Ni-Cr-Mo Alloys
General
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The prototype alloys of this group was "Hastelloy C" [/TM] (UNS N10002,
or Alloy C). Alloy C is a nickel alloy with about 15% Cr and 15% Mo
(plus about 60% Ni, 5% W, and 5% Fe), which was developed for enhanced
resistance to both oxidizing environments and strongly reducing acids.
It was so corrosion resistant that it set a standard to which other
metals and alloys were often compared. Alloy C can be produced in
wrought and cast form. Fabrication, machining and welding present no
unusual problems, except that it tends to work-harden.
Inert-gas-shielded or metal-arc-welding processes are usually
recommended. The original Hastelloy C is now almost obsolete, and is
replaced by a number of variants : the composition of the alloy has seen
several modifications that have increased metallurgical stability and
resistance to localized corrosion in the heat affected zones of welds.
In addition, other nickel-chromium-molybdenum alloys have been developed
with excellent corrosion resistance.
Alloy C-276, C-4, and C-22
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An improved version of the original Hastelloy C with a higher resistance
to intergranular corrosion is Alloy C-276 (UNS N10276). A superior
variant is Alloy C-4 (UNS N06455). Both alloys are low-carbon versions
of the original material. An even newer alloy is Alloy C-22, whose
higher chromium content extends its usefulness.
Alloy 625
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Alloy 625 is also a Ni-Cr-Mo alloy. It is an improved variant of Alloy
600 to which about 8% of molybdenum has been added and where the
chromium content has been raised to about 23%. Although this alloy is
resistant in many corrosive services (e.g., seawater), it is usuallly
somewhat less resistant than Alloy C-276. MORE on: Alloy 625
Other
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"ALLCORR" (TM Teledyne Alvac), UNS N06110 INCONEL 617 (TM Inco), UNS
N06617
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