Nickel-Chromium Alloys
Overview
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The addition of about 10% or more chromium to nickel yields alloys that
are considerably more resistant to oxidizing conditions than nickel
itself.
The prototype alloy in this category of nickel-chromium materials is
Alloy 600 (UNS N06600, "Inconel 600" /TM). It contains about 76% Ni and
15% Cr.
Note : Because, historically, ferrochromium was used to obtain the
chromium content (so as to reduce costs) there is also some iron present
and Alloy 600 nominally contains 8% iron.
Annealed UNS N06600 is a single-phase alloy with an austenitic
structure; but by the addition of small amounts of aluminum, titanium
and niobium (columbium) an age-hardenable alloy (UNS N07750, "Inconel
X-750" /TM) with mechanical properties superior to Alloy 600 may be
made.
Other
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Another variant is Alloy 601 (UNS N06601) which has a somewhat lower
carbon content than Alloy 600 and contains slightly more chromium.
There are a number of other alloys that fall into the category of
nickel-chromium alloys (e.g., Inconel 690 / TM, Cabot Alloy 214 /TM,
etc...) but not all of them are of equal importance for conventional
industrial applications.
Nickel-Chromium Castings
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Nickel-chromium castings are widely used for coil supports, etc., in
furnaces and boilers fired by fuels containing vanadium, sulfur, and
sodium compounds as impurities (e.g., residual fuel oil).
Two compositions have been widely available in the past : 50% Ni - 50%
Cr, and 40% Ni - 60% Cr. A more recent development is 49% Ni - 49% Cr
with about 1.5% niobium (Alloy IN-657). The cast equivalent of UNS
N06600 is ACI CY-40 found in ASTM specifications A494, A743 and A744.
A number of alloys have also been developed from the basic 80% Ni - 20%
Cr composition (e.g., "Nimonics" 75 or 80 /TM), and alloys like the
high-silicon Ni-Cr Alloy 705 (60.5% Ni, 15.5% Cr, 5.5% Si, and Fe 11%
max.) provide castings of high hardness with excellent galling and wear
resistance.
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