Abstract:
An iron-based alloy that contains the elements of cobalt, carbon, silicon, manganese, chromium, molybdenum, niobium, cobalt, and tungsten, and optionally also minor amounts of one or more of aluminum, nickel, vanadium, nitrogen and titanium, on the condition that the combined amount of chrome and cobalt is at least 34.5 percent by weight of the total alloy, preferably at least 36 percent by weight and at most 41.5 percent by weight of the total alloy. The alloy is particularly suitable for use in mechanical parts which are thermally highly stressed and exposed to oxidizing and/or corroding effects. The alloy preferably comprises C: 0.3-0.5% by weight, Si: 0.0-1.5% by weight, Mn: 0.0-3.0% by weight, Cr: 19-28% by weight, Mo: 1-3% by weight, Nb: 2-4% by weight, Co: 12-18% by weight, and W: 2-4% by weight measured as a percentage of total weight of the alloy.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention concerns an alloy particularly suitable for use in the manufacture of mechanical parts which are thermally highly stressed and exposed to oxidizing and/or corroding effects. The invention also concerns an exhaust-gas turbine of a turbocharger having a nozzle ring formed of the inventive alloy arranged in the inflow passage of the exhaust-gas turbine.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     Turbochargers use the exhaust gasses discharged from an internal combustion engine as a motive gas to rotate a turbine wheel which is mounted on one end of a shaft. A compressor wheel is mounted on the other end of the shaft, and is turned by the turbine wheel to compress air, which is then communicated to the engine, thereby supplying charge air to the engine for increasing engine performance. To improve operating efficiency and to extend range, it is desirable to control the flow of motive exhaust gasses into the turbine wheel. This can be done by providing a series of pivotally mounted, circumferentially spaced vanes in the entrance throat to the turbine wheel. By pivoting the vanes the nozzle area into the turbine wheel can be changed, thereby adjusting the flow of exhaust gasses into the turbine wheel. Structurally, these vanes are pivotably mounted on a ring-shaped part. This part is referred to as a nozzle ring.  
         [0003]     A nozzle ring located in an exhaust-gas turbine of a turbocharger is subjected to severe stresses due to fluctuating operating conditions, i.e., increases or reductions in the pressure and temperature of the working medium. Depending on the turbine used and the actual conditions of use, the working medium may have a large temperature gradient. Since a nozzle ring has only a small mass compared with the turbine components surrounding it, it is subjected to relatively pronounced thermal expansions.  
         [0004]     Further, since the motive exhaust gasses are extremely corrosive, the nozzle ring and the walls of the turbocharger turbine entrance throat must be of a material which is highly corrosiion resistant, to prevent sticking or binding of the vanes. Increasing longevity requirements (e.g., one million miles for commercial diesel vehicles) can only be met by advanced materials having extended durability.  
         [0005]     U.S. Pat. No. 5,411,702 (Nazmy et al) entitled “Iron-aluminum alloy for use as thermal-shock resistance material” teaches an iron-aluminum alloy which, even at temperatures between 700° and 800° C., still has mechanical properties which permit its use in components which are slightly stressed mechanically. At the same time, the alloy is described as having excellent thermal shock resistance such that it can be used particularly advantageously in those parts of thermal installations which are subject to thermal cyclic loading, such as, in particular, as a casing or casing part of a gas turbine or of a turbocharger or as a nozzle ring, in particular for a turbocharger. The iron-aluminum alloy comprises the following constituents in atom percent: 12-18 aluminum; 0.1-10 chromium; 0.1-2 niobium; 0.1-2 silicon; 0.1-5 boron; 0.01-2 titanium; 100-500 ppm carbon; 50-200 ppm zirconium; and remainder iron.  
         [0006]     While this alloy is distinguished by the fact that its constituents are limited to metals which are comparatively inexpensive and are available independently of strategic political influences, the alloy does not meet the extended life requirements expected of modern commercial diesel engines or even automobiles. Further, there is a demand for parts capable of operating at even higher temperatures.  
