Patent Publication Number: US-2013248058-A1

Title: Heat Treatment Process for Engine Ring Gear

Description:
FIELD 
     The present disclosure relates to heat treatment processes for steel parts. In particular, the disclosure relates to heat treatment processes for steel, engine ring gears. 
     BACKGROUND 
     Traditionally, steel ring gears for engines have been manufactured by one of two processes. The first process is conventionally referred to as “quench and temper.” In this process, the gear is heated above the austenitizing temperature for the steel of which the gear is made and then the gear is rapidly quenched (i.e., cooled) to room temperature. The cooled part subsequently is tempered (i.e., heated) to add toughness. The strength and hardness of the gear is generally uniform throughout the part. While this process is inexpensive, it may result in distortion from volumetric changes due to the microstructural transformation that occurs in the process, mainly from rapid quenching. As such, the part typically must be further machined after hardening adding expense to the manufacturing method. 
     The second process is referred to as “induction hardening” and is widely used for toothed gear rings. In induction hardening, only about the outer 10% of the gear is quenched and tempered (i.e., the portion of the gear comprising the teeth). For a typical gear ring, having an outer radius that is about 15-25 mm larger than an inner radius, this induction heating would involve quenching and tempering only about the outer 1.5 mm of the gear. This process, unlike conventional quench and temper, results in minimal distortion at the teeth portion. However, only the outer 1.5 mm of the ring gear is in a high strength condition and the rest of the ring gear is low strength. 
     SUMMARY 
     The present disclosure relates to methods of hardening toothed gear parts for engines. The method results in a uniformly hardened gear with minimized distortion. Gears suitable for the method typically comprises steel. In some embodiments, the carbon content of the steel is between about 0.2-0.8% carbon. 
     The first step of the method typically includes heating the gear to a temperature above the austenitizing temperature for the steel of which the gear is comprised. For most types of steel, this first step typically includes heating the gear to a temperature of greater than about 1400° F. (typically greater than about 1500° F.). The austenitizing temperature for steel is determined partially by the carbon content of the steel and is inversely proportional to the carbon content of the steel. For example, the austenitizing temperature for 0.02% carbon steel is approximately 1600° F. while the austenitizing temperature for 0.08% carbon steel is approximately 1500° F. In the disclosed methods, a suitable temperature range to which the gear is heated may include a range of about 1400-1800° F. The gear typically is heated to a temperature above the austenitizing temperature for the steel of which the gear is comprised for at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes (e.g., 2-10 minutes), depending on the thickness or height of the gear. 
     The gear may be heated by processing that include, but are not limited to, placing the gear in a molten salt bath. Multiple gears may be processed simultaneously in a molten salt bath. Preferably, the gear or gears are placed in the salt bath such that a maximum surface area of the gear or gears are in contact with molten salt. 
     Subsequently to heating the gear to a temperature above the austenitizing temperature for the steel of which the gear is comprised, the gear is quenched to a quenching temperature which may be between the Martensite start temperature (Ms temperature) for the steel of which the gear is comprised and a temperature about  20 ° F. above the Ms temperature or 15, 10, or 5° F. above the Ms temperature). For many types of steel, the Ms temperature typically is between about 250-1000° F. (more typically between about 350-650° F., even More typically between about 350-630° F.). 
     In the disclosed methods, the gear is cooled rapidly from the austenitizing temperature to the quenching temperature (which is a temperature nearly above the Ms temperature) in order to minimize transformation of the austenite structure to a ferritic structure. For example, the gear may be quenched at a rate that is more quickly than about −100, −200, −300, −400, or −500° F./second. For a medium carbon unalloyed steel, a suitable rate may be at least about −200° F./second. 
     The gear typically is held at the quenching temperature until the entire gear is completely quenched throughout to the quenching temperature (e.g., generally in a homogenous temperature condition for all elements of the gear). For example, the gear may be held at the quenching temperature for at least about 1, 2, 3, 4, 5, or 6 minutes (e.g., 3-6 minutes). Factors that may determine the minimum holding time at the quenching temperature may include the thickness or height of the gear. The gear may be quenched by processes that include, but are not limited to, placing the gear in a molten salt bath or a molten lead bath. 
