U.S. patent application number 16/135531 was filed with the patent office on 2019-04-11 for cobalt-free alloys.
This patent application is currently assigned to ROLLS-ROYCE plc. The applicant listed for this patent is ROLLS-ROYCE plc. Invention is credited to David A STEWART.
Application Number | 20190106776 16/135531 |
Document ID | / |
Family ID | 60326771 |
Filed Date | 2019-04-11 |
United States Patent
Application |
20190106776 |
Kind Code |
A1 |
STEWART; David A |
April 11, 2019 |
COBALT-FREE ALLOYS
Abstract
A cobalt-free alloy is made essentially of: an effective
concentration of up to 2.5 percent carbon; 12 to 58 percent
chromium; 7 to 9 percent manganese; 4 to 5 percent silicon; 4 to 6
percent nickel; 0.08 to 0.2 percent nitrogen; the balance being
iron plus incidental impurities.
Inventors: |
STEWART; David A; (Derby,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROLLS-ROYCE plc |
London |
|
GB |
|
|
Assignee: |
ROLLS-ROYCE plc
London
GB
|
Family ID: |
60326771 |
Appl. No.: |
16/135531 |
Filed: |
September 19, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 27/06 20130101;
B22F 2999/00 20130101; B22F 2998/10 20130101; B22F 2998/10
20130101; B22F 3/15 20130101; C22C 33/0285 20130101; B22F 7/04
20130101; C22C 38/001 20130101; C22C 38/34 20130101; C22C 38/56
20130101; C22C 33/0285 20130101; B22F 2999/00 20130101; C22C 38/58
20130101; B22F 2009/0824 20130101; B22F 2007/042 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/34 20060101 C22C038/34; C22C 38/56 20060101
C22C038/56; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 11, 2017 |
GB |
1716640.6 |
Claims
1. An alloy consisting essentially of, by weight: an effective
concentration of up to 2.5 percent carbon; 12 to 58 percent
chromium; 7 to 9 percent manganese; 4 to 5 percent silicon; 4 to 6
percent nickel; 0.08 to 0.2 percent nitrogen; the balance being
iron plus incidental impurities.
2. The alloy of claim 1, in which there is 16 percent by weight
chromium forming a first population thereof, and a second
population of chromium at a concentration such that the molar ratio
between chromium and carbon is 7:3.
3. The alloy of claim 1, in which there is 16 percent by weight
chromium forming a first population thereof, and a second
population of chromium at a concentration a concentration such that
the molar ratio between chromium and carbon is 23:6.
4. The alloy of claim 1, in which there is, by weight: 0.8 to 1.2
percent carbon.
5. The alloy of claim 1, in which there is, by weight: 1.7 to 2.0
percent carbon.
6. The alloy of claim 1, in which there is, by weight: 2.2 to 2.5
percent carbon.
7. Powdered form of the alloy of claim 1.
8. A method comprising applying the alloy of claim 1 as a component
part of a nuclear reactor.
9. An article comprising the alloy of claim 1.
10. An article having a coating comprising the alloy of claim 1.
Description
TECHNICAL FIELD
[0001] This disclosure relates to cobalt-free alloys.
BACKGROUND
[0002] Cobalt-base alloys such as the Stellites.RTM. have been used
extensively to form components or to provide hardfacing due to
their high wear resistance, especially at high temperatures, with
very good corrosion resistance. One application of such cobalt-base
alloys is as a hardfacing on valve seats.
[0003] However, in a nuclear power plant, the use of cobalt-base
alloys give rise to the potential for cobalt activation in the
neutron flux which results in the radioisotope cobalt-60. The long
half-life of cobalt-60 means that the use of cobalt-base alloys is
generally undesirable in nuclear power plants.
SUMMARY
[0004] The invention is directed towards cobalt-free alloys. One
such alloy consists essentially of, by weight:
[0005] an effective concentration of up to 2.5 percent carbon;
[0006] 12 to 58 percent chromium;
[0007] 7 to 9 percent manganese;
[0008] 4 to 5 percent silicon;
[0009] 4 to 6 percent nickel;
[0010] 0.08 to 0.2 percent nitrogen;
[0011] the balance being iron plus incidental impurities.
DETAILED DESCRIPTION
[0012] The alloy of the present invention has an iron base. The
iron is, in the solid form of the alloy, in a predominantly
austenitic crystallographic matrix. This is achieved by a
depression of the martensite start temperature. By selecting a
predominantly austenitic matrix, the formation of low-temperature
phases which may occur in duplex matrixes is inhibited.
[0013] It will be appreciated that it is acceptable for there to be
a quantity of other phases present in the solid form of the alloy
in addition to the austenite. For example, it is acceptable for
ferrite to be present, with no deleterious effects on performance.
In a specific example, up to 8 percent of the iron may be in the
form of ferrite.
[0014] The alloy further comprises carbon and chromium. As carbides
precipitate on cooling, an appropriately-controlled cooling rate
will allow the chromium to diffuse into chromium-depleted zones
formed by the carbide precipitation. This is aided by the provision
of a sufficient quantity of the chromium to, at meta-equilibrium,
form a first population dissolved in the austenite for corrosion
resistance, and a second population in the form of carbides. In one
embodiment, the carbides may be either of the form Cr.sub.7C.sub.3.
They may alternatively be in the form of Cr.sub.23C.sub.6.
Alternatively there could be combination of those carbides. Other
forms of chromium carbide may also be selectively formed.
