Nickel alloys can be joined reliably by all types of welding processes
or methods, with the exception of forge welding and oxyacetylene welding.
The wrought nickel alloys can be welded under conditions similar to those
used to weld austenitic stainless steels. Cast nickel alloys, particularly
those with a high silicon content, present difficulties in welding.
The most widely employed processes for welding the non-age-hardenable
(solid-solution-strengthened) wrought nickel alloys are gas-tungsten arc
welding (GTAW), gas-metal arc welding (GMAW), and shielded metal arc
welding (SMAW). Submerged arc welding (SAW) and electroslag welding (ESW)
have limited applicability, as does arc plasma welding (PAW). Although the
GTAW process is preferred for welding the precipitation-hardenable alloys,
both the GMAW and SMAW processes are also used.
Nickel alloys are usually welded in the solution-treated condition.
Precipitation-hardenable (PH) alloys should be annealed before welding
if they have undergone any operations that introduce high residual
Postweld Treatment. No postweld treatment, either thermal or
chemical, is required to maintain or restore corrosion resistance,
although in some cases a full solution anneal will improve corrosion
resistance. Heat treatment may be necessary to meet specification
requirements, such as stress relief of a fabricated structure to
avoid age hardening or stress-corrosion cracking (SCC) of the weldment
in hydrofluoric acid vapor or caustic soda. If welding induces
moderate-to-high residual stresses, then the PH alloys would
require a stress-relief anneal after welding and before aging.
Nickel and nickel alloys are susceptible to embrittlement by lead,
sulfur, phosphorus, and other low-melting-point elements. These materials
can exist in grease, oil, paint, marking crayons or inks, forming
lubricants, cutting fluids, shop dirt, and processing chemicals.
Work-pieces must be completely free of foreign material before they are
heated or welded. Shop dirt, oil and grease can be removed by either vapor
degreasing or swabbing with acetone or another nontoxic solvent. Paint
and other materials that are not soluble in degreasing solvents may
require the use of methylene chloride, alkaline cleaners, or special
proprietary compounds. If alkaline cleaners that contain sodium
carbonate are used, then the cleaners themselves must be removed
prior to welding. Spraying or scrubbing with hot water is recommended.
Marking ink can usually be removed with alcohol.
Processing material that has become embedded in the work metal can be
removed by grinding, abrasive blasting, and swabbing with 10% HCl solution,
followed by a thorough water wash. Oxides must also be removed from the
area involved in the welding operation, primarily because of the difference
between the oxide and base metal melting points. Oxides are normally
removed by grinding, machining, abrasive blasting or pickling.
Nickel alloys, both cast and wrought and either solid-solution-strengthened
or precipitation-hardenable, can be welded by the GTAW process. The
addition of filler is usually recommended. Direct current electrode
negative (DCEN) is recommended for both manual and machine welding.
Shielding Gas. Either argon or helium, or a mixture of the two,
is used as a shielding gas for welding nickel and nickel alloys. Additions
of oxygen, carbon dioxide, or nitrogen to argon gas will usually cause
porosity or erosion of the electrode. Argon with small quantities of
hydrogen (typically 5%) can be used and may help avoid porosity in pure
nickel, as well as aid in reducing oxide formation during welding.
Welding of Precipitation Hardenable Alloys
The PH alloys require special welding procedures because of their
susceptibility to cracking. Cracks can occur in the base-metal
heat-affected zone (HAZ) upon aging or in service at temperatures
above the aging temperature, as a result of residual welding stress
and stress induced by precipitation.
Before welding these alloys, a full-solution anneal is usually performed.
After welding, the appropriate aging heat treatment is performed. To
further improve alloy properties, a full anneal after welding, followed
by a postweld heat treatment, can be incorporated in the welding procedure.
Preweld and Postweld Treatments. Any part that has been subjected
to severe bending, drawing or other forming operations should be annealed
before welding. If possible, heating should be done in a controlled
atmosphere furnace to limit oxidation and minimize subsequent surface
General Welding Procedures. Precipitation-hardenable alloys are
usually welded by the GTAW process, but SMAW and GMAW processes are also
applicable. Heat input during the welding operations should be held to a
moderately low level in order to obtain the highest possible joint
efficiency and minimize the extent of the HAZ. For multiple-bead or
multiple-layer welds, many narrow stringer beads should be used, rather
than a few large, heavy beads. Any oxides that form during welding should
be removed by abrasive blasting or grinding. If such films are not removed
as they accumulate on multiple-pass welds, then they can become thick
enough to inhibit weld fusion and produce unacceptable laminar type oxide
stringers along the weld axis.
