Patent Application
20190106776
This disclosure relates to cobalt-free alloys.
Cobalt-base alloys such as the Stellites® 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.
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.
The invention is directed towards cobalt-free alloys. One such alloy consists 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.
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.
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.
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 Cr7C3. They may alternatively be in the form of Cr23C6. Alternatively there could be combination of those carbides. Other forms of chromium carbide may also be selectively formed.
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.
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 Cr7C3. 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 Cr23C6.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1716640.6 | Oct 2017 | GB | national |
