U.S. patent application number 16/046347 was filed with the patent office on 2019-03-07 for cobalt-free, galling and wear resistant austenitic stainless steel hard-facing alloy.
The applicant listed for this patent is The Ohio State University. Invention is credited to Sudarsanam Suresh Babu, Gregory J. Frederick, David Wayne Gandy, Lou Lherbier, Tapasvi Lolla, David Novotnak, John Albert Siefert, Ryan Thomas Smith.
Application Number | 20190071759 16/046347 |
Document ID | / |
Family ID | 54869116 |
Filed Date | 2019-03-07 |
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United States Patent
Application |
20190071759 |
Kind Code |
A1 |
Smith; Ryan Thomas ; et
al. |
March 7, 2019 |
COBALT-FREE, GALLING AND WEAR RESISTANT AUSTENITIC STAINLESS STEEL
HARD-FACING ALLOY
Abstract
A strain-hardenable stainless steel alloy includes hard
secondary phases dispersed in an austenitic primary phase, the
alloy including 0.3-0.6% nitrogen by weight.
Inventors: |
Smith; Ryan Thomas;
(Findlay, OH) ; Lolla; Tapasvi; (Columbus, OH)
; Babu; Sudarsanam Suresh; (Knoxville, TN) ;
Gandy; David Wayne; (China Grove, NC) ; Siefert; John
Albert; (Huntersville, NC) ; Frederick; Gregory
J.; (Harrisburg, NC) ; Lherbier; Lou;
(Pittsburg, PA) ; Novotnak; David; (Pittsburg,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Ohio State University |
Columbus |
OH |
US |
|
|
Family ID: |
54869116 |
Appl. No.: |
16/046347 |
Filed: |
July 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14743348 |
Jun 18, 2015 |
10094010 |
|
|
16046347 |
|
|
|
|
62014498 |
Jun 19, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16K 25/005 20130101;
C21D 2211/004 20130101; C21D 6/004 20130101; B32B 15/01 20130101;
B22F 2003/248 20130101; C22C 38/04 20130101; C22C 38/001 20130101;
Y10T 428/12979 20150115; C21D 6/005 20130101; B22F 7/08 20130101;
C21D 1/26 20130101; C21D 2241/02 20130101; C22C 38/44 20130101;
C22C 33/0285 20130101; C22C 38/00 20130101; B22F 3/15 20130101;
C22C 38/02 20130101; C21D 2211/001 20130101 |
International
Class: |
C22C 38/44 20060101
C22C038/44; F16K 25/00 20060101 F16K025/00; C22C 38/04 20060101
C22C038/04; B22F 3/15 20060101 B22F003/15; B22F 7/08 20060101
B22F007/08; B32B 15/01 20060101 B32B015/01; C21D 1/26 20060101
C21D001/26; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C22C 33/02 20060101 C22C033/02; C21D 6/00 20060101
C21D006/00 |
Claims
1. A strain-hardenable stainless steel alloy comprising hard
secondary phases dispersed in an austenitic primary phase, the
alloy including 0.3-0.6% nitrogen by weight.
2. The alloy of claim 1, wherein the nitrogen content of the alloy
is 0.44-0.55% by weight.
3. The alloy of claim 1, wherein the alloy consists essentially of,
by weight: 21.0 to 27.0% chromium; 3.0 to 7.0% manganese; 2.0 to
6.0% nickel; 1.5 to 4.0% silicon; 1.0 to 5.0% molybdenum; 0.9 to
1.3% carbon; 0.3-0.6% nitrogen; the balance iron and
impurities.
4. The alloy of claim 3, wherein the alloy consists essentially of,
by weight: 21.0 to 27.0% chromium; 3.0 to 7.0% manganese; 2.0 to
6.0% nickel; 1.5 to 4.0% silicon; 1.0 to 5.0% molybdenum; 0.9 to
1.3% carbon; 0.44-0.55% nitrogen; the balance iron and
impurities.
5. The alloy of claim 4, wherein the alloy consists essentially of,
by weight: 25.73% chromium; 4.78% manganese; 4.37% nickel; 3.34%
silicon; 2.04% molybdenum; 1.21% carbon; 0.46% nitrogen; the
balance iron and impurities.
6. The alloy of claim 1, wherein the hard phases comprise at least
one of a carbide and a nitride.
7. A hard-faced component, comprising: a metallic component having
a component surface; a layer of the hard-facing alloy according to
claim 1 applied to the component surface.
