Development of Creep-Resistant and Oxidation-Resistant Austenitic Stainless Steels for High Temperature Applications
Development of Creep-Resistant and Oxidation-Resistant Austenitic Stainless Steels for High Temperature Applications
PHILIP J. MAZIASZ 0
0 1.-Oak Ridge National Laboratory , Oak Ridge, TN 37831, USA. 2.- , USA
Austenitic stainless steels are cost-effective materials for high-temperature applications if they have the oxidation and creep resistance to withstand prolonged exposure at such conditions. Since 1990, Oak Ridge National Laboratory (ORNL) has developed advanced austenitic stainless steels with creep resistance comparable to Ni-based superalloy 617 at 800-900 C based on specially designed ''engineered microstructures'' utilizing a microstructure/composition database derived from about 20 years of radiation effect data on steels. The wrought high temperature-ultrafine precipitate strengthened (HT-UPS) steels with outstanding creep resistance at 700-800 C were developed for supercritical boiler and superheater tubing for fossil power plants in the early 1990s, the cast CF8C-Plus steels were developed in 1999-2001 for land-based gas turbine casing and diesel engine exhaust manifold and turbocharger applications at 700-900 C, and, in 2015-2017, new Al-modified cast stainless steels with oxidation and creep resistance capabilities up to 9501000 C were developed for automotive exhaust manifold and turbocharger applications. This article reviews and summarizes their development and their properties and applications.
The Oak Ridge National Laboratory (ORNL) has a
history of developing new or modified austenitic
stainless steel advanced nuclear reactor or
hightemperature creep-resistant applications.1–3 The
void-swelling-resistant D9 and Path A Prime
Candidate Alloy (PCA) of the fast breeder reactor and
magnetic fusion reactor programs, respectively, in
the 1970s and 1980s led to development of high
temperature-ultrafine precipitate strengthened
(HTUPS) steel, which was the most
void-swelling-resistant steel to date and the steel with the highest creep
resistance.3–5 The HT-UPS steel won a 1990 R&D100
Award for having creep resistance equivalent to
solid-solution Ni-based alloys like 617. In 1999, a
new cast stainless steel was developed as a laboratory
scale casting (15 lb) and then scaled up to commercial
cast heats (500 lb) based on outstanding
creep-rupture behavior at 850 C in 2001. This new steel was
designated CF8C-Plus steel and won an R&D100
Award in 2003.6 It was commercialized by Caterpillar
for regeneration of the diesel particulate filters on all
heavy-duty on-highway diesel engines from late 2006
to 2011 (550 tons), and of 45,000 units deployed for
this application, none have been reported failed to
date. This achievement received the Federal
Laboratory Consortium Award in 2009. Recently, an
aluminum-modified CF8C-Plus steel has been
developed, which has outstanding creep resistance
comparable to CF8C-Plus steel (700–1000 C), but has
oxidation resistance in air + 10% water vapor at 800–
950 C. All of these steels were designed for specific
microstructural behavior before they were tested for
other properties, and all have shown very good
properties from the first heat melted. This article
reviews their development and summarizes their
properties tested to date.
ALLOY DEVELOPMENT RESULTS—HT-UPS
In the late 1970s, the US Fusion Materials (FM)
Program selected a swelling-resistant modified 316
stainless steel called the Path A prime candidate
alloy (PCA), which was a titanium-modified
austenitic stainless steel quite similar to the D9 alloy of
the US Fast Breeder Reactor (FBR) program.7–9 As
shown in Table I, they were 14Cr-16Ni2.5Mo-2Mn
steels with Ti and C additions. In the US FM
program, more void-swelling resistance was needed
because of the higher He/dpa ratio. Special alloying
effects and microstructural engineering work were
undertaken at ORNL to further modify the PCA
alloy for improved swelling and helium
embrittlement resistance.10 The result was a triply stabilized
steel with added Ti, Nb and V in addition to small
increases in B and P, and the result was a
microstructurally stable steel termed HT-UPS steel
with extremely stable nanoscale complex MC
carbides during both reactor irradiation and
hightemperature thermal aging. Although initially
designed for irradiation effect applications, these
steels also had creep-rupture resistance for fossil
energy power applications at 650–800 C.11–13
The goal for the HT-UPS steels was to achieve
100,000 h creep rupture life at 700 C and 100 MPa,
which the strongest steels at the time, 17-14CuMo
and NF709, were not quite able to meet. Figure 1
shows the data used to win the 1990 R&D 100
Award and achieve a patent for the HT-UPS steel.14
The plot shows the American Society of Mechanical
Engineers (ASME) allowable stresses for 347H,
modified 310, NF709 and the HT-UPS steels, and
clearly the HT-UPS steel is significantly stronger.
