Patent Publication Number: US-6656293-B2

Title: Surface treatment for ferrous components

Description:
TECHNICAL FIELD 
     The invention relates generally to surface treatment and, more particularly, to methods for providing corrosion and abrasion resistance to a surface of a ferrous material. 
     BACKGROUND 
     Many of today&#39;s earthmoving, agricultural, recreational, and military machines use tracks for propulsion. The track typically includes numerous track links chained together, each track link having metal or rubber pads that contact and grip the ground. Adjacent track links are generally joined to one another at track joints by bushing assemblies. A bushing is inserted between a pin and a bore on the track link through which the bushing passes. As the tracked machine moves, the track links move around a portion of a sprocket wheel as the individual links rotate around the pin and bushing. To resist fracture under stress and withstand impact, the bushing is typically made from a plain carbon or medium alloy steel. 
     Oil or grease is typically used as a lubricant in the bushing assembly. The oil may be confined by a polymeric seal located between the end surface of the bushing and the inner surface of the track link bore. Because the polymeric seal slides against a portion of the end surface of the bushing as the track moves, the end surface of the bushing contacting the polymeric seal is typically ground and polished to provide a smooth sealing surface against which the polymeric seal can slide. The ground sealing surface, however, still abrades the polymeric seal. Furthermore, the track operates in a corrosive and abrasive environment that can exacerbate grooving of the end surface of the bushing and polymeric seal. Grooving can result in oil leakage and subsequent seizing and failure of the track. 
     Surface treatment by thermochemical diffusion processes are known to impart abrasion resistance to the surface of steels, for example, plain carbon or medium alloy steels, without affecting the tougher, impact-resistant underlying material. In particular, nitrocarburization processes, such as disclosed in U.S. Pat. No. 5,102,476, are known to provide increased wear and corrosion resistance to steel surfaces. The disclosed nitrocarburization process introduces nitrogen and carbon into the surface of steels to produce a “white” or “compound” layer. The compound layer, depending on the steel alloy and the diffusion atmosphere, contains varying amounts of γ′ (Fe 4 N), ε (Fe 2-3 N), cementite, carbides, and nitrides. Similarly, nitriding introduces nitrogen into the surface of steel to form a hardened, abrasion resistant layer. 
     While the nitrocarburized or nitrided layer provides some corrosion and wear resistance, its surface still abrades the polymeric seal thereby allowing abrasives and corrosives to get between the polymeric seal and the end surface of the bushing to cause further grooving. Grinding of the nitrocarburized or nitrided layer is generally avoided to prevent damage of the compound layer. 
     Thus, there is a need to overcome these and other problems of the prior art and to provide a surface and a method for treating a surface that avoids grooving. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a method is provided for treating a surface of a first component, wherein at least a portion of the surface of the first component contacts a surface of a second component. The method includes forming a compound layer at the at least a portion of the surface of the first component by a thermochemical diffusion treatment and isotropically finishing the at least a portion of the surface of the first component that contacts the surface of the second component. 
     In accordance with another aspect of the present invention, a method is provided for treating a surface of a track bushing wherein at least a portion of the surface of the track bushing contacts a polymeric component to form a seal. The method includes subjecting the surface of the track bushing to a thermochemical diffusion treatment to form a compound layer and isotropically finishing at least the portion of the surface of the track bushing that contacts the polymeric component to a surface roughness of Ra≦0.1 μm. 
     In accordance with another aspect of the present invention, a track bushing is disclosed. The track bushing includes a surface, wherein at least a portion of the surface is isotropically finished and includes a compound layer. 
     In accordance with yet another aspect of the present invention, a track is disclosed. The track includes a plurality of track links, each of the plurality of track links including a bore at a first end and a second end. The track further includes a plurality of bushing assemblies, wherein the plurality of bushing assemblies join adjacent track links by residing in the bore at the second end of a first track link and the bore at the first end of a second track link. Each of the plurality of bushing assemblies includes a steel bushing having an isotropically finished surface, wherein the isotropically finished surface includes a compound layer and a pin that fits in the steel bushing. The track further includes polymeric seals that contact the isotropically finished surface of the steel bushing and an inside surface of the bore of at least one of the adjacent track links. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a diagrammatic cross-section of a portion of a first component having a surface that contacts a surface of a second component. 
     FIG. 1B is a diagrammatic cross-section of a portion of a first component including a compound layer and a diffusion layer in accordance with an exemplary embodiment of the invention. 
     FIG. 2A is a diagrammatic cross-section of a portion of a first component having a surface that contacts a surface of a second component. 
     FIG. 2B is a diagrammatic cross-section of a portion of a first component including a compound layer, diffusion layer, and a physical vapor deposition layer in accordance with an exemplary embodiment of the invention. 
     FIG. 3 is a perspective partial cut-away view of a portion of a track including a bushing assembly and track links in accordance with an exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration a specific exemplary embodiment in which the invention may be practiced. This embodiment is described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense. 
