• NARROW GAP WELDING

    A nonstandard term for narrow groove welding.

  • NARROW GROOVE WELDING

    A variation of a welding process that uses multiple-pass welding with filler metal. The use of a small root opening, with either a square groove or a V groove and a small groove angle, yields a weld with a high ratio of depth to width.

    This gas metal arc welding (GMAW) process was developed to make narrow welds in thick plates. Successful welds have been made on steel plates up to 20 cm (8 in.) thick. The process is suitable for welding in all positions, and is used on a variety of heavy section carbon and low-alloy steels with minimum distortion.

    Narrow groove GMAW uses the spray transfer technique. A squared butt joint with a root opening of 6.0 to 9.0 mm (1/4 to 3/8 in.) wide is used for all plate thicknesses. A typical narrow groove joint configuration is shown in Figure N- 1.

    Using GMAW to weld joints in the narrow groove configuration requires special precautions to assure that the tip of the electrode is positioned accurately for proper fusion into the sidewalls. Numerous wire feeding methods for accomplishing this have been devised and successfully used in a production environment. Examples of some of these are shown in Figure N-2.

    Narrow groove welds have been made with electrode wires ranging from 0.9 to 1.6 mm (0.035 to 1/16in.) diameter. Out-of-position narrow groove welds are preferably made with 0.9 mm (0.035 in.) diameter electrode wires.

    Because of the narrow groove opening, relatively high travel speeds are used during welding. If the travel speed is too slow, the weld puddle becomes too large to be controlled. The first layer is deposited against a suitable backing, and because of the high travel speed, is relatively thin. Weld beads are deposited one on top of the other, with approximately 10 passes required for each 25 mm (1 inch) of plate thickness being welded. Close control over the composition of narrow groove welds can be maintained with this technique. 

    Among the many advantages of narrow groove welding are:

    (1) Improved economy because less filler metal is required

    (2) Good mechanical properties in both the weld metal and the heat-affected zone because of the relatively low heat input

    (3) Improved control of distortion

    (4) Fully automatic operation in all welding positions, including overhead, using the spray transfer technique. 

  • NATURAL GAS

    Natural gas consists of gaseous hydrocarbons which have been distilled from mineral oils stored in porous strata in the earth. It is found in all oil-producing localities all over the world. Natural gas is obtained from wells and distributed by pipelines. Its chemical composition varies widely, depending on the locality from which it is obtained. The principal constituents of most natural gases are methane (CH4) and ethane (C2H6).

    Natural gas finds its principal use in the welding industry as a fuel gas for oxygen cutting and heating operations. The volumetric requirement of natural gas is about 1-1/2 times that of acetylene to produce an equivalent amount of heat.

    Natural gas is not suitable for welding due to the oxygen-to-fuel-gas ratio which produces a highly oxidizing flame and prevents the satisfactory welding of  most metals. Many ferrous and nonferrous metals can be braze welded with careful adjustment of flame adjustment and the use of flux.

    Natural gas is also used extensively in the chemical industry for the production of acetylene (C2H2), synthetic rubber, and plastics. 

  • NAVAL BRASS

    A copper-zinc alloy with a small amount of tin added to improve mechanical properties. Nominal composition: Cu--60.0, Zn-39.25, Sn-0.75. See COPPER ALLOY WELDING. 

  • NEGATIVE

    In an external electrical circuit, the cathode, or point toward which the current flows; it is opposite to anode (positive). 

  • NEGATIVE BRUSH

    An electrical conductor made of copper strips or a block of carbon that makes sliding contact between a stationary and a moving part of a generator from which the current enters the armature; or in a motor, from which the current leaves the armature. 

  • NEGATIVE POLARITY

    In welding, the condition in which the electrode is negative in relation to the workpiece. Also called straight polarity. See DIRECT CURRENT ELECTRODE NEGATIVE (DCEN). 

  • NEODYMIUM

    (Chemical symbol: Nd). A metallic element belonging to the rare earth group.

    Neodymium is used in the electronics industry. It is also used in the ceramics industry for glazes and to add color to glass. Neodymium glass can be used as a laser material instead of ruby. It has an atomic number of 60; atomic weight: 144.24; specific gravity: 7.003; melting point 1010°C (1850°F).

  • NEUTRAL FLAME

    An oxyfuel gas jlame that has characteristics neither oxidizing nor reducing.

    See Figure A-1. See also CARBURIZING FLAME, OXIDIZING FLAME, and REDUCING FLAME.

    A neutral flame is obtained by burning a mixture of approximately 50% acetylene and 50% oxygen; it is a well balanced flame indicating complete combustion. The cone next to the tip is white hot and beyond it is a long blue streamer. The molten metal produced in welding with a neutral flame is quiet and clean, and flows well. Few sparks are produced. See OXIDIZING FLAME, CARBURIZING FLAME, and ACETYLENE, Metalworking with Acetylene. 

