Abstract:
Flanges formed by powder metallurgical methods are used for joining, for example, exhaust pipes, instead of manufacturing flanges by stamping, forging and/or machining. This offers a number of advantages, including the ability to produce flange shapes which cannot be obtained by stamping, forging and/or machining.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This is a formal application based on and claiming the benefit of U.S. provisional patent application No. 60/176,043, filed Jan. 14, 2000 and U.S. provisional patent application No. 60/194,765, filed Apr. 3, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates mainly to flanges used in exhaust systems of internal combustion engines, for example those commonly used in automobiles and trucks. In such systems, it is common to have stamped or forged and/or machined flanges which are welded to the ends of exhaust pipes which are to be joined to each other, with the flanges having flat mating surfaces which are bolted together with a gasket in between.  
           [0004]    2. Description of the Prior Art  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of the invention to improve on the existing flanges used in exhaust systems and other welded or assembled flange joints.  
           [0006]    In the invention, instead of manufacturing flanges by stamping, forging and/or machining, the flanges are formed by powder metallurgy, which offers a number of advantages, including the ability to achieve flange shapes which cannot be obtained by stamping.  
           [0007]    This permits the flange configurations of the invention to be achieved. There are two flanges in the invention, one of the end of one pipe and one for the end of the other pipe. The flanges have complementary shapes, such that one flange, hereinafter referred to as the “inner flange”, nests partially within the other flange, hereinafter referred to as the “outer flange”.  
           [0008]    In one embodiment of the invention, one flange, preferably but not necessarily the inner one, has integral mounting studs on opposing sides of the pipe, configured to extend through corresponding holes in the other flange. The studs are threaded to receive nuts which are tightened to pull the two flanges together for a secure connection.  
           [0009]    Preferably, the proximal surfaces of the flanges, i.e. the surfaces of the flanges away from their distal mating surfaces, have reinforcement ribs, which permits the overall thickness and therefore the weight of the flanges to be reduced compared to conventional flanges.  
           [0010]    As an alternative to reinforcement ribs and thin sections to reduce weight, the flanges could be of uniform thickness, with through holes formed in several locations to achieve the same weight reduction.  
           [0011]    In a further embodiment of the invention, the flanges have extensions for attachment by press-fitting, spinning, resistance welding, crimp rolling etc. to the exhaust pipes. Thus, no welding is necessary, other than as a further reinforcement of the joint.  
           [0012]    The nesting together of the flanges provides a better seal, in that there is no direct escape route for exhaust gases, thus reducing the need for expensive gaskets or perhaps eliminating the need for gaskets altogether. Also, larger diameter stud posts and matching holes with close tolerances become possible with powder metallurgy. This helps ensure a rigid joint.  
           [0013]    Another advantage of the invention is that since the mounting studs may be integral to the flanges, the assembly joint will tend to remain intact even if the nuts become loose or dislodged, whereas in typical prior art flanges, the separate bolts will fall out, permitting the joint to shift. In the invention, the nesting eliminates lateral shifting of the joint, even if the nuts are loose or missing, making the joint much more mechanically secure. Of course, if both nuts are missing, the joint may come apart longitudinally, but at least it will not separate laterally.  
           [0014]    Thus, in the invention, an exhaust flange assembly for joining ends of exhaust pipes, comprises an outer flange, having a sealing surface and a pipe attachment means arranged on a pipe attachment surface opposite to the sealing surface; and an inner flange, having a sealing surface and a pipe attachment means arranged on a pipe attachment surface opposite to the sealing surface. The flanges are made of powder metallurgically produced material.  
           [0015]    In one embodiment of the invention, the outer flange has a cavity defined in the sealing surface and the sealing surface of the inner flange is shaped to be closely received by the cavity. Further, one of the flanges advantageously has integral mounting studs extending from the sealing surface of the flange, the studs configured to extend through corresponding holes in the other the flange, the studs being threaded to receive nuts whereby the flanges may be pulled together for a secure connection by tightening the nuts. Preferably, at least one of the flanges has an annular gasket recess arranged on the sealing surface of the flange. The pipe attachment surfaces of the flanges preferably have a plurality of reinforcement ribs. The flanges may further have at least one hole therethrough for weight reduction.  
           [0016]    Preferably, the outer flange and the inner flange have bolt mounting holes arranged to receive threaded bolts onto which nuts are threadable, whereby the flanges may be pulled together for a secure connection by tightening the nuts.  
           [0017]    Alternatively, the outer flange has a curved extension protruding in a direction opposite to the cavity, and the inner flange has a curved extension protruding in a direction opposite to the sealing surface of the inner flange, the extensions arranged to be fitted into the ends of exhaust pipes, thereby deforming the exhaust pipes to form a secure joint. Preferably, the extensions have a groove surrounded by an inner ridge and an outer ridge, arranged on an outer surface of the extension. Advantageously, at least one of the flanges has an annular gasket recess arranged on the sealing surface of the flange. Optionally, the pipe attachment surfaces of the flanges have a plurality of reinforcement ribs. The flanges may further have at least one hole therethrough for weight reduction.  
           [0018]    Advantageously, the bolts have generally spherical portions facing the bolt thread, the spherical portions of the bolts arranged to cooperate with generally concave recesses arranged in the flanges around the mounting holes.  
           [0019]    Preferably, at least one of the flanges has an annular gasket recess arranged on a sealing surface of the at least one flange.  