     With reference to FIGS. 1A and 1B, a method for treating a surface of a first component, wherein at least a portion of the surface of the first component contacts a surface of a second component, in accordance with an exemplary embodiment of the present invention is disclosed. FIG. 1A depicts a portion of first component  10  having surface  15  and surface region  12  and a portion of second component  18  having surface  19 . In operation, surface  15  contacts surface  19 , as shown by, for example, arrows  17 . First component  10  includes a ferrous material. As used herein, the term “ferrous” means a metallic material having iron as a principal component, including, but not limited to, steels. FIG. 1B depicts surface region  12  including surface  15 , compound layer  13  over diffusion layer  14 , and core  11  underlying diffusion layer  14 . The microstructural composition of compound layer  13  and the thickness of the layers depends on several factors including the composition of the core material, the type of thermochemical treatment, and the parameters of the thermochemical treatment. 
     In one exemplary embodiment consistent with the present invention, compound layer  13  and diffusion layer  14  are formed by a ferritic nitrocarburization treatment. The ferritic nitrocarburization treatment diffuses nitrogen and carbon into the surface of the fererous material at temperatures completely within a ferritic phase field. The parameters for ferritic nitrocarburizing a ferrous surface in a salt bath, a furnace, and a fluidized bed are known to those of skill in the art. Ferritic nitrocarburization generally results in compound layer  13  containing varying amounts of γ′ (Fe 4 N) and ε (Fe 2-3 N) microstructures, as well as cementite and various carbides and nitrides. Diffusion layer  14  generally has the microstructure of core  11  including nitrogen in solid solution and as metal nitride (n x N) precipitates. 
     In another exemplary embodiment consistent with the present invention, compound layer  13  and diffusion layer  14  are formed by nitriding. Nitriding is a thermochemical diffusion treatment that diffuses nitrogen into the surface of a ferrous material without changing the microstructure of the material. The parameters for forming a compound layer and a diffusion layer by gas, liquid, and plasma nitriding are known to those of skill in the art. Nitriding generally results in compound layer  13  containing predominantly γ′ (Fe 4 N) or predominantly ε (Fe 2-3 N), or a mixture of γ′ and ε microstructures. Other thermochemical diffusion treatments to provide compound and diffusion layers are known to those with skill in the art and include, but are not limited to, ion nitriding, carburizing, boronizing, and carbonitriding. 
     After compound layer  13  is formed, surface  15 , the portion of first component  10  that contacts surface  19  of second component  18 , is subject to an isotropic finishing process. Isotropic finishing reduces the roughness of surface  15  to Ra≦0.1 μm without removing the compound layer. Isotropic finishing can be used to further reduce the roughness of surface  15  to Ra≦0.05 μm. The parameters for isotropic finishing are known by those with skill in the art. 
     With reference to FIGS. 2A and 2B, a method for treating a surface of a first component, wherein at least a portion of the surface of the first component contacts a surface of a second component surface, in accordance with another exemplary embodiment of the present invention is disclosed. FIG. 2A depicts a portion of first component  20  having surface  25  and surface region  22  and a portion of second component  28  having surface  29 . In operation, surface  25  contacts surface  29 , as shown by, for example, arrows  27 . First component  20  includes a ferrous material. A thermochemical diffusion treatment is used to form compound layer  23  at surface region  22  and diffusion layer  24  underlying compound layer  23 . Core  21  underlies diffusion layer  24 . As discussed above, the parameters for the thermochemical diffusion treatment of ferrous surfaces, such as, for example, nitriding and ferritic nitrocarburization, are known by those with skill in the art. 
     After formation of compound layer  23 , surface  25  of first component  20  is subject to an isotropic finishing process. Isotropic finishing reduces the roughness of surface  25  to Ra≦0.1 μm without removing the compound layer. Isotropic finishing can be used to further reduce the roughness of surface  25  to Ra≦0.05 μm. As discussed above, parameters for isotropic finishing are known by those with skill in the art. 
     Physical vapor deposition (“PVD”) layer  26  is then deposited over the isotropically finished compound layer  23 . PVD layer  26  can be formed by processes that deposit thin films in the gas phase in which the deposition material is physically transferred to compound layer  23  without chemical reaction, including, but not limited to, sputtering, electron beam, laser, vacuum evaporation, ion-beam-assisted, arc vapor, ion plating, thermal evaporation, and ion assisted deposition processes. The type of PVD layer  26  deposited by these processes include, but is not limited to, chrome nitride, metal containing diamond-like carbon, amorphous diamond-like carbon, TiCN, and TiBN. 
     With reference to FIG. 3, an example of surface treatment of an end surface of a track bushing in accordance with an exemplary embodiment of the present invention is provided. A portion of a track, generally designated by the reference numeral  30 , includes track links  31  having bore  32  at each end thereof. Adjacent track links are joined together by bushing assemblies that include pin  33 , seal  35 , and bushing  34  having end face  36 . In operation, seal  35  slides against end face  36  of bushing  34  as track  30  moves. 
