U.S. patent application number 10/829818 was filed with the patent office on 2008-10-23 for composition gradient cermets and reactive heat treatment process for preparing same.
Invention is credited to Robert Lee Antram, Narasimha-Rao Venkata Bangaru, ChangMin Chun, Christopher John Fowler, Hyun-Woo Jin, Jayoung Koo, John Roger Peterson.
Application Number | 20080257454 10/829818 |
Document ID | / |
Family ID | 36788777 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080257454 |
Kind Code |
A1 |
Chun; ChangMin ; et
al. |
October 23, 2008 |
COMPOSITION GRADIENT CERMETS AND REACTIVE HEAT TREATMENT PROCESS
FOR PREPARING SAME
Abstract
Cermets, particularly composition gradient cermets can be
prepared starting with suitable bulk metal alloys by a reactive
heat treatment process involving a reactive environment selected
from the group consisting of reactive carbon, reactive nitrogen,
reactive boron, reactive oxygen and mixtures thereof.
Inventors: |
Chun; ChangMin; (Belle Mead,
NJ) ; Bangaru; Narasimha-Rao Venkata; (Annandale,
NJ) ; Jin; Hyun-Woo; (Phillipsburg, NJ) ; Koo;
Jayoung; (Bridgewater, NJ) ; Peterson; John
Roger; (Ashburn, VA) ; Antram; Robert Lee;
(Warrenton, VA) ; Fowler; Christopher John;
(Springfield, VA) |
Correspondence
Address: |
ExxonMobil Research and Engineering Company
P.O. Box 900
Annandale
NJ
08801-0900
US
|
Family ID: |
36788777 |
Appl. No.: |
10/829818 |
Filed: |
April 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60471992 |
May 20, 2003 |
|
|
|
Current U.S.
Class: |
148/206 |
Current CPC
Class: |
C23C 8/22 20130101 |
Class at
Publication: |
148/206 |
International
Class: |
C23C 8/00 20060101
C23C008/00 |
Claims
1. A process for preparing a composition gradient cermet material
comprising the steps of: in a first step, heating a metal alloy
containing from 18 to 60 wt % chromium at a temperature in the
range of about 600.degree. C. to about 1150.degree. C. in a
hydrogen environment to form a heated metal alloy; in a second
step, exposing said heated metal alloy to a reactive carbon gaseous
environment comprising H.sub.2 and CH.sub.4, wherein the CH.sub.4
ranges from about 2 vol % to about 45 vol % in the range of about
600.degree. C. to about 1150.degree. C. for a time sufficient to
provide a reacted alloy with a reacted layer of about 1.5 mm to
about 30 mm thick on the surface or in die bulk matrix of the metal
alloy; and in a third step, cooling said reacted alloy to a
temperature below about 40.degree. C. to provide a composition
gradient cermet material, wherein one surface of the cermet is
ceramic-rich and a second unexposed surface is metal-rich, and
wherein said cooling step further comprises cooling said reacted
alloy to a temperature in the range of 500.degree. C. to
100.degree. C., holding the temperature at any temperature in the
range of 500.degree. C. 100.degree. C. for a time period between 5
minutes to 10 hours and thereafter cooling at a rate in the range
of 0.5.degree. C. per second to 25.degree. C. per second to below
about 40.degree. C.
2. The process of claim 1 wherein said metal alloy further
comprises from 0 to 10 wt % titanium, and from 30 to 88 wt % of
metals selected from the group consisting of iron, nickel, cobalt,
silicon, aluminum, manganese, zirconium, hafnium, vanadium,
niobium, tantalum, molybdenum, tungsten, and mixtures thereof.
3. The process of claim 1 wherein said metal alloy further
comprises from 0 to 10 wt % titanium, and from 30 to 88 wt %
iron.
4. (canceled)
5. The process of claim 1 wherein said exposing step is for a time
period of about 1 hour to 800 hours.
6. The process of claim 1 wherein said exposing step is for a time
period to provide a reacted alloy wherein said reacted alloy
comprises precipitated chromium-rich carbides, titanium carbides
and mixtures of chromium-rich and titanium carbides.
7. The process of claim 6 wherein said chromium-rich carbides
comprise Cr.sub.7C.sub.3, Cr.sub.23C.sub.6, (Cr.sub.0 6
Fe.sub.0.4).sub.7C.sub.3, (Cr.sub.0.6Fe.sub.0 4).sub.23C.sub.6 and
mixtures thereof.
8. The process of claim 6 wherein said titanium carbides comprise
TiC.
9. (canceled)
10. The process of claim 1 wherein said exposing step is for a time
period wherein the reacted alloy is of thickness encompassing the
entire depth of said metal alloy.
11. The process of claim 1 wherein said cooling step comprises
cooling said reacted alloy at a rate in the range of 0.5.degree. C.
per second to 25.degree. C. per second.
12. (canceled)
13. A process for preparing a composition gradient cermet material
comprising the steps of: in a first step, heating a metal alloy
containing from 18 to 60 wt % chromium at a temperature in the
range of about 600.degree. C. to about 1150.degree. C. in a
hydrogen environment to form a heated metal alloy; in a second
step, exposing said heated metal alloy to a reactive nitrogen
gaseous environment comprising H.sub.2 and ammonia, wherein the
ammonia ranges from about 2 vol. % to about 70 vol. % in the range
of about 600.degree. C. to about 1150.degree. C. for a time
sufficient to provide a reacted alloy with a reacted layer of about
1.5 mm to about 30 mm thick on the surface or in the bulk matrix of
the metal alloy; and in a third step, cooling said reacted alloy to
a temperature below about 40.degree. C. it provide a composition
gradient cermet material, wherein one surface of the cermet is
ceramic-rich and a second unexposed surface is metal-rich, and
wherein said cooling step further comprises cooling, said reacted
alloy to a temperature in the range of 500.degree. C. to
100.degree. C. holding the temperature at any temperature in the
range of 500.degree. C. to 100.degree. C. for a time period between
5 minutes to 10 hours and thereafter cooling at a rate in the range
of 0.5.degree. C. per second to 25.degree. C. per second to below
about 40.degree. C.
