Ferritic stainless steel alloy and turbocharger moving components formed from the stainless steel alloy

Abstract

This application relates to ferritic stainless steel alloys and turbocharger moving components formed from the stainless steel alloys. A ferritic stainless steel alloy and a turbocharger moving component are provided. The ferritic stainless steel alloy comprises or consists of, by weight, the following components: approximately 20% to approximately 35% chromium, less than approximately 2% nickel (i.e., 0% to approximately 2%), approximately 1% to approximately 4% carbon, approximately 1.5% to approximately 1.9% silicon, less than approximately 0.4% nitrogen (i.e., 0% to approximately 0.4%), approximately 0.5% to approximately 15% molybdenum, less than approximately 1% niobium (i.e., 0% to approximately 1%), and the balance iron, as well as other unavoidable impurities present in trace amounts. The turbocharger moving component is at least partially made of this stainless steel alloy.

Description

Technical Field

[0001] The present disclosure generally relates to iron-based alloys, such as ferritic stainless steel alloys, and fabricated articles formed therefrom. More particularly, the present disclosure relates to stainless steel alloys for use in, for example, turbine and turbocharger kinematic components, where such kinematic components exhibit improved wear resistance. Background Art

[0002] In the case of a turbine engine, a turbocharger uses the heat and volumetric flow of engine exhaust gases to pressurize or boost the intake air stream entering the combustion chamber. Specifically, the exhaust gases from the engine are introduced into the turbocharger turbine housing. A turbine is mounted within the housing, and the exhaust gas flow causes the turbine to rotate. The turbine is mounted on one end of a shaft that has a radial air compressor mounted on its opposite end. Thus, the rotational movement of the turbine also causes the air compressor to rotate. The rotational movement of the air compressor causes the intake air to enter the compressor housing and be pressurized or boosted before being mixed with fuel and burned in the engine combustion chamber.

[0003] Various systems within a turbocharger include friction interfaces, i.e., the surfaces of components that interact with each other and move relative to each other when the turbocharger is operating. Such components - commonly referred to as kinematic components - can be prone to friction and wear, even when the temperature (relative to other parts of the turbocharger) is not elevated, which reduces their service life. Examples of turbocharger systems that can include kinematic components typically include various components such as shafts, bushings, valves, etc., which are kinematic components because they interact with each other and move relative to each other and thus they are subject to frictional wear. In the prior art, a great deal of effort has been put into high-temperature wear-resistant applications where austenitic stainless steels are used, but such stainless steels have proven to be undesirable in relatively low-temperature applications due to their relatively high cost. Grade 310 stainless steel may have been used for such components, but such stainless steel has proven to be undesirable due to its relatively high cost. Therefore, effective (and less expensive) alternatives would be welcome in the art, provided that the appropriate material properties are retained. Therefore, effective (and less expensive) ferritic options would be welcome in the art, provided that the appropriate material properties are retained.

[0004] Accordingly, it is desirable to provide materials suitable for fabricating kinematic components of a turbine engine that are resistant to wear and are suitable for relatively low-temperature applications in a turbocharger. Further, in conjunction with this background of the invention and the accompanying drawings, other desirable features and characteristics of the present inventive subject matter will become apparent from the following detailed description of the present inventive subject matter and the appended claims. Summary of the Invention

[0005] A ferritic stainless steel alloy and turbocharger moving parts made from such alloys are provided.

[0006] In one embodiment, by way of example only, the ferritic stainless steel alloy comprises or consists of, by weight, the following components: approximately 20% to approximately 35% chromium, less than approximately 2% nickel (i.e., 0% to approximately 2%), approximately 1% to approximately 4% carbon, approximately 1.5% to approximately 1.9% silicon, less than approximately 0.4% nitrogen (i.e., 0% to approximately 0.4%), approximately 0.5% to approximately 15% molybdenum, less than approximately 1% niobium (i.e., 0% to approximately 1%), and the balance iron, and other unavoidable / inevitable impurities present in trace amounts.

