Abstract:
The surfaces of a stainless steel part are made harder without sacrificing corrosion resistance by subjecting the part to low temperature carburization. The soot layer formed as a by-product of carburization is then removed without removing the non-coherent metal oxide layer which also forms as a by-product. A solid lubricant is then applied to reduce the coefficient of friction of the carburized surface. The by-product metal oxide layer acts as an anchor keeping the solid lubricant in place.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This document is based on U.S. Provisional Application Ser. No. 60/354,594, filed Feb. 5, 2002, the benefit of which is hereby claimed and the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Commonly assigned U.S. Pat. No. 6,093,303 and U.S. application Ser. No. 09/494,093, filed Jan. 28, 2000, describe processes for increasing the hardness of stainless steel parts without sacrificing corrosion resistance by carburizing the parts at low temperatures, e.g., temperatures less than about 1000° F. See, also U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510, U.S. Pat. No. 5,792,282, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364). The disclosures of each of these documents is incorporated herein by reference. As taught in these patents, the corrosion resistance of such parts is maintained because formation of carbide precipitates, which occurs during conventional carburization at higher temperatures, is substantially avoided.  
           [0003]    Stainless steel is stainless because of the thin, coherent chromium oxide film which inherently forms when the steel is exposed to air. Low temperature carburization of stainless steel parts, such as those made from AISI 316 and 316L stainless steels, usually leaves the part surfaces coated with a layer of soot. Before use this soot is usually removed by washing. When carbon monoxide or other oxygen-containing gas is used as the carbon source in low temperature carburization, not only does soot form but in addition a heavy oxide film also forms. This heavy oxide film is considerably different from the coherent chromium oxide film which makes stainless steel stainless in that it is thicker and not coherent. Therefore, this film is also removed before use to uncover the part&#39;s carburized surface.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the present invention, it has been found that this heavy oxide film, because of its porous structure, serves as an ideal anchor for solid lubricants. Accordingly, it is possible in accordance with the present invention to provide stainless steel parts which are not only exceptionally hard and exceptionally corrosion resistance but which also exhibit enhanced and sustained surface slipperiness as well.  
           [0005]    Thus, the present invention provides a new article of manufacture comprising a stainless steel part having at least one carburized surface substantially free of carbide precipitates, the part further comprising an adherent, non-coherent metal oxide layer on the carbide surface and a solid lubricant on the metal oxide layer.  
           [0006]    In addition, the present invention also provides a new process of manufacture comprising applying a solid lubricant to the metal oxide layer of a stainless steel part having at least one carburized surface substantially free of carbide precipitates, the part further comprising an adherent, non-coherent metal oxide layer on the carbide surface. 
       
    
    
     DETAILED DESCRIPTION  
       [0007]    Stainless Steel Substrate  
         [0008]    The present invention is applicable to all types of stainless steels. As well known, stainless steel is a steel which forms a coherent chromium oxide coating when exposed to air. To this end, most stainless steels contain at least about 16 wt. % chromium Preferred are substantially or partially austenite. Of special interest are AISI 316, 316L, 317, 317L and 304 stainless steels, alloy 600, alloy C-276 and alloy 20 Cb, Nitronic alloy, alloy 58, alloy 825, alloy 254 SmO to name a few examples.  
         [0009]    Formation of Metal Oxide Layer  
         [0010]    As indicated above, low temperature carburization of stainless steel parts, when done using an oxygen containing gas, leaves the carburized surface carrying a layer of soot as well as a porous, i.e. non-coherent, adherent metal oxide layer. Normally, this metal oxide layer is composed of chromium oxide, although other metal oxides can be present. In conventional practice, this metal oxide layer is removed together with or independently of the soot layer which also forms. In accordance with the present invention, however, this heavy metal oxide layer is left on at least a portion of the part surfaces, since it has been found that this metal oxide layer serves as an ideal anchor for bonding subsequently applied solid lubricants.  