  • NEUTRAL FLUX, Submerged Arc Welding

    A flux that will not cause a significant change in the weld metal composition when there is a large change in the arc voltage. See also ACTIVE FLUX. 

  • NEUTRON

    An atomic particle found in the nucleus of an atom. It is electrically neutral; it has zero electrical charge. 

  • NICK-BREAK TEST

    An impact test that can be made to provide preliminary visual inspection of the weld. Visual inspection of the broken section may reveal porosity, fracture mode, incomplete fusion, or any other defects which may be present.

    A welder can use the nick-break test to check on a weld by making a test bar out of material similar to the metal being welded. If necessary, the test specimen may be cut directly out of the weld with a torch, and a new piece welded back in its place. As indicated in Figure N-3, the bar is nicked in the weld metal, 1/8 of its width on each side. It is preferable to make this nick or cut with a hacksaw, but if a hacksaw is not available, it can be made with a cutting torch.

    A sharp blow with a hammer to the specimen held in a vise will break the weld metal from nick to nick. The hammer must be heavy and the blow sufficient to make a clean break. A visual inspection for defects can then be made.

    This is used in welder performance testing to API 1104. 

  • NICKEL

    (Chemical symbol: Ni). A silvery white, hard, malleable, ductile metallic element, resistant to corrosion, used mainly in alloys and also as a catalyst. It is magnetic, a fair conductor of electricity, and belongs to the iron-cobalt group of elements. Atomic weight, 58.69; specific gravity, 8.90; melting point, 1453°C (2647°F).

    Nickel adds ductility when alloyed with steel, lowers the critical point for heat treatment, aids fatigue strength, and increases notch toughness.

    Nickel is used as an alloying agent in steel to increase strength and toughness at low temperatures. Most nickel additions are from 1 to 4%, although in some applications, the nickel content runs as high as 36% or more. In all cases, the addition of nickel will increase the strength without decreasing the toughness of the steel. Steels with a nickel content of 24% have reduced magnetism. When the nickel content is increased to 36%, the steel has a very small coefficient of expansion due to heat (up to 482°C [900oF]). 

  • NICKEL ALLOY CUTTING

    Steels clad with a nickel alloy are frequently joined by welding. Since the cladding is normally used for its corrosion resistance, the cladding alloy must be continuous over the entire surface of the structure, including the welded joints. This requirement influences joint design and welding procedure. Butt joints should be used when possible.

    Figure N-4 shows recommended weld joint designs for two thickness ranges [see (A) and (B)]. Both designs include a small root face of unbeveled steel above the  cladding to protect the cladding during welding of the steel. The steel side should be welded first with a low-hydrogen filler metal. It is important to avoid fusion of the cladding during the first welding pass.

    Dilution of the steel weld with the nickel-alloy cladding can cause cracking of the weld metal. The clad side of the joint should be prepared by grinding or chipping and welded with the filler metal recommended for cladding. The weld metal will be diluted with steel. To maintain corrosion resistance, at least two layers, and preferably three or more, should be applied.

    The strip-back method is sometimes used instead of the procedure described above. The cladding is removed from the vicinity of the joint as shown in Figure N-4 (C). The steel is then welded using a standard joint design and technique for steel, and the nickel-alloy cladding is reapplied by weld cladding. The advantage of the strip-back method is that it eliminates the possibility of cracking caused by penetration of the steel weld metal into the cladding.

    Some joints, such as those in closed vessels or tubular products, are accessible only from the steel side. In such cases, a standard joint design for steel is used, and the cladding at the bottom of the joint is welded first with nickel alloy weld metal. After the cladding is welded, the joint can be completed with the appropriate nickel alloy weld metal, or a barrier layer of carbon-free iron can be applied and the joint completed with steel weld metal. If the thickness of the steel is 8 mm (5/16 in.) or less, it is usually more economical to complete the joint with nickel alloy welding filler metal. Figure N-5 shows the most commonly used fabrication sequence when both sides are accessible. 

    Nickel alloy plate can be successfully cut by any one of several thermal cutting processes: metal powder cutting (POC), air carbon arc cutting (CAC-A), plasma arc cutting (PAC), and laser beam cutting (LBC). Other processes used are abrasive cutting and machining, and water jet cutting.  Initially, metal powder cutting (POC), which employs an oxidizing powder with an oxyfuel torch, was the only thermal method used, but the process has been superseded. Plasma arc cutting (PAC) is the most widely used of the thermal cutting processes.

    Plasma-Arc Cutting- Cutting with the plasma arc process is fast and versatile, and it produces high-quality cuts. High power concentration and gas velocity are required for cutting, so that the molten metal is blown out of the cut as the torch progresses.  Gases used for plasma cutting include argon-hydrogen mixtures, nitrogen-hydrogen mixtures, oxygen and nitrogen. The choice of gas depends on the application and the type of equipment used. The equipment manufacturer’s recommendations should be followed.

