Inconel 718 is age hardenable because of the inclusion of
columbium instead of titanium and aluminium as present in other nickel base age
hardenable alloys. Studies have described that Inconel 718 is hardened
basically by Ni3(Al-Ti-Cb), a columbium bearing gamma prime. It is special in
this fact. Due to its particular age hardening properties, Inconel 718 is
welded in the age hardened condition and in more highly restrained conditions
than the other alloys. In regions up to minimum 0.5 inch it is not essential to
use stress-relief treatments after welding. Some heat affected zone softening
is occurred when welding the hardened alloy. The sluggish precipitation of
submicroscopic age hardening columbium bearing compounds allows annealing and
welding without extreme hardening during heating and cooling.
The usual joint preparation and cleaning needs are feasible
to Inconel 718. Fusion welding often is done by the inert gas tungsten gas
tungsten arc process. Good gas coverage on face and root of the joint is
needed. Various filler metals are evaluated for Inconel 718. Inconel 718 and
Hastelloy X are some of them. When the joints are restrained, Rene 41 is the
recommended filler metal because of the higher melting point.
General electric has also evaluated the weldability of
Inconel 718 by using a circular patch weld restraint test. The outcomes showed
that the alloy could be welded and heat processed under restraint without
cracking. The material was 0.063 inch thick was the filler metal used. Tensile
and rupture properties were determined. The joint efficiencies received for
tensile and 100 hour rupture life were very good, about 92% for tension tested
and 88% for rupture tested welds. The welding procedures were not mentioned.
Welding of thick Inconel bar by a multipass welding
technique and the manual inert gas tungsten arc process is evaluated.
Sufficient weld joints were made up to 0.5 inch thick. Hastelloy R 235 filler
metal provided the best tensile and rupture properties. It was noticed that
argon gas in the torch and backup was not sufficient for thicknesses above
0.125 inch. Helium was recommended for the thicker material. Helium allowed
better penetration and provided less porosity. Double U- joints were chosen to
aid penetration. The weldability of Inconel 718 has made it an attractive alloy
for use where age hardenable nickel base alloys are required.
Resistance Welding
The standard methods
of resistance welding are useful for the welding of nickel base precipitation hardening
alloys. Low thermal conductivity and high resistivity of nickel base alloys as
compared with steel should be taken into consideration when using the welding
cycles. The complex heat treatments and high strength of nickel base alloys at
high temperatures also influence the welding factors. Cleanliness cannot be
over emphasized.
Normally these alloys can be welded using the same welding
factors as are implemented for the non-heat treatable nickel alloys. Commonly more
pressure and less current is needed. It is due to higher strength and supreme
resistance of these alloys. Inadequate pressure results into cracking.
Extensive current supply causes ejection of the molten metal.
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