Patent Publication Number: US-7718116-B2

Title: Forged carburized powder metal part and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to application Ser. No. 11/277,388 entitled “Variable Case Depth Powder Metal Gear and Method Thereof” filed simultaneously herewith and incorporated by reference herein. 
     TECHNICAL FIELD 
     The invention relates to a forged powder metal part and more particularly to a selectively non-carburized powdered metal part, including a method. 
     BACKGROUND OF THE INVENTION 
     There is continuing demand for manufacturing processes that may lessen the cost, time or steps in producing a part. More often than not, the benefits resultantly associated with improving the manufacturing process are necessitated in the first instance by customer requirements to develop and improve products to have superior dimensional, mechanical and/or performance properties. For instances a typical differential side gear may have any or all of the following performance requirements such as: the spline area requiring dimensional precision, high shear strength and brinnelling resistance; the hub and thrust faces requiring dimensional precision, surface finish and case compatibility; the gear geometry requiring dimensional precision, surface finish and optimised profile; and the tooth and core strength may require impact resistance, wear resistance, spalling resistance, and different surface and core metallurgies. Different non-compatible manufacturing processes, i.e. casting, steel forging or powder metal forging, obtain different performance requirements for the same part, advantageously or otherwise. 
     Referring to  FIG. 1 , in order to meet some of these performance requirements, a gear  10  is made by forging a powder metal  14  and then case carburizing the gear to achieve a nearly constant effective case depth  16 . The constant effective case depth  16  for each gear tooth  12  is shown in the partial cross-sectional view of  FIG. 1 . The parameters to be controlled to achieve nearly constant carburization of a fully dense part of specific hardness, case depth and carbon gradient are generally known. However, a nearly constant case depth does not necessarily achieve desired mechanical or machining properties desired in a post forged product. It would be advantageous to achieve a better-controlled balance of these performance requirements in the final product, uncompromised by the manufacturing process thereby saving time, processing or cost. 
     A manufacturing process to improve the performance requirements of powder metal parts in a process known as “sint-carb” is disclosed in U.S. Pat. No. 3,992,763 titled “Method of Making Powdered Metal Parts.” The process teaches carburization during or subsequent to sintering, and prior to forging, to enhance the case depth at a critical wall of the final forged product, eliminating the need for subsequent heat treating processes for achieving a case hardness. U.S. Pat. No. 4,002,471 titled “Method of Making a Through-Hardened Scale-Free Forged Powdered Metal Article Without Heat Treatment After Forging”, discloses a method of making forged powder iron base metal articles of high Rc hardness without need for further machining, surface treatment or heat treatment after quenching. 
     Also, U.S. Pat. No. 4,165,243 titled “Method of Making Selectively Carburized Forged Powder Metal Parts” discloses a process requiring additional steps of masking a part before sintering and removal of the masking after carburizing and before forging to obtain selected carburized surfaces on the part. 
     However, the above mentioned patents do not teach or suggest any processes in which removal of a carburized surface prior to forging may be used to achieve specific material requirements in a final forged product while providing improved dimensional precision, performance features or improved manufacturing of both carburized and noncarburized surfaces of the part. Moreover, the above mentioned patents do not teach or suggest the strategic and uncomplicated control afforded by a removal process. 
     Therefore, there is a need for a manufacturing process for removal of a carburized surface of a part prior to forging thereby leaving select carburized and noncarburized surfaces in the final forged part. Moreover, there is a need for a process that strategically removes a carburized portion of a part prior to forging and quenching thereby permitting improved post-forging operations upon the noncarburized portion of the part such that tighter tolerances, improved spline classification or improved shear resistance properties may be achieved while maintaining the beneficial forged carburized portion of the part. There is also a need for a part having improved dimensional precision or performance features. 
     SUMMARY OF THE INVENTION 
     In accordance with the above-mentioned needs, a gear and a manufacturing method for removal of a carburized surface of a part prior to forging leaving a select carburized surface having improved performance features and a select non-carburized surface having improved post forging properties for improved tolerance and classification is provided. 
