Abstract:
One exemplary embodiment includes a process for forming a hard carbide coating onto a low chromium-containing steel article via a chemical deposition process carried out on a particulate mix, in which molybdenum in the form of a compound FeMo or titanium in the form of a compound FeTi, or a mixture of FeMo and FeTi, may be added to the particulate mix used to form the coating.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/105,898 filed Oct. 16, 2008. 
     
    
     TECHNICAL FIELD 
       [0002]    The field to which the disclosure relates generally to wear resistant steel articles and, in particular, to a process for increasing adhesion of a Group 5 metal source carbide coating to a low chromium containing steel substrate to form a wear resistant steel article. 
       BACKGROUND 
       [0003]    Power transmission chains are widely used in the automotive industry not only for ignition timing, but also for transferring mechanical power to the driving wheels of a vehicle. Two types of power transmission chains are traditional roller chains and the so-called “silent chains”. Both roller chains and silent chains use steel pins as important components. 
         [0004]    During assembly and subsequent operation of a vehicle, the steel pins are subject to wear. To improve the wear resistant properties of the steel substrates, a hard coating may be applied to the steel substrate. For example, vanadium carbide (VC) coatings have been placed on small steel parts such as pins to improve wear resistance. The composition of the pin substrate steel, however, may have a significant effect on vanadium coated steel pins. For example, steel substrate materials having about 1.5 weight percent or less of chromium is thought to not form enough diffusion of carbide at the vanadium carbide coating/steel interface, which may result in poor adhesion of the vanadium carbide coating to the steel substrate. 
         [0005]    It has been found that appropriate carbon content of the substrate steel can ensure the thickness of the VC coating and impart strength and hardness, and appropriate chromium content in the substrate steel is important for good adhesion of the coating to the substrate steel pins. 
         [0006]    As a solution, pins having a hard chromium carbide layer can be made by depositing the chromium from FeCr powder surround the pin surface at 970 degrees Celsius. However, the use of ferro-chromium and elemental chromium powders is frequently foreclosed or inhibited by environmental regulation. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0007]    One exemplary method discloses a process for forming a hard carbide coating onto a low chromium-containing steel article via a chemical deposition process carried out on a particulate mix, in which molybdenum in the form of a compound FeMo may be added to the particulate mix used to form the coating. 
         [0008]    Another exemplary method discloses a process for forming a hard carbide coating onto a low chromium-containing steel article via a chemical deposition process carried out on a particulate mix, in which titanium in the form of a compound FeTi may be added to the particulate mix used to form the coating. 
         [0009]    Yet another exemplary method discloses a process for forming a hard carbide coating onto a low chromium-containing steel article via a chemical deposition process carried out on a particulate mix, in which molybdenum in the form of a compound FeMo and titanium in the form of FeTi may be added to the particulate mix used to form the coating. 
         [0010]    An exemplary particulate mix for coating a low chromium-containing steel substrate via a chemical deposition process includes a Group 5 metal source, a halide catalyst, and FeMo or FeTi, or a mixture of FeMo and FeTi. 
         [0011]    An exemplary steel article such as a chain may be formed by applying a carbide coating to a low chromium-containing steel substrate, wherein the carbide coating may be formed from the exemplary particulate mix of the previous paragraph. 
         [0012]    Other exemplary embodiments will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0014]      FIG. 1  is an idealized section of a pin coated with a carbide coating according to an exemplary embodiment; 
           [0015]      FIG. 2  is a longitudinal section view of an exemplary rotating retort containing a particulate mix for forming a coating on selected articles; 
           [0016]      FIG. 3  is an idealized end section of the retort also showing the particulate mix and selected articles; and 
           [0017]      FIG. 4  shows a portion of a silent chain generally of a prior art design but including pins as from  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0018]    The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
         [0019]    Referring now to  FIG. 1 , one exemplary embodiment includes an article  10  having a low chromium-containing steel core  12  coated along at least one surface  13  with a carbide coating  14 . 