HT-UPS steel was designed using specific rules to
control precipitation, including the (
) catalytic effects, (
) inhibitor effects and
) interference effects.1,2 In the cold-worked
condition, the HT-UPS steel had nanodispersions of
complex MC carbides that were incredibly
stable to dissolution or coarsening for creep
strength and no formation of detrimental
intermetallic compounds such as Laves and sigma
phases for long-term creep resistance (Figs. 2 and
3). Figure 2 shows the robust nano-MC with the
bright field image of analytical electron microscopy
(AEM) of the phase particle composition after
18,000 h at 700 C, and Fig. 3 shows no
intermetallics or even creep voids after > 60,000 h at
Therefore, the engineered microstructure design
for the HT-UPS steel relies on several different
creep-related microstructural mechanisms: (
formation of nanoscale dispersions of precipitates
during creep to effectively pin dislocations against
) the stability of the nanoscale
dispersions to dissolution or coarsening during
) the resistance to creep-void formation,
) the resistance of the alloy to the formation of
embrittling intermetallic phases such as Laves or
sigma, and (
) for wrought materials such as
HTUPS steel the resistance to recovery or
Commercial heats of HT-UPS tubing were made
by AMEX and by Combustion Engineering (now
Alstom/GE), and it was found to exhibit optimum
creep behavior tubes needing to be solution
annealed for 1 h at 1150–1200 C and then ‘‘mill
annealed,’’ which produces 5–15% cold work (CW).
The HT-UPS steels had only 14 Cr, so they had
oxidation problems at 750–800 C or above and had
to be clad with a higher Cr alloy, as shown in Fig. 1
with alloy 671. Lessons learned were applied to the
next generations of ORNL-modified austenitic
ALLOY DEVELOPMENT RESULTS—CAST
In 1998, ORNL formed a Cooperative Research
and Development Agreement (CRADA) project with
Caterpillar, Inc., and Solar Turbines (a Caterpillar
Company) to develop a cost-effective improved
version of standard CF8C cast austenitic stainless
steel, and in 1999, they jointly developed CF8C-Plus
stainless steel with added Mn and N. Caterpillar
needed a more heat-resistant alloy to use for diesel
exhaust components (manifolds, turbochargers,
Fig. 4) than SiMo cast iron that could operate at
750–800 C, while Solar Turbines needed a material
better than CF8C for the end-cover of the engine
(Fig. 4) for its Mercury 50 gas turbine engine. In
2001, ORNL had developed laboratory heats, and
then Caterpillar produced commercial heats of
CF8C-Plus, which showed outstanding
creep-rupture resistance at 850 C and 35 MPa and excellent
tensile, creep and fatigue resistance at 600–900 C,
and won the R&D 100 Award in 2003.6 The Mn
addition gives CF8C-Plus good metal fluidity and
castability, and the cast steel is used as-cast, with
no additional heat treatments necessary.
The excellent creep-rupture resistance of the
CF8C-Plus steel is shown in Figs. 5 and 6, and the
extraordinary microstructure that imparts these
properties is shown in Fig. 7. Extremely fine and
dense distributions of Nb-rich MC carbides and
M2N nitrides that remain stable for very long times
are the basis for the excellent creep properties of
CF8C-Plus steel (Fig. 7). The microstructures in
Fig. 7 correlate directly with the creep strain curves
in Fig. 5. The durability of the nanoscale
microstructure is remarkable, and the difference
between the CF8C-Plus steel and the standard
CF8C steel is dramatic.
The CF8C-Plus also forms no sigma phase at 700–
850 C, while the CF8C steel does form sigma phase
readily from the delta ferrite in the latter, while the
CF8C-Plus has no delta ferrite and is 100%
austenitic. Creep rupture data for CF8C-Plus steel
are given in Figs. 6 and 8 and compared to cast
irons and CF8C steel and to various solid-solution
Ni-based superalloys, respectively. CF8C-Plus steel
is orders of magnitude stronger than cast irons such
as SiMo cast iron and Ni-resist (D5S) cast iron and
is twice as strong as standard CF8C steel (Fig. 6).
