To offset the relatively high cost of solid nickel alloy construction and still provide a highly corrosion-resistant layer of alloy, weld overlaying is often viable design options.

Construction using these techniques or products is well-suited to those applications where the full metal thickness is not required for mechanical purposes or corrosion resistance. For economic reasons, the backing material is usually carbon steel, but other steels are feasible.

Weld overlay surfacing is well-suited to covering thick sections of items such as tube sheets, large diameter shafts, and the walls of thick-section pressure vessels. The substrate is usually carbon steel or, on occasion, a low alloy steel. The weld overlay may be made by a number of different welding processes; the choice is usually based on the process that gives the highest deposition rate and acceptable quality overlay for the particular application. Comments on the welding processes for overlay welding follow.

Submerged arc welding — Deposition rates using SAW are high, a 35 to 50% increase over GMAW overlay capability. SAW fluxes are commercially available for use with most of the common nickel alloy bare wire and strip filler metals. With wire electrodes, a diameter of 1.6mm (0.062 in.) yields better results than the larger diameters characteristic of steel or stainless steel SAW. Base metal dilution is normally controlled so that only two weld layers are needed unless the surface is to be machined, in which case three weld layers may be required. The as-welded surfaces are smooth enough to be dye-penetrant inspected with no special surface preparation other than wire brushing. All welding must be done in the flat position unless the equipment is specially adapted for horizontal welding.

Gas metal arc welding — GMAW overlays are usually made using the spray arc or pulsed arc mode. The spray arc mode has the advantage of higher deposition rates, but all welding must be performed in the flat position. Base metal dilution tends to be higher with GMAW welding than with other processes. The favoured method employs automatic welding with an oscillating torch movement.

Pulsed GMAW overlays are usually done with a filler wire diameter of 1.2mm (0.045 in.), compared to a wire diameter of 1.6mm (0.062 in.) used with the spray arc mode. Deposition rates are lower, but all-position welding is possible. Pulsed arc mode GMAW may be done with either manual or automatic set-up.

Shielded metal arc welding — Deposition rates are relatively low, but the process is useful in overlaying small areas and irregular, out-ofposition surfaces where automation is not justified. Facings on vessel outlets and trim on valves are good examples of suitable applications.

Weld overlay guidelines — The effect of base metal dilution and the profile of the overlay/base metal interface are two areas of concern in weld overlay work. These concerns are common to overlays made with any welding process.

Base metal dilution — Usually the objective is to provide a weld overlay in which the top weld surface has a composition equivalent to the base metal to achieve a similar corrosion resistance. This means a minimum of two weld layers and often three along with careful control of welding parameters to minimize penetration into the base metal. Each additional layer adds significantly to the total cost, so there is a strong incentive to minimize the number of weld layers. Two ways to do this follow:

• Where the overlay composition is a chromiumbearing alloy containing iron, use a low iron, high alloy filler metal. This suggestion might be implemented, for example, by substituting filler metals such as ERNiCrMo-3, ERNiCrMo-4 or ERNiCrMo-10 in an overlay weld which calls for alloys such as 825 or G-3.

• For nickel and nickel-copper overlays, do not specify lower iron limits than are needed for satisfactory corrosion performance.

Base metal interface — The second concern is the interface profile. Ideally the interface profile perpendicular to the direction of welding should be a nearly straight line, free of “spikes” of base metal between weld beads. An uneven profile is more prone to weld cracking and may fail to pass a side bend test which is often required by codes.