14. The process of claim 13 wherein said exposing step is for a
time period to provide a reacted alloy wherein said reacted alloy
comprises precipitated chromium-rich nitrides, titanium nitrides
and mixtures of chromium-rich and titanium nitrides.
15. The process of claim 14 wherein said chromium-rich nitrides
comprise Cr.sub.2N.
16. The process of claim 14 wherein said titanium nitrides comprise
TiN.
17. A process for preparing a composition gradient cermet material
comprising the steps of: in a first step, heating a metal alloy
containing from 18 to 60 wt % chromium at a temperature in the
range of about 600.degree. C. to about 1150.degree. C. in a
hydrogen environment to form a heated metal alloy; in a second
step, exposing said heated metal alloy to a reactive carbon and
nitrogen gaseous environment comprising H.sub.2 and ammonia and
CH.sub.4, wherein the CH.sub.4 ranges from about 2 vol % to about
45 vol % and the ammonia ranges from about 2 vol. % to about 70
vol. % in the range of about 600.degree. C. to about 1150.degree.
C. for a time sufficient to provide a reacted alloy with a reacted
layer of about 1.5 mm to about 30 mm thick on the surface or in the
bulk matrix of the metal alloy; and in a third step, cooling said
reacted alloy to a temperature below about 40.degree. C. to provide
a composition gradient cermet material, wherein one surface of the
cermet is ceramic-rich and a second unexposed surface is
metal-rich, and wherein said cooling step further comprises cooling
said reacted alloy to a temperature in the range of 500.degree. C.
to 100.degree. C., holding the temperature at any temperature in
the range of 500.degree. C. to 100.degree. C. for a time period
between 5 minutes to 10 hours and thereafter cooling at a rate in
the range of 0.5.degree. C. per second to 25.degree. C. per second
to below about 40.degree. C.
18. (canceled)
19. (canceled)
20. (canceled)
21. (canceled)
22. (canceled)
23. (canceled)
24. A method for protecting a metal surface exposed to an erosive
material at temperatures in the range of up to 850.degree. C., the
method comprising providing the metal surface with a cermet
composition according to any one of claims 1, 13 or 17.
25. A method for protecting a metal surface exposed to an erosive
material at temperatures in the range of 300.degree. C. to
850.degree. C., the method comprising providing the metal surface
with a cermet composition according to claim 24.
26. The method of claim 24 wherein said surface comprises the inner
surface of a fluid-solids separation cyclone.
27-32. (canceled)
Description
[0001] This application claims the benefit of U.S. Provisional
application 60/471,992 filed May 20, 2003.
FIELD OF INVENTION
[0002] The present invention is broadly concerned with cermets,
particularly composition gradient cermets and reactive heat
treatment process for preparing same.
BACKGROUND OF INVENTION
[0003] Erosion resistant materials find use in many applications
wherein surfaces are subject to eroding forces. For example,
refinery process vessel internals exposed to aggressive fluids
containing hard solid particles such as catalyst particles in
various chemical and petroleum environments are subject to both
erosion and corrosion. The protection of these vessel internals
against erosion and corrosion induced material degradation
especially at high temperatures is a technological challenge.
Refractory liners are used currently for components requiring
protection against the most severe erosion and corrosion such as
the inside walls of cyclones such as the internal cyclones in fluid
catalytic cracking units (FCCU). The life span of these refractory
liners is significantly limited by mechanical attrition of the
liner, cracking and spallation. The state-of-the-art in erosion
resistant materials is chemically bonded castable alumina
refractories. These castable alumina refractories are applied to
the surfaces in need of protection and upon heat curing harden and
adhere to the surface via metal-anchors or metal-reinforcements. It
also readily bonds to other refractory surfaces. The typical
chemical composition of one commercially available refractory is
80.0% Al.sub.2O.sub.3, 7.2% SiO.sub.2, 1.0% Fe.sub.2O.sub.3, 4.8%
MgO/CaO, 4.5% P.sub.2O.sub.5 in wt %.
[0004] Ceramic-metal composites are called cermets. Cermets of
adequate chemical stability can provide an order of magnitude
higher erosion resistance over refractory materials known in the
art. Cermets are generally produced using powder metallurgy
techniques where metal and ceramic powders are mixed, pressed and
sintered at high temperatures. Since powder metallurgically
produced cermets usually have homogeneous microstructure and
uniform composition, sophisticated attachment methods are needed to
attach cermets onto the metallic surfaces wherein erosion
resistance of the surface is desired.
[0005] Composition gradient cermets are cermets wherein one surface
of the cermet is ceramic-rich and the unexposed surface is
metal-rich. In a typical composition gradient cermet there is a
concentration gradient of the ceramic in the metal composition such
that the concentration of the ceramic decreases with depth. These
composition gradient cermets are desired and preferred for
cost-effective attachment of cermets directly onto metal or alloy
surfaces using methods such as welding due to the compatibility and
ease of welding a substantially metallic object to another
substantially metallic object. Furthermore, such composition
gradient cermets can also exhibit superior durability particularly
under conditions wherein thermal fluctuations are present. However,
there is a need for effective processes to prepare composition
gradient cermets.
[0006] One object of the present invention is to provide a process
for preparation of cermets, particularly composition gradient
cermets via reactive heat treatment of a metal alloy.
[0007] Another object of the present invention is to provide a
composition gradient cermet product derived from the reactive heat
treatment process.
[0008] These and other objects will become apparent from the
description that follows.
SUMMARY OF INVENTION
[0009] In one embodiment is a process for preparing a composition
gradient cermet material comprising the steps of: [0010] heating a
metal alloy containing at least one of chromium and titanium at a
temperature in the range of about 600.degree. C. to about
1150.degree. C. to form a heated metal alloy; [0011] exposing said
heated metal alloy to a reactive environment comprising at least
one member selected from the group consisting of reactive carbon,
reactive nitrogen, reactive boron, reactive oxygen and mixtures
thereof in the range of about 600.degree. C. to about 1150.degree.