When compared with solid-solution Ni-based
superalloys, CF8C-Plus steel has creep strength
comparable to alloys such as 625, 617 and 230 that
are 5–7 times more expensive.15–17
There have been several commercial applications
of CF8C-Plus steel. One of the first large cast
component trials for the CF8C-Plus steel in 2005
was centrifugally casting the end cover for the Solar
Turbines Mercury 50 gas turbine engine (6700 lbs)
(Fig. 9 top). This component was cut up and
inspected for casting defects and cracks, and none
were found. The mechanical properties including
creep rupture fell in line with other property studies
of this material.15–17 In 2011, two more end cover
components were centrifugally cast, and one was
installed on a Mercury 50 gas turbine engine that
was deployed to the 29 Palms Marine Base in
California in 2013.18 The gas turbine engine is
running well to date. Another much larger
commercial application was the Caterpillar Regeneration
System (CRS) burner housings that were
investment cast by IMPRO in China (Fig. 10) beginning in
the fall of 2006. The CRS units cleaned the ceramic
diesel particulate filters (DPF) that were
downstream in the exhaust gas path. The CRS units
experienced very severe thermal cycling as diesel
fuel was added to the turbocharger outlet gas,
heated up to 900 C for 20 min and then cooled.
The CRS units were deployed on every heavy-duty
on-highway diesel engine produced by Caterpillar
from 2006 to 2011; 550 tons of steel was used to
make the CRS units, and at a unit weight of 25–
29 lb, about 45,000 CRS units were made. All of
them are still in the field with no failures. This
successful commercialization of CF8C-Plus cast
steel for the CRS application won a Federal
Laboratory Consortium Award for Excellence in
Technology Transfer for ORNL and Caterpillar in 2009.
DEVELOPMENT OF ALUMINUM-MODIFIED
Wrought aluminum forming austenitic (AFA)
alloys were developed at ORNL in 2006, based on
the matrix composition of the HT-UPS steels
described earlier19–21 (Table II). The AFA alloys
won an R&D 100 Award in 2009. Recently, cast AFA
alloys (CAFA) have been developed.22,23 These AFA
alloys were initially based on Fe-25Ni-(
alloys for operation around 700–850 C, with Hf and
Y additions also needed for long-term operation in
the 900 C range. All AFA alloys have an upper
temperature limit to their oxidation and protective
alumina scale formation because of the need to
balance mechanical properties and austenite
stability with oxidation resistance.24 Cast AFA alloys
capable of 1100 C operation have also been
identified, although they are based on Fe-35Ni-25Cr
alloys with Hf and Y additions, which increase alloy
costs.24 Fe-25Ni-18Cr alloys based on wrought AFA
compositions have also been reported.25 In 2015,
aluminum-modified CF8C-Plus alloys were pursued
and developed at ORNL that have a different
chemistry, including lower Ni and higher Cr, W,
Cu and C levels than the previous wrought AFA and
CAFA alloys and were intended to be more
costeffective and have higher creep strength and
oxidation resistance at 950–1000 C for automotive
exhaust component (manifold and turbocharger)
applications (Table II).
The CF8C-Plus steel has reasonable oxidation
resistance in dry air at 800 C, but has poor
oxidation resistance in air + 10% water vapor at 800 C
and higher, so an alloy with more oxidation
resistance with added aluminum to form a protective
Al2O3 oxide scale was needed for higher
temperature applications, particularly exhaust
environments with moisture. The Al-mod. CF8C-Plus base
alloy (PJMAl-2) is a 20Cr-20Ni alloy with added Mn
for metal fluidity, Si for oxidation resistance, Nb
and W for carbide stability and strengthening, Cu
for austenite stability, C for austenite stability and
carbide strengthening, and 3.5Al for alumina scale
formation and oxidation resistance (Fig. 11).
Small heats of all the PJMAl2 alloys (Table II)
were cast as small ingots in the ORNL arc melter,
and one of the heats (PJMAl2 base) was cast as a
larger heat by the University of Missouri-Rolla
facilities to generate oxidation and mechanical
Oxidation data with 1-h cycles in air + 10% water
vapor at 950 C and 900 C are shown in Fig. 12. In
both cases, the Al-mod. CF8C-Plus with the base
PJMAl2 (3.5% Al) or PJMAl2.33 (4.0% Al) shows
excellent behavior, better than exhaust alloys based
on chromia formation or other CAFA alloys.
Preliminary oxidation testing at 1000 C shows good
oxidation resistance of the PJMAl2 alloy as well.
This oxidation resistance compares well with
Nibased alloys that contain Al, but costs 5–9 times
more. The tensile properties of the Al-mod.