    Cuts up to 150 mm (6 in.) thick have been made in high-nickel alloys. Because of the constricted plasma jet and the speed of the process, heat-affected zones are usually only 0.25 to 0.4 mm (0.010 to 0.015 in.) wide. The cut surfaces of sections thinner than 75 mm (3 in.) are superior to those produced by the powder cutting process. They are similar to sheared edges but have less bevel. For many applications they can be welded without intermediate cleaning operations. Quality of cuts in heavy sections is about equal to that of cuts produced by powder cutting.

    Air Carbon Arc Cutting (CAC-A)- This process is more effective for gouging operations than for cutting and is widely used for back-gouging on welds and for the removal of fillet welds. By controlling the depth of the groove, limited thicknesses of material can be cut. Grooves up to 25 mm (1 in.) deep can be made in a single pass, but increments of 1 mm (0.004 in.) can also be removed. The width of the groove is determined primarily by the size of electrode. Torch angle and speed affect depth of the groove and the heat-affected zone.

    Laser Beam Cutting (LBC)- Laser beam cutting is a thermal cutting process that severs material by locally melting or vaporizing, with the heat generated by a laser beam. The process is used with or without assist gas to aid in the removal of molten and vaporized material.  Laser cutting has the advantage of high speeds, narrow kerf widths, high quality edges, low heat input, and minimum workpiece distortion. It is an easily automated process that can cut most metals.  Most nickel-base alloys are intended for some form of severe service, i.e., high temperatures or corrosive environments. While these metals are easily laser-cut, it is usually necessary to examine the workpiece for such metallurgical defects as microcracking and grain growth to ensure that the part will perform properly.

    Water Jet Cutting- Water jet cutting severs metals and other hard materials using a high-velocity water jet. The water stream, with a flow rate of 0.4 to 19 L/min (0.8 to 40 ft3/h is usually manipulated by a robot or gantry system, but small workpieces maybe guided past a stationary water jet by hand. Metals and other hard materials are cut by adding an abrasive in powder form to the water stream. Higher flow rates of water are required to accelerate the abrasive particles.

    Materials are cut cleanly, without ragged edges, without heat, and generally faster than on a band saw. A smooth, narrow 0.8 to 2.5 mm (0.030 to 0.100 in.) kerf is produced. There is no problem of thermal delamination, or deformation, when water jet cutting is properly applied.

    Metal Powder Cutting (POC)- Metal powder cutting is based on the use of an oxygen jet into which finely divided powder is fed. The powder initiates an exothermic reaction that supplies the heat necessary for cutting. Nickel alloys can be readily cut with an iron powder or mixtures of iron and aluminum powders.

    In powder cutting, considerable amounts of oxide and burned material accumulate on the metal, with the greatest buildup occurring on the top surface. This slag is more adherent on nickel-copper alloys than on nickel or nickel-chromium alloys. All adherent slag, powder or dross must be removed prior to any further operation.  The depth affected by heat from powder cutting is extremely shallow. Corrosion resistance of the metal is not impaired if all discoloration is removed. 

     

  • NICKEL ALLOY WELDING

    With minor modifications, the welding procedures used for joining steel are applicable to nickel alloys, such as nickel-copper, or nickel-chromium. Since a number of processes are capable of producing satisfactory joints, the selection can be determined by the following considerations:

    (1) Corrosive environment to which the product will be exposed (to establish whether welding, silver brazing or soft soldering is applicable)

    (2) Gauge of metal

    (3) Design of the product

    (4) Design of the individual joints in the product

    Filler Metals and Fluxes

    Covered Electrodes- In most cases, the weld metal composition from a covered electrode resembles that of the base metal with which it is used. Invariably, its chemical composition has been adjusted to satisfy weldability requirements; usually additions are made to control porosity, enhance micro cracking resistance, or improve mechanical properties. Covered electrodes normally have additions of deoxidizing ingredients such as titanium, manganese, and niobium. ANSI/ AWS A5.11, Specification for Nickel and Nickel Alloy Welding Electrodes for Shielded Metal Arc Welding, is used almost universally in filler metal selection. Sometimes military specifications will apply, such as the MIL-E-22200 series, but they duplicate the AWS specification in most respects.

    Fluxes- Fluxes are available for submerged arc welding of many nickel alloys. Fluxes, in addition to protecting the molten metal from atmospheric contamination, provide arc stability and contribute important additions to the weld metal. Therefore, the filler metal and the flux must be jointly compatible with the base metal. An improper flux can cause excessive slag adherence, weld cracking, inclusions, poor bead contour, and undesirable changes in weld metal composition. Fluxes used to weld carbon steel and stainless steel are not suitable.

    Surface Preparation

    Cleanliness is the single most important requirement for successful welding of nickel alloys. At high temperatures, these alloys are susceptible to embrittlement by many low-melting substances. Such substances are often found in materials used in normal manufacturing processes. Nickel alloys are embrittled by sulfur, phosphorus, and metals with low melting points such as lead, zinc, and tin. Lead hammers, solders, and wheels or belts loaded with these materials are frequent sources of contamination. Detrimental elements are often present in oils, paint, marking crayons, cutting fluids, and shop dirt.