     Subsequently to quenching the gear to the quenching temperature, the gear is further quenched to a temperature below the Martensite finish temperature (Mf temperature) for the steel of which the gear is comprised. Mf temperatures typically are less than about 750, 500, 250, or 100° F. The gear may be quenched in this step by processes that include, but are not limited to, placing the gear in ambient air conditions (e.g., at a temperature of about 70° F. or less). Optionally, the gear treated as such subsequently may be tempered by further heating the gear. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a convention quenching and tempering process. 
         FIG. 2  is a representation of a toothed gear hardened by induction heating. 
         FIG. 3  illustrates one embodiment of the hardening process disclosed herein. 
         FIG. 4  illustrates the relationship between Martensite start temperature (Ms temperature), Martensite finish temperature (Mf temperature), and carbon content of steel. 
         FIG. 5  illustrates configuration differences for a gear hardened by induction heating and an embodiment of a gear hardened by a hardening process, as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES  
     Disclosed herein is a method for manufacturing a toothed gear part for an engine. The method may be described using several definitions as discussed below. 
     Unless otherwise specified or indicated by context, the terms “a”, “an”, and “the” mean “one or more.” In addition, singular nouns such as “gear” should be interpreted to mean “one or more gears,” unless otherwise specified or indicated by context. 
     As used herein, “about”, “approximately,” “substantially,” and “significantly” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” and “approximately” will mean plus or minus ≦10% of the particular term and “substantially” and “significantly” will mean plus or minus &gt;10% of the particular term. 
     The presently disclosed methods include. heating steps and cooling steps. As known in the art, the cooling steps may otherwise be referred to as “quenching.” 
     Ring gears are generally attached to the flywheel of an internal combustion engine and they interface via teeth with an electric starter motor to start the engine. In order to improve strength and durability of the gear, the gear is commonly subjected to a heat treatment process. 
     Conventional quenching and tempering involves heating the entire gear above the austentizing temperature for the steel of which the gear is made and quenching the entire gear (i.e., rapidly cooling the gear) to room temperature. (See  FIG. 1 ). The entire cooled part subsequently is tempered (i.e., subsequently heated) to add toughness. This process may result in distortion from volumetric changes due to the microstructural transformation that occurs as the gear is subjected to a large temperature range during the process, mainly during rapid quenching. As such, the entire part typically requires machining after hardening. 
     A more common process in industry for hardening ring gears is to selectively heat via induction the outer portion of the ring comprising the teeth to improve mechanical properties of the teeth of the gear while minimizing distortion of the gear only at the heated portion.  FIG. 1  shows a gear having been selectively heated and hardened at the outer portion comprising the teeth, where the teeth are hardened to Rockwell C 40-60, while the remainder of the gear has a hardness no higher than Rockwell C 30. 
     The presently disclosed methods may otherwise be referred to “martempering” as disclosed in  FIG. 3 . In the presently disclosed methods, the entire toothed gear is heated to a temperature above the austenitizing temperature for the steel of which the gear is comprised. Typically, this temperature is greater than about 1400, 1500, or 1600° F. 
     Subsequently, the heated gear is quenched (or cooled) to a temperature that is about the Martensite start temperature (Ms temperature) for the steel of which the gear is comprised or slightly above the Ms temperature (e.g., 20, 15, 10, or 5° F. above the Ms temperature) but to a low enough temperature to mitigate an austenite to ferrite+cementite reaction. This temperature or temperature range is referred to herein as the “quenching temperature.” For many types of steel, the Ms temperature typically is between about 250-1000° F. (more typically between about 350-650° F., even more typically between about 350-630° F.). Suitable quenching temperatures may be within a range delineated by the Ms temperature and a temperature 20, 15, 10, or 5° F., above the Ms temperature. 