         [0007]     U.S. Pat. No. 5,207,565 (Roessler) entitled “Variable geometry turbocharger with high temperature insert in turbine throat” teaches a turbocharger including a turbine housing with a variable geometry mechanism which controls the flow area of the throat through which motive exhaust gasses are communicated to the turbine wheel. The turbine housing is provided with an insert which defines one wall of the throat and which is cast in place as part of the turbine housing. The insert may be made of a relatively expensive, high nickel content material such as D5B NI-RESIST, which has a very high nickel content as compared to standard ductile iron, and on this basis has enhanced corrosion resistance properties to assure that a non-corrosive surface is provided adjacent to the vanes which are pivotally actuated to vary the area of the throat. The remainder of the housing is made of a standard ductile iron, which is substantially less expensive than the more expensive corrosion resistant high nickel content material from which the insert is manufactured. Thus, although Roessler achieves a reduction in costs, he does not achieve an increase in turbocharger life.  
         [0008]     There is thus a need for a new alloy particularly suitable for the manufacture of extended life parts of thermal machinery which are thermally highly stressed and in addition are exposed to oxidizing and/or corroding effects.  
         [0009]     There is also a need for an exhaust-gas turbine of a turbocharger having an extended life nozzle ring arranged in the inflow passage of the exhaust-gas turbine and directing the working medium onto the turbine blades.  
       SUMMARY OF THE INVENTION  
       [0010]     The above objects are attained by means of the use of an iron-based alloy that contains the elements of cobalt, carbon, silicon, manganese, chromium, molybdenum, niobium, cobalt, and tungsten, and optionally also minor amounts of one or more of aluminum, nickel, vanadium, nitrogen and titanium, on the condition that the combined amount of chrome and cobalt is at least 34.5 percent by weight of the total alloy, preferably at least 36 percent by weight and at most 41.5 percent by weight of the total alloy, the remainder being iron and metallurgically suited admixtures for the production of an alloy particularly suitable for use in mechanical parts which are thermally highly stressed and exposed to oxidizing and/or corroding effects.  
         [0011]     Another aspect of the present invention is in the manufacture of devices and machines subject to highly corrosive gasses, wherein the machine part is manufactured from an iron based alloy, preferably a stainless steel alloy, containing the elements of cobalt, carbon, silicon, manganese, chromium, molybdenum, niobium, cobalt, and tungsten, provided that the combined amount of chrome and cobalt is at least 34.5 percent by weight of the total alloy, preferably at least 36 percent by weight and at most 41.5 percent by weight of the total alloy.  
         [0012]     The alloy is characterized by fine grain structure, intermetallic phase development, the formation of cobalt oxides, and improved tribological characteristics.  
         [0013]     The iron based alloy preferably comprises C: 0.3-0.5% by weight, Si: 0.0-1.5% by weight, Mn: 0.0-3.0% by weight, Cr: 19-28% by weight, Mo: 1-3% by weight, Nb: 2-4% by weight, Co: 12-18% by weight, and W: 2-4% by weight.  
         [0014]     The iron based alloy more preferably comprises C: 0.35-0.45% by weight, Si: 0.0-1.0% by weight, Mn: 0.0-2.0% by weight, Cr: 21-25% by weight, Mo: 1.8-2.2% by weight, Nb: 2.8-3.2% by weight, Co: 13.5-16.5% by weight, and W: 2.3-3.0% by weight.  
         [0015]     As for heat treatment, a martensitic structure including at most 10% ferrite of normalized tempered steel is preferable. Included in the description is an iron-based alloy, comprising: C: 0.3-0.5% by weight, Si: 0.0-1.5% by weight, Mn: 0.0-3.0% by weight, Cr: 19-28% by weight, Mo: 2-3% by weight, Nb: 2-4% by weight, Co: 12-18% by weight, and W: 2-4% by weight; measured as a percentage of total weight of the alloy; wherein a total concentration of C and, if present, N is at no greater than about 1.2%; and the remainder of the alloy comprising iron and metallurgically suited admixtures for the production of machine parts that in their function are exposed to severe thermal stress and oxidizing or corrosive effects.  