         [0020]    For purposes herein, a low chromium-containing steel core  12  contains less than about 1.6% chromium. The term “steel core” may be used interchangeably herein with the term “steel substrate” and merely represents wherein the article includes a low chromium-containing steel surface that is to be coated with the carbide coating  14 . All percentages herein are by weight. 
         [0021]    One exemplary embodiment of a low-chromium content steel that may be utilized in the steel core  12  is AISI 52100 (UNS-G-52986) steel with the following nominal composition: 0.98-1.1 weight percent carbon; 0.25-0.45 weight percent manganese; 1.3-1.6 weight percent chromium; 0.025 weight percent or less phosphorus; 0.025 weight percent or less sulfur; 0.15-0.35 weight percent silicon; and the balance iron. 
         [0022]    In this exemplary illustration, the particulate mix  16  used for forming the carbide coating  14  may include a Group 5 metal source, a halide catalyst, and either ferrotitanium (FeTi) powder or ferromolybdenum (FeMo) powder (or a mixture thereof). Other substantially inert particulates, such as aluminum oxide, may also be included in the particulate mix  16 , and in one embodiment may be present in amounts not greater than about 50 percent of the particulate mix  16 . 
         [0023]    A Group 5 metal source includes a Group 5 metal listed on the Periodic Table of Elements in the 18-group classification designated and recommended by the International Union of Pure and Applied Chemistry. Preferably, the Group 5 metal in the particulate mix  16 , to which Vanadium and Niobium are the only members, has an atomic number no greater than 41. 
         [0024]    A non-exclusive list of available halide catalysts that may be introduced to the particulate mix  16  includes iron chloride, ammonium chloride, niobium chloride, vanadium chloride, or mixtures thereof. The halide catalyst may be used in any effective amount, wherein one embodiment may be in an amount of about 0.6% to 3% by weight of the Group 5 metal source. 
         [0025]    In one embodiment, the amount of FeTi or FeMo powder included in the particulate mix  16  may be between about 0.5 and about 4 weight percent of the Group 5 metal source. In other words, the weight ratio of FeTi, or FeMo, or a combination of FeTi and FeMo, to the Group 5 metal source may be in the range of about 0.02 to 0.04. 
         [0026]    One exemplary particulate mix  16  may include ferrovanadium (FeV) powder having a particle size of 0.8 to 3 mm and about 1% of a selected halide catalyst; here iron chloride (FeCl 3 ). In addition, the particulate mix  16  may also include ferromolybdenum (FeMo) powder. The FeMo powder may be between about 0.5 and about 4 weight percent of the FeV powder. Other substantially inert particulates, such as aluminum oxide, may be included in the particulate mix  16 , and in one embodiment in amounts not greater than about 50 percent of the particulate mix  16 . 
         [0027]    Referring now to  FIG. 2 , the method of the exemplary embodiments may be preferably implemented in a rotary container  20 , or retort  20 , having a shaft  22  held rotatably in walls  24  and  26  of furnace  28  by bushings  30  and sealed. A motor (not shown) may rotate the container  20  at a desired speed while the furnace  28  may be maintained at a temperature, in one embodiment, of about 870 to 1093 degrees Celsius (about 1600 to 2000 degrees Fahrenheit), or in another embodiment between about 927 to 1038 degrees Celsius (about 1700 to 1900 degrees Fahrenheit). Inside the container  20  may be the particulate mix  16  and at least one steel article  10 , in this case steel chain pins  10 , to be coated with the particulate mix  16  to form the carbide coating  14  of a desired thickness. The desired thickness may achieve a surface hardness of at least HV 2000, which may be associated with a thickness of about 10 to 20 microns. For the exemplary particulate mix  16  of the previous paragraph, the carbide coating  14  is a vanadium/carbide coating. 