    Arc Welding

    Nickel alloys are weldable by all the processes commonly used for steel and other base metals. Welded joints can be produced to stringent quality requirements in the precipitation-hardenable group, as well as the solid-solution group.

    Applicable Processes- Some arc welding processes broadly applicable to nickel alloys are identified by individual alloy in Table N-5. Note that the shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) processes are not applicable to the welding of the precipitation-hardenable alloys. Covered electrodes for welding the age-hardenable alloys suffer from dramatically reduced mechanical properties of the weld and interbead slag adhesion, while the GMAW process results in high heat input, to which most of the age-hardenable alloys are sensitive.

    Heat input Limitations-  High heat input during welding may produce undesirable changes in nickel alloys. Some degree of annealing and grain growth will take place in the heat-affected zone (HAZ). The heat input of the welding process and the preheat temperature will determine the extent of these changes. High heat input may result in excessive constitutional liquation, carbide precipitation, or other harmful metallurgical phenomena. These, in turn, may cause cracking or loss of corrosion resistance. 

    Fluxes- Submerged arc fluxes are available for several nickel alloys, and they are designed for use with a specific welding wire. Fluxes used to weld carbon steels and stainless steel are invariably unsuitable for welding nickel alloys. In addition to protecting the molten metal from atmospheric contamination, the fluxes provide arc stability and contribute important additions to the weld metal. 

    The flux cover should be only sufficient to prevent the arc from breaking through. An excessive flux cover can cause deformed weld beads. Slag is easily removed and should be discarded, but unfused flux can be reclaimed. However, in order to maintain consistency in the flux particle size, reclaimed flux should be mixed with an equal amount of unused flux.

    Submerged arc fluxes are chemical mixtures and can absorb moisture. Storage in a dry area and resealing opened containers are standard practice. Flux that has absorbed moisture can be reclaimed by heating. The flux manufacturer should be consulted for the recommended procedure.

    Filler Metals- Submerged arc welding employs the same filler metals used with the gas tungsten arc welding and gas metal arc welding processes. Weld metal chemical composition will be somewhat difierent as additions are made through the flux to allow the use of higher currents and larger welding wires. Welding wire diameters are usually smaller than those used to weld carbon steels. For example, the maximum size used to weld thick base metal is 2.4 mm (3/32 in.), where 1.1 mm (0.045 in.) has been used to weld thin base metal.

    Welding Current- Direct current electrode negative (DCEN) or direct current electrode positive (DCEP) may be used. DCEP is preferred for groove joints, yielding flatter beads and greater depth of fusion at low voltage (30 to 33V). DCEN is frequently used for weld surfacing, yielding higher deposition rates and reduced depth of penetration, thus reducing the amount of dilution from the base metal. However, DCEN requires a deeper flux cover and causes an increase in flux consumption. DCEN also increases the possibility of slag inclusions, especially in butt joints where the molten weld metal is thicker and solidification occurs from the sidewalls as well as the root of the weld.

    Electron Beam Welding (EBW)

    Some advantages of electron beam welding are:

    (1) Single pass welds with nearly parallel sides can be made because of the high depth-to-width ratio and full penetration of EBW.

    (2) The process is extremely efficient because it converts electrical energy directly to beam output energy.

    (3) The heat input per unit length for a given depth of penetration is less than with arc welding. This results in a narrower heat-affected zone with its attendant lower distortion and adverse thermal effects.

    (4) Rapid travel speeds are possible because of the high melting rates associated with the concentrated heat source. This increases productivity and efficiency by reducing welding time.

    Joints that can be welded include: butt, corner, lap, edge, and T-joints. Normally, fillet welds are not attempted because they are difficult to make. Square butt welds require fixturing to maintain alignment and fit-up. Without the addition of filler metal, the fit-up is more critical than for arc welding. Poor fit-up will result in lack of fill in the joint. High quality welding requires cleanliness of the parts, Weld contamination can cause porosity and cracking along with a decrease in mechanical properties.

    Usually, any metal or alloy that can be fusion welded by other welding processes can be joined by EBW. The weldability of a particular alloy or combination of alloys will depend on the metallurgical characteristics of that alloy or combination, the part configuration, joint design, process parameters and special welding procedure.

    Laser Beam Welding (LBW)

    Many of the nickel and nickel-based alloys have been successfully welded with laser beam welding. Welded joint cross sections are similar to those produced by an electron beam. Laser welding has the advantage of being done in the open, compared to the vacuum chamber required for electron beam. Some process limitations include the following:

    (1)Positioning of the weld joint must be very closely controlled.

    (2) Parts must be accurately clamped to assure alignment with the beam.

    (3) Maximum joint thickness is commonly limited to 19mm (0.75 in.).

    (4) Because of rapid solidification, some porosity may be experienced. Workpiece cleanliness is of great importance because of possible weld contamination. Joint design is important because the laser beam must have access to the weld area.