         [0016]     A process of using the iron-based alloy includes manufacturing at least one machine part from the iron-based alloy and installing the same to function under conditions of severe thermal stress and oxidizing or corrosive effects.  
         [0017]     According to the present invention, a process of making a machine part for functioning under conditions of severe thermal cycling and in a corrosive environment includes: forming the machine part from an iron-based alloy most preferably comprising: C: 0.35-0.45% by weight, Si: 0.0-1.0% by weight, Mn: 0.0-2.0% by weight, Cr: 21-25% by weight, Mo: 1.8-2.2% by weight, Nb: 2.8-3.2% by weight, Co: 13.5-16.5% by weight, W: 2.3-3.0% by weight, measured as a percentage of total weight of the alloy; optionally tempering the machine part; and further optionally forming a hardened layer on at least a portion of a surface of the machine part.  
         [0018]     The hardened layer may be made by forming a nitride layer on at least a portion of the machine part, or by forming a carbonitride layer on at least a portion of the machine part. Further, the hardened layer may be vapor deposited (PVD or CVD) as an aluminum-based carbide layer, a titanium-based carbide layer, an aluminum-based nitride layer, a titanium-based nitride layer, an aluminum-based oxide layer, or a titanium-based oxide layer. If machine parts are at least partly provided with a coating of hard material, especially favorable (i.e., low) friction values are attainable. However, for specific applications such as a nozzle ring for a turbocharger, there has not been found to be any need for a hardened layer.  
         [0019]     At the same time, the alloy according to the invention is distinguished by excellent thermal shock resistance and can therefore be used particularly advantageously in those parts of thermal installations which are subject to thermal cyclic loading, such as, in particular, as a casing or casing part of a gas turbine or of a turbocharger or as a nozzle ring, in particular for a turbocharger. Due to the superb corrosion resistance to exhaust gas flow and improved durability despite thermal stress, the present iron-based alloys are favorable for use in making turbocharger nozzle rings.  
         [0020]     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other compressor wheels for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention concerns an alloy particularly suitable for use in the manufacture of mechanical parts which are thermally highly stressed and exposed to oxidizing and/or corroding effects. The invention in particular also concerns an exhaust-gas turbine of a turbocharger having a nozzle ring formed of the inventive alloy arranged in the inflow passage of the exhaust-gas turbine.  
         [0022]     As is well known, a turbocharger is a device designed to recover energy from the exhaust gas stream and use this energy to compress charge air to an engine, to thereby increase the power output of the engine. A turbocharger housing basically comprises a center (or bearing) housing, a turbine housing mounted on one end of the center housing, and a compressor housing mounted on the opposite end of the center housing. Rotatably supported within the turbocharger center housing is a shaft. One end of the shaft extends into the turbine housing, and a turbine wheel is mounted on the turbine end of the shaft for rotation within the turbine housing. The exhaust gasses discharged by the engine serve as motive gasses for turning the turbine wheel. More specifically, exhaust gasses received in the turbine housing inlet are discharged into a circumferentially extending volute which circumscribes the turbine wheel. After passing through the turbine wheel and imparting rotation thereto, the exhaust gasses are discharged into exhaust pipe through a turbine housing outlet.  
         [0023]     The other end of the shaft extends into the compressor housing. A compressor wheel is mounted on the compressor end of shaft for rotation with the shaft. Rotation imparted to the shaft by the turbine wheel rotates the compressor wheel, thereby drawing in air through the inlet and increasing the pressure air as it passes through the turbine wheel. The pressurized air is then discharged into the line for supplying charge air to the engine.  
