| 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. |