    Resistance Welding (RW)

    This category includes spot, seam, and projection welding. The weld is made by the generation of heat at the faying surfaces of adjoining parts. Current is passed through the parts to be welded and the heat is generated by the resistance to the passage of current.

    The size and shape of the weld depends on a number of factors, some of which are: (1) the type of equipment being used, (2) the amount of current passing through the parts, (3) the length of time used to make the weld, (4) the cleanliness of the parts, and (5) the metallurgical characteristics of the materials being welded.

    Generally, nickel-base alloys are readily weldable using resistance welding processes. Some cast precipitation-hardenable, low-ductility alloys can be difficult to weld without cracking. Because nickel-base alloys have high strength at elevated temperatures, high electrode forces are needed. Surface contaminants containing lead and sulfur must be removed prior to welding because these materials can cause embrittled welds.

    Occasionally, mechanical sticking of electrodes is encountered when welding pure nickel because of its high electrical conductivity. The values of welding currents used to join various nickel-based alloys are dependent on their resistivity and strength. As the resistivity (compared to low-carbon steel) increases, less current is required to make a satisfactory weld.

    Oxyfuel Welding (OFW)

    Oxyfuel welding is seldom used for welding nickel and nickel alloys. The selection of the method is determined not by the metal but by the physical characteristics of the piece to be welded gauge of the metal, design of the workpiece and design of the individual joint. Good welding is accomplished with OFW in flat, vertical or overhead positions.  Generally, however, because OFW is slow, and because it requires fluxing and more heat input, it has been displaced by the GMAW and GTAW processes.

    Welding Dissimilar Metals

    Selecting the appropriate welding process and the filler metal requires careful consideration when joining dissimilar metals. The choice of both should be based on metallurgical factors such as differences in thermal expansion coefficients between the weld metal and base metal, the effects of dilution on the weld metal, and the possibility of changes in the structure of the materials after extended service at elevated temperatures.

    The shielded metal arc welding process has the advantage in making dissimilar metal welds in that the amount of filler metal added is less influenced by welder technique than the GTAW or GMAW processes. In GTAW, the welder can vary filler metal addition to a very large degree.  The gas tungsten arc welding process permits more control over dilution than most other processes. The gas metal arc welding (GMAW) process is sometimes used for joining dissimilar metals, but the procedure must be carefully controlled to prevent excessive dilution. The submerged arc welding (SAW) process can also be used, but again, procedures must be controlled to avoid excessive dilution from the joint sidewall.

    Filler Metals- A variety of materials can be welded using nickel alloy filler metals. Stainless and carbon steels, low-alloy steels, and high-nickel alloys are among the possibilities.

    Either covered electrodes or bare filler metals are available and can be specified to suit equipment and skills. Some of the most commonly used electrodes are listed in ANSUAWS A5.14, Specification for Nickel and Nickel Alloy Bare Welding Rods and Electrodes; and A5.11, Specification for Nickel and Nickel Alloy Welding Electrodes for Shielded Metal Arc Welding.

    Welding 9% Nickel Steel

    Nine percent nickel steel is generally specified for commercial applications in the production, handling, storage, and transportation of liquid gases, as well as related cryogenic applications. The following properties are required:

    (1) High strength and toughness

    (2) Resistance to embrittlement at temperatures as low as -196°C (-320°F)

    (3) High stress allowances of pressure vessel designs

    Electrodes and filler metals used to join 9% nickel steel are recommended in ANSVAWS A5.14, Specifications for Nickel and Nickel Alloy Bare Welding Rods and Electrodes, and also in A5.11, Specification for Nickel and Nickel Alloy Welding Electrodes for Shielded Metal Arc Welding.

    Nickel Overlays

    Weld overlays of high-nickel welding materials can be selectively applied to either large or small sections of tanks, shafts, rollers, tube sheets, vessels, valve seats, pumps, and other equipment made of various materials to increase the corrosion, heat, and wear resistance in harsh environments. Overlaying vulnerable equipment, old or new, can extend the service life of the equipment and provide easier maintenance. See NICKEL WELD CLADDING.

  • NICKEL ALLOYS

    Nickel alloys offer unique physical and mechanical properties and are useful in a variety of industrial applications, notably because of their resistance to attack in various corrosive media at temperatures from 200°C (400°F) to over 1090°C (2000"F), and their good low- and high-temperature mechanical strength. In demanding industrial environments, nickel alloy welds must duplicate the attributes of the base metal to a very high degree. Welding, heat treating, and fabrication procedures should be established with this in mind. The chemical compositions of various nickel alloys are listed in Table N- 1.

     

    High-quality weldments are readily produced in nickel alloys by commonly used welding processes. Not all processes are applicable to every alloy; metallurgical characteristics or the unavailability of matching or suitable welding filler metals and fluxes may limit the choice of welding processes.