         [0028]    In one embodiment, air is withdrawn from the rotary container  20  and the process is conducted in the sealed rotary container  20  in the substantial absence of air. In another embodiment, an inert gas, preferably argon or nitrogen, is introduced to the container  20 . During the heating and rotation of the rotary container  20 , the source of Group 5 metal in the particulate mix  16 , may be caused to dissociate, providing Group 5 metal which may be deposited at the surface of steel core  12  in the form of a halide. Carbon is drawn from the steel core  12  surface of the article  10  to displace the halide, which then reverts to the particulate mix  16  to combine with additional Group 5 metal from the source. Only a small percentage of the Group 5 metal source, estimated at 0.5 to 2% of the metal in the metal source, may consumed in the process to provide a commonly desired coating thickness of 10 to 20 microns. 
         [0029]    The molybdenum or the titanium in the FeMo or FeTi powder added to the particulate mix  16  are carbide formers that have a high solubility in the Group 5 metal and iron and therefore may increase interface bonding of the coating formed to the core steel substrate  12 . 
         [0030]    After the article or articles  10  are treated to form a hard coating  14  as described above, the particulate mix  16  and the articles  10  may be separated, and the particulate mix  16  may be returned for re-use in the rotary container  20  to be heated again in the presence of another article or articles t 10  o be coated. The particulate mix  16  need not be replenished through several iterations, but may includes the possibility of replenishing the Group 5 metal source and/or the catalyst while the bulk (at least 50%) of the particulate mix  16  in successive uses may comprise material having been used before for the purpose. Since generally less than 2% of the Group 5 metal source may be consumed in a single use, and since the halide displaced from the Group 5 metal at the surface returns to the particulate mix  16  to combine with additional Group 5 metal, the exemplary method may include the use of the same batch of particulates for at least two batches of articles  10 , and additional batches as the economics of the facility may suggest. Generally at least five uses will be quite practical. Preferably, for any given use, the ratio of Group 5 metal in the Group 5 metal source to the articles will not be below 1:2 by weight, and may be preferably 1:1 to 2:1 by weight. 
         [0031]    The article  10  including the carbide coating  14  may then be cooled and separated from the particulate mix  16 . The article  10  may then be heat-treated, in a post-production step, by subjecting the coated article  10  to at least austenitizing temperature and quenched in a conventional manner to harden the core, preferably achieving a final core hardness of Rc44-56. The article  10  may then be polished in a conventional manner. 
         [0032]      FIG. 3  is an end section of the container  20 , illustrating how the contents may be mixed, preferably with the aid of baffles  32 , during rotation of the container  20 . The particulate mix  16  and the article(s)  10  to be coated may be substantially constantly contacted during the rotation of the container  20 , therein causing the carbide coating  14  to be formed on the surface of the steel chain pins  10  at a desired thickness, wherein the desired thickness may be dictated primarily by the amount of time in which the article  10  is rotated within the rotary container  20 . The vessel, retort, or container  20  may be rocked or otherwise agitated rather than rotated. 
         [0033]    In  FIG. 4 , a portion of a typical silent chain is shown, comprising sets of plates A and B, each having two holes for pins  10 . In this configuration, parallel sets A of four plates and parallel sets B of three plates may be shaped to accommodate sprockets or otherwise to engage a force-delivering device not shown. Some of the plates A or B may articulate on the pins  10  and others may be secured to them so as not to rotate on the pins, depending on the design of the chain. In either event, whether there is articulation or not at the plate/pin interface, significant stress and wear may be engendered at the interface of the pins and the plates. 
         [0034]    A comparison of chain pins  10  made according to the exemplary process to more conventional pins showed that the hard coating on the pins  10  did not flake off the pin  10  when it was bent in a vise, whereas pins made by a conventional process flaked off. This is generally taken to mean that when the coating  14  of the pin  10  may be abraded, but will nevertheless adhere more tenaciously than the coating of the conventional pin. As indicated above, flaking or spalling of hard coatings can be very destructive to worn contact surfaces of chain parts. 
         [0035]    The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.