Abstract:
The present invention is a system of concurrent, adjacent heat treatment and vigorous cooling in section-annealing of metal workpieces. The invention process is especially advantageous in induction heating for annealing of one section of a workpiece while maintaining relatively non-annealed properties in an adjacent section.

Description:
[0001]    The present application claims priority from provisional U.S. Patent Application Ser. No. 62/360,718 filed Jul. 11, 2016, the content of which is expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is broadly directed to heat treatment of metals for annealing, and more particularly to a process for restricting annealing to a desired portion along a length of a metal workpiece. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is well known that annealing is, in general, a process of treating a metal workpiece by heating, maintaining a certain time at temperature(s), and cooling the workpiece to produce desired properties in the workpiece. Cooling should be done at a pre-determined cooling rate, so that the achieved, desired properties are not adversely affected. Annealing is an ancient process where a work piece is exposed to a temperature-time-atmosphere process steps in order to reach certain desired properties. 
         [0004]    Work pieces created with imperfections in the crystal lattice and in which the material became harder e.g. by cold working, are often re-crystallisation annealed to be able to improve or continue the material deformation. By re-crystallisation annealing new undeformed (soft) crystals are originated. The process temperature depends on the material used and the deformation ratio can be performed on all metals and alloys. 
         [0005]    Recrystallization is characterized by the gradual formation and appearance of a microscopically resolvable grain structure. The new structure is largely strain-free. There are few if any dislocations within the grains and no concentrations at the grain boundaries. 
         [0006]    Standardized annealing is used to improve irregular, inhomogeneous and coarse grained structures in, for example, castings, forgings or rolled sheet. This improves the mechanical properties of the material by obtaining a fine-grained structure with grains of roughly equal size and a round shape. 
         [0007]    Solution annealing ensures that secretions in the material lattice present in the material are dissolved. This creates a homogeneous structure. 
         [0008]    After solution annealing, secretion or precipitation hardening is often applied so that the material adjacent to the solution annealing is heated to a temperature where the material elements that are in solution lattice. This improves the mechanical properties. 
         [0009]    Aluminum and aluminum alloys, wrought and cast, are generally annealed to promote softening. Annealing is generally carried out in the range 300-410 degrees C., depending on the alloy. Exemplary heating times at temperature vary from 0.5 to 3 hours, conditional on the size of the load and the alloy type. The solution treatment temperature for aluminum and aluminum alloys is critical to the success of the procedure. It is desirable that the solution heat treatment is carried out as close as possible to the liquidus temperature in order to obtain maximum solution of the constituents. Accurate furnace temperature and special temperature variation must be controlled to within a range of ±5° C. for most alloys. Overheating must be avoided, i.e., initial eutectic melting temperatures must not be exceeded, in that early stages of overheating are not apparent but will result in a deterioration of mechanical properties of the workpiece. 
         [0010]      FIG. 1 , a graph of heat transfer coefficients versus temperature instantly indicates to those skilled in the art of annealing aluminum and aluminum alloys a critical limitation on the use of induction heating of a workpiece for annealing. While induction heating is well known and a desirable and inexpensive alternative to furnace heating of aluminum and aluminum alloys for annealing as described above, it is essentially of little use in the careful control of temperature required to prevent overheating and deterioration of mechanical properties as described above. It is seen in  FIG. 1  that the desired temperature range for annealing of aluminum and aluminum alloys is strictly within the range of the steepest part of the curve for increases of heat transfer coefficients for each degree of temperature increase. As such, heating resulting from inductive fields generated in the aluminum and aluminum alloy workpiece races upwardly and is basically out of predictive control. Undesired full melting of aluminum and aluminum alloys is known when using inductive heating for annealing a workpiece. 
         [0011]    As indicated, annealing processes in the prior art have limitations which are improved upon with the present invention 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is a system of concurrent, adjacent heat treatment and vigorous cooling in section-annealing of metal workpieces. The invention process is especially advantageous in induction heating for annealing of one section of a workpiece while maintaining relatively non-annealed properties in an adjacent end. 
         [0013]    The prior art describes many examples of furnace or induction heating of a moving section of a long metal workpiece, where portions emerging from the heating zone are cooled or rapidly quenched to obtain desired properties in the workpiece. However, the intent of such treatment is to obtain for the entire length of the workpiece essentially identical properties, such as hardness, grain structures and the like. 
         [0014]    The present invention comprises a method for obtaining an annealed section immediately adjacent to a much less or non-annealed section on the same workpiece. A specific actual example of the operation of the invention process is described below and shows that the workpiece of the invention process required annealing of one end of the workpiece while retaining non-annealed properties at another end of the workpiece. 