C. for a time sufficient to provide a reacted alloy; and cooling
said reacted alloy to a temperature below about 40.degree. C. to
provide a composition gradient cermet material.
[0012] Another embodiment is directed towards a composition
gradient cermet product obtained from the disclosed reactive heat
treatment process.
BRIEF DESCRIPTION OF THE FIGURES
[0013] FIG. 1 depicts carbon activity of an environment based on
the reaction CH.sub.4--->C+2H.sub.2 compared to austenitic
stainless steels (a.sub.c in equilibrium with Fe.sub.3C). Also
marked are the carbon activity values of gas mixtures applicable to
the instant invention.
[0014] FIG. 2 depicts the mass gain due to carbon ingression (a
measure of cermet layer formation) of 304 stainless steel
(74Fe:18Cr:8Ni in wt %) as a function of CH.sub.4 content in
H.sub.2 at 1100.degree. C. for 3 hours.
[0015] FIG. 3 depicts the thickness variation of surface cermet
structure on 304 stainless steel as a function of temperature in
37.3 vol % CH.sub.4:62.7 vol % H.sub.2 environment for 3 hours.
[0016] FIG. 4 depicts the thickness variation of surface cermet
formed on various Fe:Ni:Cr based high temperature alloys as a
function of reaction times at 1100.degree. C. in 37.3 vol %
CH.sub.4:62.7 vol % H.sub.2 environments.
[0017] FIG. 5 depicts scanning electron micrographs showing (a)
surface chromium carbide-metal cermet structure on 310 stainless
steel (54Fe:21Ni:25Cr in wt %) after reactive heat treatment at
1100.degree. C. for 3 hours in 37.3 vol % CH.sub.4:62.7 vol %
H.sub.2 environment and (b) enlarged area on the surface revealing
the Cr-rich carbide [(Cr.sub.0.6Fe.sub.0.4).sub.7C.sub.3] and
Cr-depleted steel (63Fe:31Ni:6Cr in wt %) to produce a composite
ceramic-metal two-phase structure. In this scanning electron
micrograph the Cr-rich carbides appear dark gray and the metal
appears recessed, because it has etched more deeply than the
carbides. These figures show the final product having the cermet
surface, which is the product of the process of the instant
invention.
[0018] FIG. 6 depicts optical micrographs showing M.sub.7C.sub.3
(M=Cr and Fe) carbide-metal cermet structure on (a) 55Fe:35Cr:10Ni
(in wt %) alloy, (b) 45Fe:45Cri:10Ni (in wt %) alloy and (c)
35Fe:55Cr:10Ni (in wt %) alloy after reactive heat treatment at
1100.degree. C. for 24 hours in 10 vol % CH.sub.4:90 vol % H.sub.2
environment.
[0019] FIG. 7 depicts optical micrographs showing mixed TiC and
M.sub.7C.sub.3 (M=Cr and Fe) carbide-metal cermet structure on
60Fe:25Cr:10Ni:5Ti (in wt %) alloy after reactive heat treatment at
1100.degree. C. for 24 hours in 10 vol % CH.sub.4:90 vol % H.sub.2
environment.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The first step of the process for preparing a composition
gradient cermet material comprises heating a metal alloy containing
at least one of chromium and titanium at a temperature in the range
of about 600.degree. C. to about 1150.degree. C. to form a heated
metal alloy. The metal alloy containing at least one of chromium
and titanium comprises from about 12 to 60 wt % chromium, from 0 to
10 wt % titanium, and from 30 to 88 wt % of metals selected from
the group consisting of iron, nickel, cobalt, silicon, aluminum,
manganese, zirconium, hafnium, vanadium, niobium, tantalum,
molybdenum, tungsten, and mixtures thereof. In a preferred
embodiment the major mass constituent of the alloy is iron. Thus,
stainless steels such as type 304SS, 347SS, 321SS, 310SS and the
like and iron-nickel based alloys such as Incoloy 800H are
particularly suitable for the instant process.
[0021] The second step of the process comprises exposing the heated
metal alloy to a reactive environment selected from the group
consisting essentially of reactive carbon, reactive nitrogen,
reactive boron, reactive oxygen and mixtures thereof in the range
of about 600.degree. C. to about 1150.degree. C. for a time period
sufficient to provide a reacted alloy.
[0022] When the reactive environment is a reactive carbon
environment carburization reactions are believed to occur. While
not wishing to be bound to the mechanism of the reactive heat
treatment process applicants believe that the carburization process
leads to precipitation of chromium-rich and titanium carbide phases
for example Cr.sub.7C.sub.3, Cr.sub.23C.sub.6,
(Cr.sub.0.6Fe.sub.0.4).sub.7C.sub.3,
(Cr.sub.0.6Fe.sub.0.4).sub.23C.sub.6 and TiC on the alloy surface
and within the alloy matrix resulting in a cermet and particularly
a composition gradient carbide cermet.
[0023] A reactive carbon environment is defined as an environment
in which the thermodynamic activity of carbon (a.sub.c) in the
environment is greater than that of the alloy.
(a.sub.c).sub.environment>(a.sub.c).sub.metal
The reactive carbon environment suitable for the instant invention
comprises at least one of CO, CH.sub.4, C.sub.2H.sub.6 or
C.sub.3H.sub.8. The reactive carbon environment may optionally
include H.sub.2. The reactive carbon environment may further
comprise O.sub.2, CO.sub.2, and H.sub.2O. The following reactions
[1], [2] and [3] shown below are some of the reactions that are
believed to occur under the heat treatment conditions to provide
the reactive carbon. The carbon reacts with the metal surface to
form chromium-rich and titanium-rich carbide phases.
