Abstract:
A process for treating a steel alloy component includes the steps of: providing a steel alloy component having a plurality of teeth with a root portion and a tip; and processing the steel alloy so that the root portion of the gear teeth are hardened without through hardening of the tips of the gear teeth.

Description:
BACKGROUND 
       [0001]    The present disclosure is directed to a process for treating steel alloys, such as PYROWEAR® alloys, used for components, such as gears. 
         [0002]    In the design and manufacture of steel components, there is often a need to modify properties of the material. It is well recognized that carburizing is a process suited for hardening the surface and sub-surface of the steel component. Carburizing can be broadly considered as either an atmospheric carburization process or a vacuum carburization process. In the vacuum carburization process, the component is heated to an elevated temperature within a carburizing furnace, and a carburizing gas is introduced into the environment so that carbon atoms are diffused into the surface and sub-surface of the steel material. The carbon content in the surface and near sub-surface of the component is increased while the carbon content within the core of the component remains unaltered. The characteristics of the component have this been modified to provide a hardened outer surface surrounding an interior core. 
         [0003]    It has been suggested that, when using a stainless steel, such as PYROWEAR® 675, one should first oxidize the surface of the component prior to exposure to the carburizing environment. In such a process, the component is grit blasted and placed in an air furnace at a temperature of 1300 degrees Fahrenheit for about one hour to form an oxide on its surface. Upon the component being subjected to the carburizing environment, the oxidized surface facilitates the absorption of carbon by the material. 
         [0004]    In a carburizing process, the time and temperature that the material is subjected to while in the carburizing environment will determine the surface hardness, case depth, hardness profile, and carbide microstructure of the hardened portion of the material. 
         [0005]    Gears made from steel alloys need to have the proper hardness of the gear teeth without through hardening of the tips of the gear teeth. 
       SUMMARY 
       [0006]    There is provided in accordance with the present disclosure a process for treating a steel alloy component, which broadly comprises the steps of: providing a steel alloy component having a plurality of teeth with a root portion and a tip; and processing the steel alloy so that the root portion of the gear teeth are hardened without through hardening of the tips of the gear teeth. 
         [0007]    In another and alternative embodiment, the processing step comprises: subjecting the steel alloy component to a pre-oxidation step in an air; and subjecting the steel alloy component to a carburization treatment. 
         [0008]    In another and alternative embodiment, the pre-oxidation step comprises heat treating the steel alloy component in air at a temperature in the range of from 800 to 1300 degrees Fahrenheit for one hour. 
         [0009]    In another and alternative embodiment, the processing step further comprises copper plating at least a portion of the steel alloy component prior to the pre-oxidation step. 
         [0010]    In another and alternative embodiment, the carburization treatment is an atmospheric carburization treatment. 
         [0011]    In another and alternative embodiment, the carburization treatment comprises placing the steel alloy component in a carburizing furnace and subjecting the steel alloy component to a temperature in the range of from 1650 to 1710 degrees Fahrenheit. 
         [0012]    In another and alternative embodiment, the processing step further comprises subjecting the steel alloy component to an austenitizing treatment after the carburizing treatment. 
         [0013]    In another and alternative embodiment, the austenitizing treatment is carried out at a temperature in the range of from 1650 to 1750 degrees Fahrenheit. 
         [0014]    In another and alternative embodiment, the processing step further comprises quenching the steel alloy component at a temperature in the range of from 75 to 140 degrees Fahrenheit. 
         [0015]    In another and alternative embodiment, the processing step further comprises subjecting the steel alloy component to a cold treatment at a temperature in the range of from −94 to −112 degrees Fahrenheit. 
         [0016]    In another and alternative embodiment, the processing step further comprises air warming the steel alloy component to room temperature. 
         [0017]    In another and alternative embodiment, the processing step further comprises subjecting the steel alloy component to a temper treatment at a temperature in the range of from 425 to 475 degrees Fahrenheit. 
         [0018]    In another and alternative embodiment, the steel alloy component providing step comprises providing a steel alloy component formed from an alloy consisting of from 0.05 to 0.10 wt % carbon, from 0.35 to 0.65 wt % manganese, from 0.4 to 1.0 wt % silicon, from 1.0 to 13 wt % chromium, from 2.0 to 3.0 wt % nickel, from 1.8 to 3.25 wt % molybdenum, up to 2.0 wt % copper, up to 5.4 wt % cobalt, from 0.1 to 0.6 wt % vanadium, and the balance iron. 
         [0019]    In another and alternative embodiment, the steel alloy component providing step comprises providing a gear formed from the steel alloy. 
         [0020]    Further in accordance with the present disclosure, there is provided a gear which broadly comprises the gear being formed from a steel alloy; the gear having a plurality of teeth; each tooth having a root portion and a tip; and the root portion being hardened without the tip being through hardened. 
         [0021]    In another and alternative embodiment, the steel alloy has a composition consisting of from 0.05 to 0.10 wt % carbon, from 0.35 to 0.65 wt % manganese, from 0.4 to 1.0 wt % silicon, from 1.0 to 13 wt % chromium, from 2.0 to 3.0 wt % nickel, from 1.8 to 3.25 wt % molybdenum, up to 2.0 wt % copper, up to 5.4 wt % cobalt, from 0.1 to 0.6 wt % vanadium, and the balance iron. 