8. The component according to claim 7, wherein the component is a
valve seat.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a hard-facing
alloy, and more particularly, the invention relates to a
cobalt-free hard-facing alloy.
[0002] Hard-facing alloys are used for a variety of applications
including: valve seats, valve stems, turbine blades, lawn-mower
blades, mixers, rollers, grinders, cutters, etc. These alloys offer
a variety of properties including: high galling resistance, good
wear resistance, high strength and erosion performance, corrosion
performance, and high hardness. Most hard-facing alloys fall into
one of three alloy categories: iron-based, nickel-based, and
cobalt-based alloys. Cobalt-based alloys have been the industry
standard for valve hardfacing applications for almost 50 years now,
largely in-part due to their versatility over a wide range of
applications. The two most notable of these are STELLITE 6 and 21.
Unfortunately, for nuclear applications, these alloys tend to wear
away with time and form radioactive isotopes such as Co.sup.58 and
Co.sup.60.
[0003] During the past two decades, considerable attention has been
placed on iron-based alloys, essentially modified-stainless steels,
as these alloys can eliminate the concern of radiation build-up
while still offering excellent wear, galling, and corrosion
performance. Several iron-based, cobalt-free, "modified-stainless
steels" are currently used by the nuclear industry primarily for
valve seat applications including: NOREM, GALLTOUGH PLUS, NITRON IC
60, and TRISTELLE 5183. These alloys have met with limited success
for various reasons including: solidification cracking during
welding, poor weldability, cracking in service, poor wear
properties at service temperatures, and poor acceptance by industry
in general. Compositional analysis and phase stability calculations
using today's more advanced prediction tools have described why
many of these alloys have failed to meet the rigorous industry
standards and applications.
[0004] Accordingly, there is a need for an improved alternative
cobalt-free hard-facing alloy for use in the nuclear industry as
well as other industries.
BRIEF SUMMARY OF THE INVENTION
[0005] This need is addressed by stainless steel hard-facing alloy
with high performance against wear, galling, and corrosion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The subject matter that is regarded as the invention may be
best understood by reference to the following description taken in
conjunction with the accompanying drawing figures in which:
[0007] FIG. 1 shows experimental (X-ray diffraction, dots) and
theoretical (line) stacking fault probability in a stainless steel
alloy based on contributions of nitrogen segregation in a
generalized stacking fault energy thermodynamic model;
[0008] FIG. 2 illustrates equilibrium phase balance of an alloy of
the present invention over processing temperature range predicted
by thermodynamic modeling software;
[0009] FIG. 3 shows galling results for STELLITE 6 block specimen
tested at (343.degree. C.) (650.degree. F.) and indicated stress
level;
[0010] FIG. 4 shows galling results for the alloy of the present
invention (1102.degree. C. Anneal) block specimen tested at
343.degree. C. (650.degree. F.) and indicated stress level;
[0011] FIG. 5 shows surface galling wear produced for the alloy of
the present invention during loading at 30 ksi at 350.degree. C.;
and
[0012] FIG. 6 shows surface galling wear produced for a NOREM alloy
during loading at 35 ksi at 350.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0013] A new alloy will provide high galling, wear, and erosion
resistance. The new alloy has been manufactured in a powder-form
and may be applied via powder metallurgy-hot isostatic processing
to a component to be protected from wear, such as a surface of a
valve seat. The powder may be applied to a surface of a component
and bonded thereto using conventional powder metallurgy
processes.
[0014] As noted above, several existing hard-facing alloys are on
the market today. Today, only a few of these are used in the power
industry and are targeted primarily at improved galling resistance.
An extensive review of the literature suggests that galling
resistance in stainless steel iron-based alloys is achieved through
two key contributions: high strain-hardening rate and high volume
fraction of hard secondary phases. High strain-hardening is
achieved by modification of the plastic deformation mechanism
through lowering of stacking fault energy ("SFE") by nitrogen
addition. Of course, the exact properties are dependent on the
composition, processing, and resulting initial microstructure of
the alloy.
[0015] The present invention is focused on understanding the role
of each of these contributors in existing hard-facing iron-based
alloys and then designing an optimized alloy that employs the
contributors thereby minimizing potential detrimental phases. The
results of the program have produced an alloy that has been
designed around three significant attributes: (1) a high nitrogen
super-saturation in the matrix to lower stacking fault energy (SFE)
and alter strain-induced martensitic transformation; (2) a high
volume fraction of hard secondary phases (carbides and nitrides);
and (3) the use of proper processing using powder metallurgy-hot
Isostatic processing (PM-HIP) and an optimized heat treatment.