           [0020]    The inner flange advantageously comprises a first substantially flat part and a second annular sealing part. The first part has a first recess and the second part has a second recess, the recesses having complimentary shapes so that the first recess fits in the second recess. Preferably, the first recess and the second recess have multiple steps.  
           [0021]    Still a further embodiment of an exhaust flange assembly according to the invention comprises a straight flange cooperating with a curved flange, where an outer edge of the curved flange is bent away from a surface of the curved flange which contacts the straight flange, so that, when the straight flange is mounted to the curved flange by fasteners, the curved flange deflects towards the flat flange to form a flat sealing surface, thus enhancing the seal between the curved flange and the straight flange, the flanges being made of powder metallurgically produced material. Preferably, the straight flange and the curved flange have mounting holes and the fasteners comprise mounting bolts, for placing through the mounting holes, and mounting nuts to be tightened onto each the mounting bolt, for fastening the straight flange to the curved flange. Advantageously, the straight flange and the curved flange have gasket recesses, for accommodating gaskets. Preferably, the curved flange is substantially weakly bowl-shaped.  
           [0022]    Preferably, the outer flange is made of one material and the inner flange is made of another material.  
           [0023]    A preferred method of producing two-part exhaust flanges, comprises the steps of:  
           [0024]    a) press-forming metal powder to shape a first substantially flat part of an inner flange;  
           [0025]    b) press-forming metal powder to shape a second annular sealing part of the inner flange;  
           [0026]    c) fitting the first part onto the second part; and  
           [0027]    d) sintering the first part and the second part to thereby bond them together and form the inner flange.  
           [0028]    Advantageously, the method further comprises the step of pre-sintering the first part and the second part after press-forming but before fitting the first part onto the second part.  
           [0029]    Preferably, the materials used for making the flanges contain 0.75 to 1 weight percent of hexagonal boron nitride (BN), which enhances the corrosion resistance properties of the powder metallurgical materials used.  
           [0030]    Further features will be described or will become apparent in the course of the detailed description which follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The invention will now be described in greater detail, with reference to the accompanying drawings of the preferred embodiment, in which:  
         [0032]    [0032]FIG. 1 is an exploded side view showing the inner and outer flanges welded on ends of exhaust pipes to be joined;  
         [0033]    [0033]FIG. 2 is a sectional side view corresponding to FIG. 1;  
         [0034]    [0034]FIG. 3 is a side view showing the assembled exhaust pipes;  
         [0035]    [0035]FIG. 4 is a proximal end view of the outer flange;  
         [0036]    [0036]FIG. 5 is a side view of the outer flange;  
         [0037]    [0037]FIG. 6 is a distal end view of the outer flange;  
         [0038]    [0038]FIG. 7 is a distal end view of the inner flange;  
         [0039]    [0039]FIG. 8 is a side view of the inner flange;  
         [0040]    [0040]FIG. 9 is a proximal end view of the inner flange;  
         [0041]    [0041]FIG. 10 is a sectional view of the inner flange at A-A on FIG. 9;  
         [0042]    [0042]FIG. 11 is a top view of the inner flange;  
         [0043]    [0043]FIG. 12 is a distal end view of an alternative embodiment of the outer flange;  
         [0044]    [0044]FIG. 13 is a side view of the alternative embodiment of the outer flange;  
         [0045]    [0045]FIG. 14 is a proximal end view of the alternative embodiment of the outer flange;  
         [0046]    [0046]FIG. 15 is a proximal end view of the outer flange according to yet another embodiment;  
         [0047]    [0047]FIG. 16 is a side view of the outer flange according to FIG. 15;  
         [0048]    [0048]FIG. 17 is a distal end view of the outer flange of FIG. 15;  
         [0049]    [0049]FIG. 18 is a proximal end view of the inner flange corresponding to and cooperating with the outer flange according to FIG. 15;  
         [0050]    [0050]FIG. 19 is a side view of the inner flange according to FIG. 18;  
         [0051]    [0051]FIG. 20 is a distal end view of the inner flange according to FIG. 18;  
         [0052]    [0052]FIG. 21 is an exploded side view showing the inner and outer flanges, according to FIGS. 15 and 18, pushed on ends of exhaust pipes to be joined;  
         [0053]    [0053]FIG. 22 is a sectional side view corresponding to FIG. 21; showing the assembled flange joint;  
         [0054]    [0054]FIG. 23 is a side view showing the assembled exhaust pipes, for flanges according to FIGS. 15 and 18;  
         [0055]    [0055]FIG. 23A is an exploded side view of still a further embodiment of a flange joint according to the invention;  
         [0056]    [0056]FIG. 23B is an exploded side view of yet a further embodiment of a flange joint according to the invention;  
         [0057]    [0057]FIG. 24 is an end view of a flange joint according to a further embodiment of the invention; showing a flange having reinforced areas;  
         [0058]    [0058]FIG. 25 is a sectional side view of the flange of FIG. 24 along line C-C;  
         [0059]    [0059]FIG. 26 is a side view of a flange joint according to a yet further embodiment of the invention; showing a flange having a curved half;  
         [0060]    [0060]FIG. 27 is an exploded partially sectioned side view of a flange joint according to a further embodiment of the invention; showing a flange having a gasket;  
         [0061]    [0061]FIG. 28 is an exploded partially sectioned side view of a flange joint according to still a further embodiment of the invention; showing a flange having a further embodiment of a gasket;  