[0007] Regarding the foregoing alloy embodiments: the amount of chromium can be limited to approximately 22% to approximately 33%, or approximately 24% to approximately 31%, or approximately 26% to approximately 29%; or alternatively, or in addition, the amount of nickel can be limited to approximately 0.1% to approximately 1.5%, or approximately 0.2% to approximately 1%; or alternatively, or in addition, the amount of carbon can be limited to approximately 1.5% to approximately 3.5%, or approximately 2% to approximately 3%; or alternatively, or in addition, the amount of silicon can be limited to approximately 1.6% to approximately 1.8%; or alternatively, or in addition, the amount of nitrogen can be limited to approximately 0.05% to approximately 0.3%, or approximately 0.1% to approximately 0.2%; or alternatively, or in addition, the amount of niobium can be limited to approximately 0.05% to approximately 0.7%, or approximately 0.1% to approximately 0.5%; and, or alternatively, or in addition, the amount of molybdenum can be limited to approximately 2% to approximately 13%, or approximately 4% to approximately 11%, or approximately 6% to approximately 9%.

[0008] In another embodiment, by way of example only, at least part of the turbocharger moving parts are made using a ferritic stainless steel alloy that comprises or consists of, by weight, the following components: approximately 20% to approximately 35% chromium, less than approximately 2% nickel (i.e., 0% to approximately 2%), approximately 1% to approximately 4% carbon, approximately 1.5% to approximately 1.9% silicon, less than approximately 0.4% nitrogen (i.e., 0% to approximately 0.4%), approximately 0.5% to approximately 15% molybdenum, less than approximately 1% niobium (i.e., 0% to approximately 1%), and the balance iron, and other unavoidable / inevitable impurities present in trace amounts.

[0009] Regarding the foregoing embodiments of the moving components of a turbocharger, particularly the ferritic stainless steel alloy for manufacturing the same: the amount of chromium can be limited to about 22% to about 33%, or about 24% to about 31%, or about 26% to about 29%; or alternatively or in addition, the amount of nickel can be limited to about 0.1% to about 1.5%, or about 0.2% to about 1%; or alternatively or in addition, the amount of carbon can be limited to about 1.5% to about 3.5%, or about 2% to about 3%; or alternatively or in addition, the amount of silicon can be limited to about 1.6% to about 1.8%; or alternatively or in addition, the amount of nitrogen can be limited to about 0.05% to about 0.3%, or about 0.1% to about 0.2%; or alternatively or in addition, the amount of niobium can be limited to about 0.05% to about 0.7%, or about 0.1% to about 0.5%; and, or alternatively or in addition, the amount of molybdenum can be limited to about 2% to about 13%, or about 4% to about 11%, or about 6% to about 9%.

[0010] In a specific embodiment of the present disclosure, there is disclosed a moving component of a turbocharger that includes, at least as part of its constituency, a ferritic stainless steel alloy, wherein the ferritic stainless steel alloy comprises or consists of, by weight, the following components: about 24% to about 31% chromium, about 0.2% to about 1% nickel, about 2% to about 3% carbon, about 1.6% to about 1.8% silicon, about 0.1% to about 0.2% nitrogen, about 4% to about 11% molybdenum, about 0.1% to about 0.5% niobium, and the balance iron, and other inevitable / unavoidable impurities present in trace amounts.

[0011] This summary is provided to introduce a selection of concepts in a simplified form that will be further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to help determine the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The subject matter of the present invention will be described hereinafter in connection with the following drawings, in which like reference numerals denote like elements, and in which:

[0013] The figure is a system view of an embodiment of a turbocharged internal combustion engine according to the present disclosure. DETAILED DESCRIPTION

[0014] The following detailed description is merely exemplary and is not intended to limit the invention or the application and use of the invention. As used herein, the word "exemplary" means "used as an example, instance or illustration". Therefore, any embodiment described herein as "exemplary" is not necessarily interpreted as being more preferred or more advantageous than other embodiments. All embodiments described herein are exemplary embodiments, which are provided to enable those skilled in the art to make or use the invention, rather than to limit the scope of the invention defined by the claims. In addition, there is no intention to be bound by any express or implied theory presented in the previous technical field, background technology, summary of the invention or the following detailed description.

[0015] Unless specifically stated or apparent from the context, as used herein, the term "about" is understood to be within the normal tolerance in the art, such as within 2 standard deviations of the mean. "About" may be understood to be within 10%, 5%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. "About" may alternatively be understood to mean the exact value stated. Unless it is clear from the context that this is not the case, all numerical values provided herein are modified by the term "about".

[0016] All ferritic stainless steel alloys described herein may be understood as: (1) "comprising" the recited elements in their various percentages in the open-ended case, or (2) "consisting of" the recited elements in their various percentages in the closed-ended case. Alternatively, the ferritic stainless steel alloys described herein may be understood as (3) "consisting essentially of" the recited elements in their various percentages, wherein other elements may be present in amounts that do not affect the novel / non-obvious properties of the alloy. Therefore, as used herein, the terms "comprising," "consisting of," and "consisting essentially of" should be understood to apply to all ranges of alloy compositions disclosed herein.

