Patent Publication Number: US-6217677-B1

Title: Method for annealing stamped components

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for annealing stamped components, and more particularly to a method for locally annealing sharp radii in precision formed housings, the housings being formed by a stamping process and made from a carbon or HSLA steel. 
     DESCRIPTION OF THE PRIOR ART 
     In an automatic transmission, several components are formed from carbon steel or HSLA steel. Torque transmission members include a number of drums which are stamped and splines are rolled into the cup-shaped outer periphery. Normally, the stamping displaces the grain structure of the steel thereby work hardening the part. This work hardening causes high stresses at the radius and slivering may occur within such drums. Typically, such parts are annealed in a furnace, wherein the entire part is heated to the appropriate temperature and the entire part is annealed, although only the stress areas require such annealing. This process requires a long cycle time to heat the entire part and anneals portions of the part for which it is not desired to be annealed and therefore softened. 
     The work hardening creates further problems in that the hardened portion creates areas of high stress that may contribute the fatigue and/or failure such as cracking, splintering or slivering of the material. Methods to control these problems include additional die stations to control and minimize the amount of displacement in a particular stamping operation. Alternatively, a steel having a lower carbon content may be used. Or, alternatively, a nonselected stress relief process may be used to anneal the entire component in a furnace or oven. Each of the above may require additional cost or produce undesirable characteristics in the part. 
     One such component is a hub for a reverse and low gear, one-way clutch in the Ford CD4E automatic transmission. It would therefore be desirable to produce a precision stamped component and anneal the areas of high stress in the stamped component. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a stamped component is locally annealed using a magnetic heating process. By doing so, stresses in the part are reduced so as to avoid splintering, slivering, or other defects during subsequent operations. 
     A process for forming a precision formed cup-shaped member is provided, including stamping a blank into a cup-shaped member. The cup-shaped member is positioned onto a magnetic heating machine. A magnetic core is positioned adjacent a radius of the cup-shaped part on both a top side and a bottom side of the cup-shaped part. The part is heated with the magnetic heating machine so as to anneal the cup-shape part at the radius. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional side view of a precision stamped component according to the present invention. 
     FIG. 2 is a partial sectional side view of the component of FIG. 1 having a further process step formed thereon to form splines in an outer surface thereof. 
     FIG. 2A is a partial end view of the component of FIG.  2 . 
     FIG. 3 is a partial sectional side view of the component of FIG. 1 having yet further operations formed thereon. 
     FIG. 4 is a schematic representation of a machine for use with the method according to the present invention. 
     FIG. 5 is a schematic representation of a part in a machine for use with the method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     As shown in FIG. 1, a precision stamped member  10  in a preferred embodiment has a cup-shaped form. The member  10  is formed from flat blank of sheet metal. The flat blank has a radius  12  formed therein to cause the cup-shaped part  10  to acquire its shape. During the formation of the radius  12 , a high stress region results thereat. Further, a second radius  14  is formed in the cup-shaped member  10 , also causing a residual stress in the part  10 . 
     As shown in FIG. 2, the cup-shaped part  10  has a later process step formed thereon to form a plurality of splines  16 , as better illustrated in the view shown in FIG. 2 a . As shown in FIG. 3, a plurality of apertures  20 ,  22  are formed in the member. Similarly, a plurality of oil vanes  24  may be formed therein. Each of the subsequent operations illustrated in FIGS. 2-3, if performed on the part  10  in a high stress area (i.e. after forming to the shape shown in FIG. 1 without heat treating), would result in frequent defects in the form of fracturing or slivering in this region of the part. 
     As shown in FIG. 4, a magnetic heating machine  40  according to the present invention is illustrated schematically. As described in “Patented Heat Treating System Uses Magnetic Fuel Technology for Through-Heating of Metal Parts”, by D. Keith Patrick, in  Industrial Heating,  March 1998, pp. 61-68, a magnetic heating process is described, the article incorporated herein by reference. A machine  40  according to this article is further described in U.S. Pat. No. 5,025,124, and  The Principles of Uniform Magnetic Heating  ( UMH ), by Mitsubishi and Core-Flux, both of which are incorporated herein by reference. 