    Welding procedures for nickel alloys are similar to those used for stainless steel, except the molten weld metal is more sluggish, requiring more accurate weld metal placement in the joint. Thermal expansion characteristics of nickel alloys approximate those of carbon steel and are more favorable than those of stainless steel. Thus, warping and distortion are not severe during welding.

    The mechanical properties of nickel alloy base metals will vary depending on the amount of hot or cold work remaining in the finished form (sheet, plate, or tube). Some modification in the procedures may be needed if the base metal is not in the fully annealed condition.

    In general, the properties of welded joints in fully annealed nickel alloys are comparable to those of the base metals. Postweld treatment is generally not needed to maintain or restore corrosion resistance in most nickel alloys. In most media, the corrosion resistance of the weld metal is similar to that of the base metal. Welds made on Ni-Mo alloy NlOOOl and Ni-Si cast alloys commonly are solution annealed after welding to restore corrosion resistance to the heat-affected zone (HAZ).

    Over-alloyed filler metals are often used (sometimes in lieu of postweld heat treatment) to fabricate components for very aggressive corrosive environments. The over-matching composition offsets the effects of weld metal segregation when using a matching composition. Examples are the use of filler metal NiCrMo-3 products to weld the “super” stainless alloys, containing 4 to 28% molybdenum, and the use of filler metal NiCrMo-10 to fabricate components of the base metal Ni-Cr-Mo alloy C-276 (UNS N10276).

    Postweld heat treatment may be required for precipitation hardening in specific alloys. Postweld stress relief may be necessary to avoid stress-corrosion cracking in applications involving hydrofluoric acid vapor or certain caustic solutions. For example, Ni-Cu alloy 400 (UNS N04400) immersed in hydrofluoric acid is not sensitive to stress-corrosion cracking, but it is when exposed to the aerated acid or the acid vapors.

    The choice of welding process will be based on the following:

    (1) Alloy to be welded

    (2) Thickness of the base metal

    (3) Design conditions of the structure (such as temperature, pressure, or type of stresses)

    (4) Welding position

    (5) Need for jigs and fixtures

    (6) Service conditions and environments

    Metal Characteristics

    Nickel has a face-centered-cubic (FCC) structure up to its melting point. Nickel can be alloyed with a number of elements without forming detrimental phases.

    Nickel in some aspects bears a marked similarity to iron, its close neighbor in the periodic table. Nickel is only slightly denser than iron, and it has similar magnetic and mechanical properties. The crystalline structure of pure nickel at room temperature, however, is quite different from that of iron. Therefore, the metallurgy of nickel and nickel alloys differs from that of iron alloys.

    Alloy Groups

    Nickel alloys can be classified into four groups:

    (1) Solid-solution-strengthened alloys

    (2) Precipitation-hardened alloys

    (3) Dispersion-strengthened alloys

    (4) Cast alloys

    Solid-Solution-Strengthened Alloys

    All nickel alloys are strengthened by solid solution. Additions of aluminum, chromium, cobalt, copper, iron, molybdenum, titanium, tungsten, and vanadium contribute to solid-solution strengthening. Aluminum, chromium, molybdenum, and tungsten contribute strongly to solid-solution strengthening while others have a lesser effect. Molybdenum and tungsten improve strength at elevated temperatures.

    Pure Nickel- Nickel 200 and the low-carbon version, nickel 201, are most widely used where welding is involved. Of these, the low-carbon nickel (201) is preferred for applications involving service exposure to temperatures above 315°C (600°F) because of its increased resistance to graphitization at elevated temperatures. This graphitization is the result of excess carbon being precipitated intergranularly in the temperature range of 315 to 760°C (600 to 1400’F) when nickel 200 is held there for extended time.

    Major applications for the two alloys are food processing equipment, caustic handling equipment, laboratory crucibles, chemical shipping drums, and electrical and electronic parts.

    Nickel-Copper Alloys- Nickel and copper form a continuous series of solid solutions with a face-centered-cubic crystal structure. The principal alloys in this group are alloy 400 and the free-machining version of it, R-405. These alloys have high strength and toughness, and they are important in industry primarily because of their corrosion resistance. The alloys have excellent resistance to sea or brackish water, chlorinated solvents, glass etching agents, sulfuric acids, and many other acids and alkalis.

    Nickel-copper alloys are readily joined by welding, brazing, and soldering with proper precautions. To improve strength and to eliminate porosity in the weld metal, filler metals that differ somewhat in chemical composition from the base metal may be used. Welding without the addition of filler metal is not recommended for manual gas tungsten arc welding. Most automatic or mechanized welding procedures require the addition of filler metal, but a few do not. Welding filler metals applicable to this alloy group are also widely used to weld copper alloys.

    Nickel-Chromium Alloys- Nickel alloys 600, 601, 690,214, 230, G-30, and RA-330 are commonly used. Alloy 600, which is the most widely used, has good corrosion resistance at elevated temperatures along with good high-temperature strength. Because of its resistance to chloride-ion stress-corrosion cracking, it finds wide use at all temperatures and has excellent room-temperature and cryogenic properties. 