Welding of Cast Nickel Alloys
Cast nickel alloys can be joined by the GTAW, GMAW and SMAW processes. For
optimum results, casting should be solution annealed before welding to
relieve some of the casting stresses and provide some homogenization of
the cast structure. Light peening of solidified metal after the first pass
will relieve stresses and, thus, reduce cracking at the junction of the
weld metal and the cast metal. The peening of the subsequent passes is of
little, if any, benefit. Stress relieving after welding is also desirable.
Minimizing Weld Defects
The defects and metallurgical difficulties encountered in the
arc welding of nickel include:
- Susceptibility to high-temperature embrittlement by sulfur
and other contaminants
- Cracking in the weld bead, caused by high heat input
and excessive welding speeds
- Stress-corrosion cracking in service.
Oxygen carbon dioxide, nitrogen, or hydrogen can cause
porosity in welds. In the SMAW and SAW processes porosity can be minimized
by using electrodes that contain deoxidizing or nitride forming elements,
such as aluminum and titanium. These elements have a strong affinity for
oxygen and nitrogen and form stable compounds with them. Presence of
deoxidizers in either type of electrode serves to reduce porosity. In
addition, porosity is much less likely to occur in chromium-bearing nickel
alloys than in non-chromium-bearing alloys.
In the GMAW and GTAW processes, porosity can be avoided by preventing the
access of air to the molten weld metal. Gas backing on the underside of
the weld is sometimes used. In the GTAW process the use of argon with up
to 10% H2 as a shielding gas helps to prevent porosity. Bubbles of hydrogen
that form in the weld pool gather the diffusing hydrogen. Too much
hydrogen (>15%) in the shielding gas can result in the hydrogen porosity.
Cracking. Hot shortness of welds can result from contamination by
sulfur, lead, phosphorus, cadmium, zinc, tin, silver, boron, bismuth, or
any other low-melting-point elements, which form intergranular films and
cause severe liquid-metal embrittlement at elevated temperatures. Many of
these elements are found in soldering and brazing filler metals.
Hot cracking of the weld metal usually results from such contamination.
Cracking in heat-affected zone is often caused by intergranular penetration
of contaminants from the base-metal surface. Sulfur, which is present
in most cutting oils used for machining, is a common cause of cracking
in nickel alloys.
Weld metal cracking also can be caused by heat input that is too high,
as a result of high welding current and low welding speed. Welding speeds
have a large effect on the solidification pattern of the weld. High welding
speeds create a tear-drop molten weld pool, which leads to uncompetitive
grain solidification at the center of the weld. At the weld centerline,
residual elements will collect and cause centerline hot cracking or lower
transverse tensile properties.
In addition, cracking may result from undue restraint. When conditions
of the high restraint are present, as in circumferential welds that are
self-restraining, all bead surfaces should be slightly convex. Although
convex beads are virtually immune to centerline splitting, concave beads
are particularly susceptible to centerline cracking. In addition,
excessive width-to-depth or depth-to-width ratios can result in cracking
may be internal (that is subface cracking).
Stress Corrosion Cracking. Nickel and nickel alloys do not
experience any metallurgical changes, either in the weld metal or in
the HAZ, that affects normal corrosion resistance. When the alloys are
intended to contact substances such as concentrated caustic soda,
fluorosilicates, and some mercury salts, however, the welds may need
to be stress relieved to avoid stress corrosion cracking. Nickel alloys
have good resistance to dilute alkali and chloride solutions. Because
resistance to stress-corrosion cracking increases with nickel content,
the stress relieving of welds in the high-nickel-content alloys is not
Effect of slag on weld metal. Because fabricated nickel alloys are
ordinarily used in high-temperature service and in aqueous corrosive
environments, all slag should be removed from finished weldments. If slag
is not removed in these type of application, then crevices and accelerated
corrosion can result. Slag inclusions between weld beads reduce the
strength of the weld. Fluorides in the slag can react with moisture
or elements in the environment to create highly corrosive compounds.