     Specifically, a method for obtaining a selectively non-carburized powder metal part is provided. The method includes compacting, sintering, removing, forging and cooling. A metal powder is compacted to form a preform having at least one first surface portion in which a forged part is required to have a case hardness depth and at least one second surface portion in which a carburized portion is required to be removed prior to forging. The preform is then sintered and carburized. After carburizing the at least one second surface portion of the preform is removed and subsequently forged and cooled. The forged part has at least one second surface portion having improved shear resistance properties and at least one first surface having surface hardness properties. 
     A part made from the present method is also provided. The part includes a noncarburized portion in which post forging operation may be accomplished with less machining expense or time, while providing for improved tolerancing or dimensional control due to the nature of the softer noncarburized material portion. The part also includes a carburized portion having the beneficial attributes such as improved wear resistance, load bearing, impact resistance or bending fatigue resultantly obtained by the near net-shaped finished sint-carb forging manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this invention, reference should now be made to the inventive aspects illustrated in greater detail in the accompanying drawings and described below. 
         FIG. 1  shows a partial cross-sectional view of a case carburized gear. 
         FIG. 2  shows a partial cross-sectional view of a differential side gear having a variable case depth profile used to advantage in accordance with an embodiment of the invention. 
         FIG. 3  shows the microstructure below the effective case depth of the gear shown in  FIG. 2 . 
         FIG. 4  shows the microstructure within the effective case depth of the gear shown in  FIG. 2 . 
         FIG. 5  shows an isometric view of a preform after sintering representative of the inventive aspects of the invention required to obtain the inventive product after forging. 
         FIG. 6A  shows a partial cross-sectional view of the representative preform of  FIG. 5  after carburization. 
         FIG. 6B  shows a partial cross-sectional view of a carburized portion to be removed from the preform of  FIG. 6A  by a removing process prior to forging. 
         FIG. 7  shows an isometric view of the differential side gear made from the preform of  FIG. 6B  in accordance with an embodiment of the invention. 
         FIG. 8  shows a schematic layout of the process to obtain a variable case depth powder metal gear in accordance with an embodiment of the invention. 
         FIG. 9  shows a schematic layout of an embodiment of a process according to the invention to obtain a forged carburized powder metal part. 
     
    
    
     DETAILED DESCRIPTION 
     In all figures, the same reference numerals are used to identify like parts in the various views. Thus, simultaneous reference to the various figures is appropriate. In some instances, for clarity, equivalent parts in different figures may have different item numbers. 
       FIG. 2  shows a partial cross-sectional view of a differential side gear  50  having a variable case depth profile  58  in accordance with an advantage of the invention.  FIG. 7  shows an isometric view of the differential side gear  50  made from the preform  184  of  FIG. 6B  in accordance with the invention. Before turning to the inventive aspects of obtaining a selectively non-carburized powdered metal part, discussion will first be made of the gear  50  having a variable case depth profile  58  usable to advantage in one aspect of the invention. 
     The differential side gear  50  includes plurality of teeth  52  and a variable case depth profile  58  formed in a carburized portion of the gear  50 . Each tooth of the plurality of teeth  52  has a first surface  54  and a tooth core or root  56 . The differential side gear  50  has a rotational axis  60 , wherein the teeth  52  extend radially in the same general direction as the rotational axis of the gear, but are inclined with respect to the rotational axis. The differential side gear  50  further includes an axially splined internal section  62  axially aligned with the rotational axis  60 . The splined internal section  62  is formed in a noncarburized portion of the gear  50  that was selectively obtained by removing a carburized portion of a preform prior to forging as is discussed herein. 
     The variable case depth profile  58  is formed in the plurality of teeth  52 . The variable case depth profile  58  advantageously provides a gear having greater tooth wear resistance on the first surface  54  and greater impact resistance in the tooth root  56 . The variable case depth profile  58  is representative of the effective case depth profile achieved after forging, by carbon diffusion prior to forging the gear. The variable case depth profile  58  resultantly achievable by the forging process is discussed herein. 
