What is Nickel Alloy?
High-quality joints are readily produced in nickel alloys by conventional welding processes. However, some of the characteristics of nickel alloys necessitate the use of somewhat different techniques than those used for commonly encountered materials such as carbon and stainless steels.
The choice of welding process is dependent upon many factors. Base metal thickness, component design, joint design, position in which the joint is to be made, and the need for jigs or fixtures all must be considered for a fabrication project. Service conditions and corrosive environments to which the joint will be exposed and any special shop or field construction conditions and capabilities which might be required are also important.
The first consideration in designing joints for nickel alloys is to provide proper accessibility. The joint opening must be sufficient to permit the torch, electrode, or filler metal to extend to the bottom of the joint.
In addition to the basic requirement of accessibility, the characteristics of nickel alloy weld metal necessitate the use of joint designs that are different than those commonly used for ferrous materials. The most significant characteristic is the sluggish nature of the molten weld metal. Nickel alloy weld metal does not flow or spread as readily as steel weld metal. The operator must manipulate the weld puddle so as to direct the weld metal to the proper location in the joint. The joint must, therefore, be sufficiently open to provide space for movement of the torch or filler metal. The importance of producing slightly convex beads cannot be overemphasized. The joint design chosen must allow for the first weld bead to be deposited with a convex surface. Small included angles and narrow roots induce concave beads and often lead to centerline cracking.
Another different characteristic is the lower weld penetration encountered when welding nickel alloys. This is caused by the physical properties of nickel alloys and must be considered in the weld design. The lower penetration makes necessary the use of smaller lands in the root of the joint. Increases in weld current will not significantly increase the penetration of the arc. Excessive weld current when shielded metal arc welding can cause overheating of covered electrodes such that the flux spalls off and the deoxidizers in the flux are destroyed. The use of excessive heat with gas shielded processes results in weld spatter and overheating of the welding equipment. With proper joint selection and design, the welding product can be effectively used within the recommended current ranges and a sound, full penetration weld deposited.