##STR00001##
[0024] When heat treatment follows reaction [3], the carbon
activity (a.sub.c) in the environment is
a.sub.c=e.sup.-G.degree./RT(P.sub.CH4/P.sup.2.sub.H2)
where G.degree. is the free energy of activation, R is the gas
constant, T is the temperature in Kelvin units and P is the partial
pressure of the respective gases methane and hydrogen. Carbon
activities as a function of (P.sub.CH4/P.sup.2.sub.H2) are plotted
in FIG. 1 wherein is indicated the preferred range of
P.sub.CH4/P.sup.2.sub.H2 for the process of the instant
invention.
[0025] When a mixture of methane and hydrogen are used to provide
the reactive carbon environment, the methane content in the gaseous
mixture of methane and hydrogen can range from about 1 vol % to
about 99 vol %, preferably about 2 vol % to about 45 vol %. This is
depicted in FIG. 2, where the mass gain due to carbon ingression (a
measure of cermet layer formation) of 304 stainless steel
(74Fe:18Cr:8Ni in wt %) at 1100.degree. C. exposed for 3 hours is
plotted as a function of CH.sub.4 content in H.sub.2. The preferred
methane content in the gaseous mixture of methane and hydrogen
corresponds to the plateau region of the curve. In this range, the
reaction times are shorter to obtain a specific thickness of
cermet. Gas mixtures in which the methane content in the gaseous
mixture of methane and hydrogen is greater than 45 vol % can also
be used. However, in these ranges, solid carbon deposition on the
alloy surface can be encountered as indicated by the rapid increase
of mass gain in FIG. 2.
[0026] When a mixture of CO and hydrogen are used to provide the
reactive carbon environment, the CO content in the gaseous mixture
of CO and hydrogen can range from about 0.1 vol % to about 5 vol %,
preferably about 0.1 vol % to about 2 vol %.
[0027] When the reactive environment is a reactive nitrogen
environment, nitridation reactions are believed to occur. While not
wishing to be bound to the mechanism of the reactive heat treatment
process applicants believe that the nitridation process leads to
precipitation of chromium-rich and titanium nitride phases for
example Cr.sub.2N and TiN on the alloy surface and within the alloy
matrix resulting in a cermet and particularly a composition
gradient nitride cermet.
[0028] A reactive nitrogen environment is defined as an environment
in which the thermodynamic activity of nitrogen (a.sub.N) in the
environment is greater than that of the alloy.
(a.sub.N).sub.environment>(a.sub.N).sub.metal
Since molecular nitrogen is relatively inert in terms of
nitridation of an alloy, ammonia-bearing atmospheres are preferred.
Ammonia is metastable and dissociates into molecular N.sub.2 and
molecular H.sub.2 when heated to elevated temperatures. The
preferred composition of the reactive nitrogen environment
comprises at least one of air, ammonia and nitrogen. The
composition can further comprise H.sub.2, He, and Ar. In such a
reactive nitrogen environment at temperatures in the range of
600.degree. C. to 1150.degree. C. alloys containing elements such
as Cr and Ti which have strong chemical affinities for nitrogen
undergo rapid nitridation reactions. In order to increase nitrogen
absorption by the alloy, molecular NH.sub.3 is preferred to
dissociate on the alloy surface, thus allowing dissociated atomic
nitrogen to dissolve at the surface and diffuse into the bulk
interior of the metal alloy. Similar to carburization process,
nitridation can lead to the formation of surface nitrides, internal
nitrides in the matrix and at grain boundaries near the alloy
surface.
[0029] When a mixture of ammonia and hydrogen are used to provide
the reactive nitrogen environment, the ammonia content in the
gaseous mixture of ammonia and hydrogen can range from about 1 vol
% to about 99 vol %, preferably about 2 vol % to about 70 vol
%.
[0030] The preferred temperature range for accomplishing the
conversion of a metal alloy containing chromium, titanium and
mixtures thereof to a nitride cermet is in the range of about
600.degree. C. to about 1150.degree. C.
[0031] When the reactive environment comprises a mixture of
reactive carbon and reactive nitrogen a mixed composition gradient
cermet comprising carbide, nitride, carbonitride and mixtures
thereof results. When the reactive environment is a reactive carbon
and nitrogen environment, carbonitridation reactions are believed
to occur. While not wishing to be bound to the mechanism of the
reactive heat treatment process applicants believe that the
carbonitridation process leads to precipitation of chromium-rich
and titanium carbonitride phases for example Cr.sub.2CN and TiCN on
the alloy surface and within the alloy matrix resulting in a cermet
and particularly a composition gradient carbonitride cermet.
[0032] A reactive carbon and nitrogen environment is defined as an
environment in which the thermodynamic activity of carbon (a.sub.c)
and nitrogen (a.sub.N) in the environment is greater than that of
the alloy. The preferred composition of the reactive carbon and
nitrogen environment comprises at least one of ammonia and nitrogen
and at least one of CO, CH.sub.4, C.sub.2H.sub.6 or C.sub.3H.sub.8.
The composition can further comprise H.sub.2, He, and Ar. In such a
reactive carbon and nitrogen environment at temperatures in the
range of 600.degree. C. to 1150.degree. C. alloys containing
elements such as Cr and Ti which have strong chemical affinities
for carbon and nitrogen undergo rapid carbonitridation reactions.
Similar to carburization or nitridation process, carbonitridation
can lead to the formation of surface carbonitride, internal
carbonitride in the matrix and at grain boundaries near the alloy
surface.
[0033] When the reactive environment is a reactive boron
environment, boridation reactions are believed to occur. While not
wishing to be bound to the mechanism of the reactive heat treatment
process applicants believe that the boridation process leads to
precipitation of chromium-rich and titanium boride phases for
example Cr.sub.2B and TiB.sub.2 on the alloy surface and into the
alloy matrix resulting in a cermet and particularly a composition
gradient boride cermet.
[0034] A reactive boron environment is defined as an environment in
which the thermodynamic activity of boron (a.sub.B) in the
environment is greater than that of the alloy. The preferred
composition of the reactive boron environment comprises for example
at least one of diborane (B.sub.2H.sub.6), BCl.sub.3, and BF.sub.3.