         [0062]    [0062]FIG. 29 is an assembled partially sectioned side view of the flange joint of FIG. 28;  
         [0063]    [0063]FIG. 30A is an exploded side view of a flange joint according to a further embodiment of the invention, similar to the embodiment of FIG. 28; having the composite gasket bonded onto the inner flange;  
         [0064]    [0064]FIG. 30B is an assembled sectioned side view of the flange joint of FIG. 30A;  
         [0065]    [0065]FIG. 31A is an end view of a flange joint according to a further embodiment of the invention, showing a gasket recess;  
         [0066]    [0066]FIG. 31B is a sectioned side view of the flange joint of FIG. 31A;  
         [0067]    [0067]FIG. 32A is an end view of a flange joint according to still a further embodiment of the invention, showing a further variation of a gasket recess;  
         [0068]    [0068]FIG. 32B is a sectioned side view of the flange joint of FIG. 32A;  
         [0069]    [0069]FIG. 33 is a sectioned side view of a further embodiment of a flange joint of the invention, similar to the flange of FIG. 30B with the addition of a gasket recess on the composite gasket;  
         [0070]    [0070]FIG. 34 is a sectioned side view of a still further embodiment of a flange joint of the invention, similar to the flange of FIG. 30B with the addition of a gasket recess on the composite gasket, which is an integral part of the inner flange;  
         [0071]    [0071]FIG. 35 is a sectional side view of yet a further embodiment of a flange joint of the invention, showing a pair of flanges where the inner flange has a sealing extension for cooperation with a sealing recess arranged in the outer flange;  
         [0072]    [0072]FIG. 36 is a partially sectioned side view of another embodiment of a flange joint of the invention, showing a pair of flanges where the inner flange has a gasket holding ridge for cooperation with a gasket holding recess arranged in the outer flange and a gasket;  
         [0073]    [0073]FIG. 37 is a sectioned side view of the gasket of FIG. 36;  
         [0074]    [0074]FIG. 38 is a front view of the gasket of FIG. 37;  
         [0075]    [0075]FIG. 39A is a front view of a further embodiment of an outer flange according to the invention, having a conical rib on its sealing side;  
         [0076]    [0076]FIG. 39B is a sectioned side view of the outer flange as seen from line F-F of FIG. 39A;  
         [0077]    [0077]FIG. 40A is a front view of a further embodiment of an inner flange according to the invention, having a conical recess on its sealing side;  
         [0078]    [0078]FIG. 40B is a sectioned side view of the inner flange as seen from line G-G of FIG. 40A;  
         [0079]    [0079]FIG. 41 is a sectioned side view of another embodiment of a flange joint of the invention, showing the outer flange of FIG. 39A and the inner flange of FIG. 40A;  
         [0080]    [0080]FIG. 42 is a side view of the embodiment of a flange joint of FIG. 41, showing the outer flange of FIG. 39A and the inner flange of FIG. 40A joined to respective pipes and the connecting bolts used to clamp the flange joint together;  
         [0081]    [0081]FIG. 43A is a front view of yet a further embodiment of an outer flange according to the invention, having a recess for cooperating with a further part of the flange and two bolt mounting holes;  
         [0082]    [0082]FIG. 43B is a sectioned side view of the outer flange according to FIG. 43A;  
         [0083]    [0083]FIG. 44A is a front view of still a further embodiment of an outer flange according to the invention, having a stepped recess for cooperating with a further part of the flange and three bolt mounting holes;  
         [0084]    [0084]FIG. 44B is a sectioned side view of the outer flange according to FIG. 44A;  
         [0085]    [0085]FIG. 45 is a sectioned side view of a further embodiment of an outer and an inner flange according to the invention, where the outer flange is manufactured in two parts;  
         [0086]    [0086]FIG. 46 is a sectioned side view of the outer flange according to FIG. 45, after assembly;  
         [0087]    [0087]FIG. 47 is a sectioned side view of another embodiment of a flange joint of the invention, similar to the embodiment shown in FIGS. 41 and 42, but having an integral stand-off on the inner flange;  
         [0088]    [0088]FIG. 48A is a sectioned side view of an outer flange according a further embodiment of the invention, where the flange is manufactured in two parts; and  
         [0089]    [0089]FIG. 48B is a sectioned side view of the outer flange according to FIG. 48A after assembly.  
     
    
     DETAILED DESCRIPTION  
       [0090]    [0090]FIG. 1 shows an exhaust flange assembly, having two flanges  1  and  2  made of powdered metal welded to ends of exhaust pipes  3  to be joined. One flange, namely the “outer” flange  1 , has a cavity  4  defined in its distal surface. The other flange, i.e. the “inner” flange  2 , has a distal surface  5  with a shape complementing the shape of the cavity. Preferably, that shape involves a generally flat surface  5 , with a rounded edge  6  to match a rounded inner edge  7  of the cavity  4  and thereby provide a sealing seat. In view of this rounding, there is no direct escape path for exhaust, which reduces or eliminates the need for a gasket. If a gasket is still needed, it may be that it could be a less expensive gasket in view of the reduced sealing needs.  
         [0091]    Other edge shapes are certainly possible. For example, there could be a sharp edge, a radius, chamfer or domed seal, or combinations thereof.  
         [0092]    One of the flanges, for example the inner flange  2 , has integral mounting studs  8  on opposing sides, configured to extend through corresponding holes  9  in the other flange, i.e. the outer flange  1 . After production of the flange by a typical powder metallurgy process, the studs are tapped to provide threads  10 . Preferably, the studs are also provided with bolstered heads, to provide extra strength. Nuts  11  are installed to pull the flanges together for a secure connection by tightening the nuts.  