[0017] All embodiments and implementations of the ferritic stainless steel alloy, turbocharger moving parts and methods of making the same described herein are exemplary embodiments, which are provided to enable those skilled in the art to make or use the invention, rather than to limit the scope of the invention as defined by the claims. Of course, the described embodiments should not be considered limited to such parts, but rather they may be considered applicable to any manufactured product in which an iron alloy or stainless steel alloy may be used. In addition, there is no intention to be bound by any express or implied theory presented in the previous technical field, background technology, summary of the invention or the following detailed description.

[0018] As described above, the present disclosure relates to ferritic stainless steel alloys for moving components of turbochargers (for various vehicles and other applications) for wear purposes related to the use and implementation of such moving components. Thus, for the sake of completeness of the description, reference is made to Figure 1 , an exemplary embodiment of a turbocharger 101 having a radial turbine and a radial compressor includes a turbocharger housing and a rotor configured to rotate about a rotor axis of rotation 103 within the turbocharger housing on a thrust bearing and two sets of journal bearings (one for each respective rotor impeller) or alternatively on other similar supporting bearings during operation of the turbocharger. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 (i.e., a central housing that houses the bearings) that connects the turbine housing to the compressor housing. The rotor includes a radial turbine impeller 111 located substantially within the turbine housing 105, a radial compressor impeller 113 located substantially within the compressor housing 107, and a shaft 115 that extends along the rotor axis of rotation 103 and passes through the bearing housing 109 to connect the turbine impeller 111 to the compressor impeller 113.

[0019] The turbine housing 105 and the turbine impeller 111 form a turbine configured to circumferentially receive a high-pressure, high-temperature exhaust gas stream 121 from an engine, such as an exhaust manifold 123 of an internal combustion engine 125. The turbine impeller 111 (and thus the rotor) is driven to rotate about the rotor axis of rotation 103 by the high-pressure, high-temperature exhaust gas stream, which becomes a low-pressure, low-temperature exhaust gas stream 127 and is axially released into an exhaust system (not shown).

[0020] The compressor housing 107 and the compressor impeller 113 form a compressor stage. The compressor impeller (driven to rotate by the turbine impeller 111 driven by the exhaust gas) is configured to compress axially received input air (such as ambient air 131, or in a multi-stage compressor, already pressurized air from a previous stage) into a pressurized air stream 133, which is circumferentially ejected by the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature that is higher than the temperature of the input air.

[0021] Optionally, the pressurized air stream may be directed through a convectively cooled chargeair cooler 135 configured to dissipate heat from the pressurized air stream and increase its density. The resulting cooled and pressurized output air stream 137 is directed into an intake manifold 139 on the internal combustion engine or into a subsequent stage series compressor. Operation of the system is controlled by an ECU 151 (engine control unit) that is connected via a communication link 153 to the remainder of the system.

[0022] Exemplary embodiments of the present disclosure exist in motor vehicles equipped with gasoline or diesel-powered internal combustion engines and turbochargers. The turbocharger is equipped with a unique combination of features that, in various embodiments, can provide efficiency benefits by relatively restricting the amount (and kinetic energy) of secondary flow in the turbine and / or compressor compared to similar unmodified systems. The stainless steel alloy used for the turbocharger can have an operating temperature of up to approximately 800 °C (or up to approximately 850 °C), for example. Some embodiments of the present disclosure relate to stainless steel alloys that contain iron alloyed with various alloying elements, as described in more detail below in weight percentages based on the total weight of the alloy. As described below, the description of the specific effects of materials containing certain weight percentages is specific to the alloys of the present disclosure and should not be understood to apply to any other alloys. Additionally, the description of the specific effects of materials containing certain weight percentages is not intended to limit the scope or content of the present disclosure.

[0023] Thus, in one embodiment, the stainless steel alloy of the present disclosure contains from about 20% to about 35% chromium (Cr), such as from about 22% to about 33% Cr, such as from about 24% to about 31% Cr, or from about 26% to about 29% Cr. Chromium hardens and toughens the steel and improves its corrosion resistance. It has been found that if Cr is added in excess, large primary carbides of Cr are formed, resulting in extremely high brittleness. Therefore, the content of Cr is preferably limited to a maximum of about 35% to maintain an appropriate volume fraction within the stainless steel to obtain corrosion resistance.

