• D-C ARC Welding

    An arc welding process using a power source that supplies a direct current to the welding arc.

  • DC or D-C

    An arc welding process using a power source that supplies a direct current to the welding arc. 

  • DCEN or DC (-)

    Abbreviation for direct current electrode negative.

  • DCEP or DC (+)

    Abbreviation for direct current electrode positive.

  • DECARBURIZATION

    The removal of carbon (usually referring to the surface of solid steel) by the action of media which reacts with the carbon to cause its oxidation.

  • DEEP DRAWING

    As it refers to sheet metal, drawing is a process of forming flat sheet metal into hollow shapes by means of a punch that causes the metal to flow into a die cavity. If the depth of the formed part (die cavity) is one or more times the sheet thickness, the process is called deep drawing.

    Examples of deep drawing are found in shell case forming, the forming of deep pans, and some automobile body panels and other parts. Alloys used for this purpose are required to have high ductility. The stock must be fine-grained, since a coarse-grained material will exhibit a very rough surface after forming, due to localized yielding, and the ductility of such material is generally too low to permit such extensive drawing without cracking.

  • DEEP WELDING

    Deep welding is a term applied to a shielded metal arc welding (SMAW) technique which utilizes higher welding speeds than conventional methods, uses the benefits of greater arc penetration to obtain the required weld strength, and thereby decreases the cost of the welding operation. When applied to fillet welding, it is often called deep-fillet welding.

    Basic Idea:

    For years, sound welds have been made by conventional methods in the accepted belief that deeper penetration was produced by slower arc speeds. In fact, however, faster speeds, within limits, result in greater penetration, while slower speeds tend to build up more of the weld metal on the surface. A fillet weld with greater penetration resulting from faster travel speed appears smaller, but its strength actually is as great or greater than the weld made at slow speed, which sacrifices penetration for buildup. Since increased penetration reduces the amount of deposited metal needed, the speed of welding can be increased without impairing the strength.

    Travel Speed-Penetration

    The key factor in applying arc force is making the arc travel fast enough to utilize the penetrating power of the arc. An analogy would be to squirt a stream of water through the nozzle of a hose to dig into the ground. The digging action of the stream of water is effective only when the stream is directed at the digging point in the dirt, not when directed into the pool of water that soon accumulates. To maintain the digging action, the of stream of water must be kept moving rapidly enough to stay ahead of the pool, because when it is directed into the pool, its force is expended in merely displacing and churning the water in the pool, not in digging into the ground.

    The same principle can be applied in welding. When the arc is moved slowly, the pool of molten metal buffers the arc, and its force is expended in the molten pool instead of penetrating into the parent metal at the root of the joint. This molten metal merely flows along the joint under the weld without fusing to the parent metal below the depth of arc penetration. When the arc is moved forward rapidly enough, the arc force digs into the base metal and the result is good penetration.

    When conventional arc speeds are used, there is usually a small puddle of molten metal under the arc, dissipating the arc force and preventing full penetration. The limiting speed is usually the highest speed at which the surface appearance remains satisfactory. See Figure D-1 for a comparison of arc penetration at conventional and high travel speeds. Note that deposited weld metal is minimized.

    An increase in current increases the arc force, which increases penetration, just as an increase in the analogous volume of water through the same size hose nozzle increases the digging power of the stream of water. To use higher currents, larger size electrodes

    may be needed. In general, the first indication of excess current is a poor surface appearance of the weld.

    Effect of Arc Length

    In a further comparison of the arc to the stream of water from a hose, to dig deeply into the dirt the nozzle must be kept as close to the ground as possible in

    order to avoid letting the stream of water spread out into an ineffective spray. In welding, when a long arc is held, heat is dissipated into the air, the stream of molten metal from the electrode to the work is scattered in the form of spatter, and the arc force is spread

    over a large area. The result is a wide, shallow bead instead of a narrow bead with deep penetration.

    The advantages of deep-welding are: (1) less deposited metal, (2) increased rate of deposit, and (3) lower costs and simplified process.

    Less Deposited Metal

    By getting deeper penetration, the welded joint is comprised of more fused base metal and less deposited metal than in conventional welding. Since the deposited metal is relatively costly and the fused base metal can be utilized at practically no additional cost (other than labor to make the weld), the deep-welded joint is made at a proportionately reduced cost.

    Greater penetration also allows changing joint preparation from a V-butt in 3/8-inch plate to a plain square-edge butt joint, reducing the amount of filler metal deposited by about 50%. This, in turn, reduces labor by almost 80%.Figure D-2 shows the use of arc penetration to reduce plate edge preparation.