[0015] The carbon is provided at an effective concentration of up
to 2.5 percent by weight. The chromium is provided at a
concentration of between 12 and 58 percent by weight. The lower
bound of the chromium concentration is selected so as to ensure a
sufficient quantity (a first population) is dissolved in the
austenitic matrix for corrosion resistance. A second population of
chromium combines with the carbon to form chromium carbides. The
chromium carbides are provided at an effective concentration for
galling resistance. They also provide general wear resistance.
[0016] In one embodiment, the first population of chromium is
provided at a concentration of 16 percent by weight. The second
population of chromium is provided at a concentration such that the
molar ratio between chromium and carbon is 7:3. This allows the
formation of Cr.sub.7C.sub.3. Alternatively, the second population
of chromium is provided at a concentration such that the molar
ratio between chromium and carbon is 23:6. This allows the
formation of Cr.sub.23C.sub.6.
[0017] In one embodiment, the alloy has, by weight, 0.8 to 1.2
percent carbon. This produces an alloy which has comparable carbide
content to Stellite 6. In another embodiment, the alloy has, by
weight, 1.7 to 2.0 percent carbon. This produces an alloy which has
comparable carbide content to Stellite 12. In another embodiment,
the alloy has, by weight, 2.2 to 2.5 percent carbon. This produces
an alloy which has comparable carbide content to Stellite 3.
[0018] It will be appreciated that chromium is a ferrite
stabiliser, and indeed in high concentrations can completely
eliminate the austenitic phase. This effect is offset however by
provision of a sufficient concentration of austenite stabilisers to
ensure the austenitic matrix is metastable at all temperatures that
the alloy will be exposed to during service.
[0019] The austenite stabilisers maintain a predominantly
austenitic crystallographic matrix in a metastable condition so as
to prevent formation of ferrite during service. In this way, the
quantity of dissolved chromium remains sufficient to maintain
corrosion resistance.
[0020] The austenite stabilisers are nickel, manganese and
nitrogen, with the overall concentration of these elements being
selected so as to effect the required degree of austenite
stabilisation.
[0021] The concentration of nickel required to depress the
martensite start temperature sufficiently also increases the
stacking fault energy of the austenitic matrix. The increase in
stacking fault energy results in the matrix having a lower
tolerance to defects introduced by deformation, such as galling.
Thus, the nickel-equivalents manganese and nitrogen are also used
so as to reduce the required nickel concentration.
[0022] Nitrogen also operates to reduce stacking fault energy
throughout the entire austenitic matrix. This complements the
carbide population in terms of providing resistance to galling.
[0023] It will be appreciated that chromium and manganese both
increase the solubility of the nitrogen in the austenite, whilst
having only a moderate propensity to form nitrides, and/or
carbonitrides, compared with, for example, titanium which is a
strong nitride former. In this way, higher concentrations of
nitrogen may be used, thereby further reducing the amount of nickel
required and further reducing stacking fault energy.
[0024] Furthermore, manganese not only acts as an austenite
stabiliser and as a nitrogen solubility modifier, but it also acts
to increase the hardenability of the alloy. Again, this acts to
increase the galling resistance of the alloy.
[0025] Nickel also improves the corrosion resistance of the alloy
over and above that achievable by provision of solely chromium for
such a purpose. Manganese is abundant, as is nitrogen, thereby
reducing difficulties in terms of sourcing supply of nickel.
[0026] The austenite stabilisers are provided in the following
concentrations by weight: 7 to 9 percent manganese; 4 to 6 percent
nickel; 0.08 to 0.2 percent nitrogen.
[0027] Additional carbide formers may be provided to assist in
maintaining the concentration of chromium dissolved in the matrix
constant. In one embodiment, one or more additional carbide formers
over and above the chromium are included whose carbides have a
temperature of formation greater than that of chromium carbide. In
a specific embodiment, the one or more additional carbide formers
are selected from the group consisting of titanium and vanadium.
Other additional carbide formers are possible singularly or in
combination, for example hafnium. The provision of additional
carbide formers may reduce the degree of sensitisation.
[0028] However, in another embodiment, the carbide population is
solely chromium-based. It has been found that whilst this may
require measures to be taken to reduce sensitisation, it increases
the corrosion resistance of the alloy relative to those embodiments
where the additional carbide formers are provided.
[0029] In addition, it has been found that in gas atomisation
production processes, the fast-quench from the melt does not give
time for chromium carbides to form. Thus, the chromium and carbon
content in the powder form of the alloy of the present invention
remains in solution. The chromium carbides then precipitate during
a HIP (hot isostatic pressing) process to produce or coat an
article. Other carbides such as titanium carbide and vanadium
carbide have been found to form and precipitate at higher
temperatures, for example whilst the iron is still liquid, which in
certain gas atomisation apparatus can cause nozzle blockages.
[0030] In order to facilitate production of a powdered form of the
alloy, a melt fluidity promotor is included in the form of silicon
at a concentration of 4 to 5 percent by weight. The powdered form
of the alloy may be produced by gas atomisation.
[0031] It is envisaged that the alloys described herein may be
provided in powdered form. Thus they may be used in, for example, a
HIP process or a weld deposition process. The alloys may be used in
a component part of a nuclear reactor. More generally, the alloys
may constitute an article, or may constitute a coating of an
article, for example a hardfacing. Additional heat treatments may
be performed, for example to increase the quantity of
austenite.
[0032] It will be understood that except where mutually exclusive,
any of the features of the invention may be employed separately or
in combination with any other features and the disclosure extends
to and includes all combinations and sub-combinations of one or
more features described herein.
* * * * *