Patent Description:
In the context of turbine engines, turbochargers use heat and volumetric flow of engine exhaust gas to pressurize or boost an intake air stream into a combustion chamber. Specifically, exhaust gas from the engine is routed into a turbocharger turbine housing. A turbine is mounted inside the housing, and the exhaust gas flow causes the turbine to spin. The turbine is mounted on one end of a shaft that has a radial air compressor mounted on an opposite end thereof. Thus, rotary action of the turbine also causes the air compressor to spin. The spinning action of the air compressor causes intake air to enter a compressor housing and to be pressurized or boosted before the intake air is mixed with fuel and combusted within the engine combustion chamber.

Various systems within turbochargers include tribological interfaces, that is, surfaces of components that interact with and move relative to one another while the turbocharger is in operation. Such components, which are commonly referred to as kinematic components, may be susceptible to friction and wear, especially at elevated temperatures, which reduces their service life. Examples of turbocharger systems that may include kinematic components include waste-gate systems, which divert exhaust gasses away from the turbine to regulate airflow to the turbine, and variable geometry systems, which include a row of moveable inlet vanes to accomplish the same purpose. These systems commonly include various components such as shafts, bushings, valves, and the like, which are kinematic components because they interact and move relative to one another, and are thus subject to friction wear. Documents cited during prosecution include <CIT>.

Accordingly, it is desirable to provide materials that are suitable for use in fabricating kinematic components for turbine engines that exhibit reduced friction during motion relative to one another and that can resist wear during elevated temperature operations. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.

Stainless steel alloys, and turbocharger kinematic components fabricated from such alloys (for example by sintering), are provided.

In an embodiment, by way of example only, a stainless steel alloy includes, by weight, about <NUM>% to about <NUM>% chromium, about <NUM>% to about <NUM>% nickel, about <NUM>% to about <NUM>% cobalt, about <NUM>% to about <NUM>% molybdenum, about <NUM>% to about <NUM>% carbon, about <NUM>% to about <NUM>% silicon, about <NUM>% to about <NUM>% niobium, and a balance of iron and other inevitable/unavoidable impurities.

In another embodiment, by way of example only, a turbocharger kinematic component is fabricated from an alloy that includes a stainless steel alloy, which itself includes, by weight, about <NUM>% to about <NUM>% chromium, about <NUM>% to about <NUM>% nickel, about <NUM>% to about <NUM>% cobalt, about <NUM>% to about <NUM>% molybdenum, about <NUM>% to about <NUM>% carbon, about <NUM>% to about <NUM>% silicon, about <NUM>% to about <NUM>% niobium, and a balance of iron and other inevitable/unavoidable impurities.

The inventive subject matter will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:.

As used herein, numerical ordinals such as "first," "second," "third," etc., such as first, second, and third components, simply denote different singles of a plurality unless specifically defined by language in the appended claims. All percentages herein are given by weight of the overall alloy.

All of the stainless steel alloys described herein may be understood as "consisting of" the listed elements in their various percentages, in a closed-ended context.

All of the embodiments and implementations of the stainless steel alloys, turbocharger kinematic components, and methods for the manufacture thereof described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention, which is defined by the claims. Of course, the described embodiments should not be considered limited to such components, but may be considered applicable to any articles of manufacture where an iron alloy, or a stainless steel alloy may be employed.

As noted above, the present invention is directed to stainless steel alloys for use in kinematic components of a turbocharger for purposes for reducing friction and wear with regard to the use and implementation of such kinematic components. As further noted above, a variable geometry turbocharger (among other possible turbocharger systems) employ such kinematic components. Accordingly, for completeness of description, <FIG> illustrates a portion of a variable geometry turbocharger (VGT) <NUM> comprising a turbine housing <NUM> having a standard inlet <NUM> for receiving an exhaust gas stream, and an outlet <NUM> for directing exhaust gas to the exhaust system of the engine. A volute is connected to the exhaust inlet and an integral outer nozzle wall is incorporated in the turbine housing casting adjacent the volute. A turbine wheel <NUM> and shaft assembly <NUM> is carried within the turbine housing <NUM>. Exhaust gas, or other high energy gas supplying the turbocharger, enters the turbine housing through the inlet <NUM> and is distributed through the volute in the turbine housing for substantially radial delivery to the turbine wheel through a circumferential nozzle entry <NUM>.