         [0093]    Preferably, the proximal surfaces of the flanges, i.e. the surfaces of the flanges away from their distal mating surfaces, have reinforcement ribs  12 , which permits the overall thickness and thus weight of the flanges to be reduced compared to conventional flanges.  
         [0094]    FIGS.  12 - 14  show an example of the above-mentioned alternative to reinforcement ribs and thin sections to reduce weight, in which the flanges are of uniform thickness, with through holes  13  formed in several locations to achieve the same weight reduction. Only the outer flange is shown, but preferably the inner flange would have corresponding aligned holes as well.  
         [0095]    The invention provides a number of advantages, which include that the integral mounting studs avoid the need for separate studs, that the use of powdered metal permits reduced thickness and weight by permitting the use of reinforcing ribs, that the sealing configuration avoids a direct escape path for exhaust gases, thereby potentially reducing the need for gaskets and potentially reducing emissions.  
         [0096]    FIGS.  15  to  17  show a further embodiment of an “inner” flange  2 ′, made of powdered metal and attached by press-fitting, spinning, resistance welding, crimp rolling etc. to ends of exhaust pipes  3 ′ to be joined. In FIGS.  18  to  20 , an “outer” flange  1 ′ is shown, which is cooperating with the “inner” flange of the embodiment according to FIGS.  15  to  17 . The “outer” flange has a cavity  4 ′, defined in its distal surface. The “inner” flange  2 ′ has a distal surface  5 ′ with a shape complementing the shape of the cavity  4 ′. Preferably, the distal shape  5 ′ involves a generally flat surface, with a rounded edge  6 ′, to match a rounded inner edge  7 ′ of the cavity  4 ′ and thereby provide a sealing seat. In view of this rounding, there is no direct escape path for exhaust, which reduces or eliminates the need for a gasket. If a gasket is still needed, it may be that it could be a less expensive gasket in view of the reduced sealing needs.  
         [0097]    Other edge shapes are certainly possible. For example, there could be a sharp edge, a radius, chamfer or domed seal, or combinations thereof.  
         [0098]    The “inner” flange  2 ′ further has an extension  17  arranged on the side of the flange opposite the distal shape  5 ′. The extension has a co-axial hole, corresponding to the hole in the flange, through which exhaust gases are flowing when the flange combination is in use. The extension has a substantially smooth inner surface and a grooved outer surface, preferably having a groove  18  surrounded by an inner ridge  20  and an outer ridge  19 . The outer diameter of the extension is larger than the inner diameter of the exhaust pipe  3 ′. When the exhaust pipe is attached by press-fitting, spinning, resistance welding, crimp rolling etc. onto the flange  2 ′, the exhaust pipe expands and generally forms itself corresponding to the extensions outer profile, i.e. the groove  18 , inner ridge  20  and outer ridge  19  (see FIGS.  21  to  23 ). In this way, the exhaust pipe is held securely to the flange. The extension may be tapered, having a larger outer diameter at a distal end of the extension as seen from the flange. In this case, the groove  18  is optional. Pressing the exhaust pipe onto the extension may be performed using a lubricant or inductively heating the pipe prior to installation. The grooves  18  may have a variety of profile shapes, to accommodate fitting of the pipes either by induction heating or resistance welding. The groove shape is thus determined by the assembly process or method.  
         [0099]    The “outer” flange  1 ′ further has an extension  13  arranged on the side of the flange opposite the cavity  4 ′. The extension has a co-axial hole, corresponding to the hole in the flange, through which exhaust gases are flowing when the flange combination is in use. The extension has a substantially smooth inner surface and a grooved outer surface, preferably having a groove  14  surrounded by an inner ridge  15  and an outer ridge  16 . The outer diameter of the extension is larger than the inner diameter of the exhaust pipe  3 ′. When the exhaust pipe is attached by press-fitting, spinning, resistance welding, crimp rolling etc. onto the flange  1 ′, the exhaust pipe expands and generally forms itself corresponding to the extensions outer profile, i.e. the groove  14 , inner ridge  15  and outer ridge  16  (see FIGS.  21  to  23 ). In this way, the exhaust pipe is held securely to the flange. The extension may be tapered, having a larger outer diameter at a distal end of the extension as seen from the flange. In this case, the groove  14  is optional. Pressing the exhaust pipe onto the extension may be performed using a lubricant or inductively heating the pipe prior to installation. The grooves  14  may have a variety of profile shapes, to accommodate fitting of the pipes either by induction heating or resistance welding. The groove shape is thus determined by the assembly process or method.  
         [0100]    The “inner” flange  2 ″ is preferably made of powdered metal and attached by press-fitting, spinning, resistance welding, crimp rolling etc. to ends of the exhaust pipes  3 ′ to be joined, forming a joint  22 ″. The “outer” flange  1 ″ is also preferably made of powdered metal and attached by press-fitting, spinning, resistance welding, crimp rolling etc. to ends of the exhaust pipes  3 ′ to be joined, forming a joint  21 ″.  
         [0101]    The flanges have mounting holes  8 ′,  9 ′, respectively, for mounting a bolt  10 ′ and nut  11 ′. Alternatively, one flange has integral mounting studs on opposing sides, configured to extend through corresponding holes in the other flange, as described in conjunction with the previous embodiment. The nuts  11 ′ are installed to pull the flanges together for a secure connection by tightening the nuts.  
