Nickel and Nickel Alloys

Honestly, this week is very busy and very hard week for me, but some how I have to spend some times to post this Piping & Fabrication blog, and now we comes to Nickel and Nickel Alloys. Nickel and its alloys can be welded by SMAW, GTAW, and GMAW. SAW is limited to certain compositions. Welding is similar to austenitic stainless steels except that the molten metal is more sluggish and does not wet as well. Larger groove angles may be required. Preheat is not required, but welding at temperatures below 60°F (16°C) in the presence of moisture is not recommended. A low interpass temperature is suggested. For GTAW welding shielding gas is normally argon, but helium or an argon-helium mix may be used. The inside surface of GTAW root welds should be shielded with an inert gas. GMAW in the spray, pulsed, globular, or short-circuiting modes may be used with argon or argon-helium mixtures as shielding. Postweld heat treatment is not usually required. Many nickel and nickel alloys may be used down to -325°F (-199°C).

Titanium. Titanium and its alloys are normally welded using the GTAW and GMAW processes. It is vital that the HAZ and molten metal be protected from the atmosphere by a blanket of inert gas during welding. Most welding is done in a protective chamber purged with an inert gas or by using trailing shields. Precleaning is extremely important. Use of degreasers, stainless steel wire brushes, or chemical solutions may be required. Preheating or postweld heat treatment are not normally required. 

Dissimilar Metals. Until now we have discussed welding where both items being joined are essentially the same material and are joined with a filler metal of similar chemistry and physical properties. Occasions arise where metals of different chemical composition and physical properties must be joined. In joining dissimilar metals, normal welding techniques may be employed if the two base metals have melting temperatures within about 200°F (95°C) of each other. Otherwise different joining techniques are required. In designing a welding procedure for dissimilar metals, a great many factors must be considered. Service conditions such as temperature, corrosion, and the degree of thermal cycling may apply. The effects of dilution of the two base metals by the filler and each other must be evaluated to assure a sound weld with suitable chemical, physical, metallurgical, and corrosion-resistant properties. Similarly, preheat and postweld heat treatment requirements for one base metal may not be suitable for the other. It is usually necessary to qualify a separate welding procedure for the particular combination of base metals and filler material. ASME Section IX should be consulted for specifics. As a general rule, when welding within a family such as ferritic to ferritic, austenitic to austenitic, or nickel alloy to nickel alloy, the filler metal may be of the same nominal composition as either of the base metals or of an intermediate composition. The filler metal normally used to weld the lower alloy is most often preferred.

The previous advice may not always hold true. It has been noted that when welding P 22 (21⁄4Cr-1Mo) to P 91 (9Cr-1Mo-V) using 21⁄4Cr filler metal at high temperatures, carbon migration from the 21⁄4Cr weld metal to the 9Cr base metal can produce a carbon-denuded zone at the interface, resulting in a weakened area. One recommendation is to ‘‘butter’’ the 9Cr side with a 5Cr filler metal, heat-treat the buttered segment, and complete the weld with 21⁄4Cr. Bear in mind that the 5Cr filler may not have high-temperature properties similar to the 21⁄4Cr, and design the weldment accordingly. In welding dissimilar materials, selection of preheating and postweld heat treatment requires a great deal of care. What is desirable for one metal may be detrimental to another. Some compromise may be required. Establishing a welding procedure for welding ferritic to austenitic steels requires careful consideration of the service conditions. For moderate service temperatures (below 800°F or 427°C), where the thickness of the ferritic side does not require postweld heat treatment, austenitic stainless steel electrodes are often the choice. Some prefer electrodes such as type 309 or 310 because of their higher chrome content. Because of the thickness involved, the ferritic member may require some type of postweld heat treatment. In this case the preferred method is to butter the ferritic weld surface with a nickel-chrome-iron (NiCrFe) filler metal such as ERNiCrMo-3 (see ASME Section II Part C SFA-5.14) and postweld heat-treat the buttered section as required for the ferritic composition. The buttered section is then prepared for welding, set up with the austenitic side, and the weld between the butter and austenitic base metal is completed with NiCrFe filler metal without subsequent postweld heat treatment. For high-temperature service (above 800°F or 427°C) the buttering procedure just described is also recommended. There is a difference in coefficients of expansion between the ferritic and austenitic metals. This difference will result in expansion stresses above the yield point at the weld juncture while at operating temperature.

At higher temperatures there is also greater probability of diffusion of carbon from the ferritic side to the austenitic side. The NiCrFe ‘‘butter’’ minimizes the carbon diffusion problem and has an expansion coefficient which is intermediate between the two base metals, thus reducing but not eliminating the thermal stress at the interface. Where a transition from ferritic to austenitic steels is required in high temperature applications involving cyclic services, a transition piece of a highnickel alloy such as UNS N06600 with two welds is often used to reduce thermal fatigue damage. In welding nonferrous metals to ferrous or other nonferrous metals, a filler metal with a melting point comparable to the lower melting point base metal is usually recommended.

Nickel and nickel alloys are invariably welded to ferrous metals with nickelalloy filler metals. Sulfur embrittlement can be a problem with nickel to ferritic welds, just as it is in nickel-to-nickel welds. Copper-nickel and nickel-copper alloys should not be joined with filler materials containing iron or chromium since hot cracking may result. Copper and copper alloys can be welded to carbon steel with silicon bronze or aluminum bronze electrodes, but the preferred method is to butter the carbon steel side with nickel and weld the copper to the nickel butter with nickel filler. This will preclude hot cracking of the copper because of iron dilution. The copper side may require preheat. Copper can easily be welded to nickel, copper-nickel, or nickelcopper filler metal. When welding nickel alloys which contain iron or chromium to copper, the nickel alloy should be buttered with nickel. Aluminum and titanium generally cannot be welded to ferrous or other nonferrous metal using currently available welding procedures, and special joining procedures must be employed.