Multiple vanes <NUM> are mounted to a nozzle wall <NUM> machined into the turbine housing using shafts <NUM> that project perpendicularly outwardly from the vanes. The shafts <NUM> are rotationally engaged within respective openings <NUM> in the nozzle wall. The vanes each include actuation tabs <NUM> that project from a side opposite the shafts and that are engaged by respective slots <NUM> in a unison ring <NUM>, which acts as a second nozzle wall. The tabs <NUM>, slots <NUM>, and other described components move relative to one another, and as such it would be desirable to reduce the friction therebetween for tribological purposes.

<FIG> illustrates the general movement pattern of conventional vanes <NUM>, as used in the VGT described and illustrated above, when actuated by the unison ring <NUM>. Each vane tab <NUM> is disposed within a respective elongated slot <NUM> of a unison ring <NUM>. In a closed position "A", the vane tab <NUM> is positioned adjacent a first end <NUM> of the slot <NUM>. This position is referred to as a closed position because the vane is not flared radially outward, thereby serving to limit the flow of exhaust gas to the turbine. At an intermediate position "B" the unison ring <NUM> has been rotated a sufficient amount such that the vane tab <NUM> is moved within the slot <NUM> away from the first slot end <NUM> (as opposed to second slot end <NUM>) towards a middle position of the slot. Again, it would be desirable to reduce friction as the components of the vanes <NUM> move relative to the components of the unison ring <NUM>, for tribological purposes.

Additionally, as noted above, waste-gate systems may also include tribological components, and as such, for completeness of disclosure, <FIG> illustrates an exemplary waste-gate system. Specifically, <FIG> shows a cutaway view of an example of an assembly <NUM> that includes a turbine housing component <NUM> and a waste-gate <NUM>. In the assembly <NUM>, the turbine housing component <NUM> includes an opening <NUM>, for example, as defined by a surface <NUM> of a substantially cylindrical wall portion <NUM> of the turbine housing component <NUM>. As shown, the wall <NUM> extends to an edge (e.g., defining the opening <NUM>) and then flattens joining a relatively flat surface <NUM>, which may be referred to as a waste-gate seat. In the example of <FIG>, the surface <NUM> defines a relatively short passage, for example, having an axis (e.g., a z-axis), from which the surface <NUM> is disposed at a radial distance (e.g., an r-axis). Extending away from the opening <NUM>, the seat <NUM> descends along another surface <NUM> (e.g., of the substantially cylindrical wall portion <NUM>) to a floor <NUM> of an exhaust chamber formed in part by the turbine housing component <NUM>, for example, in combination with a wall surface <NUM>. As shown in <FIG>, the wall surface <NUM> of the turbine housing component <NUM> rises to an edge that defines an opening <NUM> of the exhaust chamber and then extends outwardly to a relatively flat surface <NUM>, which may include one or more apertures, etc., such as an aperture <NUM>, for example, to attachment of another component to the turbine housing component <NUM>.

In the example of <FIG>, the waste-gate <NUM> includes a plug portion <NUM> that is connected to a waste-gate arm <NUM>. The plug portion <NUM> includes a lower surface <NUM>, a stem <NUM> that extends upwardly to an upper end <NUM> of the plug portion <NUM> and a rim surface <NUM> (e.g., disposed at a radius about the stem <NUM> and having an axial height). As shown, the stem <NUM> is received by a bore <NUM> of the waste-gate arm <NUM> where the bore <NUM> extends between a lower surface <NUM> and an upper surface <NUM> of the waste-gate arm <NUM>. In the example of <FIG>, a clamping washer <NUM> clamps to the stem <NUM> of the plug portion <NUM> to thereby prevent the stem <NUM> from sliding through the bore <NUM> of the waste-gate arm <NUM>. Accordingly, as the waste-gate arm <NUM> pivots, the lower surface <NUM> of the plug portion <NUM> is positioned with respect to the seat <NUM> of the turbine housing component <NUM> for opening and closing of the waste-gate <NUM>.

