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 from 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 source, 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)

When nickel-copper or copper-nickel cladding is to be applied to steel, a barrier layer of nickel filler metal ER61 must be applied first. Nickel weld metal will tolerate greater iron dilution without fissuring. When cladding is applied manually, the iron content of the first bead will be considerably higher than that of subsequent beads. The first bead should be applied at a reduced travel speed to dissipate much of the penetrating force of the arc in a large weld pool and reduce the iron content of the bead. The iron content of subsequent beads, as well as the surface contour of the cladding, can be controlled by elimination of weaving and maintaining the arc at the edge of the preceding bead.

Such a procedure will result in a 50% overlap of beads, and the weld metal will wet the steel without excessive arc impingement. The welding gun should be inclined up to 5″ toward the preceding bead so that the major force of the arc does not impinge on the

steel.

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.

Shielded Metal Arc Cladding

Shielded metal arc cladding on cast and wrought steels is widely used for such applications as facings on vessel outlets and trim on valves. The procedures outlined for shielded metal arc joining should be followed, except that special care must be taken to control dilution of the cladding. Excessive dilution can result in weld metal that is crack sensitive or has reduced corrosion resistance. The amperage should be in the lower half of the recommended range for the electrode. The major force of the arc should be directed at the edge of the previous bead so that the weld metal will spread onto the steel with only minimum weaving of the electrode. If beads with feather edges are applied, more layers will be required, and the potential for excessive dilution will be greater. The weld interface contour of the cladding should be as smooth as possible. A scalloped weld interface contour can result in excessive iron dilution, with subsequent cracking as the weld specimen is subjected to a 180-degree longitudinal bend test.

Hot-wire Plasma Arc Cladding

High-quality cladding can be produced at high deposition rates with the hot-wire plasma arc process. The process offers precise control of dilution, and dilution rates as low as 2% have been obtained. For optimum uniformity, however, a dilution rate in the 5 to 10% range is recommended. High deposition rates result from the use of two filler metal wires, which are  resistance heated by a separate ac power source. The filler metal is in a nearly molten state before it enters the weld pool. Deposition rates for nickel-alloy weld metal are 16 to 18 kg/h (35 to 40 lb/h), approximately double those obtained with submerged arc weld cladding. Welding conditions for hot-wire plasma arc cladding are given in Table N-4.

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.

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.

 

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