[0016] For comparison, several prior art alloys are shown in Table
1a.
TABLE-US-00001 TABLE 1a TRISTELLE NITRONIC NOREM GALLTOUGH EPRI H'
Alloy 5183 60 Carbon 0.85-1.4 0.25max 0.3-0.85 1.7-2.0 0.001 to
0.25 Manganese 5.0-13.0 2.0-7.0 3.0-5.0 NA 6.0-16.0 Silicon
1.5-5.5.sup. 1.0-5.0 1.5-4.5 5.25-5.75 2.0-7.0 Nickel 4.0-12.0
2.0-7.75 3.0-7.0 8.5-10.5 3.0-15.0 Chromium 18.0-27.0.sup.
12.0-20.0 20.0-25.0 19.0-22.0 10.0-25.0 Molybdenum 0-6.0 3.0max
1.0-3.0 NA 4.0max Nitrogen 0.1-0.3.sup. 0.35max 0.1-0.3 NA
0.001-0.4 Vanadium 0-1.0 NA NA NA 0.2 Niobium 0-1.0 NA NA 8.0-9.0
0.1 Titanium 0-1.0 NA NA 0.3-0.5 NA Tantalum 0-1.0 NA NA NA NA
Copper NA 3.0max NA NA 4.0max Iron Bal Bal Bal Bal Bal
[0017] The new alloy chemistry and proposed ranges are shown in
Table 1b.
TABLE-US-00002 TABLE 1b Inventive Inventive Alloy Alloy Range
Example Carbon 0.9-1.3 1.21 Manganese 3.0-7.0 4.78 Silicon 1.5-4.0
3.34 Nickel 2.0-6.0 4.37 Chromium 21.0-27.0 25.73 Molybdenum
1.0-5.0 2.04 Nitrogen 0.30-0.60 0.46 Vanadium NA NA Niobium NA NA
Titanium NA NA Tantalum NA NA Copper NA NA Iron Bal Bal
[0018] Together, these attributes have produced an alloy that
provides excellent galling and sliding wear properties at room
temperature and all the way up through the 343.degree. C.
(650.degree. F.) nuclear plant operating temperatures. Furthermore,
the alloy rivals galling and slide wear performance of cobalt-based
alloys such as STELLITE 6 and 21 across the operating range up
through 343.degree. C. (650.degree. F.). Each of the alloy
attributes are described more fully in the following
paragraphs.
High Nitrogen Concentration
[0019] Nitrogen is traditionally considered to be an austenite
stabilizer in stainless steels and to raise the stacking fault
energy. In conventional 18-8 stainless steels, such as 304 or 316,
where typical nitrogen concentrations are low, nitrogen stabilizes
the austenite phase and indeed raises the stacking fault energy. At
high concentrations (>0.2 wt % N), however, nitrogen may
dramatically lower the energy required to form stacking faults at a
microscopic level, FIG. 1. The optimal range was found to be
0.44-0.55 wt % nitrogen in the present alloy. This effect has been
attributed to a segregation or clustering effect of N to the
stacking faults, accounting for its nonlinear effect on SFE.
[0020] Mechanistically nitrogen provides a significant influence on
the plastic deformation mechanisms in austenitic stainless steels,
primarily by altering the effective SFE. SFE in face-centered cubic
("FCC") metals is known to control the plastic deformation
mechanism, with "forest" dislocation hardening occurring at high
values, and progressing though extended stacking faults, twinning,
and an FCC to hexagonal close-packed ("HCP") martensitic phase
transformation as the SFE is lowered. As the plastic deformation
mechanism changes with decreasing SFE, the strain hardening rate
increases concurrently. Indeed, this same process occurs in
cobalt-based alloys, which have a low SFE over a wide range of
temperatures and compositions, and is posited to be a cause of
their good wear and galling performance. Indeed, a higher
work-hardening rate has been generally correlated with
galling-resistance in stainless steels as well. In the present
alloy, the presence of deformation twinning in high-temperature
worn surfaces demonstrates that high concentrations of matrix
nitrogen lower SFE substantially (e.g. to about 20-50 mJ/m.sup.2)
even at 343.degree. C.