    Travel Speed

    On welds where penetration is the major consideration, such as square-edge butt welds and fillet welds made by deep-welding procedures, the travel speed is not proportional to the current, since the limiting factor for travel speed is the rate at which the slag will follow and cover the weld. Thus, the travel speed with this type of joint is determined by the slag covering characteristics of the coated electrode, rather than by the melt-off rate.

     

  • DEFECT

    A discontinuity or discontinuities that by nature or accumulated effect (for example, total crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability. See DISCONTINUITY and FLAW.

    Defects in welds are points, areas or volumes of a weld that are unsound, indicating that there is either a geometric or metallurgical discontinuity in the structure. Such defects may involve regions where metal is absent and there is no solid present (e.g., pores, voids, cracks), regions where there are low-density (compared to the weld metal) non-metallic inclusions (e.g., entrapped slag), regions where there are high-density (compared to the weld metal) inclusions (e.g., tungsten inclusions), or various geometric discontinuities (eg, lack of penetration, missed seam, mismatch, or undercut).

    Defects in welds can be caused by one or more of the following:

    (1) Improper joint design, preparation, alignment, or fit-up

    (2) Inherent base or filler metal characteristics

    (3) Process characteristics

    (4) Environmental factors

    Regardless of origin, defects almost always act as points of stress concentration, often reduce the cross sectional load-bearing area, and sometimes degrade the properties of the metal, especially ductility and toughness.

     

    Joint-Induced Defects

    Improper or inappropriate joint design, preparation, alignment, or fit-up can lead to the following types of defects:

    (1) Lack of complete penetration of the joint groove or seam because of improper design or inappropriate process or parameter selection

    (2) Mismatch or surface offset due to misalignment of joint elements

    (3) Severe distortion caused by unbalanced masses or excessive heat input

    (4) Porosity as a result of entrapped air or volatile contaminants

    (5) Shrinkage, voids or cracks as a result of poor fit-up or excessive restraint

    (6) Underfill caused by poor fit

    (7) Excessive dilution as a result of improper design or process selection.

     

    Fusion-Zone Defects

    Potential defects that can occur in the fusion zone of a weld include:

    (1) Porosity caused by dissolved gases being released on solidification

    (2) Entrapped slag within or between passes caused by the coatings of electrodes, the cores of flux-cored wires, or other origins, in processes employing slag

    (3) Solidification hot cracks as a result of low-melting constituents at grain boundaries being pulled open by shrinkage stresses

    (4) Severe macro-segregation as a result of gross unmixed dissimilar base metals or unmatched fillers and base metals

    (5) Cold cracks caused by hydrogen embrittlement

    (6) High-density inclusions as a result of contamination by non-consumable tungsten electrodes used in gas tungsten arc welding.

     

    Partially-Melted Zone Defects

    The three major defects in the partially-melted zone in fusion welds are solidification hot cracks, back filled hot cracks, and hydrogen cold cracks.

     

    Heat-Affected Zone Defects

    Defects in the heat-affected zone of fusion welds include:

    (1) Hydrogen cold cracks

    (2) Liquation, reheat, or strain-age cracks

    (3) Stress-corrosion cracks, weld decay cracks, or knife-line attack cracks (e.g., in sensitized stainless steels).

    (4) Lamellar tears in base metals containing extensive non-metallic inclusions in the form of stringers 

     

  • DELAYED CRACKING

    A nonstandard term when used for cold cracking caused by hydrogen embrittlement. See COLD CRACK.

  • DELONG DIAGRAM

    Named after W. T. DeLong. The DeLong Diagram is a method of calculating the Ferrite Number (FN) of a stainless steel weld deposit from its chemical composition. The DeLong Diagram is a modified Schaeffler Diagram predicting the Ferrite Number up to a maximum of 18. Ferrite is important in a weld because it is known to be beneficial in reducing cracking or fissuring in weld metal. See SCHAEFFLER DIAGRAM, WRC 1992 (FN) Diagram, and ANSUAWS A5.22, Specification for Stainless Steel Electrodes for Flux Cored Arc Welding and Stainless Steel R.ods for Gas Tungsten Arc Welding.

  • DELTA IRON

    A term applied to iron which assumes the body- centered cubic structure between 1535 and 1400°C  (2796 and 2552°F). When the temperature of the iron is reduced to 1400°C (2552"F), a transformation occurs and the iron below that temperature assumes a face-centered cubic structure and is called gamma iron. See METALLURGY

  • DEMAGNETIZATION

    The process of removing the magnetic fields of force from a magnetized substance. Demagnetization can be accomplished by: 

    (1) heating to a red heat,

    (2) by violent jarring, or

    (3) by holding the magnetized substance in the magnetic field of a solenoid operated on alternating current, and then gradually removing it.