[0024] In one embodiment, the stainless steel alloy of the present disclosure minimizes nickel to a feasible extent because nickel is associated with the formation of the austenite phase. Thus, the stainless steel alloy contains less than about 2% nickel (Ni) (i.e., from about 0% to about 2% nickel), such as from about 0.1% to about 1.5% Ni, such as from about 0.2% to about 1% Ni. Just in terms of containing nickel, it can have some benefits regarding formability, weldability, and ductility.

[0025] In one embodiment, the stainless steel alloy of the present disclosure contains from about 0.5% to about 15% molybdenum (Mo), such as from about 2% to about 13% Mo, such as from about 4% to about 11% Mo, or from about 6% to about 9% Mo. Molybdenum is a ferrite stabilizer and is thus included in the stainless steel alloy of the present disclosure to obtain a ferritic alloy. Additionally, molybdenum has the benefit of providing pitting and corrosion resistance to the alloy.

[0026] In one embodiment, the stainless steel alloy of the present disclosure comprises from about 1% to about 4% carbon (C), such as from about 1.5% to about 3.5% C, such as from about 2% to about 3% C. C functions to improve the sinterability of the alloy. When present within the relatively high ranges disclosed herein, C also forms eutectic carbides with niobium (which may also be included in the alloy as discussed in more detail below), which improves wear resistance. In order to effectively exhibit these functions, the amount of C should be 1% or more. Additionally, C effectively strengthens the material through solid solution strengthening. To maximize corrosion resistance, the C content is reduced to about 4% and lower.

[0027] In one embodiment, the stainless steel alloy of the present disclosure comprises from about 1.5% to about 1.9% silicon (Si), such as from about 1.6% to about 1.8% Si. A particular embodiment may employ about 1.7% Si. Si has the effect of enhancing the stability of the alloy's metallic structure and its oxidation resistance. Additionally, when present in an amount greater than about 1.5%, Si functions as a deoxidizer and is also effective in improving castability and reducing pinholes in the resulting sintered product. If the Si content is too high, Si deteriorates the mechanical properties of the stainless steel, such as impact toughness. Thus, the content of Si is preferably limited to about 1.9% and lower.

[0028] In one embodiment, the stainless steel alloy of the present disclosure comprises less than about 0.4% nitrogen (N) (i.e., from about 0% to about 0.4%), such as from about 0.05% to about 0.3% N, or from about 0.1% to about 0.2% N. If desired, adding nitrogen to the alloy in the foregoing amounts can improve ductility, enabling the alloy to be cast into the desired form (i.e., turbocharger moving parts). Nitrogen, if included, should be limited to not more than about 0.4% to avoid brittleness in the formed alloy. Thus, the alloy of the present disclosure may contain nitrogen in the foregoing amounts.

[0029] In one embodiment, the ferritic stainless steel alloy of the present disclosure optionally comprises less than about 1% niobium (Nb) (i.e., from about 0% to about 1%), such as from about 0.05% to about 0.7% Nb, such as from about 0.1% to about 0.5% Nb. The wear-resistant ferritic steel of the present disclosure, if it contains Nb, can provide some castability benefits by forming eutectic carbides of Nb and may also provide benefits in terms of strength and ductility. However, since Nb is relatively expensive, if Nb is included, it can be minimized within the foregoing amounts.

[0030] Certain inevitable / unavoidable impurities may also be present in the stainless steel alloy of the present disclosure, such as those described below with respect to phosphorus and sulfur (the amounts of these and other impurities are minimized as much as possible).

[0031] In one embodiment, phosphorus (P) may be present in the alloy, but is minimized to about 0.04% or less. P is seeded at grain boundaries or interfaces and may deteriorate corrosion resistance and toughness. Therefore, the content of P is reduced as low as possible. Preferably, considering the efficiency of the refining process, the upper limit content of P is limited to 0.04%. The content of harmful impurities such as P is as small as possible. However, due to cost issues associated with removing these impurities, the P content is limited to 0.04%.

[0032] In one embodiment, sulfur (S) may be present in the alloy, but is minimized to about 0.01% or less. S in steel deteriorates hot workability and can form sulfide inclusions that adversely affect pitting corrosion resistance. Therefore, it should be limited to less than 0.01%. S deteriorates hot formability, thereby deteriorating corrosion resistance. Therefore, the content of S is reduced as low as possible. The content of harmful impurities such as S (sulfur) is as small as possible. However, due to cost issues associated with removing these impurities, the S content is limited to about 0.01%.

