Inconel 600.pdf

MMPDS-01
31 January 2003
6.3 NICKEL-BASE ALLOYS
6.3.0 GENERAL COMMENTS — Nickel is the base element for most of the higher temperature heatresistant
alloys. While it is more expensive than iron, nickel provides an austenitic structure that has greater
toughness and workability than ferritic structures of the same strength level.
6.3.0.1 Metallurgical Considerations
Composition — The common alloying elements for nickel are cobalt, iron, chromium, molybdenum,
titanium, and aluminum. Cobalt, when substituted for a portion of the nickel in the matrix, improves hightemperature
strength; small additions of iron tend to strengthen the nickel matrix and reduce the cost;
chromium is added to increase strength and oxidation resistance at very high temperatures; molybdenum
contributes to solid solution strengthening. Titanium and aluminum are added to most nickel-base heat
resistant alloys to permit age-hardening by the formation of Ni 3 (Ti, Al) precipitates; aluminum also contributes
to oxidation resistance.
The nature of the alloying elements in the age-hardenable nickel-base alloys makes vacuum melting
of these alloys advisable, if not mandatory. However, the additional cost of vacuum melting is more than
compensated for by the resulting improvements in elevated-temperature properties.
Heat Treatment — The nickel-base alloys are heat treated with conventional equipment and fixtures
such as would be used with austenitic stainless steels. Since nickel-base alloys are more susceptible to sulfur
embrittlement than are iron-base alloys, it is essential that sulfur-bearing materials such as grease, oil, cutting
lubricants, marking paints, etc., be removed before heat treatment. Mechanical cleaning, such as wire brushing,
is not adequate and if used should be followed by washing with a suitable solvent or by vapor degreasing.
A low-sulfur content furnace atmosphere should be used. Good furnace control with respect to time and
temperature is desirable since overheating some of the alloys as little as 35EF impairs strength and corrosion
resistance.
When it is necessary to anneal the age-hardenable-type alloys, a protective atmosphere (such as
argon) lessens the possibility of surface contaminations or depletion of the precipitation-hardening elements.
This precaution is not so critical in heavier sections since the oxidized surface layer is a smaller percentage
of the cross section. After solution annealing, the alloys are generally quenched in water. Heavy sections
may require air cooling to avoid cracking from thermal stresses.
In stress-relief annealing of a structure or assembly composed of an aluminum-titanium hardened
alloy, it is vitally important to heat the structure rapidly through the age-hardening temperature range, 1200EF
to 1400EF (which is also the low ductility range) so that stress relief can be achieved before any aging takes
place. Parts which are to be used in the fully heat-treated condition would have to be solution treated, air
cooled, and subsequently aged. In this case, the stress-relief treatment would be conducted in the solutiontemperature
range. Little difficulty has been encountered with distortion under rapid heating conditions, and
distortion of weldments of substantial size has been less than that observed with conventional slow heating
methods.
6.3.0.2 Manufacturing Considerations
Forging — All of the alloys considered, except for the casting compositions, can be forged to some
degree. The matrix-strengthened alloys can be forged with proper consideration of cooling rates, atmosphere,
etc. Most of the precipitation-hardenable grades can be forged, although heavier equipment is required and
a smaller range of reductions can be safely attained.
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