  • DENDRITIC GROWTH

    Growth of a crystalline solid (e.g., metal) from a melt along certain preferred crystallographic orientations or easy growth directions, resulting in a tree-like appearance in the grain. Dendrites typically contain primary and secondary branches or arms, and may even contain tertiary branches, all of which are aligned with easy growth directions. In welds made in alloys, dendritic growth can exhibit any or all of several substructures including: equiaxed dendritic, columnar dendritic, and cellular dendritic. See METALLURGY and WELD METAL. 

  • DENSENESS

    Compactness or soundness; the absence of porosity in a material or weld. See SPECIFIC GRAVITY.

  • DENSITY

    The ratio of the mass of a homogeneous portion of matter to its volume. The density of solids is compared to water at 16.7"C (62"F), and gases are compared to air at 15.6"C (60°F) at a pressure of 762 mm (30in.) of mercury (101 kPa [14.7 psi])

  • DEOXIDIZED COPPER

    Usually a 99 to 99.9% pure copper with a fractional percentage of one or more deoxidizing agents such as phosphorus, silicon, manganese, cadmium, zinc or aluminum. All of these agents act to reduce the cuprous oxide and thus entirely purge the metal of oxygen. Deoxidized copper is preferred when the metal is to be welded because weaknesses due to the cuprous oxide are avoided. See COPPER ALLOY WELDING.

  • DEOXIDIZING AGENT

    Deoxidizing agents are elements such as aluminum, silicon, or titanium, which, when added to filler metals, eliminate oxygen and ensure sound welds free from oxide inclusions and porosity, or blowholes. 

  • DEPOSIT

    A nonstandard term when used for THERMAL SPRAY DEPOSIT.

  • DEPOSITED METAL ZONE

    In a fusion weld, the portion or area of the weld metal zone external to the original surface or edge planes of the base metal, and consisting substantially of deposited weld metal. For metal deposited by a non-fusion process, the deposited metal zone is the portion comprised of the metal added by friction. 

  • DEPOSITED METAL, Surfacing

    Surfacing metal that has been added during surfacing. 

  • DEPOSITED METAL, Welding, Brazing and Soldering

    Filler metal that has been added during welding, brazing or soldering. Deposited metal refers to metal which has been added by any of these fusion processes or a non-fusion welding process (using friction) to apply a surface overlay during surfacing.

  • DEPOSITION EFFICIENCY

    See ARC WELDING DEPOSITION EFFICIENCY and THERMAL SPRAYING DEPOSITION EFFICIENCY.

    Deposition efficiency is the ratio of the weight of deposited metal to the net weight of electrodes or wire consumed, exclusive of any loss from stubs, or cut off.

    An effective method for calculating the deposition efficiency for a given process is to use that process to deposit a measurable electrode weight on a clean plate of known weight, remove the slag and spatter, and re-weigh. (If wire is a factor, the method is the same as above; wire weight is determined by weighing the reel of wire and subtracting the weight of the reel). The ratio of weight added to the plate to the weight of the electrode used is the deposition efficiency. Typical deposition efficiencies for various processes are shown in Table D- 1. See also ARC WELDING DEPOSITION EFFICIENCY.

  • DEPOSITION RATE

    The weight of material deposited in a unit of time. Deposition rate is a direct measure of the amount of weld metal deposited in kg/h (lb/h) or kg/min (Ib/min) under a given set of conditions.

    The deposition rate of a specific electrode varies according to the type of power source. In a test using E6012 electrodes, the deposition rate with a d-c motor generator welding machine was about 9% greater than the transformer-rectifier type, and 15% greater than one powered with an a-c transformer. The deposition rate of an electrode is always less than the melting rate because of losses by spatter and fumes.

    The melting rate of an electrode, sometimes called the “burn-off rate,” is the rate at which the electrode of a specific type and size is melted by a specific welding current. It is usually expressed in cdmin (in./min.). The melting rate increases rapidly as the current is increased, especially for small diameter electrodes. 

  • DEPOSITIONOR DEPOSIT SEQUENCE

    A nonstandard term when used for WELD PASS SEQUENCE.

  • DEPTH OF BEVEL

    The perpendicular distance from the base metal surface to the root edge or the beginning of the root face.