Typical embodiments of the present disclosure reside in a motor vehicle equipped with a gasoline or diesel powered internal combustion engine and a turbocharger. The turbocharger is equipped with a unique combination of features that may, in various embodiments, provide efficiency benefits by relatively limiting the amount of (and kinetic energy o9 secondary flow in the turbine and/or compressor, as compared to a comparable unimproved system. Stainless steel alloys for use in turbochargers may have operating temperatures up to about <NUM> (or up to about <NUM>), or greater. Some embodiments of the present disclosure are directed to stainless steel alloys that include iron alloyed with various alloying elements, as are described in greater detail below in weight percentages based on the total weight of the alloy.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% chromium (Cr), for example from about <NUM>% to about <NUM>% Cr, such as about <NUM>% to about <NUM>% Cr, for example about <NUM>% to about <NUM>% Cr. A specific embodiment may utilize about <NUM>% Cr. Also possible are ranges from about <NUM>% to about <NUM>% Cr, about <NUM>% to about <NUM>% Cr, about <NUM>% to about <NUM>% Cr, about <NUM>% to about <NUM>% Cr, about <NUM>% to about <NUM>% Cr, about <NUM>% to about <NUM>% Cr, or about <NUM>% to about <NUM>% Cr. It has been discovered that if Cr is added excessively, coarse primary carbides of Cr are formed, resulting in extreme brittleness. As such, the content of Cr is limited to a maximum of about <NUM>% so as to maintain an appropriate volume fraction within the stainless steel for corrosion resistance.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% nickel (Ni), for example about <NUM>% to about <NUM>% Ni, for example about <NUM>% to about <NUM>% Ni. A specific embodiment may utilize about <NUM>% Ni. Also possible are ranges from about <NUM>% to about <NUM>% Ni, about <NUM>% to about <NUM>% Ni, and about <NUM>% to about <NUM>% Ni. Ni, together with optional nitrogen (which is described in greater detail below), is an element to stabilize the austenite phase. To reduce a production cost, if the content of expensive Ni is lowered, the decrement of Ni can be replaced by increasing the content of optional nitrogen that form an austenite phase. However, if the content of Ni is excessively lowered, optional nitrogen would be excessively needed so that the corrosion resistance and the hot formability characteristics are deteriorated. Thus, the content of Ni ranges from about <NUM>% to about <NUM>%.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% molybdenum (Mo), such as about <NUM>% to about <NUM>% Mo, for example about <NUM>% to about <NUM>% Mo. A specific embodiment may utilize about <NUM>% Mo. Also possible are ranges from about <NUM>% to about <NUM>% Mo, about <NUM>% to about <NUM>% Mo, and about <NUM>% to about <NUM>% Mo. As molybdenum also increases the risk of intermetallic phase formation, the level should be maximized to <NUM>%, and preferably less than <NUM>%. If the content of Mo is excessive, Mo is likely to form the sigma phase when it is annealed, thereby deteriorating the corrosion resistance and impact resistance, which is deleterious to the tribological properties of the kinematic components of a turbocharger described herein.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% cobalt (Co), for example about <NUM>% to about <NUM>% Co, for example about <NUM>% to about <NUM>% Co. A specific embodiment may utilize about <NUM>% Co. Also possible are ranges from about <NUM>% to about <NUM>% Co, about <NUM>% to about <NUM>%Co, and about <NUM>% to about <NUM>% Co. As found herein, in amounts above about <NUM>%, The presence of Co in the stainless steel improves its durability and hardness at higher temperatures, and thus are included herein in amounts of <NUM>% or more. However, stainless steels with Co in an amount of greater than about <NUM>% have a somewhat greater tendency for decarburization and are more sensitive to cracking when exposed to abrupt temperature changes, and as such, the present disclosure is limited to amounts of Co of about <NUM>% or less.

As compared with stainless steel alloys known in the prior art, it should be appreciated that the stainless steel alloys disclosed herein typically include a relatively low amount of Ni and a relatively low amount of Co, and a relatively high amount of Mo. Thus, as between Ni, Co, and Mo, specific embodiments may employ amounts of Ni that include amounts/ranges of <NUM>% or below, such as amounts/ranges of <NUM>% or below; these embodiments may also employ amounts of Co that include amounts/ranges of <NUM>% or below, such as amounts/ranges of <NUM>% or below; further, these embodiments may also employ amounts of Mo that include amounts/ranges of <NUM>% or above, such as amounts/ranges of <NUM>% or above.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% carbon (C), for example about <NUM>% to about <NUM>% C. A specific embodiment may employ about <NUM>% C. C has a function of improving the sintering ability of the alloy. C, when present in the relatively-high disclosed range, also forms a eutectic carbide with niobium (which, as discussed in greater detail below, may also be included in the alloy), which improves wear resistance. To exhibit such functions effectively, the amount of C should be <NUM>% or more. Further, C is effective for strengthening a material by solid solution strengthening. To maximize the corrosion resistance, the content of C is lowered to about <NUM>% and below.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% silicon (Si), for example about <NUM>% to about <NUM>% Si. A specific embodiment may employ about <NUM>% Si. Si has effects of increasing the stability of the alloy metal structure and its oxidation resistance. Further, Si has functions as a deoxidizer and also is effective for improving castability and reducing pin holes in the resulting sintered products, when present in an amount greater than about <NUM>%. If the content of Si is excessive, Si deteriorates the mechanical property such as impact toughness of stainless steel. Therefore, the content of Si is limited to about <NUM>% and below.