The composition can further comprise H.sub.2, He, and Ar. In such a
reactive boron environment at temperatures in the range of
600.degree. C. to 1150.degree. C. alloys containing elements such
as Cr and Ti which have strong chemical affinities for boron
undergo rapid boridation reactions. Similar to carburization or
nitridation process, boridation can lead to the formation of
surface borides, internal borides in the matrix and at grain
boundaries near the alloy surface.
[0035] When the reactive environment is a reactive oxygen
environment, oxidation reactions are believed to occur. While not
wishing to be bound to the mechanism of the reactive heat treatment
process applicants believe that the oxidation process leads to
precipitation of chromium-rich and titanium oxide phases for
example (Cr,Fe).sub.2O.sub.3, Cr.sub.2O.sub.3 and TiO.sub.2 on the
alloy surface and within the alloy matrix resulting in a cermet and
particularly a composition gradient oxide cermet.
[0036] A reactive oxygen environment is defined as an environment
in which the oxygen potential in the environment is greater than
the oxygen partial pressure in equilibrium with the oxide. The
preferred composition of the reactive oxygen environment comprises
at least one of air, oxygen and CO.sub.2. The composition can
further comprise H.sub.2, He, and Ar. In such a reactive oxygen
environment at temperatures in the range of 600.degree. C. to
1150.degree. C. alloys containing elements such as Cr and Ti which
have strong chemical affinities for oxygen undergo rapid oxidation
reactions. Similar to carburization or nitridation process,
oxidation can lead to the formation of surface oxides, internal
oxides in the matrix and at grain boundaries near the alloy
surface.
[0037] The third step of the process is cooling of the reacted
alloy. The cooling step can include a variety of cooling rates
and/or an intermediate temperature hold before cooling to below
about 40.degree. C. In one embodiment the cooling step comprises
cooling the reacted alloy at a rate in the range of 0.5.degree. C.
per second to 25.degree. C. per second. In another embodiment the
cooling step comprises cooling said reacted alloy to a temperature
in the range of 500.degree. C. to 100.degree. C., holding the
temperature at any temperature in the range of 500.degree. C. to
100.degree. C. for a time period between 5 minutes to 10 hours and
thereafter cooling at a rate in the range of 0.5.degree. C. per
second to 25.degree. C. per second to below about 40.degree. C.
Applicants believe this preferred cooling profile has process and
product advantages.
[0038] The exposure time (the time period the heated alloy is
exposed to the reactive environment) can vary in the range of about
1 hour to 800 hours to achieve various thickness of the carbide,
nitride, carbonitride, boride or oxide cermet on the surface the
metal alloy. An example for carbide cermet is depicted in FIG. 4
where the thickness of the surface carbide cermet formed on various
Fe:Ni:Cr high temperature alloys is plotted as a function of
exposure time at conditions of 1100.degree. C. in 37.3 vol %
CH.sub.4:62.7 vol % H.sub.2 environment. Thus, this example shows
that the process of the instant invention can be used to obtain any
thickness of carbide cermet resulting in a composition gradient
carbide cermet. Alternately, the process can also be used to
completely convert the entire bulk of the chromium, titanium or
mixture of chromium and titanium comprising alloy to a composition
gradient cermet wherein the gradient traverses the entire thickness
of the bulk alloy.
[0039] The thickness of cermet layers can be controlled by the
composition of the reactive environment, the temperature and the
exposure time. Exposure times can be determined experimentally as
depicted in FIG. 4 for a carbide cermet. For thinner layers, the
exposure time will be less, and for thicker layers the exposure
time will be greater. Typical exposure times for a carbide cermet
can range from about 1 hour to about 500 hours, preferably from
about 5 hours to about 300 hours, and more preferably from about 10
hours to about 200 hours. Thus, the exposure time and temperature
are two variables that can provide a desired thickness of cermet
and a desired composition gradient cermet. For a nitride cermet,
typical exposure times can range from about 1 hour to about 800
hours, preferably from about 5 hours to about 500 hours, and more
preferably from about 10 hours to about 300 hours. Thus, the
exposure time and temperature are two variables that can provide a
desired thickness of nitride cermet and a desired composition
gradient nitride cermet.
[0040] Typical layer or cermet structure thickness can range from
at least about 100 microns up to the thickness of the metal alloy
being acted on, preferably from about 5 mm to about 30 mm, more
preferably from about 5 mm to about 20 mm. Layer thickness can be
determined by electron microscopy techniques known to one of
ordinary skill in the art of electron microscopy.
[0041] The instant invention is also applicable to an article
consisting of an amount of chromium-rich or titanium-rich carbide,
nitride, carbonitride, boride, and oxide in combination with a
chromium and titanium containing metal alloy.
[0042] The reactive heat treatment process of the instant invention
results in a composition gradient cermet having erosion resistance
superior to that of the untreated alloy containing chromium,
titanium and mixtures thereof as shown in Example 4. This is
because the erosion resistance of the alloy improves as the cermet
layer develops and provides hardening. In the instant invention,
the amount of reactive carbon, reactive nitrogen, reactive boron,
reactive oxygen diffusing into the metal alloy containing chromium,
titanium and mixtures thereof from the respective reactive
environment is utilized to produce the composition gradient cermet.
The portion of the alloy containing chromium, titanium and mixtures
thereof not converted to cermet, is unchanged and maintains the
physical properties it possessed prior to treatment in accordance
with the instant invention. This composition gradient structure is
particularly advantageous when one desires to use welding as an
attachment method of the carbide cermet to a surface. Furthermore,
a composition gradient cermet can have a superior thermal expansion
match with the underlying metallic substrate with superior
durability under thermal fluctuations. Thus, the cermet layer
provides erosion resistance while retaining physical properties for
the attachment and mechanical reliability of the alloy.
