Patent Publication Number: US-2012043490-A1

Title: Pre-oxidation of engine valves and seat inserts for improved life

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This disclosure was made in part with Government support under CRADA NFE-07-00995 between Caterpillar Inc. and UT-Battelle, LLC under its Prime Contract with the U.S. Department of Energy for the operation of the Oak Ridge National Laboratory. The Government may have certain rights in this disclosure. 
    
    
     TECHNICAL FIELD 
     This patent disclosure relates generally to internal combustion engine valve assemblies. More particularly this disclosure relates to wear resistant valve seat inserts, including a pre-applied oxide coating for use in combination with valves for selectively opening and closing ports to cylinder combustion chambers in an internal combustion engine. 
     BACKGROUND 
     Internal combustion engines are high-temperature environments that may incorporate valve assemblies to control the flow of fuel mixtures and exhaust products into and out of cylinder combustion chambers during combustion cycles. Such valve assemblies may incorporate an angled valve seat insert that engages a corresponding valve head to form a desired sealing relationship. During multiple engagement cycles between the valve and the valve seat insert, the contacting surfaces may experience wear, eventually resulting in the need to replace the valve seat insert and/or the valve. The valve seat inserts may require replacement sooner than the corresponding valves. Moreover, the rate of wear of the valve seat inserts tends to be greatest during an initial start-up period following installation 
     In the past, use of oxidation treatments to protect titanium-based valve components in internal combustion engines was advocated. For example, a surface treatment method of a titanium alloy valve is taught by Japanese Laid-open Patent Publication Number 11-117056 (&#39;7056), in which the titanium alloy valve is oxidized in order to produce a wear resistant hard oxide film on its surface and to eliminate the need for use of a copper series valve seat. In &#39;7056, an engine valve made from a metastable β titanium alloy is exemplified as the titanium part, because it has been generally known that when an α-β titanium alloy is oxidized, its fatigue strength is reduced. However, as best understood the teachings in this reference are limited to valves formed from a titanium which readily forms a stable passive oxide coating. There is no recognition of using oxidation treatments to protect valve seats formed from cobalt alloys or valves formed from nickel alloys (i.e. Group VIII elements), which have a more positive free energy of formation than oxides of titanium. 
     SUMMARY 
     The present disclosure describes, in one aspect, a valve seat insert. The valve seat insert comprises a cobalt alloy base including at least about 40 wt % Co in combination with at least about 5 wt % Cr. Further, the valve seat insert includes a wear face having an oxide scale coating disposed thereon, the oxide scale coating having a thickness of less than about 1000 nm and a surface composition including chromium, oxygen and cobalt. 
     The present disclosure describes, in another aspect, a valve assembly adapted to selectively open and close a passage to a cylinder in an internal combustion engine during a plurality of combustion cycles. The valve assembly comprises a valve seat insert having a cobalt alloy base including at least about 40 wt % Co and at least about 5 wt % Cr. The valve assembly also comprises a moveable valve including a valve head, the valve head being at least partially covered with a cobalt alloy overlay. The valve seat insert further includes a wear face having a first oxide scale coating disposed thereon, the first oxide scale coating having a thickness of less than about 1000 nm and a surface composition including less than about 10 wt % Co. 
     The present disclosure describes, in yet another aspect, a method for extending the useful life of a valve seat insert for use in conjunction with a valve adapted to selectively open and close a passage to a cylinder in an internal combustion engine during a plurality of combustion cycles. The method comprises forming the valve seat insert from a cobalt alloy including at least about 40% Co and least about 5% Cr. The method also comprises subjecting the valve seat insert to an oxidizing heat treatment to form an oxide scale coating disposed on a valve seat insert wear face that is adapted to engage the valve, wherein the oxide scale coating has a thickness up to about 1000 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a diagrammatic illustration of a machine according to an exemplary and disclosed embodiment. 
         FIG. 2  is a diagrammatic illustration of an exemplary internal combustion engine. 
         FIG. 3  is a diagrammatic illustration of an exemplary valve assembly for use in an internal combustion engine. 
         FIG. 4  is a perspective sectional view of an exemplary valve seat insert for use in a valve assembly in an internal combustion engine. 
         FIG. 5  is a diagrammatic illustration showing an oxide scale coating on a wear face of an exemplary valve seat insert. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  that includes a frame  12 , an operator station  14 , one or more traction devices  16 , and an internal combustion engine  20  that combusts fuel and generates an exhaust stream. Although  FIG. 1  shows machine  10  as a truck, machine  10  could be any type of machine having an internal combustion engine  20 . Accordingly, the traction devices  16  may be any suitable type of traction device such as, for example, wheels (as shown in  FIG. 1 ), tracks, belts, or combinations thereof. The machine  10  may also be substantially non-mobile, such as an electric generator or a well-service rig, which incorporates an internal combustion engine as a power source. 
