Patent Publication Number: US-10316694-B2

Title: Stainless steel alloys, turbocharger turbine housings formed from the stainless steel alloys, and methods for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/447,772, filed on Jul. 31, 2014. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to stainless steel alloys. More particularly, the present disclosure relates to stainless steel alloys used for casting applications, for example turbine and turbocharger housings, exhaust manifolds, and combustion chambers, that exhibit oxidation resistance at elevated temperatures, and method for manufacturing the same. 
     BACKGROUND 
     During operation, automotive or aircraft turbocharger housings are subjected to elevated operating temperatures. These housings must be able to contain a turbine wheel generating very high rotational speeds. Exhaust gas from the automotive or aircraft engine initially contacts the turbocharger in metal sections, such as the gas inlet area of the turbocharger, at elevated temperatures. As high-speed performance improves through exhaust temperature increase, there have been attempts to gradually raise the exhaust temperature of the engine. Due to these high temperatures, the thermal load on the parts such as the exhaust manifold and the turbine housing becomes very great. 
     Various problems have been encountered by these increased exhaust gas temperatures contacting metal sections of the turbocharger. For example, one problem caused by the exhaust temperature rise is the problem of thermal deformation of the material, wherein the turbine housing and exhaust manifold, which alternates between regions of high temperature and low temperature is accompanied by thermal expansion and thermal shrinkage depending on the situation, which can cause surface oxidation wrinkles by such thermal deformation, and which can progress and develop into a penetration crack. 
     In order to overcome the challenges associated with higher operating temperatures, prior art alloys used in turbocharger applications have included alloys of higher nickel content such as commercially available high nickel ductile iron casting alloys. Examples of these are NiResist™ developed by the International Nickel Company, or HK30, a chromium-nickel-iron stainless steel alloy containing approximately 25% chromium and 20% nickel, with the balance essentially iron. The HK series stainless steel alloys in general have about 18-22% nickel and are fully austenitic. 
     The HK stainless steel alloys are strong stainless steel casting alloys, in terms of creep strength. However, while meeting the high temperature property requirements for turbocharger housings, they are quite expensive because of their high nickel content. Further, due to the sudden rise of the Ni price recently, there has been substantial concern over the cost of materials that have a high nickel content. 
     Accordingly, there is a need for stainless steel alloys useful in turbocharger applications that are able to withstand the higher operating temperatures produced by modern engines, but that minimize the expensive nickel content. 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. 
     BRIEF SUMMARY 
     Stainless steel alloys, turbocharger turbine housings, and methods of manufacturing turbocharger turbine housings are provided. 
     In an embodiment, by way of example only, an austenitic stainless steel alloy includes, by weight, about 22% to about 28% chromium, about 3.5% to about 6.5% nickel, about 1% to about 6% manganese, about 0.5% to about 2.5% silicon, about 0.3% to about 0.6% carbon, about 0.2% to about 0.5% nitrogen, and a balance of iron. Molybdenum, niobium, and tungsten are excluded. 
     In another embodiment, by way of example only, a turbocharger turbine housing includes an austenitic stainless steel alloy that includes, by weight, about 22% to about 28% chromium, about 3.5% to about 6.5% nickel, about 1% to about 6% manganese, about 0.5% to about 2.5% silicon, about 0.3% to about 0.6% carbon, about 0.2% to about 0.5% nitrogen, and a balance of iron. Molybdenum, niobium, and tungsten are excluded. 
     In yet another embodiment, a method of fabricating a turbocharger turbine housing include forming the turbocharger turbine housing from an austenitic stainless steel alloy that includes, by weight, about 22% to about 28% chromium, about 3.5% to about 6.5% nickel, about 1% to about 6% manganese, about 0.5% to about 2.5% silicon, about 0.3% to about 0.6% carbon, about 0.2% to about 0.5% nitrogen, and a balance of iron. Molybdenum, niobium, and tungsten are excluded. 
