MATERIALS Q&A BY THOMASSPENCE
QUESTION: How do I know
what the best type of Ni-Cr-Mo
alloy will be for a particular
application?
ANSWER: This is not an easy question
to answer since each alloy has its own
strengths and weaknesses. Ni-Cr-Mo
alloys are best known as the Hastelloy C
type alloys in the wrought form, but they
are less recognizable when their ASTM
cast designations are used. These C type
alloys are nickel-based and contain 15%
to 24% chromium and 12.5% to 20%
molybdenum with the percentages of
chromium and molybdenum a little different for each alloy. Some of the C type
alloys also contain small percentages of
tungsten, vanadium or copper.
Confusion over which grade is best
stems from the fact there are so many
grades of the C type alloys. In the
wrought form, there is UNS N10276
(Hastelloy C-276), UNS N06455
(Hastelloy C- 4), UNS N06022 (
Hastelloy C- 22), UNS N06200 (Hastelloy C-
2000), and then there is UNS N06059
(Alloy 59) from ThyssenKrupp VDM. For
castings, ASTM specification A494 for
nickel-based castings gives us
CW12MW, CW6M, CW2M, CW2MW
and CX2M. This expanse of choices
makes it difficult for a user to know what
grade they really need. Likewise, for suppliers, it is not good business practice to
stock or to offer all these grades. Let’s
look at each alloy individually.
CW12MW is the old original cast
Hastelloy C alloy. Its higher carbon limit
of 0.12% maximum along with its higher
iron and tungsten levels makes this alloy
inferior to any of the other grades in corrosion resistance and ductility.
CW12MW is not the cast equivalent for
UNS N10276.
CW6M is the next cast Ni-Cr-Mo alloy
developed. It was introduced in 1949 so
there is a long, successful history for this
alloy. While it does have a higher allowable carbon and silicon limit than the
newer C types listed below, it is typically
made with much lower carbon and silicon
levels, thus giving it good corrosion
resistance to both oxidizing and reducing
acids.
CW2M is the cast equivalent of UNS
N06455 developed in 1976 to provide
better metallurgical stability than UNS
N10276 at high temperatures. An added
benefit was that it is a better cast grade
than CW12MW. Its improved performance over CW12MW was a result of
reduced amounts of intermetallic phases
in its microstructure. This was accomplished by lowering its carbon, silicon,
iron, chromium and molybdenum levels
and eliminating the tungsten addition.
CX2MW is the cast equivalent of UNS
N06022. This alloy has a higher chromium content, but a lower molybdenum
content than the other Hastelloy C type
alloys. The higher chromium content does
give this alloy better corrosion resistance
to hot oxidizing environments such as
bleach and chlorinated brines. However,
the lower molybdenum content sacrifices
some resistance to reducing services
where these alloys are also commonly
used, e.g., sulfuric and hydrochloric
acids.
CX2M is the cast equivalent to UNS
N06059. It has slightly higher chromium
and molybdenum than CX2MW. The
higher chromium can provide good resistance to strong oxidizing services and its
higher molybdenum content can provide
good resistance to sulfuric and
hydrochloric acids.
UNS N06200 (Hastelloy C-2000) is the
newest C type alloy, but it is only available as a wrought component because its
extremely low carbon and silicon content
mean it’s not beneficial for making a cast
alloy. UNS N06200 contains a chromium content similar to UNS N06022, but
it has more molybdenum along with a
small amount of copper. The higher
molybdenum content along with the copper gives the alloy better resistance to
reducing acids like sulfuric and
hydrochloric.
CONSIDER CASTING
QUALITY
So which grade is better overall? The
answer cannot be found without first considering casting quality. These are fairly
complex alloys, and if their chemistry
and heat treatment are not optimized,
their corrosion resistance will not be sufficient. A widely held—but mistaken—
belief is that if an alloy meets the ASTM
requirements, it must be a good alloy. For
most alloys and in particular these higher, more complex alloys, an optimum
chemistry and heat treatment will give
the alloy its best corrosion resistance.
Therefore, it takes a competent foundry
to produce the best quality C type alloy.
In summary, no one alloy will perform
the best in both reducing and oxidizing
environments. Generally, the higher
chromium grades will do better in oxidizing conditions and the higher molybdenum alloys will do better in reducing
environments. Also, keep in mind that the
performance of any alloy is dependent
upon the competency of the foundry producing it, and that is particularly true for
these high nickel base alloys. VM
THOMAS SPENCE is director of materials
engineering for Flowserve Corp.
( www.flowserve.com), Dayton, OH. Reach him
at tspence@flowserve.com.