[0043] The composition gradient cermets produced by the process of
instant invention can be used in the temperature range of
300.degree. C. to 800.degree. C. to protect any steel or any other
alloy surface exposed to severe erosion and abrasion. Some
non-limiting examples of these applications include protective
linings, lining tiles for fluid-solids separation cyclones as in
the cyclone of Fluid Catalytic Cracking Unit used in refining
industry, wear plates, nozzle and grid hole inserts, turbine blades
and components subject to erosion flow streams. In these
applications composition gradient cermets prepared by the process
of the instant invention offer a combination of erosion resistance
and toughness as well as an optimization of thermal stresses within
the component. Compared to conventional cermets prepared via powder
metallurgy method, it affords attachment via conventional welding
techniques and a better matching of thermal expansion to the base
steel. It also could provide a superior method of protecting
turbine blades from both oxidation and erosion.
[0044] Another embodiment of the invention is directed to a
composition gradient cermet product prepared by the process
comprising: [0045] heating a metal alloy containing at least one of
chromium and titanium at a temperature in the range of about
600.degree. C. to about 1150.degree. C. to form a heated metal
alloy; [0046] exposing said heated metal alloy to a reactive
environment comprising at least one member selected from the group
consisting of reactive carbon, reactive nitrogen, reactive boron,
reactive oxygen and mixtures thereof in the range of about
600.degree. C. to about 1150.degree. C. for a time sufficient to
provide a reacted alloy; and [0047] cooling said reacted alloy to a
temperature below about 40.degree. C.
[0048] The process of the instant invention can be applied to any
surface. For example the internal surface of any chemical or
petroleum processing reactor comprised of a metal selected from the
group consisting essentially of chromium, titanium and mixtures
thereof at a temperature can be heated to a temperature in the
range of about 600.degree. C. to about 1150.degree. C. and then
exposed to a reactive environment selected from the group
consisting essentially of reactive carbon, reactive nitrogen,
reactive boron, reactive oxygen and mixtures thereof in the range
of about 600.degree. C. to about 1150.degree. C. for a time period
sufficient to provide a reacted internal surface. Upon cooling to
temperatures below about 40.degree. C. a composition gradient
cermet material is formed on the internal surface of the reactor.
The internal surface of the rector comprising the composition
gradient cermet can exhibit enhanced erosion resistance. One
non-limiting illustrative example of this use is the cyclone
separator of a Fluid Catalyst Cracking Unit in oil refining.
[0049] As another example, the surface of any object, for example
the blades of a turbine, can be made of a metal selected from the
group consisting essentially of chromium, titanium and mixtures
thereof at a temperature, heated to a temperature in the range of
about 600.degree. C. to about 1150.degree. C. and then exposed to a
reactive environment selected from the group consisting essentially
of reactive carbon, reactive nitrogen, reactive boron, reactive
oxygen and mixtures thereof in the range of about 600.degree. C. to
about 1150.degree. C. for a time period sufficient to provide a
heat treated object. Upon cooling to temperatures below about
40.degree. C. a composition gradient cermet material is formed on
the surface of the object exposed to the reactive environment.
[0050] The cermet compositions prepared by the process of the
instant invention possess enhanced erosion and corrosion
properties. The erosion rates were determined by the Hot Erosion
and Attrition Test (HEAT) as described in the examples section of
the disclosure. The erosion rate of the gradient cermets prepared
by the process of the instant invention is less than
1.0.times.10.sup.-6 cc/gram of SiC erodant. The corrosion rates
were determined by thermogravimetric (TGA) analyses as described in
the examples section of the disclosure. The corrosion rate of the
gradient cermets prepared by the process of the instant invention
is less than 1.times.10.sup.-10 g.sup.2/cm.sup.4 sec.
[0051] The cermet compositions prepared by the process of the
instant invention possess fracture toughness of greater than about
3 MPam.sup.1/2, preferably greater than about 5 MPam.sup.1/2, and
more preferably greater than about 10 MPam.sup.1/2. Fracture
toughness is the ability to resist crack propagation in a material
under monotonic loading conditions. Fracture toughness is defined
as the critical stress intensity factor at which a crack propagates
in an unstable manner in the material. Loading in three-point bend
geometry with the pre-crack in the tension side of the bend sample
is preferably used to measure the fracture toughness with fracture
mechanics theory. The cermets of the instant invention can be
affixed to metal surfaces by mechanical means or by welding.
EXAMPLES
[0052] The following non-limiting examples are included to further
illustrate the invention.
Example 1
Reactive Heat Treatment of Commercial Alloys
[0053] The reactive heat treatments were conducted on the selected
chromium containing commercial alloys, 304SS, 310SS, Haynes HR120
and Inconel 353MA. The nominal compositions are given below.
TABLE-US-00001 TABLE 1 Compositions of Chromium Containing
Commercial Alloys UNS Alloys No. Composition (wt %) 304 Stainless
Steel S30400 Bal Fe:18.5Cr:9.6Ni:1.4Mn:0.6Si 310 Stainless Steel
S31000 Bal Fe:25.0Cr:21.0Ni:1.5Si:2.0Mn Haynes HR120 N08120 Bal
Fe:33.0Cr:37.0Ni:2.5Mo:2.5W:0.6Si Inconel 353MA S35315 Bal
Fe:24.8Cr:34.8Ni:1.6Si:1.4Mn
[0054] The samples had rectangular geometry with dimensions of
about 1.25 cm.times.1.25 cm.times.1 cm. The sample surfaces were
ground to a 600 grit SiC finish and cleaned ultrasonically in
acetone. The procedure used in the invention was to establish the
kinetics of carburization of the selected alloys in a purely
carburizing environment (CH.sub.4--H.sub.2), which was determined
thermogravimetrically in a Cahn 1000 thermogravimetric unit. The
investigations were carried out in the temperature range,
800.degree. C. to about 1160.degree. C. A coupon was heated to a
temperature of 1100.degree. C. in a hydrogen environment in a
vertical quartz reactor tube and held at that temperature for
approximately 5 minutes. Thereupon, the environment was changed to
37.3 vol % CH.sub.4-62.7 vol % H.sub.2. After 3 hours of exposure,
lowering the furnace surrounding the quartz reactor cools the metal
sample. After the sample has attained room temperature, the surface
microstructure was examined by scanning electron microscopy. By
"Bal" is meant balance of metal in the constituent composition.