         [0011]    The particular conditions under which this metal oxide layer can be formed are already well known in the art, since this formation occurs to a greater or lesser degree during low temperature carburization when carbon monoxide or other oxygen containing gas is present in the carburizing gas. Especially heavy (i.e., thick) metal oxide layers can be obtained by using carbon monoxide as the predominant carbon source for carburization. Carburization gases in which carbon monoxide accounts for at least about 75%, 85%, 95% or even 100% of the carbon source for carburization are especially interesting.  
         [0012]    Other approaches that can be used for fostering the formation of this metal oxide coating during carburization are small percentages of CO 2 , H 2 O, etc.  
         [0013]    Normally, the oxide layer will be about 200 to 400 Angstroms thick, although thinner and thicker layers can also be used so long as they serve to anchor the applied solid lubricant in the manner indicated herein.  
         [0014]    Removal of Soot Layer  
         [0015]    Removal of the soot layer which forms during low temperature carburization can be done in a conventional manner. For example, washing the carburized part aqueously with ultrasonics will effectively remove all or substantially all the soot without adversely affecting the metal oxide layer in any significant way.  
         [0016]    Solid Lubricants  
         [0017]    In accordance with the present invention, a solid lubricant is applied to the metal oxide layer formed by low temperature carburization.  
         [0018]    For this purpose, essentially any solid lubricant can be used. Many such solid lubricants are well known. Some are particulate in form while other are supplied larger in size. A few examples are graphite, molybdenum disulfide, tungsten disulfide, UHMWPE (ultra high molecular weight polyethylene), halogenated polymers such as PFA, PTFE, PCTFE and the like. Examples of commercially available solid lubricants include Dow Corning® 321 Dry Film Lubricant available from Dow Coming Corporation of Midland, Mich. and Slickote® Dry Lube 100 available from Trans Chem Coatings, of Monrovia, Calif.  
         [0019]    These lubricants can be used not combined with another material, or mixed with another material such as a resinous carrier or the like. In addition, they can be used in essentially any solid form including powders, granules, pastes and bulk solids.  
         [0020]    Application of Solid Lubricants  
         [0021]    The solid lubricants of the present invention can be applied to the metal oxide layer by any standard method such as by hand, such as by rubbing, by aerosol or air spraying or by automatic equipment. Any coating thickness can be used which will provide lubricating properties. Solid lubricant thicknesses exceeding standard class 2 thread clearances are usually not required.  
         [0022]    If appropriate, the lubricant can also be heated to enhance its adhesion. For example, some lubricants, especially those supplied in a resinous binder, can be heated to effect cure of the binder. For example, Slickote® Dry Lube 100 can be heated following manufacturer&#39;s instructions to 300° F. for 1 hour, for example.  
         [0023]    Utility  
         [0024]    The present invention can be used anywhere it is desirable to enhance the surface slipperiness of case hardened, corrosion resistant stainless steel parts. Particular examples are nuts, bolts, gears, valves, connectors, fasteners, ferrules and the like whose bearing surfaces have been case hardened by low temperature carburization. Because of the lubricant, these surfaces slide easier, thereby reducing stress imparted by resistance to sliding movement. Moreover, because of the anchoring nature of the metal oxide layer, the lubricant remains effective far longer than would be the case if the lubricant were applied to a comparable product whose metal oxide layer were removed first.  
         [0025]    The present invention finds particular utility in making stainless steel nuts having case hardened threads lubricated in accordance with the present invention. Because of the lubricant, less pull-up torque is required during tightening as compared to a similar case hardened nut without lubricant. Moreover, this enhanced lubricating effect is retained longer than a lubricated nut made in a conventional manner, i.e. by removing the metal oxide layer before applying the lubricant, because the metal layer anchors the lubricant in place even after repeated use. Thus, nuts made in accordance with the present invention can withstand repeated fitting remakes (i.e., loosenings and retightenings of the nut) without being removed or replaced.  
         [0026]    Although only a few embodiments of the present invention have been described above, it should be appreciated that many modifications can be made without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of the present invention, which is to be limited only by the following claims: