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AWS - Chapter 4 - Nickel and Cobalt Alloys

Organization: AWS
Publication Date: 1 January 2015
Page Count: 74
scope:

Nickel and cobalt alloys provide unique combinations of physical and mechanical properties and exceptional resistance to corrosion attack. If the need for these extraordinary properties were not critical to a variety of industrial applications, these alloys probably would not be manufactured because of high cost. Nickel and cobalt alloys resist corrosive attack in various media at temperatures up to 1100°C (2000°F) and maintain good mechanical strength in both lowtemperature and high-temperature environments. For use in these demanding service conditions, welds in nickel and cobalt must duplicate the attributes of the base metal to a very high degree; thus, conditions for welding, heat treating, and fabrication procedures must be established with these attributes included. High-quality weldments are readily produced in nickel-base and cobalt-base alloys by commonly used welding processes, although 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 and cobalt alloys are similar to those used for stainless steel, except that the molten weld metal is more sluggish, which requires more accurate placement of the weld metal in the joint. The thermal expansion characteristics of nickel and cobalt alloys approximate those of carbon steel but they are more favorable than those of stainless steel. Warping and distortion during welding is not severe. The mechanical properties of nickel-base and cobaltbase metals vary, depending on the amount of hot work or cold work applied to the finished form (sheet, plate, or tube). Some modifications in the welding procedures can be made if the base metal is not in the fully annealed condition. The properties of welded joints in fully annealed nickel and cobalt alloys should be similar to those of the base metals. Post-weld treatment is usually not needed to maintain or restore corrosion resistance in most nickel and cobalt alloys. Welds made in nickel-molybdenum Alloy B (N10001) and nickel-silicon cast alloys are commonly solution annealed after welding to restore corrosion resistance to the heat-affected zone (HAZ).1, 2 Rapid cooling from the solution annealing temperature is necessary to avoid formation of detrimental metallurgicalphases, which can have an effect on mechanical properties and corrosion resistance. Filler metals that are more highly alloyed than the base metals are often used (sometimes in lieu of postweld heat treatment) to fabricate components for service in very aggressive corrosive environments. The overmatching composition offsets the effects of weld metal segregation that occurs when a matching composition is used. Examples are the use of nickel-chromium- molybdenum NiCrMo-14 (N06686) filler metal to weld the super austenitic stainless steel alloys (those containing 4% to 6% molybdenum) and Alloy C-276 (N10276) base metal. Nickel and nickel-alloy filler metal designations for welding electrodes for shielded metal arc welding and bare welding electrodes and rods in this chapter are based on American Welding Society specifications.3, 4 Post-weld heat treatment may be required for precipitation- hardening (also called age hardening) in specific alloys. Post-weld stress relief may be necessary to avoid stress-corrosion cracking in applications involving hydrofluoric acid vapor or certain caustic solutions. For example, nickel-copper Alloy 400 (N04400) immersed in hydrofluoric acid is not sensitive to stress-corrosion cracking, but becomes sensitive when exposed to the aerated acid or the acid vapors Most of the commonly used joining processes are appropriate for nickel and cobalt alloys. The choice of welding process should be based on the following parameters: 1. The specific alloy to be welded; 2. Thickness of the base metal; 3. Design requirements of the structure (i.e., temperature, pressure, and type of stresses); 4. Welding position; 5. Need for fixtures and positioners; and 6. Service conditions and environment.

abstract:

Nickel and Cobalt Alloys

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