     An exemplary embodiment of internal combustion engine  20  is illustrated in  FIG. 2 . For the purposes of the present disclosure, internal combustion engine  20  is depicted and described as a four stroke diesel engine. One skilled in the art will recognize, however, that internal combustion engine  20  may be any other type of internal combustion engine, such as, for example, a gasoline or natural gas internal combustion engine. 
     As illustrated in  FIG. 2 , internal combustion engine  20  includes an engine block  28  that defines a plurality of cylinders  22 . A piston  24  is slidably disposed within each cylinder  22  to define a combustion chamber  23 . In the illustrated embodiment, internal combustion engine  20  includes six cylinders  22  and six associated pistons  24 . One skilled in the art will readily recognize that the internal combustion engine  20  may include a greater or lesser number of pistons  24  and that pistons  24  may be disposed in an “in-line” configuration, a “V” configuration, or any other conventional configuration. As also shown in  FIG. 2 , internal combustion engine  20  includes a crankshaft  27  that is rotatably disposed within engine block  28 . Connecting rod  26  connects each piston  24  to crankshaft  27 . Each piston  24  is coupled to the crankshaft  27  so that a sliding motion of piston  24  within respective cylinder  22  results in a rotation of crankshaft  27 . Similarly, a rotation of crankshaft  27  will result in a sliding motion of pistons  24 . 
     The internal combustion engine  20  also includes a cylinder head  30 . The cylinder head  30  defines an exhaust passageway  41  that leads to at least one exhaust port  36  for each cylinder  22 . Cylinder head  30  may further define two or more exhaust ports  36  for each cylinder  22 . An exhaust valve  32  is disposed within each exhaust port  36 . Each exhaust valve  32  includes an exhaust valve head  40  configured to selectively block the respective exhaust port  36 . A valve stem  46  extends away from exhaust valve head  40 . As will be readily appreciated by those of skill in the art, each exhaust valve  32  may be actuated to move or “lift” to thereby open the respective exhaust port  36 . In a cylinder  22  having a pair of exhaust ports  36  and a pair of exhaust valves  32 , the pair of exhaust valves  32  may be actuated by a single valve actuation assembly or by a pair of valve actuation assemblies. 
     The cylinder head  30  also defines at least one intake port  38  for each cylinder  22 . Each intake port  38  leads from an intake passageway  43  to the respective cylinder  22 . Cylinder head  30  may further define two or more intake ports  38  for each cylinder  22 . An intake valve  34  is disposed within each intake port  38 . Each intake valve  34  includes an intake valve head  48  that is configured to selectively block the respective intake port  38 . As with exhaust valves  32 , each intake valve  34  may be actuated to move or “lift” to thereby open the respective intake port  38 . In a cylinder  22  having a pair of intake ports  38  and a pair of intake valves  34 , the pair of intake valves  34  may be actuated by a single valve actuation assembly or by a pair of valve actuation assemblies. 
       FIG. 3  illustrates an exemplary arrangement for operating an exhaust valve  32  in a cylinder  22 . As will be appreciated, a similar arrangement may be used for operating a corresponding intake valve. Exhaust passageway  41  leads from an exhaust manifold opening  87  to exhaust port  36  and into combustion chamber  23 . In addition, internal combustion engine  20  includes an exhaust manifold  88  that may be engaged with cylinder head  30 . Exhaust gases may be directed from combustion chamber  23  through intake passageway  41  and to exhaust manifold  88 . 
     Exhaust valve head  40  is configured to selectively engage a valve seat insert  50  in exhaust port  36 . As will be described further hereinafter, valve seat insert  50  may be formed from a cobalt based alloy that has been pre-oxidized to provide enhanced surface protection. Exhaust valve head  40  may be moved between a first position where exhaust valve head  40  engages valve seat insert  50  to prevent a flow of fluid relative to exhaust port  36  and a second position (as illustrated in  FIG. 3 ) where exhaust valve head  40  is away from the valve seat insert  50  to allow a flow of fluid relative to the exhaust port  36 . A similar seating arrangement may be used in relation to intake valves  34 . 
     Internal combustion engine  20  also includes a series of valve actuation assemblies  44 . One valve actuation assembly  44  may be provided to move exhaust valve  32  between the first and second positions. Another valve actuation assembly  44  may be provided to move intake valve  34  between the first and second positions. Valve actuation assembly  44  may also include a valve spring  72 . Valve spring  72  may act on valve stem  46  through a locking nut  74 . Valve spring  72  may act to move exhaust valve  32  relative to cylinder head  30 . In the illustrated embodiment, valve spring  72  acts to bias exhaust valve  32  into the first position, where exhaust valve head  40  engages valve seat insert  50  to prevent a flow of fluid relative to exhaust port  36 . 