     While a process is described with respect to a differential side gear  50 , it is anticipated that the variable case depth profile  58  may be achieved on other parts or gears, including bevel, differential or pinion gears, without limitation. 
     The differential side gear  50  may be made from a low alloy, fully compacted, ferrous powder metal material. However, it is anticipated that the gear may be made of various other types of forged powder metal steels. 
     Turning to  FIG. 2 , the first surface  54  of each tooth of the differential side gear  50  includes a tip surface  64 , a pitch line surface  66 , a root fillet surface  68  and a rootdiameter or land surface  70 . The variable case depth profile  58  is substantially represented by effective case depth of: 2.4 mm at the tip surface  64 ; 1.9 mm at the pitch line surface  66 ; 0.4 mm at the root fillet surface  68 ; and 0.8 mm at the root land surface  70 . This results from the carbon diffusion and subsequent forging of a preform. While specific numbers are presented in the present embodiment, it is recognized that the variable case depth may have any non-constant effective case depth profile over a particular surface cross-section and is not limited to the specific profile here presented. 
     The variable case depth profile  58  may also be represented by a case depth ratio. The effective case depth ratio is given by comparing case depths measured at the tip surface  64  to the root fillet surface  68 , the pitch line surface  66  to the root fillet surface  68 , or the root land surface  70  to the root fillet surface  68 . For example, the variable case depth ratio for the tip surface  64  to the root fillet surface  68  is 6:1, the pitch line surface  66  to the root fillet surface  68  is 19:4, and the root land surface  70  to the root fillet surface  68  is 2:1. A case depth ratio of nearly 1:1 is considered to be within the effective range of a constant case depth  16  of the gear  10  shown in  FIG. 1 . 
     Advantageously, the case depth ratio may be 6:1 over the variable case depth profile  58  from the greatest depth to the shallower depth of effective case hardness, thereby achieving greater mechanical properties such as tooth wear and impact resistance. 
     The tooth root  56  of the gear  50  may include a mid-tooth section  74  having hardness of about 43 HRC, a root section  76  having hardness of about 31 HRC and a core section  78  having hardness of about 32 HRC. While these hardness numbers are only representative of a gear having improved mechanical properties, a core hardness ratio is obtained between the mid-tooth section  74  and the root or core sections  76 ,  78  of nearly 4 to 3. A higher core hardness ratio is representative of a gear having greater tooth impact resistance, i.e. ductility. Whereas a gear, like the one represented in  FIG. 1 , would have nearly a 1 to 1 core hardness ratio and thus, less ductility. 
       FIG. 3  shows the microstructure below the effective case depth of the gear shown in  FIG. 2 , and  FIG. 4  shows the microstructure within the effective case depth of the gear shown in  FIG. 2 . The depth boundary is the point where the effective carbon content of the material becomes nearly constant and may be effectively represented by the variable case depth profile  58 . 
     Turning to the method of making the powder metal gear with variable case depth, a process is shown in  FIG. 8 . The process begins with the steps of mixing  20  and continues with some of the possible steps of filling  22 , compacting  24 , sintering  26 , carburizing  28 , preheating  30 , variable forging  32 , and cooling  34 . Post forging operations  36  may also be used to further enhance the gear. For brevity, and because some of these process steps are well known to those in the art of forging powder metals, only certain aspects of the inventive process are discussed below. In this regard, material selection, temperature processing and compaction pressures are discussed only briefly. 
     The mixing step  20  readies the metal powder, including any needed binders or lubricants, by mixing until a nearly uniform mixture is achieved ready for filling into a compacting form during the filing step  22 . The compacting step  24  comprises compacting a metal powder into a preform having a nearly uniform initial carbon content throughout the preform. The initial carbon content is achieved by mixing of the metal powder with constituent amounts of graphite together with necessary binders or lubricants to make the preform. The preform includes at least one cross-sectional surface in which the final forged part resultantly obtains a variable case depth profile, as discussed herein. 