    Precipitation-HardenableAlloys

    These alloys are strengthened by controlled heating, which precipitates a second phase known as gamma prime, from a supersaturated solution. Precipitation occurs upon reheating a solution-treated and quenched alloy to an appropriate temperature for a specified time. Each alloy will have an optimum thermal cycle to achieve maximum strength in the finished aged condition. Some cast alloys will age directly as the solidified casting cools in the mold.

    The most important phase from a strengthening standpoint is the ordered face-centered-cubic gamma prime that is based upon the compound Ni3A1. This phase has a high solubility for titanium and niobium; consequently, its composition will vary with the base-metal composition and temperature of formation. Aluminum has the greatest hardening potential, but this is moderated by titanium and niobium. Niobium has the greatest effect on decreasing the aging rate and improves weldability.

    Nickel-Copper Alloys- The principal alloy in this group is K-500. Strict attention to heat-treating procedures must be followed to avoid strain-age cracking. Its corrosion resistance is similar to the solid-solution alloy 400. The alloy has been in commercial existence for well over 50 years and is routinely welded, using proper care, with the gas tungsten arc welding process. Weld metal properties using filler metals of matching composition seldom develop 100% joint efficiencies, thus a common consideration by the designer is to locate the weld in an area of low stress. ERNiFeCr-2 filler metal has been used to join this alloy, but an evaluation of service environment and the differing aging temperatures between the two alloys must be made. The base metal supplier should be consulted forrecommendations for filler materials.

    Dispersion-Strengthened Alloy

    Nickel and nickel-chromium alloys can be strengthened to very high strength levels by the uniform dispersion of very fine refractory oxide (Tho2) particles throughout the alloy matrix. This is done using powder metallurgy techniques during manufacture of the alloy. When these metals are fusion welded, the oxide particles agglomerate during solidification. This destroys the original strengthening afforded by dispersion within the matrix. The weld metal will be significantly weaker than the base metal. The high strength of these base metals can be retained with processes that do not involve melting the base metal. Contact the base metal supplier for recommendations for specific conditions.

    Cast Alloys

    Casting alloys, like wrought alloys, can be strengthened by solid-solution or precipitation hardening. Precipitation-hardening alloys high in aluminum content, such as alloy 713C, will harden during slow cooling in the mold and are considered unweldable by fusion processes. However, surface defects and service damage are frequently repaired by welding. It should be understood that a compromise is being made between the convenience of welding and the cast strength and ductility. Most nickel cast alloys will contain significant amounts of silicon to improve fluidity and castability. Most of these cast alloys are weldable by conventional means, but as the silicon content increases, so does weld-cracking sensitivity. This cracking sensitivity can be avoided using welding techniques that minimize base metal dilution.

    Nickel castings that are considered unweldable by arc welding methods may be welded using the oxyacetylene process and a very high preheat temperature. Cast nickel alloys containing 30% copper are considered unweldable when the silicon exceeds 2% because of their sensitivity to cracking. However, when weldable grade castings are specified, weldability is quite good, and such welds will pass routine weld-metal inspections using methods such as radiography,  liquid penetrant testing, and pressure tests.

  • NICKEL ALLOYS, Weld Cladding

    Nickel alloy weld metal is readily applied as cladding on carbon steels, low-alloy steels, and other base metals to increase the service life of the workpiece or to provide a corrosion-resistant surface. One of the benefits of this procedure, for example, is the cost saving realized by cladding a steel vessel with a thin corrosion-resistant layer of nickel alloy rather than making the whole vessel of nickel alloy.

    Nickel-alloy cladding can be applied to cast iron, but a trial cladding should be made to determine whether standard procedures can be used. The casting skin, or cast surface, must be removed by a mechanical means such as grinding. Cladding on cast irons with high sulfur or phosphorus content may crack because of embrittlement by those elements. Cracking can often be eliminated by applying a barrier layer of AWS ENiFe-CI welding electrode or AWS ENiFeT3-CI cored wire. These filler metals were especially developed for welding cast iron, and the weld metal is more resistant to cracking caused by phosphorus, sulfur, and carbon dilution. When cladding is applied directly to cast iron without a barrier layer, amperage should be the minimum that provides proper arc characteristics in order to hold dilution at the lowest level.

    Gas Metal Arc Cladding

    Gas metal arc welding (GMAW) with spray transfer is successfully used to apply nickel-alloy cladding to steel. The cladding is usually produced with mechanized equipment and with weaving of the electrode. Argon is often used as the shielding gas. The addition of 15 to 25% helium, however, is beneficial for cladding with nickel and nickel-chromium-iron. Wider and flatter beads and reduced depth of fusion result as the helium content is increased to about 25%. Gas-flow rates are influenced by welding technique and will vary in a range of 15 to 45 Wmin (35 to 100 ft3AI). As welding current is increased, the weld pool will become larger and require larger gas nozzles for shielding.