     Bushing  34  may be any medium carbon steel or medium carbon low alloy steel. Bushing  34  may be, for example, made of an austenitized and direct hardened steel alloy having a composition of 0.26-0.31 wt % C, 0.50-0.70 wt % Mn, a maximum of 0.015 wt % P, a maximum of 0.010 wt % S, 1.45-1.80 wt % Si, 1.60-2.00 wt % Cr, 0.30-0.40 wt % Mo, 0.70-0.12 wt % V, 0.010-0.025 wt % Al, 0.03-0.05 wt % Ti, 0.005-0.013, and the balance Fe. Other steels suitable for bushing  34  include, but are not limited to, compositions including 0.38-0.43 wt % C, 0.75-1.00 wt % Mn, 0.035 wt % maximum of P, 0.040 wt % maximum of S, 0.15-0.35 wt % Si, 0.80-1.10 wt % Cr, 0.15-0.25 wt % Mo, and the balance Fe, and compositions including 0.28-0.33 wt % C, 0.90-1.20 wt % Mn, 0.035 wt % maximum of P, 0.050-0.080 wt % S, 0.15-0.35 wt % Si, 0.90-1.20 wt % Cr, 0.05-0.10 wt % V, 0.08-0.13 wt % Al, and the balance Fe. 
     Bushing  34  may be subject to a ferritic nitrocarburization treatment that includes an initial etch with phosphoric acid. As an alternative, nitric acid can be used for this etch. Bushing  34  can then be placed into an integral quench furnace at a temperature of about 570° C. An endothermic gas of 40% H 2 , 40% N 2 , and 20% CO may flow into the integral quench furnace at about 160 cubic feet per hour (“cfh”) to serve as a carrier gas for ammonia. Ammonia gas may flow into the integral quench furnace at about 200 cfh and air may flow into the integral quench furnace at about 400 cfh. After approximately 3 hours, bushing  34  may be removed from the integral quench furnace and quenched in oil. The resultant compound layer will be approximately 5-30 μm and include γ′ (Fe 4 N) and ε (Fe 2-3  N) microstructures. 
     End face  36  of bushing  34  may then be isotropically finished. Bushing  34  may be placed into a part container of a vibratory bath. In an initial cut stage, an abrasive may include ceramic media about 25 mm square and 8 mm thick in an acidic bath of a dilute oxalic acid solution, such as, for example, Feromill 575 made by REM Chemical. Bushing  34  may remain in the cut stage for approximately 5 minutes. A subsequent burnishing stage may use similar ceramic media and a potassium phosphate solution, such as, for example, Feromill FBC 295. Bushing  34  may remain in the burnishing stage for approximately 5 minutes. After removal from the vibratory bath, the surface roughness (Ra) of end face  36  will be about 0.05 μm or less. 
     With further reference to FIG. 3, an example of surface treatment of an end surface of a track bushing in accordance with another exemplary embodiment of the present invention is provided. Bushing  34 , including end face  36 , may be subject to a ferritic nitrocarburization treatment, such as, for example, a Trinide® process. Alternatively, the ferritic nitrocarburization treatment can include, for example, placing bushing  34  into a furnace at a temperature of about 565° C. and an atmosphere of about 500 cfh of Nx (endothermic) gas. An exothermic gas, nominally about 11% CO and 13% H 2  with the balance N 2  and CO 2 , may be used with an ammonia flow of about 350 cfh. Bushing  34  may be held in the furnace for about 330 minutes, whereupon the ammonia flow may be stopped. Bushing  34  may be held for about an additional 30 minutes before being removed from the furnace and quenched in oil. 
     End face  36  of bushing  34  may then be isotropically finished to a surface roughness Ra≦0.05 μm or less as described above. A chrome nitride PVD coating may then be deposited on the isotropically finished, ferritic nitrocarburized end face  36 . The chrome nitride coating can be about 2-6 μm thick. 
     INDUSTRIAL APPLICABILITY 
     The disclosed methods provide surface treatments for ferrous components. Although the methods have wide application to surface treat most ferrous materials, the present invention is particularly applicable to providing corrosion and abrasion resistant layers on plain carbon and medium alloy steels that serve as sealing surfaces. Plain carbon and medium alloy steels are typically used because of their toughness and impact resistance. A thermochemical diffusion layer provides a corrosion and abrasion resistant layer on these materials without affecting the impact resistance of the underlying steel, but the surface roughness of the layer, even after grinding, is difficult to seal against. The present invention provides a method that preserves the corrosion and abrasion resistant layer on the impact resistant underlying steel while further treating the surface to permit sealing, for example, by a polymeric seal. The method accomplishes this by use of a thermochemical diffusion process coupled with an isotropic finishing process that avoids the problems associated with other surface treatments, such as, grinding. 
     While the present invention has applicability in a number of fields, it is known to provide a surface with improved sealability in track joints of a tracked machine. This provides improved performance and lower warranty and repair costs. 
     It will be readily apparent to those skilled in this art that various changes and modifications of an obvious nature may be made, and all such changes and modifications are considered to fall within the scope of the appended claims. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.