7121 Welding

The welding of nickel base alloys requires some basic precautions regarding cleanliness of the weld zone, and one should take into account the more sluggish nature of welding products along with the low penetration in the base metal. Cleanliness of the weld joint is the single most important parameter for producing a sound weldment. Lack of thorough cleaning has accounted for a majority of the problems encountered in the industry, that is, cracking, porosity and accelerated corrosion. The contaminants to watch out for prior to welding are carbon, oxides, sulfur, lead, phosphorous, and other elements that form low-melting-point eutec-tics with nickel such as tin, zinc, bismuth, antimony, and arsenic. These could come from a variety of sources such as marking crayons, temperature-indicating sticks, machining oil, grease, oil mist from compressors, shop dirt, and other sources. The importance of thorough cleaning prior to welding cannot be overemphasized. Cleaning of the base metal in the weld area (both sides of the weld joint) should be carried with acetone or other suitable cleaners. Nowadays, due to environmental restrictions, the use of trichloroethylene (TRI), perchloroethy-lene (PER), and carbon tetrachloride (TETRA) is prohibited in many countries. Grinding with a clean alumina grinding wheel, at least 2 in. from either side of the weld joint has also been used for cleaning prior to welding. It is very important that the grinding wheels used are brand new and have not been previously used on either stainless steel or carbon steel.

Due to the viscous nature of molten nickel-alloy weld metal, increased joint angle and using relatively thinner land to compensate for low penetration are often necessary to produce good-quality welds. A misconception by welders unfamiliar with nickel alloys is to attempt to make the weld-metal flow by increasing the amperage (heat input) above the recommended range. Excessive amperage will not improve the welding characteristics or flowability of the weld metal but may cause mechanically unsound and metallurgically poor corrosion-resistant welds. Table 7.19 gives the basic guidelines of the weld parameters for various welding

TABLE 7.19 Welding Parameter Guidelines for Alloy 59 and Other Alloys

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