[0055] FIG. 5a reveals that a chromium carbide-metal cermet layer
of 400 micron thickness has formed on 310 stainless steel
(54Fe:21Ni:25Cr in wt %) surface after reactive heat treatment at
1100.degree. C. for 3 hours in 37.3 vol % CH.sub.4:62.7 vol %
H.sub.2 environment. A magnified view of cermet microstructure,
revealing the Cr-rich carbide [(Cr.sub.0.6Fe.sub.0.4).sub.7C.sub.3]
and Cr-depleted steel (63Fe:31Ni:6Cr in wt %) to produce a
composite ceramic-metal two-phase structure, is depicted in FIG.
5b. Cr-rich is meant that the metal Cr is of a higher proportion on
a weight basis than the other constituent metals comprising M,
where M is 54Fe:21Ni:25Cr in wt %. In this scanning electron
micrograph the Cr-rich carbides appear dark gray and the metal
appears recessed, because it has etched more deeply than the
carbides. These figures show the final product having the cermet
surface, which is produced in accordance with this invention.
Changing the duration of exposure to the carbon gaseous environment
changes the thickness of the cementite layer. This is shown by the
graph in FIG. 4.
Example 2
Reactive Heat Treatment of Commercial Alloys
[0056] The chromium containing alloys listed above were reactively
heat treated in a tube furnace for 24 hours at 1100.degree. C. in
10 vol % CH.sub.4:90 vol % H.sub.2 environment. Samples were heated
to a temperature of 1100.degree. C. in a hydrogen environment and
held at that temperature for approximately 5 minutes. After 24
hours of exposure, the alloy samples were cooled down. After the
samples reached room temperature (25.degree. C.), the surface
microstructure and the thickness of cermet layer formed on various
alloy surfaces were investigated by cross sectional scanning
electron microscopy. Chemical compositions of M.sub.7C.sub.3
carbide phase and Cr-depleted binder phase were investigated by
semi-quantitative energy dispersive x-ray spectroscopy. The
tendencies of Fe and Ni to partition between the metal matrix and
the carbide precipitates are expected to be different. The
thickness of cermet layers, Cr and Fe contents in M.sub.7C.sub.3
carbide phase and composition of Cr-depleted metal matrix phase
within cermet layers are summarized below.
TABLE-US-00002 TABLE 2 The Thickness, Cr and Fe Contents in
M.sub.7C.sub.3 Carbide Phase and Composition of Cr-depleted Metal
Matrix Phase within Cermet Layers after Reactive Heat Treatment of
Selected Chromium Containing Commercial Alloys Thickness Cr and Fe
of cermet Contents Composition of layer in M.sub.7C.sub.3 Carbide
Cr-depleted metal Alloys (mm) Phase (wt %) matrix phase (wt %) 304
Stainless Steel 2.13 27.0Cr:73.0Fe 76.6Fe:3.6Cr:19.8Ni 310
Stainless Steel 1.90 52.0Cr:48.0Fe 63.0Fe:5.8Cr:30.2Ni Haynes HR120
1.79 58.0Cr:42.0Fe 36.7Fe:4.4Cr:58.9Ni Inconel 353MA 1.50
58.0Cr:42.0Fe 37.4Fe:4.1Cr:58.5Ni
Example 3
Reactive Heat Treatment of Custom-Made Alloys
[0057] Alloys containing different concentrations of Fe, Ni, Cr and
Ti were prepared by arc melting. The arc-melted alloy buttons were
annealed at 1100.degree. C. overnight in inert argon atmosphere and
furnace-cooled to room temperature. Cubical samples of about 1.25
cm.times.1.25 cm.times.0.75 cm were cut from the buttons. The
sample faces were polished to 600-grit finish and cleaned in
acetone. The specimens were exposed to a 10 vol % CH.sub.4:90 vol %
H.sub.2 gaseous environment at 1100.degree. C. for 24 hours.
[0058] Detailed electron microscopy and chemical analysis of the
alloys after exposure indicated that specific alloy compositions in
the Fe--Ni--Cr system generate cermet structure with M.sub.7C.sub.3
carbide and metal phase. The thickness of cermet layers, Cr and Fe
contents in M.sub.7C.sub.3 carbide phase and compositions of
Cr-depleted metal matrix phase within cermet layers are summarized
in Table 3. By contrast to the example of selected commercial
alloys, relatively thick cermet layer was obtained and the
concentration of Cr in metal matrix phase formed in the Fe--Ni--Cr
system was relatively enriched. Higher Cr concentration in metal
phase enhances oxidation resistance at higher temperatures. The
optical microscopic image shown in FIG. 6 indicates the size and
morphology of M.sub.7C.sub.3 (M=Cr and Fe) carbide-metal cermet
structure in the surface regions after reactive heat treatment at
1100.degree. C. for 24 hours in 10 vol % CH.sub.4:90 vol %
H.sub.2.
[0059] An alloy of composition 60Fe:25Cr:10Ni:5Ti (in wt %)
generates cermet structure with mixed TiC and M.sub.7C.sub.3
carbide and metal phase. The thickness of cermet layers, Cr and Fe
contents in M.sub.7C.sub.3 carbide phase and compositions of
Cr-depleted metal matrix phase within cermet layers are summarized
in Table 3. The optical microscopic image shown in FIG. 7 indicates
the size and morphology of mixed TiC and M.sub.7C.sub.3 (M=Cr and
Fe) carbide-metal cermet structure in the surface regions after
reactive heat treatment at 1100.degree. C. for 24 hours in 10 vol %
CH.sub.4:90 vol % H.sub.2.