     The sintering and carburizing steps  26 ,  28  may be accomplished simultaneously or the carburizing step may be completed after sintering of the preform. Sintering the preform binds the metal powder. Carburizing the preform substantially increases the initial carbon content in developing a carbon gradient from the surface of the preform into the core. The carbon gradient is produced by providing a controlled carbon atmosphere and maintaining the preform in the controlled atmosphere for a predetermined period of time. It is necessary to obtain a substantially constant carbon case depth in the preform in order to enhance critical flow of metal during forging for achieving the desired variable case depth profile in the post forged part. Of course, density gradient, part geometry and carburizing conditions dictate the uniformity of the carburizing process. The case depth of carbon necessary in the preform is determined by the preform geometry and the desired areas of critical metal flow during forging. To achieve the variable case depth profile in gear  50  mentioned above as measured by the ratios presented, the preform is carburized to a case depth of ¼ the tooth height, but may also be satisfied by carburizing to a case depth of 1/20 the tooth height or to ⅞ the tooth height. It is anticipated that too little case depth in the preform may result in non-carburized areas. It is also anticipated that too much case depth in the preform may result in a nearly constant case depth profile.  FIG. 6A  shows a partial cross-sectional view of the carburized preform  85  of the representative preform  84  of  FIG. 5  after carburization process. The preform  85  has substantially constant carbon case depth  86  achieved after sintering and carburizing the preform. 
     The variable forging step  32  comprises forging the carburized preform at a forge temperature and a forge pressure to obtain a substantially dense, net shape, part. The variable case depth profile for the gear results in nearly symmetrical profiles for each tooth because of the symmetrical nature of the forging dies and the carburized preform. However, it is recognized that different carburization schemes and forging steps may be used to obtain multiple variable case depth profiles. 
     The variable case depth profile is achieved by utilizing a die set of the forge to variably enhance critical flow of the carburized metal portion during the forging process. Essentially, the constant case depth of the carburized powder metal preform is strategically compressed into the die sections, wherein portions of the preform are stretched and thinned during forging and other portions of the preform are thickened and deepened with the carburized powder metal. Again, case depth that is too shallow or too deep in the carburized powder metal preform prior to forging will not produce the variable case depth profile in the final product. 
     The cooling step  34  allows the forged part to obtain a particular metallurgy resulting in a gear having the desired variable case depth profile. Cooling of the forged part may be by quenching in oil, water, air or by other methods suitable to the powder metal forging process. 
     Prior to cooling, including a dwelling step of the forged part for a dwell period may allow for enhanced properties by allowing temperature stabilization of the material of the part. 
     The optional preheating step of the preform to a pre-forge temperature prior to forging may enhance the desired metal flow during the forging process. 
     Optional post forging operations step  36  may include, turning, facing, surface grinding, splining, and broaching of the product depending upon final specification requirements, thereby being ready for washing, packing, or shipping. 
     With proper selection and combinations of powder metal, compaction dies, processing times, processing temperatures, processing pressures, forging dies, and cooling method a near net shape, fully dense product may be obtained having the variable case depth profile, thereby requiring minimal if any machining operations facilitating cost savings and performance improvements. 
     Turning to the inventive method of making a forged carburized powder metal part, an embodiment of a process is shown in  FIG. 9 . The process begins with the steps of mixing  120  and continues with the steps of filling  122 , compacting  124 , sintering  126 , carburizing  128 , removing  130 , forging  132 , and again ending with cooling  134 . Post forging operations  136  may also be used to further enhance the gear. Compatible processing steps mentioned above in regards to the process shown in  FIG. 8  that are also compatible to the current method of  FIG. 9  may also be used to further enhance the powdered metal part. For brevity, and because some of these process steps are well known to those in the art of forging powder metals, only certain aspects of the invention are primarily discussed below. 