     This summary is provided to introduce a selection of concepts in a simplified form that are 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 as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The inventive subject matter will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a system view of an embodiment of a turbocharged internal combustion engine in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Furthermore, 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 of the embodiments and implementations of the stainless steel alloys, turbocharger turbine housings, 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. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description. 
     The turbocharger turbine housing, usually a cast stainless steel or cast iron, is the most expensive component of the turbocharger. Reduction in cost of the housing will have a direct effect on the cost of the turbocharger. In order to withstand the high operating temperatures commonly produced by exhaust gasses impinging on the turbine housing, turbine housing materials are usually alloyed with elements such as chromium and nickel in addition to other carbide forming elements, resulting in increased cost. Reducing the content and/or eliminating these expensive alloying elements will have a direct effect on the cost of the turbine housing. 
     Typical embodiments of the present disclosure reside in a motor vehicle equipped with a gasoline powered internal combustion engine (“ICE”) 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 of) secondary flow in the turbine and/or compressor, as compared to a comparable unimproved system. 
     With reference to  FIG. 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 within the turbocharger housing around an axis of rotor rotation  103  during turbocharger operation on thrust bearings and two sets of journal bearings (one for each respective rotor wheel), or alternatively, other similarly supportive bearings. The turbocharger housing includes a turbine housing  105 , a compressor housing  107 , and a bearing housing  109  (i.e., a center housing that contains the bearings) that connects the turbine housing to the compressor housing. The rotor includes a radial turbine wheel  111  located substantially within the turbine housing  105 , a radial compressor wheel  113  located substantially within the compressor housing  107 , and a shaft  115  extending along the axis of rotor rotation  103 , through the bearing housing  109 , to connect the turbine wheel  111  to the compressor wheel  113 . 
     The turbine housing  105  and turbine wheel  111  form a turbine configured to circumferentially receive a high-pressure and high-temperature exhaust gas stream  121  from an engine, e.g., from an exhaust manifold  123  of an internal combustion engine  125 . The turbine wheel  111  (and thus the rotor) is driven in rotation around the axis of rotor rotation  103  by the high-pressure and high-temperature exhaust gas stream, which becomes a lower-pressure and lower-temperature exhaust gas stream  127  and is axially released into an exhaust system (not shown). 
     The compressor housing  107  and compressor wheel  113  form a compressor stage. The compressor wheel, being driven in rotation by the exhaust-gas driven turbine wheel  111 , is configured to compress axially received input air (e.g., ambient air  131 , or already-pressurized air from a previous-stage in a multi-stage compressor) into a pressurized air stream  133  that is ejected circumferentially from the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature over that of the input air. 
     Optionally, the pressurized air stream may be channeled through a convectively cooled charge air cooler  135  configured to dissipate heat from the pressurized air stream, increasing its density. The resulting cooled and pressurized output air stream  137  is channeled into an intake manifold  139  on the internal combustion engine, or alternatively, into a subsequent-stage, in-series compressor. The operation of the system is controlled by an ECU  151  (engine control unit) that connects to the remainder of the system via communication connections  153 . 
     Turbochargers can be designed to operate at a variety of temperatures, depending on the configuration of the turbocharger and the desired output. As used herein, the term operating temperature refers to the maximum temperature of exhaust gas designed to be experienced by the turbine housing and blade components of the turbocharger. Stainless steel 1.4826, well-known in the art, with its specification for nickel between 8% and 12% is an exemplary prior art material for turbine housing applications up to about 980° C. Besides Ni, the presence of expensive carbide forming elements such as niobium (Nb), tungsten (W) and molybdenum (Mo) increase the cost of the material. Thus alloys with less expensive materials but that retain the same beneficial properties are desirable. TABLE 1, set forth below, provides the specifications for stainless steel 1.4826. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of 1.4826 Stainless Steel 
               