[0021] Thus the alloy described herein results in a twinning
induced plasticity ("TWIP") steel at high temperature which
represents a novelty in Co-free hard-facing alloys. Additionally,
the incorporation of low-temperature strain-induced martensite,
high volume-fraction of second phases, and a hot isostatic pressing
("HIP") process for weld-free hardfacing fabrication result in a
novel finished product that has uniquely superior galling
performance at high temperature.
Strain-Induced Martensitic Transformation
[0022] In austenitic stainless steels, additionally, a higher
strain hardening rate can be achieved at low temperatures through a
strain-induced FCC to BCC martensitic transformation. Some
austenitic stainless steel alloys are based on an FCC phase which
is metastable at room temperature. When strain or deformation is
introduced to this family of austenitic alloys, the microstructure
may undergo a transformation to a stronger, martensitic
microstructure. The crystal structure of these martensitic
structures can be HCP or base-centered cubic ("BCC") or body
centered tetragonal ("BCT") or combinations thereof. Martensitic
microstructures are known to provide increased erosion protection,
wear resistance, and galling performance.
[0023] Two forms of martensitic structures have been observed to
result from the transformation from austenite, including a
martensitic BCC structure and an epsilon martensitic HCP. It is
known that the c-martensite transformation will result in a stable
phase that is directly connected to a low, stacking fault energy.
Correspondingly, the a-martensite formation results in a very
stable BCC structure, which also provides good, wear resistance. In
either case, the ability to readily form martensite under loading
(strain/stress) is believed to be extremely beneficial in providing
galling, wear, and erosion resistance to austenitic stainless steel
alloys, primarily by affecting the strain-hardening rate. Indeed,
the loss of the alpha martensite transformation, without a low SFE,
has been connected with the degradation of wear properties with
increasing temperature in some stainless-steel hardfacing.
[0024] Alpha martensite formation in stainless steels is considered
a nucleation limited process, and thus requires high-energy defect
sites for the transformation to occur. The generation of these
defect sites is controlled by the underlying matrix deformation
mechanism, which can be controlled by stacking fault energy.
Nitrogen modification of the matrix has been shown to increase
transformation kinetics in in-situ tensile testing. Thus, nitrogen
changes to SFE can control deformation mechanisms over the entire
temperature range of the alloy (room temperature to 350.degree.
C.), and increase strain hardening rate over this temperature
range.
[0025] Nitrogen therefore acts to increase the strain-hardening
rate across a range of temperatures by different plastic
deformation mechanisms. At low temperature, a strain-induced FCC to
BCC martensite is observed in deformed wear surfaces, whereas at
high temperature (343.degree. C.) the deformation mode changes to
twinning induced plasticity. In each case, nitrogen acts through
its effect on the stacking fault energy, which changes the
micro-mechanics of the plastic deformation process and results in a
higher strain-hardening rate over the entire range of considered
temperatures. This results in a very small strain-hardened layer
near the surface, which reduces the overall wear volume and delays
the onset of the galling process to higher stresses.
High Volume of Secondary Phases
[0026] Heterogeneous microstructures composed of hard particles in
a ductile matrix are known to improve abrasive and adhesive wear
resistance. Overall wear rates are reduced by lowering interfacial
adhesion, preventing surface deformation, and providing low-energy
paths for wear particle formation. Additionally, the partitioning
of plastic strain to the more ductile matrix increases the
strain-induced martensite transformation and increases the
strain-hardening rate. This increases the resistance to strain
localization and increases galling resistance. These effects are
commonly exploited in cermet (ceramic particles embedded in a metal
matrix) cladding materials. In the alloy described herein, however,
the hard second phases are engineered into the alloy chemistry and
at an optimal volume fraction. This results in comparative
long-term high-temperature stability, increased toughness, and
improved thermal expansion matching (for part fabrication) while
still retaining high wear and galling resistance at elevated
temperatures.
Processing of the Hard-Facing Alloy
[0027] The hard-facing alloy described herein is applied using a
combination of powder metallurgy-HIP and an optimized solution
annealing heat treatment to generate an appropriate microstructure.
Powder metallurgy techniques offer superior microstructure,
composition, and defect control compared to traditional
weld-cladding or thermal-spray techniques. An initial step in
hard-facing a component is to apply and bond a layer of the alloy
in powder form to a component surface of the component, using
conventional powder metallurgy techniques, and then to subject the
component with the applied layer to a conventional HIP process.