         [0102]    Preferably, the proximal surfaces of the flanges, i.e. the surfaces of the flanges away from their distal mating surfaces, have reinforcement ribs  12 ′, which permits the overall thickness and thus weight of the flanges to be reduced compared to conventional flanges.  
         [0103]    [0103]FIG. 21 shows the “outer” flange  1 ′ and the “inner” flange  2 ′ attached by press-fitting, spinning, resistance welding, crimp rolling etc. onto exhaust pipes  3 ′ and ready for having bolts  10 ′ inserted into holes  8 ′ and  9 ′ and nuts  11 ′ threaded onto the bolts, to tighten the flange joint. A tightened flange joint is shown in FIGS. 22 and 23.  
         [0104]    [0104]FIG. 23A shows an “outer” flange  1 ″ and an “inner” flange  2 ″ pressed onto exhaust pipes  3 ′ and ready for having bolts  10 ″ inserted into holes  8 ″ and  9 ″, respectively, and nuts  11 ″ threaded onto the bolts, to tighten the flange joint. The “inner” flange  2 ″ is preferably made of powdered metal and press-fitted to ends of the exhaust pipes  3 ′ to be joined, forming a joint  22 ″. The “outer” flange  1 ″ is also preferably made of powdered metal and attached by press-fitting, spinning, resistance welding, crimp rolling etc. to ends of the exhaust pipes  3 ′ to be joined, forming a joint  21 ″. The “outer” flange  1 ″ cooperates with the “inner” flange. The “outer” flange has a cavity  4 ″, defined in its distal surface. The “inner” flange  2 ″ has a distal surface  5 ″ with a shape complementing the shape of the cavity  4 ″. Preferably, the distal shape  5 ″ involves a generally flat surface, with a rounded edge  6 ″, to match a rounded inner edge  7 ″ of the cavity  4 ″ and thereby provide a sealing seat. In view of this rounding, there is no direct escape path for exhaust, which reduces or eliminates the need for a gasket. If a gasket is still needed, it may be that it could be a less expensive gasket in view of the reduced sealing needs. Other edge shapes are certainly possible. For example, there could be a sharp edge, a radius, chamfer or domed seal, or combinations thereof. The “inner” flange  2 ″ has a projection  24  arranged on its surface, which faces the “outer” flange  1 ″. A recess  23 , formed on a surface of the “outer” flange  1 ″, which faces the “inner” flange  2 ″, has a shape generally complementary to the projection  24 , to facilitate the alignment of the two flanges with each other. Advantageously, the projections  24  and the recesses  23  are arranged concentrically to the holes  8 ″ and  9 ″, respectively.  
         [0105]    [0105]FIG. 23B shows an alternative embodiment to the embodiment shown in FIG. 23A. The “outer” flange  1 ′″ is essentially the same as shown in FIG. 23B, except it does not have any recesses, the holes  9 ′″ are thus of one diameter. Instead, the “inner” flange  2 ′″ has holes  8 ′″ which have recesses  25  arranged on the side of the “inner” flange which faces away from the “outer” flange  1 ′″. The recesses  25  have generally concave surfaces, to cooperate with bolts  10 ′″, which have heads with generally spherical lower portions. Thus, the bolt is allowed to swivel in the holes  8 ′″ and  9 ′″, respectively, to facilitate alignment of the flanges. Advantageously, the “inner” flange  2 ′″ has an increased thickness, compared to the “outer” flange  1 ′″, to provide strength to compensate for the enlarged hole  8 ′″. Further, the bolts  10 ′″ advantageously have a substantially cylindrical extension  40  of the bolt head. The extension has a diameter which is larger than the diameter of the threaded portion of the bolt. A bolt recess  40 ′ arranged in the hole  8 ′″ of the “inner” flange  2 ′″ has a shape corresponding to the extension  40  of the bolt head, to securely hold the bolt once it has been tightened into the hole.  
         [0106]    [0106]FIGS. 24 and 25 show another embodiment of a flange  300  according to the invention. The flange has raised areas  301  arranged between mounting studs  320  having mounting holes  321 . The raised areas provide reinforcement of the flange, to prevent deflection of the flange when tightening the flange to another flange using fastening means (not shown). The flange further has a substantially cylindrical flange extension  310 , for attachment of the flange to an exhaust pipe (not shown). The flange extension has an inner diameter  311 .  
         [0107]    [0107]FIG. 26 shows still a further flange pair combination. A straight flange  400 , having mounting holes  404  and gasket recesses  402 , cooperates with a curved flange  401 , having mounting holes  405  and gasket recesses  403 . The curved flange is substantially weakly bowl-shaped, the outer edge of the flange bent away from the surface of the curved flange which contacts the straight flange. Thus, when mounting bolts  407  are placed through the mounting holes  404  and  405 , respectively, and a mounting nut  406  tightened onto each mounting bolt, the curved flange will deflect towards the flat flange to form a flat sealing surface. This enhances the seal between the two flanges and prevents the further outward deflection of the flange after tightening of the mounting bolts.  