     As shown in  FIG. 2 , a controller  100  may be connected to each valve actuation assembly  44 . Controller  100  may include an electronic control module that has a microprocessor and a memory. As is known to those skilled in the art, the memory is connected to the microprocessor and stores an instruction set and variables. Associated with the microprocessor and part of electronic control module are various other known circuits such as, for example, power supply circuitry, signal conditioning circuitry, and solenoid driver circuitry. 
     As will be appreciated by those of skill in the art, when exhaust valve head  40  of exhaust valve  32  engages the corresponding valve seat insert  50  to establish a sealing relation, the engaging surfaces may be subject to abrasion and wear. Eventually, as wear progresses, replacement of valve seat insert  50  and/or exhaust valve  32  may be required. Similar wear and eventual replacement is also associated with operation of intake valve heads  48  and corresponding valve seat inserts  50  that are configured to operate in the same manner. It has been found that wear of contacting surfaces may be particularly acute during an initial break-in period. Break-in periods for interfacing components are generally understood in the art, and may be defined, at least in part, as a tribological concept for the time necessary to allow interfacing surfaces to conform to one another such that they are adequately seated against one another. The duration of any given break-in period is highly dependent on the operating conditions, but generally, for the engine components highlighted herein, a suitable break-in period is less than about 250 hours of engine operation. It should be understood that a break-in period may be longer than about 250 hours, and that exemplary suitable break-in periods typically fall under 250 hours. It is contemplated that reducing wear during the initial break-in period may increase the overall life of the valve components and reduce the frequency of replacement. 
     In order to increase high temperature wear resistance, it is known to use nickel-based alloys to form portions of exhaust valves  32  and intake valves  34 . It is also known to use cobalt-based alloys to form complementary valve seat inserts. The use of cobalt alloys may be particularly beneficial for valve seat inserts  50 , which engage exhaust valve heads  40 , due to the high temperatures of the exhaust gases. While such a combination of materials is beneficial, the cobalt-based valve seat inserts may nonetheless exhibit relatively higher levels of wear during an initial break-in period. Accordingly, the valve seat inserts tend to require replacement sooner than the nickel-based valve elements. The use of cobalt-based valve seat inserts exhibits some desirable mechanical and tribological properties. Likewise, nickel-based valves exhibit some beneficial traits related to structural integrity at high temperatures and reasonable cost. 
     As such, the present disclosure provides cobalt-based valve seat inserts  50  that have been subjected to forced pre-use oxidation before installation. Such pre-use oxidation provides a relatively thin oxide scale coating incorporating relatively small atomic percentages of cobalt oxides at the surface of the valve seat inserts prior to use. In this regard, it will be understood that the term “scale” denotes a film or layer made up predominantly of mixed metallic oxides that have been formed on the surface of an alloy by reacting the metallic constituents with oxygen at elevated temperatures. It has been discovered that wear resistance is greatly enhanced despite the relative thinness and scale character of the oxide scale surface coating. Optionally, complementary nickel-based valves that contact the valve seat inserts also may be provided with a cobalt alloy covering and be subjected to forced, pre-use oxidation before installation. 
     Referring jointly to  FIGS. 3 ,  4 , and  5 , in one exemplary embodiment, valve seat insert  50  has a ring structure and includes angled wear face  52 , which engages exhaust valve head  40  repeatedly as exhaust port  36  is opened and closed during the combustion cycle. As shown, the valve seat insert has an oxide surface scale coating  54  disposed on insert substrate  58  at least at wear face  52 . By way of example only, in one embodiment a valve seat insert  50  may comprise an insert substrate  58  comprising an alloy having at least about 40 wt % Co, at least about 5 wt % Cr, and at least about 20 wt % Cr+Mo. For example, insert substrate  58  may have between about 40-65 wt % Co, such as between about 40-57 wt % Co, or between about 40-45 wt % Co. Further, the composition of Cr in the alloy for insert substrate  58  may be between about 5-35 wt %, or between about 7.5-32 wt %. Moreover, the combined amount of Cr+Mo in the allow for insert substrate  58  may be between about 20-50 wt % Cr+Mo, such as between about 25-40 wt % Cr+Mo). 