         [0024]     To improve the performance and/or to extend the operating range of the turbocharger, it is desirable to control the flow of motive exhaust gasses into the turbine wheel. This can be done by providing a series of pivotally mounted, circumferentially spaced vanes in the entrance throat to the turbine wheel. By pivoting the vanes the nozzle area into the turbine wheel can be changed, thereby adjusting the flow of exhaust gasses into the turbine wheel. Structurally, these vanes are mounted on an axially extending pivot pin which extends through and is pivotably supported in a ring-shaped part referred to as a nozzle ring. The present invention is concerned with an alloy which has properties specifically adapted for use in the manufacture of such a nozzle ring. Control of the angle of the vanes will increase or decrease the nozzle area of the throat area, thereby controlling the flow of motive exhaust gasses through the throat and into the turbine wheel. Details of the vanes and the actuating mechanism therefore can be found for example in U.S. Pat. No. 4,659,295 and, since they form no part of the present invention, will not be disclosed in detail herein.  
         [0025]     As can be readily appreciated by those skilled in the art, in order to effectively control the flow of motive gasses through the throat, the clearance between the vanes and the walls of the throat must be carefully controlled. As will also be readily appreciated, the motive exhaust gasses communicated through the throat are at a very high temperature and are extremely corrosive. Accordingly, the nozzle ring must be made of an extremely corrosive resistant material. Also, since the throat wall defined by the turbine housing opposite the nozzle ring is exposed to the same corrosive gasses, at least the exposed surface of this wall should preferably be made of the very corrosive resistant material of the present invention.  
         [0026]     Stainless steel relating to the present invention can be either cast steel, forged steel, or rolled steel, but cast steel is preferable. Cast parts have the advantage of undergoing an essentially isometric change in shape upon a change of temperature (i.e., they expand and contract isometrically). This is due to the fact that the cast parts have a largely random microstructure and a homogeneous microscopic hard-particle distribution, such as, e.g., carbide distribution, and an equally homogeneous soft matrix configuration. This kind of microstructure is especially advantageous at frictional surfaces, and it decreases both the forces of friction and abrasion of material, especially if the surface is inadequately or poorly and/or incompletely coated with lubricant. Slide bearings, cylinder bushings, piston rings and similar machine parts that are exposed to severe stress from sliding friction are therefore primarily produced from cast or sintered materials, especially iron-based alloys.  
         [0027]     The present alloys are especially favorable for use under corrosive conditions, since the reduced carbon content and high homogeneous chromium concentration, along with the elements of nitrogen and molybdenum has been found to synergistically bring about the corrosion resistance of the material by stabilizing the surface passive layer, despite frictional stress.  
         [0028]     Due to the superb corrosion resistance to exhaust gas flow and improved durability despite thermal stress, the present iron-based alloys are favorable for use in making turbocharger nozzle rings. If, as has also been found, the nozzle rings made of the aforementioned alloy are at least partly provided with a coating of hard material, especially favorable (i.e., low) friction values are attainable.  
         [0029]     Considering now the elemental constituents of the inventive alloy, chromium (Cr) is a ferrite forming element that improves the hardenability of the steel material, accelerates the spheroidization of carbide and produces a chromium oxide layer of not less than 5 nm. Cr is, a necessary element for improving corrosion resistance and for obtaining the martensitic base, and is added in a range of 19-28 wt %, preferably 21-25 wt %. If the Cr content is less than 19 wt %, a martensitic structure is formed in an austenitic phase, and if more than 28%, the amount of delta ferrite increases, and accordingly, ductility, erosion resistance the corrosion resistance is not sufficient, and toughness is decreased. Especially, a content of at least 21% is preferable, and more preferably in the range of 21-25%.  
         [0030]     When the parts are additionally subject to corrosive stresses, as in the case, for instance, with plunger pistons or piston rings in internal combustion engines, as a result of positive displacement media, condensates or the like, it is necessary to provide high chromium content in the alloy. However, the other properties of the material that are needed for an appropriate function of the part must not be disadvantageously affected by the inclusion of chromium.  