    When weaving is used, a trailing shield may be necessary for adequate shielding. In any case, the nozzle should be large enough to deliver an adequate quantity of gas under low velocity to the welding area. Representative chemical compositions of automatic gas metal arc cladding are shown in Table N-2. The cladding in this table was produced with the following welding conditions:

    (1) Torch gas, 24 L/min (50 ft3//h) argon

    (2) Trailing shield gas, 24 L/min (50 ft3/h) argon

    (3) Electrode extension, 19 mm (3/4 in.)

    (4) Power - DCEP

    (5) Oscillation frequency, 70 cycles/min

    (6) Bead overlap, 6 to 10mm (1/4 to 3/8 in.)

    (7) Travel speed, 110mm/min (4-1/2 in./min)

     

    Submerged Arc Cladding

    The submerged arc welding (SAW) process produces high-quality nickel-alloy cladding on carbon steel and low-alloy steel. The process offers several advantages over gas metal arc cladding:

    (1) High deposition rates, 35 to 50% increase with 1.6 mm (0.062 in.) diameter surfacing metal, and the ability to use larger electrodes.

    (2) Fewer layers are required for a given cladding thickness. For example, with 1.6 mm (0.062 in.) surfacing metal, two layers applied by the submerged arc process have been found to be equivalent to three layers applied by the gas metal arc welding process.

    (3) The welding arc is much less affected by minor process variations such as welding wire condition and electrical welding fluctuations.

    (4) Welded surfaces of submerged arc cladding are smooth enough to be liquid-penetrant inspected with no special surface preparation other than wire brushing.

    (5) Increased control provided by the submerged arc process yields fewer defects and requires fewer repairs.

    Chemical compositions of specific submerged arc weld claddings are shown in Table N-3. The power supply for all weld cladding applied using weaving techniques is direct current electrode negative (DCEN) with constant voltage. DCEN produces an arc with less depth of fusion, which reduces dilution. Direct current electrode positive (DCEP) results in improved arc stability and is used when stringer-bead cladding is needed to minimize the possibility of slag inclusions.

     

    Welding of Nickel Alloy Clad Steel

    Steels clad with a nickel alloy are frequently joined by welding. Since the cladding is normally used for its corrosion resistance, the cladding alloy must be  continuous over the entire surface of the structure, including the welded joints. This requirement influences joint design and welding procedure. Butt joints should be used when possible. Figure N-4 shows recommended weld joint designs for two thickness ranges [see (A) and (B)]. Both designs include a small root face of unbeveled steel above the cladding to protect the cladding during welding of the steel. The steel side should be welded first with a low hydrogen filler metal. It is important to avoid fusion of the cladding during the first welding pass. Dilution of the steel weld with the nickel-alloy cladding can cause cracking of the weld metal. The clad side of the joint should be prepared by grinding or chipping and welded with the filler metal recommended for cladding. The weld metal will be diluted with steel. To maintain corrosion resistance, at least two layers, and preferably three or more, should be applied.

  • NICKEL SILVER

    An alloy of copper, zinc and nickel. See COPPER ALLOY WELDING. 

  • NICKEL STEEL

    A steel alloyed with nickel to obtain characteristics such as high strength, toughness, corrosion resistance, and other properties of nickel. See STEEL, Alloy. 

  • NIOBIUM

    (Chemical symbol: Nb) A ductile metallic element used in alloys, tools and dies and superconductor magnets. Also known as columbium (Cb). It is used as a major alloying element in nickel-base, high-temperature alloys and as an important additive to high-strength structural steel. Atomic number, 41; atomic weight, 92.906; melting point, 2468°C (4474°F); specific gravity, 8.57 at 20°C (68°F). 

  • NITRIDE OF IRON

    A compound closely resembling cementite when etched, caused by the nitrogen of the air combining with iron at a very high temperature. 

  • NITRIDING

    A process by which certain steels can be surface hardened. The workpieces are placed in a nitriding box in a furnace. Ammonia gas is passed through the box and the furnace is kept at a temperature of about 510°C (950°F) for periods from two to ninety hours, depending on the depth of hardness required. See HEAT TREATMENT. 

  • NITROGEN

    (Chemical symbol: N). A gaseous element that occurs freely in nature and constitutes about 78% of the atmosphere. It is a colorless, odorless and relatively inert gas, although it combines directly with magnesium, lithium and calcium when heated with them. Nitrogen occurs in all living things as an essential element. When mixed with oxygen and subjected to electric sparks, it forms nitrogen peroxide. Atomic weight, 14.008; melting point, -210.5"C (-347°F); boiling point, -195°C (-3 19°F); specific gravity, 0.967 (air).

    Nitrogen is produced either by liquefaction and fractional distillation of air, or by heating a water solution of ammonium nitrate (a mixture of ammonium chloride and sodium nitrite). For a description of the liquefaction process, see OXYGEN PRODUCTION. 

  • NON-CONDUCTOR

    A material which does not readily conduct electric current: an insulator. 

  • NON-INDUCTIVE

    A material having little or no inductance.