     In one exemplary embodiment, valve seat insert  50  may be provided that is formed with insert substrate  58  having an approximate alloy composition of about 2.2-2.6 wt % C, 0-3.0 wt % Ni, 29.0-32.0 wt % Cr, 0.4-1.0 wt % Si, 0-3.0 wt % Fe, 11.0-14.0 wt % W, 0-1.0 wt % Mn, with 0-5.0 wt % combined Mo+Fe+Ni and the balance being Co and incidental impurities. In another exemplary embodiment, valve seat insert  50  may be provided which is formed with insert substrate  58  having an alloy composition of about 26.5-29.5 wt % Mo, 7.5-8.5 wt % Cr, 2.2-2.6 wt % Si, 0-3.0 wt % combined Fe+Ni, 0-0.03 wt % P, 0-0.03 wt % S, 0-0.08 wt % C, and the balance being Co and incidental impurities. Moreover, other base alloys may be used according to this disclosure so long as they include at least about 40 wt % Co in combination with at least about 5 wt % Cr. As indicated previously, valve seat insert  50  has an outer angled wear face  52  with a relatively thin oxide scale surface coating  54  adapted to engage a complementary exhaust valve head  40  after the heat treating operation. 
     In one exemplary embodiment, oxide surface scale coating  54  on wear face  52  is characterized by a surface composition including less than about 10 wt % Co, such as less than about 9 wt % Co, less than about 8 wt % Co, less than about 7 wt % Co, less than about 6 wt % Co, or between about 1-5 wt % Co. Further, scale coating  54  has a surface composition including at least about 50 wt % O, such as, e.g., about 50-70 wt % O. The balance of the surface composition of scale coating  54  includes not more than about 5 wt % C, such as less than about 4 wt % C, less than about 3 wt % C, or even less than about 2 wt % C. Oxide surface scale coating  54  covering wear face  52  of valve seat insert  50  may have a thickness of up to about 1000 nm, such as, e.g., between about 1-1000 nm, between about 100-1000 nm, between about 100-500 nm, or between about 200-400 nm. In this regard, it should be understood that the oxygen content tends to diminish progressively as distance from the surface increases. Accordingly, for purposes of evaluating the thickness of oxide surface scale coating  54 , coating thickness is defined as the depth at which total oxygen drops below about 5 wt %. According to the disclosure, oxide surface scale coating  54  is present after any preliminary formation or treatment practices, but before activation of an internal combustion cycle. The chemical composition may be evaluated by any suitably precise analytical technique as known to those of skill in the art. By way of example only, one suitable analytical technique for evaluation of surface chemistry is X-ray photoelectron spectroscopy (XPS), which is also known as electron spectroscopy for chemical analysis (ESCA). Such a technique is sensitive to a depth of about 10 nm. Such a system measures chemical composition as well as the electronic states of materials. 
     To form oxide surface scale coating  54 , the valve seat insert is subjected to a heat treatment in an oxidizing atmosphere before installation of valve seat insert  50  at cylinder  22 . The heat treatment may be carried out in air at an appropriate temperature, such as, e.g., between about 700-850° C., for an appropriate time, such as, e.g., between about 3-5 hours. However, the heat treatment may also be conducted in an oxygen enriched atmosphere if desired, which would likely reduce the amount of time required to form oxide surface scale coating  54 . Likewise, temperatures and treatment times may be adjusted as desired. In one example, valve seat insert  50  was subjected to heat treatment in an air furnace at about 785-790° C. for a period of about 4 hours. Following the heat treatment, the surface of the valve seat insert  50 —including wear face  52 —was covered with a Cr-rich oxide scale coating having a thickness of about 250 nm. The resulting chemical composition at the surface of the oxide scale coating was measured to be about 4 wt % Co, 30 wt % Cr, 64 wt % O, 1 wt % C, and 1 wt % trace elements. 
     Exhaust valve head  40  and/or intake valve head  48  may also undergo a hardfacing treatment or comprise another overlay of a cobalt alloy held in place by a metallurgical bond. In such a configuration, the valve head having an overlay may be subjected to an oxidizing heat treatment to form an oxide scale coating having a thickness up to about 1000 nm, as previously described. The presence of such an oxide scale coating at the valve head may provide additional wear resistance. 
     By way of example only, exhaust valve  32  and/or intake valve  34  may be formed from a Ni-based age hardenable superalloy. In accordance with one exemplary embodiment, such Ni-based superalloys may include between about 50-75 wt % Ni, such as between about 55-65 wt % Ni, in combination with between about 15-30 wt % Cr, such as between about 20-25 wt % Cr, the balance including Fe, Ti, Al, and other minor additives and impurities. For example, one such material has a composition of about 57 wt % Ni, about 23 wt % Cr, about 14 wt % Fe, about 2 wt % Mo, about 2.3 wt % Ti, about 1.3 wt % Al, about 0.9 wt % Nb, about 0.005 wt % B, about 0.05 wt % Zr, about 0.04 wt % C, less than about 0.5 wt % Mn, less than about 0.2 wt % Si, less than about 0.02 wt % P, and less than about 0.005 wt % S. 