[0033] In some embodiments, high-cost elements included in stainless steels in the prior art are specifically excluded from the alloy (except for unavoidable amounts of impurities). These elements that can be excluded are, for example, Mn, W, Co, and V. In various embodiments, any number or combination of the foregoing elements can be excluded.

[0034] The disclosed alloy, which is a stainless steel alloy, further comprises the balance of iron (Fe). As used herein, the term "balance" refers to the amount remaining to reach 100% of the entire alloy by weight. It should be understood that if the embodiment "comprises" the element, "consists of" the element, or "consists essentially of" the element and the balance is Fe, the amount may vary.

[0035] The manufactured articles described herein, such as turbocharger moving parts made of the above-described stainless steel alloy, can be formed using a sintering process. For example, as is known in the art, sintering refers to a process of compressing and forming a solid entity of a material by heat and / or pressure (without melting the material to the liquefaction point). The articles can also be manufactured using a casting process or a metal injection molding (MIM) process, or they can be forged.

[0036] Accordingly, embodiments of the present disclosure provide materials suitable for manufacturing turbine engine moving components capable of resisting wear, where operation at relatively high temperatures is not required. As described above, examples of turbocharger systems can include shafts, bushings, valves, and the like. Of course, the described embodiments should not be considered limited to such components, but rather they can be considered applicable to any manufactured article in which ferrous alloys or stainless steel alloys can be employed. The described materials can provide an effective and low-cost alternative to austenitic alloys, where operation at relatively high temperatures is not required.

[0037] Although at least one exemplary embodiment has been presented in the foregoing detailed description of the subject matter of the present invention, it should be understood that there are numerous variations. It should also be understood that the exemplary embodiment or exemplary embodiments are merely examples and are not intended to limit in any way the scope, applicability, or configuration of the subject matter of the present invention. On the contrary, the foregoing detailed description will provide those skilled in the art with a convenient guide for implementing the exemplary embodiments of the subject matter of the present invention. It should be understood that various changes can be made to the functions and arrangements of the elements described in the exemplary embodiments without departing from the scope of the subject matter of the present invention as set forth in the appended claims.

Claims

1. A ferritic stainless steel alloy, comprising by weight: 20% to 35% chromium, 0.1% to 1.5% nickel, 0.5% to 15% molybdenum, 1% to 4% carbon, 1.5% to 1.9% silicon, 0.05% to 0.3% nitrogen, 0.05% to 0.7% niobium, less than 0.01% by weight of sulfur and less than 0.04% by weight of phosphorus, and the balance being iron, and other inevitable / unavoidable impurities present in trace amounts.

2. The ferritic stainless steel alloy according to claim 1, comprising 22% to 33% chromium.

3. The ferritic stainless steel alloy according to claim 1, comprising 2% to 13% molybdenum.

4. The ferritic stainless steel alloy according to claim 1, comprising 1.6% to 1.8% silicon.

5. The ferritic stainless steel alloy according to claim 1, comprising 1.5% to 3.5% carbon.

6. A turbocharger moving part, at least as part of its composition, comprising: a ferritic stainless steel alloy, wherein the ferritic stainless steel alloy comprises by weight: 20% to 35% chromium, 0.1% to 1.5% nickel, 0.5% to 15% molybdenum, 1% to 4% carbon, 1.5% to 1.9% silicon, 0.05% to 0.3% nitrogen, 0.05% to 0.7% niobium, less than 0.01% by weight of sulfur and less than 0.04% by weight of phosphorus, and the balance being iron, and other inevitable / unavoidable impurities present in trace amounts.

7. The turbocharger moving part according to claim 6, wherein the ferritic stainless steel alloy comprises 22% to 33% chromium.

8. The turbocharger moving part according to claim 6, wherein the ferritic stainless steel alloy comprises 2% to 13% molybdenum.

9. The turbocharger moving part according to claim 6, wherein the ferritic stainless steel alloy comprises 1.6% to 1.8% silicon.

10. The turbocharger moving part according to claim 6, wherein the ferritic stainless steel alloy comprises 1.5% to 3.5% carbon.

11. The turbocharger moving part according to claim 6, wherein the turbocharger moving part includes a shaft, a bushing or a valve.

12. A turbocharger comprising the turbocharger moving part according to claim 6.

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