         [0108]    [0108]FIG. 27 shows yet a further embodiment of a flange joint, having a gasket element  504  arranged between a fifth flange  500  and a sixth flange  501 . The fifth flange has a recess  502 , with sloping side walls  513 , which generally conforms in shape to a curved surface  505  of the gasket element. The fifth flange further has an inner diameter  508  and mounting holes  510 . Similarly, the sixth flange  501  has a recess  503 , with curved side walls  512 , which generally conforms in shape to the curved surface  505  of the gasket element. The sixth flange further has an inner diameter  509  and mounting holes  511 . The gasket element  504  has an inner diameter  507 , substantially the same as the inner diameters  508  and  509 , respectively, of the flanges. The gasket element further has a flange gap defining protrusion  506  arranged along its circumference. When mounting bolts  515  are inserted into the mounting holes  510  and  511 , respectively, and a mounting nut  514  is tightened onto each mounting bolt, the two flanges compress the gasket element  504 , which will allow only a limited amount of float (relative movement of the flanges) to provide an enhanced leak resistance for the flange joint.  
         [0109]    [0109]FIGS. 28 and 29 show a further embodiment of a flange joint, having a composite gasket element  504 ′ arranged between a sixth flange  501  (as described in conjunction with FIG. 27) and a inner flange  500 ′. The inner flange has mounting holes  510 ′ and a substantially cylindrical flange extension  517 , for attachment of the flange to an exhaust pipe (not shown). Further, the inner flange  500 ′ has a backing plate portion  516  arranged on the same side of the flange as the flange extension. The backing plate portion has an inner diameter  508 ″ and an inner surface  519 , facing away from the flange extension side of the flange. The inner flange has a general inner diameter  508 ′, which is larger than the inner diameter  508 ″ of the backing plate portion  516 . The composite gasket element  504 ′ has a curved surface  505 ′, generally conforming in shape to the recess  503 , with sloping side walls  512  of the sixth flange. The composite gasket element further has an inner diameter  508 ′, substantially the same as the inner diameters  508 ′ and  509 , respectively, of the flanges. The gasket element further has an outer surface  517  and a substantially flat surface facing the inner flange  500 ′. When mounting bolts (not shown) are inserted into the mounting holes  510 ′ and  511 , respectively, and a mounting nut (not shown) is tightened onto each mounting bolt, the two flanges compress the gasket element  504 ′, which will allow only a limited amount of float (relative movement of the flanges) to provide an enhanced leak resistance for the flange joint. The backing plate portion  516  retains the composite seal between the inner flange  500 ′ and the composite gasket element  504 ′.  
         [0110]    [0110]FIGS. 30A and 30B show a further embodiment of a flange joint similar to the embodiment shown in FIGS.  27  to  29 , except that a composite gasket element  504 ″ is bonded to a inner flange  500 ″, for example by pressing and sintering the two in one sintering step, or sintering the inner flange first then inserting the pressed gasket element and sintering again. The composite gasket element  504 ″ is thus arranged between a sixth flange  501  (as described in conjunction with FIG. 27) and the inner flange  500 ″. The inner flange has mounting holes  510 ″ and a backing plate portion  516 ′ arranged on the same side of the flange as the flange extension. The backing plate portion has an inner diameter  508   IV  and an inner surface  519 ′, facing away from the flange extension side of the flange. The inner flange has a general inner diameter  508   V , which is larger than the inner diameter  508   IV  of the backing plate portion  516 ′. The composite gasket element  504 ″ has a curved surface  505 ″, generally conforming in shape to the recess  503 , with sloping side walls  512  of the sixth flange. The composite gasket element further has an inner diameter  508   VI , substantially the same as the inner diameters  508   IV  and  509 , respectively, of the flanges.  
         [0111]    [0111]FIGS. 31A and 31B show a preferred embodiment of a flange  400 ′ having an annular gasket recess  402 ′, which has an inner wall  411  and an outer wall  410 . Further, the flange has mounting holes  414 , and an inner diameter  415 . The inner wall  411  and the outer wall  410  form at least one squeeze area  412 , where the width of the gasket recess, the distance between the outer wall and the inner wall, is substantially narrower than a maximum gasket recess width  411 . In this way, the gasket (not shown) will be held in the gasket recess after insertion of the gasket. Preferably, two such squeeze areas  412  are defined in the gasket recess  402 ′.  
         [0112]    [0112]FIGS. 32A and 32B show an alternative preferred embodiment of a flange  400 ″ having an annular gasket recess  402 ″, which has an inner wall  411 ′ and an outer wall  410 ′. Further, the flange has mounting holes  414 ′, and an inner diameter  415 ′. At least one protrusion  416  is arranged along the inner wall  411 ′ and/or (not shown) the outer wall  410  to form at least one squeeze area  412 ′, where the width of the gasket recess, the distance between the outer wall and the protrusion, is substantially narrower than a maximum gasket recess width  411 ′. In this way, the gasket (not shown) will be held in the gasket recess after insertion of the gasket. Preferably, two, three or four such squeeze areas  412 ′ are defined in the gasket recess  402 ″, by arranging two, three or four protrusions  416 , respectively, on the inner wall  411 ′.  
         [0113]    [0113]FIG. 33 shows a further embodiment of a flange joint similar to the embodiment shown in FIGS. 30A to  30 B. A composite gasket element  504 ′″ is bonded to a inner flange  500 ″, for example by pressing and sintering the two in one sintering step, or sintering the inner flange first then inserting the pressed gasket element and sintering again. The composite gasket element  504 ′″ is thus arranged between a sixth flange  501  (as described in conjunction with FIG. 27) and the inner flange  500 ″. The inner flange has mounting holes  510 ″ and a backing plate portion  516 ′ arranged on the same side of the flange as the flange extension. The backing plate portion has an inner diameter  508   IV  and an inner surface  519 ′, facing away from the flange extension side of the flange. The inner flange has a general inner diameter  508   V , which is larger than the inner diameter  508   IV  of the backing plate portion  516 ′. The composite gasket element  504 ′″ has a curved surface  505 ″, generally conforming in shape to the recess  503 , with sloping side walls  512  of the sixth flange. The composite gasket element further has an inner diameter  508   VI , substantially the same as the inner diameters  508   IV  and  509 , respectively, of the flanges. Additionally, the composite gasket element  504 ′″ has a gasket recess  402 ′ arranged on the curved surface  505 ″, to receive an additional annular sealing gasket (not shown).  
