a skilled welder the opportunity to vary the current substantially by changing the arc length. This could be useful for out-of-position welding because it would enable the welder to control the electrode melting rate and molten pool size. Generally, however, less skilled welders would prefer the current to stay constant if the arc length should change.

    Constant Voltage

    A typical volt-ampere curve for a constant-voltage power source is shown in Figure V-2. This power source does not have true constant-voltage output. It has a slightly downward (negative) slope because internal electrical impedance in the welding circuit causes a minor voltage drop in the output. Changing that impedance will alter the slope of the volt-ampere curve.

    Starting at point B in Figure V-2, the diagram shows that an increase or decrease in voltage to A or C (5 V or 25%) produces a large change in amperage (100 A or 50%).This V-A characteristic is suitable for constant-feed electrode processes, such as gas metal arc, submerged arc, and flux cored arc welding, in order to maintain a constant arc length. A slight change in arc length (voltage) will cause a fairly large change in welding current. This will automatically increase or decrease the electrode melting rate to regain the desired arc length (voltage). This effect has been called self regulation. Adjustments are sometimes provided with constant-voltage power sources to change or modify the slope or shape of the V-A curve. If done with inductive devices, the dynamic characteristics will also change. 

    Combined Constant-Current and Constant-Voltage

    Electronic controls can be designed to provide either a constant-voltage or constant-current output from a single power source so that it can be used for a variety of welding and cutting purposes.

    Electronically controlled outputs can also provide output curves that are a combination of constant current and constant voltage, as shown in Figure V-3. The top part of the curve is essentially constant current; below a certain trigger voltage, however, the curve switches to constant voltage. This type of curve is beneficial for shielded metal arc welding (SMAW) to assist starting and to avoid electrode stubbing (sticking in the puddle) if a welder uses too short an arc length. 

  • VOLTAGE DROP

    The difference in voltage between two points in an electric circuit caused by resistance opposing the flow of current. 

  • VOLTAGE REGULATOR

    An automatic electrical control device for maintaining a constant voltage supply to the primary of a welding transformer.