         [0022]    Other details of the process for treating steel alloys for gears are set forth in the following detailed description and the accompanying drawing wherein like reference numerals depict like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic representation of a gear having a plurality of teeth; 
           [0024]      FIG. 2  is an enlarged view of the gear teeth; and 
           [0025]      FIG. 3  is a schematic representation of a process for hardening the gear teeth in accordance with the present disclosure 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  illustrates a gear  10  having a plurality of teeth  12 . The gear  10  and the teeth  12  may be formed from a steel alloy such as a PYROWEAR® alloy. The steel alloy may have a composition which consists of from 0.05 to 0.10 wt % carbon, from 0.35 to 0.65 wt % manganese, from 0.4 to 1.0 wt % silicon, from 1.0 to 13 wt % chromium, from 2.0 to 3.0 wt % nickel, from 1.8 to 3.25 wt % molybdenum, up to 2.0 wt % copper, up to 5.4 wt % cobalt, from 0.1 to 0.6 wt % vanadium, and the balance iron. 
         [0027]    Referring now to  FIG. 2 , there is shown a plurality of the teeth  12  in the gear  10 . Each of the teeth  12  has a root  14  and a tip  16 . The tip  16  may be formed as a top land which does not act as a working surface. Each tooth  12  further has a drive flank  18  which is a working surface. Adjacent teeth  12  may be connected by a fillet  20  which is also a non-working surface. Each tooth  12  further has a non-driving flank  22 , which may be a non-working surface, and end faces  24  which are non-working surfaces. 
         [0028]    It is important to have a process for treating the material forming the gear  10  so that the root  14  of the each tooth  12  has the proper hardness without there being through hardening of the tips  16  of the gear teeth  12 . 
         [0029]    Referring now to  FIG. 3 , there is shown a process for treating a gear  10  having teeth  12  to arrive at the desired hardness properties. In step  102 , a rough machine gear  10  having a plurality of teeth  12  is provided. The rough machine gear  10  may be made from a steel alloy such as those discussed above. In step  103 , rough machining of the gear  10  may occur. 
         [0030]    In step  104 , portions of the gear  10 , such as the tips  16  and the end faces  24 , may be selectively plated with a metal, such as copper or a copper alloy, to prevent over-carburization. If desired, the entire gear  10  may be copper plated. The plating metal may be deposited in a layer having a thickness in the range of from 0.010 mm to 0.030 mm. The plating metal may be deposited on the desired portions of the gear  10  using any suitable technique such as an electroless plating technique or an electroplating (galvanic) technique. The metal deposit allows selective carburizing. 
         [0031]    In step  106 , the gear may be subjected to a pre-oxidation heat treatment to activate the steel alloy to accept the carbon which comes from a subsequent carburization process. The pre-oxidation heat treatment comprises placing the gear in a furnace having an air atmosphere and heating the gear for one hour at a temperature in the range of from 800 to 1300 degrees Fahrenheit. It has been found that this pre-oxidation heat treatment helps form the desired hardness properties in the root  14  while avoiding through hardness of the tip  16 . When the pre-oxidizing treatment is done at 1300 degrees Fahrenheit, it provides careful control of the oxide film that is formed. After the pre-oxidation heat treatment is completed, the gear  10  may be removed from the furnace and allowed to cool. 
         [0032]    In step  108 , the plated gear may be placed into a furnace and subjected to a carburization treatment. Carburization in general includes subjecting the gear  10  to an environment wherein carbon atoms can be diffused into the material through the outer surface of the gear  10 . The carburization treatment may be an atmospheric treatment where a carburizing gas is introduced into the furnace. The carburizing gas may be propane, methane, acetylene, and combinations thereof. During the carburization treatment, the plated gear may be placed in a carburizing furnace and heated to a carburizing temperature in the range of from 1650 to 1710 degrees Fahrenheit for a time which is dependent on the case depth requirement. 
         [0033]    In step  110 , the gear  10  may be subjected to an austenitizing treatment where it is heated to an austenitizing temperature in the range of from 1650 to 1750 degrees Fahrenheit in a furnace such as an air furnace or a plasma furnace. During the austenitizing treatment, ferrite particles in the steel forming the gear  10  are transformed to austenite particles. 
         [0034]    In step  112 , the gear  10  may be quenched at a temperature in the range of from 75 to 140 degrees Fahrenheit at the beginning of the quenching operation. The quenching step may be carried out in tanks which allow continuous quenching and free movement of the gear  10  within the quench media. The quench media may be oil or water. 
         [0035]    In step  114 , the gear  10  may be cold treated, using a cryogenic deep freezing technique, at a temperature in the range of from −94 to −112 degrees Fahrenheit for a minimum of 2.0 hours. After this step is performed, the gear  10  may be air warmed to room temperature. 
         [0036]    In step  116 , the gear  10  may be subjected to a temper/retemper treatment in an air furnace at a temperature in the range of from 425 to 475 degrees Fahrenheit for a minimum of 2.0 hours. 
         [0037]    In step  118 , if needed, the gear  10  may be subjected to a stress relief heat treatment which may be carried at a temperature in the range of from 350 to 400 degrees Fahrenheit for a time period in the range of from 1.0 to 2.0 hours. 
         [0038]    The heat treatment described herein may be used for any highly stressed gear with a small root radius that could not have the gear teeth through hardened, since that may make the gear teeth brittle and cause a fracture. 
         [0039]    The gear  10  described herein is merely illustrative and is not intended to be limiting. The process described herein may be applicable for use on any type of gear with no limitation intended based on the specific type of gear. The process herein may also be used on other types of components formed from a steel alloy. 
         [0040]    While one of the embodiments described herein is directed to a specific family of steel alloys, the description is exemplary in nature and should not be construed in a limiting sense. 
         [0041]    There has been provided a heat treatment for steel alloys for gears. While the heat treatment has been described in the context of specific embodiments thereof, unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.