TABLE-US-00003 TABLE 3 The Thickness, Cr and Fe Contents in
M.sub.7C.sub.3 Carbide Phase and Composition of Cr-depleted Metal
Matrix Phase within Cermet Layers after Reactive Heat Treatment of
Fe--Ni--Cr--Ti system Cr and Fe Thickness Contents of cermet in
M.sub.7C.sub.3 Composition of layer Carbide Cr-depleted metal
Alloys (wt %) (mm) Phase (wt %) matrix phase (wt %) 55Fe:35Cr:10Ni
3.17 48.0Cr:52.0Fe 65.3Fe:7.9Cr:26.8Ni 45Fe:45Cr:10Ni 3.35
77.1Cr:22.9Fe 67.6Fe:13.8Cr:18.6Ni 35Fe:55Cr:10Ni 1.00
79.0Cr:21.0Fe 52.1Fe:7.0Cr:40.9Ni 60Fe:25Cr:10Ni:5Ti 2.50
66.3Cr:33.7Fe 74.5Fe:9.1Cr:16.4Ni
Example 4
Erosion Testing
[0060] The reactive heat treatments were conducted on commercial
310SS to prepare samples for Hot Erosion and Attrition Test (HEAT).
The 310SS samples had rectangular geometry with dimensions of about
2.0 inch.times.2.0 inch.times.0.5 inch. One sample was reactively
heat treated in a tube furnace for 138 hours at 1100.degree. C. in
10 vol % CH.sub.4:90 vol % H.sub.2 environment and named as
C310SS1100. The other sample was reactively heat treated in a tube
furnace for 96 hours at 1150.degree. C. in 10 vol % CH.sub.4:90 vol
% H.sub.2 environment and named as C310SS1150.
[0061] Erosion Rate was measured as the volume of cermet,
refractory, or comparative material removed per unit mass of
erodant particles of a defined average size and shape entrained in
a gas stream, and had units of cc/gram (e.g., <0.001 cc/1000
gram of SiC). Key defined erosion test conditions are erodant
material and size distribution, velocity, mass flux, angle of
impact of the erodant as well as erosion test temperature and
chemical environment.
[0062] Erosion Loss of Cermet was measured by the Hot Erosion and
Attrition Test (HEAT). The carrier gas and atmosphere, simulating
the intended use, but preferably air, were heated to the same
temperature. HEAT tests were preferably operated as follows. In the
preferred operation of the HEAT test, the cermet specimen blocks
(C310SS1100 and C310SS1150) of about 2 inch square and about 0.5
inch thickness were weighed to an accuracy of .+-.0.01 mg. The
center of one side of the bock was subjected to 1200 g/min of SiC
particles entrained in an air jet exiting from a riser tube with a
0.5 inch diameter where the end of the riser tube was 1 inch from
the target disk. The 58 .mu.m angular SiC particles used as the
erodant were 220 grit #1 Grade Black Silicon Carbide (UK Abrasives,
Inc., Northbrook, Ill.). The erodant velocity impinging on cermet
targets was 45.7 m/sec (150 ft/sec) and the impingement angle of
the gas-erodant stream on the target was 45.degree..+-.5.degree.,
preferably 45.degree..+-.2.degree. between the main axis of the
riser tube and the surface of the specimen disk. The carrier gas
was air for all tests. The erosion tests in the HEAT unit were
performed at 732.degree. C. (1350.degree. F.) for 7 hours. After
testing the cermet specimen were again weighed to an accuracy of
.+-.0.01 mg, to determine the weight loss. The erosion rate was
equal to the volume of material removed per unit mass of erodant
particles entrained in the gas stream, and has units of cc/gram.
Improvement in Table 4 is the reduction of weight loss due to
erosion compared to a value of 1.0 for the standard RESCOBOND.TM.
AA-22S (Resco Products, Inc., Pittsburgh, Pa.). AA-22S typically
comprises at least 80.0% Al.sub.2O.sub.3, 7.2% SiO.sub.2, 1.0%
Fe.sub.2O.sub.3, 4.8% MgO/CaO, 4.5% P.sub.2O.sub.5 in wt %.
Micrographs of the eroded surface were electron microscopically
taken to determine damage mechanisms. Table 4 summarizes the
erosion loss of selected cermets as measured by the HEAT
TABLE-US-00004 SUMMARY OF HEAT RESULTS Starting Finish Weight Bulk
Improvement Weight Weight Loss Density Erodant Erosion [(Normalized
Sample (g) (g) (g) (g/cc) (g) (cc/g) erosion).sup.-1] C310SS1100
246.6146 243.4477 3.1669 7.30 5.04E+5 8.6076E-7 1.2 C310SS1150
247.5390 244.7651 2.7739 7.37 5.04E+5 7.4678E-7 1.4
[0063] The HEAT test measures very aggressive erodant particles.
More typical particles are softer and cause lower erosion rates.
For example FCCU catalysts are based on alumina silicates which are
typically softer than aluminas which are typically much softer than
SiC.
Example 5
Corrosion Testing
[0064] Each of the cermets of Examples 4 was subjected to an
oxidation test. The procedure employed was as follows:
[0065] 1) A specimen cermet of about 10 mm square and about 1 mm
thick was polished to 600 grit diamond finish and cleaned in
acetone.
[0066] 2) The specimen was then exposed to 100 cc/min air at
800.degree. C. in thermogravimetric analyzer (TGA).
[0067] 3) Step (2) was conducted for 65 hrs at 800.degree. C.
[0068] 4) After 65 hours the specimen was allowed to cool to
ambient temperature.
[0069] 5) Thickness of oxide scale was determined by cross
sectional microscopy examination of the corrosion surface.
[0070] The thickness of oxide scale was ranging about 0.5 .mu.m to
about 1.5 .mu.m. The cermet compositions exhibited a corrosion rate
less than about 1.times.10.sup.-11 g.sup.2/cm.sup.4s or an average
oxide scale of less than 30 .mu.m thickness when subject to 100
cc/min air at 800.degree. C. for at least 65 hours. These represent
superior corrosion resistance.
* * * * *