Welding-nickelfor special benefits.SOLUTIONS with Effective, Practical AdviceWelding-nickel and nickel alloys are important subjects because welding is required for fabrication of specific applications, resistant to Heat and/or Corrosion, using any of the large number of alloys that were developed for their important properties. A previous page dedicated to heat resisting alloys includes also a discussion of some Cobalt alloys that have suitable properties for certain applications. See Heat Resisting Alloys Welding. Nickel and nickel alloys in wrought form are quite ductile, but certain alloys, developed primarily for high temperature service, resist deformation even when hot (they were designed that way). Cast nickel alloys may be difficult to weld, especially those with high silicon. Cast nickel alloy blades for gas turbine engines, with directionally solidified or single crystals and air cooling channels cannot be welded by traditional means. Surface restoration of damaged expensive parts has been attempted with high energy beams. See the Laser fused metal powders article, published (2) in issue 43 of Practical Welding Letter for March 2007. Welding-nickel is done mostly with arc welding and with high energy processes. Oxyacetylene welding should not be attempted. Two general classes of nickel alloys are available. Those that derive their strength properties from their composition are known as Solid Solution alloys and are generally serviceable in as welded condition. The Precipitation Hardenable alloys develop their remarkable mechanical properties in consequence of delicately balanced heat treatments performed in view of developing suitable microstructures capable of exceptional strength even at high heat. These alloys (usually containing small amounts of aluminum and titanium), if welded, need that the complete heat treatment cycle be performed, including solutioning and precipitation, in order to exhibit the special properties they are capable of. Cleaning is most important in Welding-nickel, as at molten metal temperature many contaminants like sulfur, lead and others, as well as oil, paint and dirt if present on the surface, may find their way into the weld with harmful consequences. Oxides should also be removed. Preheating is usually not needed except for avoiding water condensation in cold weather. Postweld treatment is normally not required to restore corrosion resistance, however full solution treatment is sometimes recommended. Joint design is quite normal except that special attention should be paid to accessibility, so that the opening is sufficient for electrode or torch to reach the bottom of the joint. As penetration tends to be lower, when Welding-nickel alloys are compared to steels, thinner lands should be designed in the joint root. Beveling is needed only for material thicker than about 2.4 mm except if Welding-nickel with high energy processes (Electron- and Laser-Beam), or with Plasma arc over a certain thickness. Fixtures and clamping tools should be used to keep aligned components in place, minimize buckling and reduce distortion. Backing bars should be made of copper, and include a contoured groove to permit penetration and avoid gas entrapment. If thin nickel alloy sheet metal parts are fixtured with firm hold-down force, the thermal expansion may set in the weld favorable compressive stresses. For Shielded Metal Arc Welding-nickel, (SMAW), weaving of the electrode is recommended, to melt and wet the sides of the groove. This is because molten nickel metal is not free-flowing as iron base alloys, and therefore it must be deposited where needed. All beads should show slightly convex profile. Gas Tungsten Arc Welding-nickel alloys (GTAW), both manual and mechanized, is performed with Direct Current with Electrode Negative (DCEN - Straight Polarity). Argon is the shielding gas usually selected, while helium or mixtures of the two may be used in special cases. Filler metal are normally similar in composition to the base metals to join. 2% Thoriated tungsten electrodes are preferably selected. They should not make contact the base metal. High frequency circuits permit arc starting without touching and contaminating the electrode. Arc length should be kept as short as practicable. The hot end of filler metal should be kept always in the shielding gas. The underside of groove welds has to be protected from oxidation by having shielding gas flowing from the backing bar. Gas Metal Arc Welding-nickel alloys is used when greater deposition rates are needed than available with GTAW. The metal transfer mode should be selected as suitable for the application. Direct Current with Electrode Positive (DCEP - Reverse Polarity) should be used. Filler metal should be selected as required by application and depending on the type of base metal. Flux-Cored Arc Welding uses special filler metal wires that contain a flux inside. They may provide better oxidation protection and increased productivity. For Welding-nickel, shielding gas is generally used as an additional precaution against air contamination, besides the flux. Plasma Arc Welding (PAW) is used when special benefits can be obtained from using the keyhole technique, without adding filler metal up to about 9 mm (0.3 in). Overlaying is the process of Welding-nickel alloys on the surface of less expensive material like steel. Iron dilution should be kept to a minimum, by using buffer layers. High deposition rate processes are used, like GMAW and Submerged Arc Welding (SAW). This last process must be performed by keeping the weld flat, so that rotating positioners must be employed for the internal overlaying of large cylinders. If iron dilution is not kept under control, cracking may occur in the weld. If this happens, all precautions must implemented to reduce it or to better insulate the last layers from the firsts. The following American Welding Society documents classify the nickel base welding filler metals: ANSI/AWS A5.11/A5.11M:2005 ANSI/AWS A5.14/A5.14M:2005 ANSI/AWS A5.34/A5.34M-2007
Specification for Nickel-Alloy Electrodes for Flux Cored ArcWelding The following Tables report selected designations and approximate nominal compositions. For exact limits and requirements refer to the official AWS documents.
S=0.015-0.03
S=0.015-0.04 An Article on Filler Metals for Welding Nickel was published (4) in Issue 62 of Practical Welding Letter for October 2008. An Article on Welding Nitinol was published (8) in Issue 66 of Practical Welding Letter for February 2009. Click on PWL#066 to see it. A new page on Welding Nitinol was recently added to this Website. Click on the link to read it.
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