         [0031]     Carbon (C) is a necessary element for improving the hardenability of the metal on heat treatment and for increasing the toughness by making the structure martensitic, and concurrently, carbon is an effective element for increasing the strength and hardness of the material and improving erosion resistance. However, if the content is less than 0.3% the above effects cannot be sufficiently realized because the martensitic phase is yielded to the austenitic matrix, and regions hardened by the local generation of induced martensitic phase are effective for improving erosion resistance. On the other hand, if the content exceeds 0.5%, although it may change somewhat depending on balance with Co and Cr contents, the proportion of carbide and in particular of Cr carbides increases and as a result the corrosion resistance of the material is substantially reduced, and further the risk of generation of cracks at high temperature increases. Therefore, in consideration of both erosion resistance and stress strength, carbon content is preferably in a range of 0.3-0.5%, more preferably in a range of 0.35-0.45%.  
         [0032]     Silicon (Si) is an element that accelerates the delay in the texture change and improves the hardenability of the steel material. Si is a necessary element for forming ferrite and in order to decrease the generation of micro-bubbles, and normally, is added to at most 1.5%. Below about 0.2 wt % casting de-oxidation is remarkably reduced (thus necessitating inert gas conditions for casting). As an excess addition of Si increases delta ferrite and decreases toughness of the material, the Si component in the present invention is restricted to at most 1%, the same as for Si content in normal austenitic alloys. Especially, a range of 0.2-0.6% is preferable.  
         [0033]     Manganese (Mn) is an element that improves the hardenability of the steel material. Mn acts not only as a de-oxidation agent with Si, but also as an agent for preventing segregation of S by stabilizing S in the molten metal by combining with it to form MnS. Further, Mn increases the strength and toughness of martensite after heat treatment (taking the place of or supplementing Ni, as will be explained later). If the Mn content is less than 0.1%, the above effects are hardly realized, and if it exceeds 3%, it increases the amount of austenite remaining, decreases flowing characteristics of molten metals and decreases the strength. Especially, a content of at least 0.25% is preferable, and more preferably in the range of 0.5-2.0%.  
         [0034]     Nickel (Ni) is conventionally used to stabilize austenite in a martensitic matrix, and to improve hardenability and strength and toughness after heat treatment. However, in the alloy of the present invention Ni is not needed.  
         [0035]     Molybdenum (Mo) is an element that renders the steel material resistant to temper softening and is bonded to carbon to produce molybdenum carbide. In order to allow a finely particulate molybdenum carbide having a particle diameter in the range of 50 nm to 300 nm to be stably deposited and distributed in the matrix, it is necessary that the content of molybdenum be not less than 1.1 wt %. The presence of such a finely particulate molybdenum carbide provides a surface hardness that improves the abrasion resistance and exerts the foregoing effect of shielding from hydrogen. Mo is an effective component for improving the corrosion resistance and strength of martensite, and for preventing embrittlement caused by heat treatment. However, Mo is hardly effective if the added amount is less than 0.5%, and and if the additive amount exceeds 3.0%, toughness is decreased and brittlement increases, and accordingly, the addition must be at most 3.0%. Accordingly, in the present invention, the Mo content was restricted in to a range of 1-3 wt %. The range is preferably from 1.8% to 2.2%. Vanadium can be used in addition to molybdenum for a high hardness and strength.  
         [0036]     By means of a nitrogen (N) content of at least 0.1 wt %, the friction wear is markedly reduced, with the most favorable values having been found with approximately 0.2 wt % of nitrogen in the alloy. However, excessive nitrogen nitride, and effects the toughness and corrosion resistance. Therefore, if present, N content was restricted to a range of 0.05-0.20%. Because of the nitrogen content, a particularly fine-grained microstructure is also brought about, and growth of grains is largely prevented even at high hardness temperatures.  