In an embodiment, the stainless steel alloy of the present disclosure includes from about <NUM>% to about <NUM>% niobium (Nb), for example from about <NUM>% to about <NUM>% Nb. It has been discovered that the friction-resistant, wear-resistant, stainless steel of the present disclosure is provided with a high castability by forming eutectic carbides of Nb as well as a high strength and ductility, and as such, in an embodiment, Nb is present in an amount of about <NUM>%. However, as indicated by the <NUM>% lower range-end, embodiments are contemplated wherein Nb is not present.

Certain inevitable/unavoidable impurities may also be present in the stainless steel alloy of the present disclosure, for example as described below with regard to phosphorous, sulfur, and nitrogen (the amounts of such described impurities (and others) are minimized as much as practical).

In an embodiment, phosphorus (P) may be present in the alloy, but is minimized to about <NUM>% or less. P is seeded in the grain boundary or an interface, and is likely to deteriorate the corrosion resistance and toughness. Therefore, the content of P is lowered as low as possible. The upper limit content of P is limited to <NUM>% in consideration of the efficiency of a refining process. The contents of harmful impurities, such as P are as small as possible. However, due to cost concerns associated with removal of these impurities, and the P content is limited to <NUM>%.

In an embodiment, sulfur (S) may be present in the alloy, but is minimized to about <NUM>% or less. S in steels deteriorates hot workability and can form sulfide inclusions that influence pitting corrosion resistance negatively. It should therefore be limited to less than <NUM>%. S deteriorates the hot formability, thereby deteriorating the corrosion resistance. Therefore, the content of S is lowered as low as possible. The contents of harmful impurities, such as S (sulfur), are as small as possible. However, due to cost concerns associated with removal of these impurities, the S content is limited to about <NUM>%.

Nitrogen (N) is an element capable of improving the high-temperature strength and the thermal fatigue resistance like C, and such effects can be obtained when the amount of N is <NUM>% or more. However, N is optional, and need not be included in any amount. On the other hand, to insure the production stability and to avoid the brittleness due to the precipitation of Cr nitrides, the upper limit of N should be about <NUM>%. N, together with Ni, is one of elements that contribute stabilization of an austenite phase. As the content of N increases, the corrosion resistance and high strengthening are achieved. However, when the content of N is too high, the hot formability of steel is deteriorated, thereby lowering the production yield thereof. Therefore, the content of N ranges up to a maximum of about <NUM>%, though it need not be included at all.

Additionally, it should be appreciated that, although completely optional, boron (B), calcium (Ca), and cerium (Ce), may be added/(present) in very small quantities in the disclosed steels to improve hot workability. The levels for B, Ca, and Ce, if included at all, are, individually, less than about <NUM>% each.

Other elements, which may be included in some stainless steels known in the prior art, are specifically excluded (or substantially and/or effective excluded) from the stainless steel alloys of the present disclosure. These excluded elements are manganese (Mn), tungsten (W), and vanadium (V).

The disclosed alloys, being stainless steel alloys, also include a balance of iron (Fe). As used herein, the term "balance" refers to the amount remain to achieve <NUM>% of a total alloy, in terms of weight.

The articles of manufacture described herein, such as the kinematic components of a turbocharger fabricated with the above-described stainless steel alloys, may be formed using sintering processes. For example, as is known in the art, sintering refers to a process of compacting and forming a solid mass of material by heat and/or pressure without melting the material to the point of liquefaction.

Claim 1:
A stainless steel alloy, comprising, by weight:
about <NUM>% to about <NUM>% chromium;
about <NUM>% to about <NUM>% nickel;
about <NUM>% to about <NUM>% cobalt;
about <NUM>% to about <NUM>% molybdenum;
about <NUM>% to about <NUM>% carbon;
about <NUM>% to about <NUM>% silicon; about <NUM>% to about <NUM>% niobium;
about <NUM>% or less sulfur;
about <NUM>% or less phosphorus;
about <NUM>% or less nitrogen;
less than about <NUM>% boron;
less than about <NUM>% calcium;
less than about <NUM>% cerium;
and a balance of iron and inevitable impurities, wherein the alloy contains no manganese, tungsten and vanadium.