            
           
           
               
               
               
            
               
                   
                 1.4826 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Elements 
                 Min (%) 
                 Max (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Carbon 
                 0.3 
                 0.50 
               
               
                   
                 Silicon 
                 1 
                 2.50 
               
               
                   
                 Chromium 
                 18 
                 23.0 
               
               
                   
                 Nickel 
                 8 
                 12.0 
               
               
                   
                 Molybdenum 
                 0 
                 0.50 
               
               
                   
                 Manganese 
                 0 
                 2.00 
               
               
                   
                 Niobium 
                 1.3 
                 1.70 
               
               
                   
                 Phosphorous 
                 0 
                 0.04 
               
               
                   
                 Sulphur 
                 0 
                 0.04 
               
               
                   
                 Nitrogen 
                 — 
                 — 
               
            
           
           
               
               
               
               
            
               
                   
                 Iron 
                 Rest 
                   
               
               
                   
                   
               
            
           
         
       
     
     As such, embodiments of the present disclosure are directed to improvements over the currently available stainless steel alloys for use in turbochargers having operating temperatures up to about 980° C. In particular, embodiments of the present disclosure are directed to stainless steel alloys that have a nickel content that is less than stainless steel 1.4826 for cost considerations, and eliminates the need for other expensive materials such as W and Mo. The stainless steel alloys described herein 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 0.3% to about 0.6% carbon (C), for example about 0.4% to about 0.5% C. C has a function of improving the fluidity and castability of a melt. C also has a function of improving the castability by forming eutectic carbides. To exhibit such functions effectively, the amount of C should be 0.3% 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 into about 0.6% and below. 
     In an embodiment, the stainless steel alloy of the present disclosure includes from about 0.5% to about 2.5% silicon (Si), for example about 1.0% to about 2.0% Si. Si has effects of increasing the stability of its metal structure and its oxidation resistance. Further, it has a function as a deoxidizer and also is effective for improving castability and reducing pin holes in the resulting cast products, when present in an amount greater than about 0.5%. If the content of Si is excessive, Si deteriorates the mechanical property such as impact toughness of steel. Therefore, the content of Si is preferably limited to about 2.5% and below. 
     In an embodiment, the stainless steel alloy of the present disclosure includes from about 22% to about 28% chromium (Cr), for example about 23% to about 27% Cr, such as from about 24% to about 26% Cr. On the other hand, if it is added excessively, coarse primary carbides of Cr are formed, resulting in extreme brittleness. When the content of Cr increases, the corrosion resistance increases, but the content of expensive Ni should be also increased to maintain the volume fraction. As such, the content of Cr is preferably limited to a maximum of about 28% so as to maintain the volume fraction of the stainless steel and the corrosion resistance. 
     In an embodiment, the stainless steel alloy of the present disclosure includes from about 3.5% to about 6.5% nickel (Ni), for example about 4.0% to about 6.0% Ni, such as from about 4.5% to about 5.5% Ni. Ni, together with manganese and nitrogen (which as described in greater detail below are included in the alloy of the present disclosure), 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 manganese and nitrogen that form the austenite phase. However, if the content of Ni is excessively lowered, manganese and nitrogen would be excessively needed so that the corrosion resistance and the hot formability characteristics are deteriorated. Thus, the content of Ni preferably ranges from about 3.5% to about 6.5%. 
     In an embodiment, the stainless steel alloy of the present disclosure preferably excludes (or at least substantially excludes) molybdenum (Mo), which is a relatively expensive material, to further reduce costs. 
     In an embodiment, the stainless steel alloy of the present disclosure includes from about 1.0% to about 6.0% manganese (Mn), for example about 1.5% to about 5.5% Mn, such as from about 2.0% to about 5.0% Mn. Mn is effective like Si as a deoxidizer for the melt, and has a function of improving the fluidity during the casting operation. To exhibit such function effectively, the amount of Mn is about 6.0% or less, preferably 5.0% or less. Mn generally has a content of greater than about 1.0% to adjust a metal flow rate. However, when the content of Mn is excessive, Mn is combined with sulfur of the steel and forms excessive levels of manganese sulfide, thereby deteriorating the corrosion resistance and the hot formability. Thus, the upper limit content of Mn is limited to 6.0%. 
     In an embodiment, the stainless steel alloy of the present disclosure preferably excludes (or at least substantially excludes) tungsten (W), which is a relatively expensive material, to further reduce costs. 
     In an embodiment, the stainless steel alloy of the present disclosure preferably excludes (or at least substantially excludes) niobium (Nb), which is a relatively expensive material, to further reduce costs. 
     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 0.2% or more. Accordingly, in an embodiment, the stainless steel alloy includes, in various alternative embodiments, from about 0.2% to about 0.5% N, for example from about 0.3% to about 0.5% N, from about 0.4% to about 0.5% N, from about 0.3% to about 0.4% N, from about 0.2% to about 0.4%, or from about 0.2% to about 0.3%. To ensure the production stability and to avoid the brittleness due to the precipitation of Cr nitrides, the upper limit of N should be about 0.5%. 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 preferably ranges up to a maximum of about 0.5%. 
     Certain impurities may also be present in the stainless steel alloy of the present disclosure. The amounts of such impurities are minimized as much as practical. In an embodiment, phosphorus (P) may be present in the alloy, but is minimized to about 0.03% 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. Preferably, the upper limit content of P is limited to 0.03% 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 0.03%. 
     In an embodiment, sulfur (S) may be present in the alloy, but is minimized to about 0.03% 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 0.03%. S deteriorates the hot formability, or forms MnS together with Mn, 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 0.03%. 
     Boron (B), calcium (Ca), and cerium (Ce) can optionally be added in very small quantities in steels to improve hot workability. The preferred levels are for B, Ca, and Ce are less than about 0.005%. 
     TABLE 2 sets forth the composition of an exemplary embodiment of the present disclosure, in accordance with the description provided above. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Composition of Exemplary Stainless Steel Alloy 
               
            
           
           
               
               
               
               
            
               
                   
                 Elements 
                 Min (%) 
                 Max (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Carbon 
                 0.4 
                 0.5 
               
               
                   
                 Silicon 
                 1.0 
                 2.0 
               
               
                   
                 Chromium 
                 24.0  
                 26.0 
               
               
                   
                 Nickel 
                 4.5 
                 5.5 
               
               
                   
                 Molybdenum 
                 — 
                 — 
               
               
                   
                 Manganese 
                 2.0 
                 5.0 
               
               
                   
                 Tungsten 
                 — 
                 — 
               
               
                   
                 Niobium 
                 — 
                 — 
               
               
                   
                 Phosphorous 
                   
                 0.03 
               
               
                   
                 Sulphur 
                   
                 0.03 
               
               
                   
                 Nitrogen 
                 0.3 
                 0.5 
               
               
                   
                 Boron 
                   
                 0.005 
               
            
           
           
               
               
               
               
            
               
                   
                 Iron 
                 Rest 
                   
               
               
                   
                   
               
            
           
         
       
     
     As such, embodiments of the present disclosure provide numerous benefits over the prior art, such as stainless steel 1.4826. For example, in accordance with the present disclosure, the presently described embodiments lower the nickel content in as compared to stainless steel 1.4826. Further, the amount of molybdenum, niobium, and tungsten are reduced to zero. Thus, due to lower cost, alloys in accordance with the present disclosure are a suitable lower cost option for turbine housing materials for operation up to about 980° C. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the inventive subject matter, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the inventive subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the inventive subject matter. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the inventive subject matter as set forth in the appended claims.