     As described in the &#39;124 patent, and as illustrated schematically in FIG. 4, a machine  40  includes a C-frame  42  which is used to magnetically heat the parts. The C-frame  42  includes a first, or upper, core  46  positioned adjacent the workpiece  10  opposite a second core  44  positioned the workpiece  10  on the opposite side thereof. The cores  46 ,  44  are strategically positioned so as to magnetically heat the part  10  at a localized area thereof  12 , previously indicated to be a high stress area due to the forming operation formed thereon. By so locally applying this magnetic field, the part  10  may be annealed at this local radius  12 , or locally at any other portion, in order to soften the material to prevent damage during further forming operations performed on the part  10  and/or use of the part  10  in a vehicle. In a preferred embodiment, the part  10  is made of a soft steel, such as SAE 1020, and has a hardness of about 85 Rb at the radius  12  after cold forming. The process anneals the part  10  at the radius  12  to a hardness of about 60 Rb, approximately equivalent to the hardness of the remainder of the part  10 . 
     As shown in FIG. 5, the cores  44 ,  46  are placed near the radius  12 . Thus, the lines of flux  47 ,  48  which travel through the part  10  are concentrated at the radius  12 , thereby heating the part at the radius  12  to approximately a minimum temperature of 400° F. This enables the part  10  to be annealed locally as would be appreciated by one skilled in the art upon reading this disclosure. As described in the Mitsubishi publication, the UMH system operates on the basis of a hysteresis loss system. Preferably, the frequency of the power supply is adjusted to optimize the efficiency of the heating process, so as to heat the part at about the resonance of the part  10 . In a preferred embodiment, a part  10  of about 173 MM diameter having a height of about 60 mm and weighing about 0.45 kg. is best heated with a power supply frequency of about 140 Hz. The current flow is adjusted in a similar manner to optimize the process to achieve the desired temperature. 
     The machine  40  includes a number of details to heat, locate and support the part  10  as described below. The upper and lower cores comprise a laminated material, such as a grain oriented silicon directional steel, known to one skilled in the art. Attached to the upper core  46  is a plate  41 . The plate  41  is provided to support a hoop  43 . The plate  41  and hoop  43  in a preferred embodiment comprise a low carbon steel material. The hoop  43  is provided to direct the flux  47  at the local area, such as the radius  12 , to be heated. The hoop  43  circumferentially surrounds the radius  12  and is provided in light contact therewith. 
     The lower core  44  includes an insulator  45  provided on a top surface thereof. The insulator  45  provides a horizontal surface to support the part  10  vertically. The insulator  45  may also provide features to locate the part horizontally, such as a vertical projection  45 ′ to protrude through an opening provided on the part  10  and provide an interference fit or small clearance to locate the part  10 . A second insulator  49  is provided about the outer portion of the lower core  44 . The second insulator is used to insulate the inner circumference of the part  10  from the lower core  44 . 
     Subsequent to the annealing step above, the process steps of forming the splines  16  on the outer surface of the part  10 , as indicated in FIG. 2 at  16 , is performed using the GROB process as is also known to one skilled in the art. One skilled in the art appreciates the GROB process is one preferred cold working rolling process for cold forming the splines, and alternative methods exist to form these splines. The subsequent operations of forming the holes and oil dams  20 ,  22 ,  24 , as indicated in FIG. 3, are performed using typically pressing, punching, piercing, and forming operations known to one skilled in the art. The annealing process permits one to locally anneal the part  10  and thereby prevent damage of the part, such as slivering, during the subsequent forming operations. 
     In a preferred embodiment, the annealing process is performed in a nitrogen atmosphere to eliminate oxidation and scaling during this operation within a chamber. Alternatively, this operation could be performed without the use of a cover gas, but the resultant oxidation would be produced. 
     Although the preferred embodiments of the present invention have been described, it will be apparent to a person skilled in the art that variations may be made to the process that is described herein without departing from the scope of the invention as defined by the following claims.