CF8CPlus alloys are shown in Fig. 13 [yield strength (YS)
and total elongation (TE)], and clearly the Al-mod
CF8C-Plus steels have more strength than the
Fig. 12. Graphs of mass change as a function of time for coupons of
various heats of steel, including the PJM-Al2 alloys, for testing in
air + 10% water vapor at 950 C up to 350 h and 900 C up to 900 h
(data of B.A. Pint at ORNL).
standard CF8C-Plus steel and 10% or more TE over
the temperature ranges of interest. The YS at 700–
800 C compares well with that of the solid-solution
Ni-based alloy 617, and preliminary creep data
show that the rupture life of the Al-mod. CF8C-Plus
steel is 2–3 times more than standard CF8C-Plus
steel at 850–950 C (Fig. 14).
Currently, ORNL is scaling up heats PJM-Al2
and PJM-Al2.33 at MetalTek International to
produce about 500-lb heats to make keel bars for
tensile/creep/fatigue testing, weld plates for welding
tests and molds to test the metal fluidity to measure
the castability for future component trials.
Applications for the Al-modified CF8C-Plus should include
(but are not limited to) automotive and diesel
exhaust components, including exhaust manifolds
and turbocharger components, gas-turbine
components, including turbine end covers and other
exhaust hot-gas path pieces, fuel cell balance of
plant and heat exchanger cover components, A-USC
steam turbine parts, including turbine casing and
steam valve components, and heat-recovery steam
generators for combine cycle gas/steam turbine
systems. External, protective alumina scale
formation should be beneficial for resistance to
carburizing, coking and metal dusting environments
including chemical and petrochemical tubing and
piping, as well as supercritical CO2 turbine systems.
A methodology exists that enables one to design
alloys with significantly improved heat resistance
and long-term durability by carefully engineering
the microstructure so that the stainless steels and
alloys can successfully perform in various extreme
environments. The rules that govern precipitation
behavior in the alloy matrix include (
) catalytic effects, (
) inhibitor effects and
(d) interference effects of the various alloying
element additions. These in turn produce an alloy
matrix with (
) nanoscale precipitation of carbide
and/or nitride phases, (
) robust nanodispersions
that are resistant to dissolution or coarsening, (
resistance to embrittling intermetallic phases such
as sigma, chi and Laves, (
) resistance to
radiationinduced or thermal creep voids and (
) for wrought
materials, resistance to recovery or
recrystallization. This engineered microstructure method
produced the 14Cr-16Ni HT-UPS steel with creep
strength equivalent to Ni-based superalloy 617 at
600–800 C because of triply stabilized MC carbides,
but lacked oxidation resistance at those
temperatures. It also produced the CF8C-Plus cast
austenitic stainless steel modified with Mn and N to
produce a matrix resistant to the adverse effects of
aging and creep strength at 600–900 C similar to
superalloys 617 and 230 because of nanodispersions
of Nb-rich MC carbides and M2N nitrides. However,
CF8C-Plus lacked moisture-enhanced oxidation
resistance at 800 C and above. Finally, recent
application of the engineered microstructure
method produced the 20Cr-20Ni
aluminum-modified CF8C-Plus cast steels with more creep strength
and outstanding oxidation resistance in air plus
moisture at 900–950 C and above. Alloy design of
cost-effective stainless steels has transformed steels
that usually cannot perform in extreme
environments into materials with performance and
durability that cannot fail.
NOTICE OF COPYRIGHT
This manuscript has been authored by
UT-Battelle, LLC, under Contract No.
DE-AC0500OR22725 with the US Department of Energy.
The United States Government retains and the
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acknowledges that the United States Government
retains a non-exclusive, paid-up, irrevocable,
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Department of Energy will provide public access to these
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Fig. 14. A plot of creep rupture life for various creep conditions for
as-cast PJM-Al2 and standard CF8C-Plus steel.
Thanks to Dr. Bruce Pint at ORNL for being a
coinventor of the Al-mod. CF8C-Plus steels and for
generating the oxidation data at ORNL. Thanks to
Christopher Stevens and Jeremy Moser at ORNL
for tensile and creep testing of the CF8C-Plus and
Al-modified CF8C steels. Thanks to Dr. Yukinori
Yamamoto and Dr. Michael Brady at ORNL for
reviewing the manuscript. Thanks to the US DOE
Office of Energy Efficiency and Renewable Energy,
Vehicle Technologies Office, Propulsion Materials
Program and the ORNL Technology Transfer and
Maturation Funds for funding this research.
This article is distributed under the terms of the
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which permits unrestricted use, distribution, and
reproduction in any medium, provided you give
appropriate credit to the original author(s) and the
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license, and indicate if changes were made.
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