         [0037]     Nb and W are included as carbide forming elements, and are preferably included in an amount of from 2-4 wt % each, preferably Nb: 2.8-3.2 wt %; W 2.3-3.0 wt %. The alloy can contain one or more additional strong carbide forming elements such as V, Ti, Hf, and Zr up to a total of at most 0.2%. Preferably a lower limit of 0.02% is desirable for the additional elements. Vanadium (V) is an element that is deposited on the crystal boundary to inhibit the increase of the size of crystalline particles and is bonded to carbon to produce a finely particulate vanadium carbide.  
         [0038]     The residual consists of Fe and accompanying impurities including P, S, As, Sb, and the like. However, the impurities are desirably as little as possible because the above elements deteriorate ductility and toughness. The content of P is preferably at most 0.025%, and the content of S is preferably at most 0.015%.  
         [0039]     Preferred embodiments of the machine parts include a nozzle ring comprising a hardened layer on at least a portion of an outer surface thereof, wherein the hardened layer is formed from the group consisting of: a titanium-based carbide layer formed by PVD; a titanium-based nitride layer formed by PVD; a titanium-based oxide layer formed by PVD; a titanium-based carbide layer formed by CVD; a titanium-based nitride layer formed by CVD; and a titanium-based oxide layer formed by CVD. The hardened layer comprising a nitride layer having a thickness of at least 0.05 mm, and preferably at least 0.2 mm.  
         [0040]     The alloy according to the invention surpasses comparably usable alloys according to the prior art, not only in terms of the mechanical strength at temperatures above 700° C., and as high as 1000 to 1050° C., but also in terms of thermal shock resistance. The alloy according to the invention can therefore be used particularly advantageously as a material for components of thermal installations, which at temperatures of at least 700° C. to 800° C. and still have a relatively high mechanical strength, and which, like gas turbine casings, are subject to strong thermal cyclic loading.  
       EXAMPLE 1  
       [0041]     A nozzle ring was manufactured using standard techniques, except that the alloy used was the alloy according to the present invention having the following composition (with all percentages being weight percent of the total alloy):  
         [heading-0042]     Alloy A  
         [none]    
       
          C: 0.4% by weight,  
          Si: 0.6-1.0% by weight,  
          Mn: 0-2.0% by weight,  
          Cr: 21-25% by weight,  
          Mo: 1.8-2.2% by weight,  
          Nb: 2.8-3.2% by weight,  
          Ni: 0.5  
          Co: 11.5-16.5% by weight,  
          W: 2.2-3.0% by weight.  
       
     
         [0052]     The alloy (46-50 HRC) was solution annealed 1 h at 1150-1180° C., then hardened for 2 h at 700° C. The product had the following characteristics: 
        Grain Structure: Duplex steel with very, very fine grain, with 3-4 phases in the matrix (tribologically very good)     Hardness: 32-35 HRC     Thermal Coefficient of Expansion: (alpha value) less than 15×10 −6       Dynamic Test: 1500 hours under performance at 100% of nominal value of a commercial truck without problems. Test 2: 7 hour at 1000° C. (pascar range). and nozzle ring appeared new, no detectable mechanical distortion     Brake Test: Iveco EuroTech Engine Test Cell (Arbon, CH)—satisfactory     Less particulate        
 
         [0059]     The results observed in testing Alloy A were superior to results conducted with Tribaloy 400, PL 34 and GV 006 (Pleuco, Germany)  
         [0060]     The same procedures and tests were carried out with the following alloys:  
         [0061]     Although a turbocharger nozzle ring has been described herein with great detail with respect to an embodiment suitable for the automobile or truck industry, it will be readily apparent that the novel alloy and the process for production of machine parts using the alloy are suitable for use in a number of other applications, such as turbine engines, power plants, etc. Although this invention has been described in its preferred form with a certain of particularity with respect to an automotive internal combustion turbocharger nozzle ring, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of structures and the composition of the combination may be resorted to without departing from the spirit and scope of the invention.  
         [0062]     Now that the invention has been described,