         [0114]    [0114]FIG. 34 shows still a further embodiment of a flange joint similar to the embodiment shown in FIG. 33. A inner flange  500 ′″ has an inner diameter  508   VII , mounting holes  510 ′″ and a protrusion  504   IV . The protrusion has a curved surface  505 ′″, which cooperates with a sixth flange  501  (as described in conjunction with FIG. 27) in that it generally conforms in shape to the recess  503 , with sloping side walls  512  of the sixth flange. Additionally, the protrusion  504   IV  has a gasket recess  402 ″ arranged on the curved surface  505 ′″, to receive an additional annular sealing gasket (not shown).  
         [0115]    [0115]FIG. 35 shows a further embodiment of a flange joint according to the invention, where a inner flange  602  has an exhaust mounting extension  22 ′″, mounting holes  604 , gasket recesses  607  and a sealing extension  603 . A outer flange  601  has an exhaust mounting extension  21 ′″, mounting holes  605 , gasket recesses  606  and a sealing recess  608  having an outer diameter substantially corresponding to the outer diameter of the sealing extension  603 . The sealing extension and the sealing recess thus cooperate to provide a positive seal and enhanced alignment between the two flanges. The flanges are fastened together using bolts  10 ″ and nuts  11 ″.  
         [0116]    FIGS.  36  to  38  show yet a further embodiment of a flange joint according to the invention, where an “inner” flange  702  has an exhaust mounting extension  706 , mounting holes  703  and an annular gasket holding ridge  713  with a base  714 . An “outer” flange  701  has an exhaust mounting extension  705 , mounting holes  704  and an annular gasket holding recess  712  with an annular ring  711 . The gasket holding ridge and the gasket holding recess thus cooperate to positively hold a shaped gasket  710  to provide a positive seal and enhanced alignment between the two flanges. The flanges are fastened together using bolts  10 ″ and nuts  11 ″. The gasket is shown in detail in FIGS. 37 and 38, showing the gasket inner diameter  717 , which is substantially the same or larger as the inner diameters of the exhaust openings of the flanges. The gasket has five sealing surfaces on its “hat”-like cross-section (brim, side, top, other side and other brim), and this shape gives no direct escape for gases. The gasket is made of a resilient material, and preferably laminated and/or spirally wound.  
         [0117]    [0117]FIGS. 39A to  42  show a further advantageous embodiment of a flange joint according to the invention. FIGS. 39A and 39B show an outer flange  800 , having a conical rib  810  on a sealing surface  801  of the outer flange, which cooperates with an inner flange  850  (FIG. 40A). The outer flange further has a pipe attachment surface  802  having a pipe attachment means  830 . An exhaust through hole  840  is arranged in the outer flange, for passage of exhaust gas when the flange is assembled as part of an exhaust system flange joint. Mounting holes  820  are also arranged through the outer flange. FIGS. 40A and 40B show the inner flange  850 , having a conical recess  860  on a sealing surface  851  of the inner flange. The inner flange further has a pipe attachment surface  852  having a pipe attachment means  880 . An exhaust through hole  890  is arranged in the inner flange, for passage of exhaust gas when the flange is assembled as part of an exhaust system flange joint. Mounting holes  870  are also arranged through the outer flange. FIG. 41 shows a flange joint made of the outer flange  800  and the inner flange  850 . FIG. 42 shows the flange joint of FIG. 41, with the outer flange  800  and the inner flange  850  joined to respective pipes  3  and connecting bolts  10 ′ with nuts  11 ′, used to clamp the flange joint together. When an inner and an outer flange are fastened together, the conical rib  810  and the conical recess  860  form a compression fitting to eliminate leaks in the flange joint. The rigidity of the joint is also enhanced by the compression fitting of the invention.  
         [0118]    [0118]FIGS. 43A to  46  show a further embodiment of a flange joint of the invention. The inner flange  850 ′, cooperating with an outer flange  501  as described in FIGS. 30A and 30B or an outer flange as described in FIGS. 48A and 48B below, is manufactured in two parts, a flat part (backing plate) which has mounting holes  870 ′, a central through hole  890 ′ and a recess  855  with an end wall  855 ″, and a sealing part  504 ′″. The flange material is preferably sintered material, possibly different material in the two different parts, for instance using material with enhanced sealing properties for the sealing part and material with enhanced strength for the backing plate. The sealing part has an outwardly curved surface  505 ′″ generally corresponding to the recess  503 , with sloping side walls  512  of the outer flange  501 . The sealing part further has a recess  519 ″ having an inner annular surface  520 . The sealing part also has an outer annular sealing surface  518 . The sealing part  504 ′″ further has an inner diameter  508   VII , smaller than the inner diameters (central through hole)  890 ′ and  509 , respectively, of the inner flange and the outer flange. The two parts of the inner flange  850 ′ are made in separate pressing operations of a sintering manufacturing process. The required press force is lower, making it possible to use smaller and cheaper presses. Both parts are optionally pre-sintered at relatively low temperatures and then pressed together and subjected to a final sintering step, during which the two parts bond together. When the two parts are pressed together, the inner flange recess  855  contacts the sealing part recess  519 ″, and the inner flange recess end wall  855 ″ contacts the outer annular sealing surface  518 , whilst the sealing part inner annular surface  520  contacts the surface of the central through hole  890 ′, forming mating surfaces for the two parts. Thus, the inner flange recess has a shape cooperating with and corresponding to the shape of the sealing part recess. For the two-piece inner flange described above, a bonding agent/welding flux may be applied to the mating surfaces after an optional pre-sintering step but before final sintering, to enhance the bond between the two parts after sintering. Further, an optional weld may be applied to the inner flange after sintering, to additionally strengthen the joint between the sealing part and the backing plate. The weld would be applied on the side of the inner flange where the sealing part extends from the backing plate, and either be in the form of a tack weld or a continuous weld along the full joint between the sealing part and the backing plate.  