     A metallurgically bonded overlay of a Co-based alloy may be applied on at least a portion of exhaust valve head  40  and/or intake valve head  48 . Such a Co-based alloy does not need to be similar to the Co-based alloy formed on valve seat insert  50 . In one embodiment, the Co-based alloy applied to the valve head may have a composition including between about 2.2-2.6 wt % C, between about 0-3.0 wt % Ni, between about 29.0-32.0 wt % Cr, between about 0.4-1.0 wt % Si, between about 0-3.0 wt % Fe, between about 11.0-14.0 wt % W, between about 0-1.0 wt % between about Mn, and between about 0-5.0 wt % combined Mo+Fe+Ni, the balance being Co and incidental impurities. In another exemplary embodiment, the Co-based alloy applied to the valve head may have a composition including between about 26.5-29.5 wt % Mo, between about 7.5-8.5 wt % Cr, between about 2.2-2.6 wt % Si, between about 0-3.0 wt % combined Fe+Ni, between about 0-0.03 wt % P, between about 0-0.03 wt % S, and between about 0-0.08 wt % C, the balance being Co and incidental impurities. Moreover, other base alloys may be used according to this disclosure so long as they include at least about 40 wt % Co in combination with at least about 5 wt % Cr. 
     The Co-based alloy may be applied to the valve head by any suitable hardfacing techniques, such as plasma transferred arc (PTA) or laser welding. In such techniques, a powder feedstock form of the Co-based alloy is dropped into a molten weld pool that is developed progressively on the surface of the valve head being treated. The powder feedstock melts into the weld pool, then resolidifies to form the overlay. Alternatively, the overlay may be formed on the valve head by tungsten inert gas (TIG) welding, which uses a welding rod as the feedstock to build up the overlay. In this regard, various Co-based alloys as previously described may be used in wire form. In yet another alternative, the Co-based overlay may be applied by techniques such as thermal spray coating, including high velocity oxygen fuel (HVOF) or the like. 
     Regardless of the particular technique used to apply the Co-based overlay, the result is the formation of a metallurgically bonded layer including cobalt and chromium. Upon being subjected to an oxidizing heat treatment as described herein, the oxide scale coating previously described is developed on the overlay. The presence of this oxide scale coating at the valve head may further improve wear resistance during an initial start-up period. Portions of the valves without the Co-based overlay may also develop a Cr-rich oxide coating. 
     INDUSTRIAL APPLICABILITY 
     In accordance with one exemplary practice, the present disclosure is applicable to valve seat inserts  50  formed from cast Co-based alloys including at least about 40 wt % Co and at least about 5 wt % Cr, which are useful in internal combustion engines in conjunction with exhaust valves  32  and/or intake valves  34 . Prior to instillation in the engine, valve seat inserts  50  are subjected to a forced oxidation heat treatment in air or another oxidizing atmosphere to establish oxide surface scale coating  54 . The oxide surface scale coating  54  covers a wear face  52  on an angled segment of valve seat inserts  50 . 
     In use, the valve seat inserts  50  are installed in an internal combustion engine to engage exhaust valve heads  40  and/or intake valve heads  48 . The presence of oxide surface scale coating  54  on wear face  52  prior to the initiation of an internal combustion cycle provides protection against undue wear during an initial break-in period for valve seat inserts  50 . Despite the thin, scale character of the coating, the low percentage of cobalt, and high percentage of chromium relative to the base alloy, it has been discovered that the presence of the coating substantially enhances the life span of valve seat insert  50 . In this regard, testing has shown that a valve seat insert  50  that is formed from the cobalt-based alloy disclosed herein and subjected to the oxidation heat treatment described herein to form a Cr-rich oxide scale coating before installation into an engine yields an average lifespan, i.e., before requiring replacement, of about 20% longer than the same insert without an oxide treatment coating. Accordingly, the frequency of required replacement is substantially diminished. 
     Further, exhaust valves  32  and/or intake valves  34  of an internal combustion engine may be provided with a Co-based alloy overlay and be subjected to some level of oxidizing heat treatment before installation. After forming an oxide scale coating on the cobalt alloy overlay, further wear reduction during an initial break-in period may be observed. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Moreover, all steps in the methods described herein may be performed in any suitable order, unless otherwise indicated herein or otherwise clearly contradicted by context.