     For a proper appreciation of the process of  FIG. 9 , attention is directed momentarily to the preform  184  of  FIG. 6B .  FIG. 6B  shows a partial cross-sectional view of a carburized portion  188  to be removed from the preform  85  of  FIG. 6A  by a removing process prior to forging.  FIG. 6A  shows a partial cross-sectional view of the representative preform  84  of  FIG. 5  after carburization. The preform  184  includes at least one first surface  185 , a case depth  186 , at least one second surface  187 , and a carburized portion  188 . The carburized portion  188  is selectively chosen to be removed from the preform prior to forging. By selectively removing the carburized portion prior to forging results in a final part having superior machining capabilities for secondary operations thereby obtaining, for instance, higher spline classification or tighter broaching tolerances in the relatively soft non-carburized material. Additional benefit is further derived from the strategically improved carburized portion resultantly obtained during the sint-carb and forging processes. 
     Returning to the process of  FIG. 9 , a selectively non-carburized powdered metal part requires a compacting step of a metal powder into a preform. 
     The simultaneous or subsequent steps of sintering and carburizing, or sintering and then carburizing the preform to obtain preform  184  may be accomplished with the sintering and carburizing processes mentioned above. 
     As is known in the art, a typical sintering temperature for steel metal powder is from about 2000.degree. F. to 2100.degree. F. while the initial carbon content of the preform may be less than 0.22% by weight. By carburizing, a final carbon content in the at least one first surface may be in the range of 0.22% to 0.37% by weight. 
     The step of removing the carburized portion on the at least one second surface of the preform is performed prior to forging. Removing at least one carburized portion advantageously results in a forged part having a carburized portion on at least one first surface with its beneficial case hardness while at the same time providing at least one second surface in which the material is not case hardened. The removing step allows for the strategic removal of the carburized portion  188  of the second surface  187  from the sintered preform prior to forging. The removal process may be done with known methods, such as punching, machining, or grinding, for example. One advantage to removing the carburized portion is that it avoids the need for complex and complicated masking and unmasking operations to control the diffusion of carbon during the sint-carb processes. Another added advantage to removing the carburized portion in the sintered preform during its relatively soft state is improved tool life thereby obtaining higher performance characteristic for items such as spline classification. Moreover, typically the splining operation is completed when the part is considered to be in its hard state after forging causing unnecessary tool wear and reduced spline tolerance or performance. 
     The forging and cooling steps are then completed to obtain the part having at least one second surface having shear resistance properties and at least one first surface having surface hardness properties. Forging and cooling may be done in accordance with accepted methods known to those in the art of powder metal forging, typically being at a temperature for steel powder metal being from about 1600.degree.F. to 1800.degree.F. It is also desirable to obtain a substantially fully dense part having at least 99.6% of theoretical density by applying a forging pressure typically ranging between 50 and 70 tons per inch. 
     If a final part is to be near-net shape after forging, the material being removed during the removing step should be accounted for in the compaction step by making an appropriate or oversize preform. 
     The process as given in  FIG. 9  may also advantageously utilize the variable forging step as given in the process of  FIG. 8 . Specifically, a variable case depth profile may be achieved in the final part during forging of the carburized portion of the at least one first surface of the preform after the removing step. In this instance, if the part is a gear made in accordance with this invention, then it may have the variable case depth profile as represented in  FIG. 2 , while simultaneously having the preferable soft internal diameter required for greater spline class and shear strength. 
     The part made in accordance with the inventive process of  FIG. 9  may include any part having required hard and soft surfaces. Specifically, the inventive process of  FIG. 9  is particularly advantageous for a toothed gear. 
     While various process steps have been presented, they are intended only to be limited in scope or order as indicated in the claims of this invention. Further, while the invention has been described in connection with several embodiments, it should be understood that the invention is not limited to those embodiments. Thus, the invention covers all alternatives, modifications, and equivalents as may be included in the spirit and scope of the appended claims.