     Gears manufactured by the disclosed methods typically comprise steel, and in particular steel that is comprised mainly of iron with a carbon content of about 0.2 0.8% carbon. Medium carbon steel having a carbon content of about 0.3-0.6% is particularly suitable. However, suitable steel for the disclosed methods may include, but is not limited to, steel designated by the Society of Automotive Engineers (SAE) under the following designation numbers: 1050, 1065, 1066, 1084, 1086, 1090, 4095, 1350, 4063, 4150, 4365, 5140, 5160, 8750, and 50100. 
     The Ms temperature is indirectly proportional to carbon content of the steel. (See  FIG. 4 ). Specific Ms temperatures of SAE designated steel are as follows: 1050 steel—610° F. (320° C.); 1065 steel—525° F. (275° C.); 1066 steel—500° F. (260° C.); 1084 steel—395° F. (200° C.); 1086 steel—420° F. (215° C.); 1095 steel—410° F. (210° C.); 1350 steel—450° F. (235° C.), 4063 steel—475° F. (245° C.); 4150 steel—545° F. (285° C.); 4365 steel—410° F. (210° C.); 5140 steel—630° F. (330° C.); 5160 steel—490° F. (255° C.); and 8750 steel—545° F. (285° C.). In some embodiments, Ms temperatures may be calculated according to a formula Ms (° F.)=1002−793 (% C)−87(% Mn)−64(% Ni)−54(% Cr)−45(% Mo)+{50(% Co)−45(% Si)} or Ms (° C.)=539−423 (% C)−30.4(% Mn)−17.7(% Ni)−12.1(% Cr)−75(% Mo)+{10(% Co)−7.5(% Si)}. 
     Subsequently to quenching the heated gear to the quenching temperature, the gear is further quenched below the Martensite finish temperature (Mf temperature). For many types of steel, the Mf temperature is less than about 700° F. or less than about 500, 250, 100, or 50° F. (or less than about 400, 300, 100, 50, or 25° C.). The Mf temperature, like the Ms temperature is indirectly proportional to carbon content of the steel. (See  FIG. 4 ). The gear may be quenched in this step by processes that include, but are not limited to, placing the gear in ambient air conditions (e.g., at room temperature or at a temperature of about 70° F. or less). 
     The disclosed methods typically produce a toothed gear having a uniform hardness with limited distortion. Typically, the gear has a uniform hardness in the range of about Rockwell C 40 to Rockwell C 60. 
     The presently disclosed methods may be utilized to manufacture gears having a uniform hardness while using a reduced amount of raw material (i.e., steel) to produce the gear. For a gear hardened by induction heating, the gear must have a root diameter (RD=diameter measured from the base of a tooth) and inner diameter OD) of suitable dimensions to provide strength for the gear. (See  FIG. 5 ). Because a gear hardened by induction heating is not hardened throughout, the gear must have a smaller inner diameter in order to provide a suitable width for the gear ((RD−ID)/2=width) to compensate for the lack of uniform hardness. For a gear hardened by the process disclosed herein, the inner diameter may be larger because the gear has: a uniform hardness throughout. As such, a gear hardened by the process disclosed herein may have a width (i.e., (RD−ID)/2) that is less than the width for a gear hardened by induction heating. In some embodiments, the inner diameter of a gear hardened by the methods disclosed herein may be 5, 10, or 15% larger than the inner diameter of a gear hardened by induction heating. In some embodiments, the width ((RD−ID)/2) of a gear hardened by the methods disclosed herein may be 10, 20, 30, 40, or 50% less than the width of a gear hardened by induction heating. In some embodiments, gear hardened by the process disclosed herein may have a mass that is 5, 10, 15, 20, 25, 30, or 35% less than the mass for a gear hardened by induction heating. 
     In the present description, certain terms have, been used for, brevity, clearness and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes only and are intended to be broadly construed. The different systems and methods described herein may be used alone or in combination with other systems and methods. Various equivalents, alternatives and modifications are possible within the scope of the appended claims. Each limitation in the appended claims is intended to invoke interpretation under 35 U.S.C. §112, sixth paragraph only if the terms “means for” or “step for” are explicitly recited in the respective limitation. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art, to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.