         [0119]    As is shown in FIGS. 43A and 43B, the inner flange  850 ′ may have two mounting holes  870 ′ and a single step recess  855 , as described above. An alternative embodiment of a inner flange  850 ″ is shown in FIGS. 44A and 44B. The inner flange has three mounting holes  870 ″ and a multi-stepped recess  855 ′ corresponding in shape to a multi-stepped recess of the sealing part (not shown). The larger number of mounting holes makes a secure attachment of the inner flange and a outer flange easier, and the multi-stepped recess enlarges the surface area between the two parts of the inner flange, which enhances the bonding between the two parts after final sintering.  
         [0120]    As is shown in FIGS. 48A and 48B, also the outer flange  501 ′ may be of two-piece construction (as inner flange of FIGS. 43A to  46 ). The outer flange is thus manufactured in two parts, a flat part (backing plate) which has mounting holes  511 ′, a central through hole  890 ″ and a recess  855 ′″ with an end wall  855   IV , and a sealing part  504   IV . The flange material is preferably sintered material, possibly different material in the two different parts, for instance using material with enhanced sealing properties for the sealing part and material with enhanced strength for the backing plate. The sealing part has an inwardly curved surface  505   IV . The sealing part further has a recess  519 ′″ having an inner annular surface  520 ′ and an outer annular surface  518 ′. The sealing part  504   IV  further has an inner diameter  508   VIII , smaller than the central through hole  890 ″. The two parts of the outer flange  501 ′ are made in separate pressing operations of a sintering manufacturing process. The required press force is lower, making it possible to use smaller and cheaper presses. Both parts are optionally pre-sintered at relatively low temperatures and then pressed together and subjected to a final sintering step, during which the two parts bond together. When the two parts are pressed together, the outer flange recess  855 ′″ contacts the sealing part recess  519 ′″, and the outer flange recess end wall  855   IV  contacts the outer annular sealing surface  518 ′, whilst the sealing part inner annular surface  520 ′ contacts the surface of the central through hole  890 ″, forming mating surfaces for the two parts. Thus, the outer flange recess has a shape cooperating with and corresponding to the shape of the sealing part recess. For the two-piece outer flange described above, a bonding agent/welding flux may be applied to the mating surfaces after an optional pre-sintering step but before final sintering, to enhance the bond between the two parts after sintering. Further, an optional weld may be applied to the outer flange after sintering, to additionally strengthen the joint between the sealing part and the backing plate. The weld would be applied on the side of the outer flange where the sealing part extends from the backing plate, and either be in the form of a tack weld or a continuous weld along the full joint between the sealing part and the backing plate.  
         [0121]    The invention provides a number of advantages, which include that the use of powdered metal permits reduced thickness and weight by permitting the use of reinforcing ribs or the formed lightening holes, that the sealing configuration avoids a direct escape path for exhaust gases, thereby potentially reducing the need for gaskets and potentially reducing emissions and to facilitate the alignment of the different parts of the exhaust system.  
         [0122]    Preferably, the outer flange is made of one material and the inner flange is made of another material. In this way, the heat expansion of the flange can be regulated to compensate for differences in heating of the flanges (the flange closest to the engine will theoretically be heated more than the flange further away). By choosing a material having a lower heat expansion for the flange closest to the engine, and a material having a higher heat expansion for the other flange, both flanges can be made to expand equally much during use, thus enhancing the fit and seal of the flange assembly.  
         [0123]    Preferably, the materials used for making the flanges contain between 0.75 to 1 weight percent of hexagonal boron nitride (BN), which enhances the corrosion resistance properties of the powder metallurgical materials used, as disclosed in U.S. Pat. No. 6,103,185.  
         [0124]    Using flanges as described in the different embodiments of the invention will enhance the sealing and rigidity properties of the flange joint, compared to known flange assemblies.  
         [0125]    It will be appreciated that the above description relates to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described. For example, when assembling exhaust pipes it might be economical to use standard length bolts, which might be too long for the application and possibly interfere with the pipe if it has a sharp bend adjacent the flange. To accommodate longer bolts, it is foreseen to produce the sintered flanges having a stand-off sleeve integrally formed on the surface of the flange which faces away from the sealing surface of the flange, as shown in FIG. 47. When using a two-piece flange, having a separate sealing part, the two parts may be fixed to each-other by low-strength glue, or similar, to secure the sealing part in place until the flange is used with another flange to form a flange assembly. After the flanges are joined, the sealing part will be held in place by the joining forces.