Considerable thermodynamic and phase modeling have been performed
to establish proper alloy chemistry and heat treatment conditions.
It was realized that producing a fully (or near fully) austenitic
FCC matrix structure with a dispersed secondary hard phase at the
HIP processing temperature would be desirable. Phase modeling
suggested that a near fully austenitic FCC matrix structure could
be produced at the HIP processing temperature, provided that a
rapid cooling (quench) could readily be obtained following a
certain time at the processing temperature, FIG. 2. Unfortunately
most HIP units are air cooled and thus can take hours to reach room
temperature.
[0028] As a result, it was elected to process the hard-facing alloy
through the HIP processing temperature, for example about
1050.degree. C., allow it to cool in air (as normal), reheat the
alloy back to the solution annealing temperature (above
1100.degree. C.), followed by a rapid water quench. The latter two
steps (solution anneal and quench) permitted the alloy to form a
fully austenitic microstructure upon cooling with a supersaturation
of matrix nitrogen.
[0029] Subsequently upon application of load (stress) as one might
find on the surface of a valve seat, the austenitic structure is
readily strain-hardened along a thin layer at the surface of the
seat, FIG. 5, thereby confining deformation to a smaller surface
layer and raising galling resistance. This is much improved
compared to the layer produced by the NOREM alloy, FIG. 6. Thus,
the alloy of the present invention provides a structure that has
superior wear and galling performance. This structure (together
with the proper alloying) readily produces galling resistance
across the entire operating range of a nuclear power plant (room
temperature through 350.degree. C.).
[0030] No other Cobalt-free hard-facing alloy covers the range from
room temperature to 350.degree. C. operating temperature while
providing good galling and wear performance. At elevated
temperature 343.degree. C. (650.degree. F.), sliding wear
conditions (such as a valve disc to seat), the alloy of the present
invention shows comparable behavior to a standard Co-based
hard-facing material (STELLITE 6). Additionally, and in reference
to processing, the increased annealing temperature (1102.degree. C.
versus 1065.degree. C.) further increases the sliding wear
resistance, as shown in Table 2. For the tested conditions at
ambient and elevated temperatures, the alloy of the present
invention shows behavior that is vastly improved over traditional
Fe-based hard-facing materials and is nearly equivalent in wear
resistant as Co-based hard-facing materials like STELLITE 6.
TABLE-US-00003 TABLE 2 Volume Loss (mm.sup.3) Inventive Inventive
ASTM G133 Sliding Wear Alloy Alloy Test Conditions (1065.degree. C.
(1102.degree. C. STEL- Temperature Stress Anneal) Anneal) LITE 6
68.degree. F., 20.degree. C. 15 ksi, 103.4 MPa 0.023 0.026 0.013
650.degree. F., 343.degree. C. 15 ksi, 103.4 MPa 0.086 0.058
0.040
[0031] To verify the resistance to galling under sliding wear
conditions, ASTM G98 galling tests were conducted and are provided
in FIGS. 3 and 4. As shown, the wear scars on the block specimens
for the alloy of the present application (1102.degree. C. anneal)
and STELLITE 6 are comparable in appearance and in the magnitude of
the surface damage. A threshold galling stress is not easily
determined for the alloy of the present application as there was no
tendency to macroscopic galling deformation under the indicated,
applied stress values in FIG. 4. These results confirm the
superiority of the alloy of the present application for demanding
valve applications where the applied service conditions (i.e.
temperature and stress) may induce significant galling in seats and
discs for less-resistant materials.
[0032] The foregoing has described a cobalt-free hard-facing alloy.
All of the features disclosed in this specification (including any
accompanying claims, abstract and drawings), and/or all of the
steps of any method or process so disclosed, may be combined in any
combination, except combinations where at least some of such
features and/or steps are mutually exclusive.
[0033] Each feature disclosed in this specification (including any
accompanying claims, abstract and drawings) may be replaced by
alternative features serving the same, equivalent or similar
purpose, unless expressly stated otherwise. Thus, unless expressly
stated otherwise, each feature disclosed is one example only of a
generic series of equivalent or similar features.
[0034] The invention is not restricted to the details of the
foregoing embodiment(s). The invention extends any novel one, or
any novel combination, of the features disclosed in this
specification (including any accompanying claims, abstract and
drawings), or to any novel one, or any novel combination, of the
steps of any method or process so disclosed.
* * * * *