         [0015]    The cooling required in one section of the metal workpiece during the invention process is not typical for quenching and cooling of prior art heat treating processes, as the cooling applied to the cooled section of the invention process is intended not for quenching or to achieve a desired property in the workpiece. In fact, the opposite is obtained, i.e., the properties of the cooled section of the invention process are essentially retained as they were before an induction heat treatment was applied to the heated section. 
         [0016]    An unexpected benefit of the invention process has been, for at least aluminum and aluminum alloys, an extremely narrow transition zone between a fully annealed heated section and an essentially non-annealed cooled section. It appears that the vigorous cooling applied to the cooled section results in basically a thermal “dam” forming an interface of the heated section and the cooled section. 
         [0017]    Various objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings submitted herewith constitute a part of this specification, include exemplary embodiments of the present invention, and illustrate various objects and features thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a graph of heat transfer coefficients versus temperature for aluminum and steel. 
           [0019]      FIG. 2  is a generalized cross section of a metal workpiece, preferably aluminum, showing heat treated and cooled zones. 
           [0020]      FIG. 3  is a top perspective view of an actual specific aluminum workpiece, showing application of the coils of an induction heating device for the heat treated zone and a container of dry ice (frozen carbon dioxide) simultaneously cooling a cooled zone. 
           [0021]      FIG. 4  is a top view of the workpiece of  FIG. 3 . 
           [0022]      FIG. 5  is a side cross section view of the workpiece of  FIG. 3  side by side with the generalized workpiece of  FIG. 2 , showing corresponding sections for both. 
           [0023]      FIGS. 6 and 7  are each a top view and side view of a vertical sections of cylindrical workpieces  10 A and  10 B, which are to that of  FIGS. 4 and 5  respectively. 
           [0024]      FIGS. 8 and 9  are each a top view and side view of a vertical sections of cylindrical workpieces  20 A and  20 B, which are to that of  FIGS. 4 and 5  respectively. 
           [0025]      FIG. 10  shows three views of an actual workpiece  10 A as in  FIG. 6 , where a left item is a side view of a first top half cross section, a central item is a side view of a second top half cross section, and a right item is a side view. 
           [0026]      FIG. 11  is a top view of the workpiece of the right item of  FIG. 10 , showing internal walls and temperature indicator substances after the invention heat and cooling treatment. 
           [0027]      FIG. 12  shows a table of hardness measurements at spots A, B, C and D for the aluminum workpiece shown and treated according to the objects of the invention. 
           [0028]      FIG. 13  shows hardness testing results for the workpieces shown in  FIGS. 6-9  for sections of the workpieces  10 A,  10 B,  20 A and  20 B. 
           [0029]      FIG. 14  shows a prior art graph of thermal conductivity versus temperature for aluminum and other metals, demonstrating the efficacy of the present invention. 
           [0030]      FIG. 15  is a top perspective view of the invention process being performed, with induction heating elements heating a top portion of the workpiece and dry ice cooling a bottom portion of the workpiece. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The invention is now discussed with reference to the drawing figures and specific examples. 
         [0032]      FIG. 2  is a generalized cross section of a metal workpiece  10 , exemplary aluminum, showing heat treated zone  11 , a cooling medium  14 , a lower cooled zone  15 , an upper cooled zone  16 , and a transition zone  12 , a relatively narrow band in workpiece  10 . In transition zone  12 , physical properties (such as hardness and related properties) of the workpiece transition far more rapidly than in zone  11  above and zone  13  below transition zone  12 . In the invention process, heat transfer to an upper part of or all of zone  11  results in a significant change in the physical properties of at least an upper part of zone  11 , resulting in rapid heating of zone  11 . Heat transfer from zone  11  would rapidly take place into upper cooling zone  16 , but for application of cooling medium  14  to a lower cooling zone  15  of cooling zone  13 . Cooling medium  14  may be dry ice, cooled or liquid gases (such as nitrogen), glycol or similar cooling or chilling devices or (any or all chilling mediums and devices in combination) processes achieved by processes adapted to accomplish the objects of the invention. In the specific examples disclosed, roughly, crushed dry ice at or around minus 78 degrees C. was used as the cooling medium  14 . 
         [0033]    Transition zone  12  is the result of the application of the invention process, in that extreme cooling rates occur in zone  12  which prevent significant heat treatment of portions of workpiece  10  below zone  12 , thereby providing for relaxation of stresses and weakening of aluminum in zone  11  by way of heat treatment and preservation of non-heat treated properties in zones  15  and  16  below transition zone  12 . 
         [0034]      FIG. 3  is a top perspective view of an actual specific cylindrical aluminum workpiece  20 , showing application of the coils of an induction heating device  19  within and outside of the walls of the heat treated zone  26 , which operation of the device  19  and simultaneous application of cooling medium  14   a  established a transition zone  27  and an upper cooling zone  28 . A container of dry ice (frozen carbon dioxide) was provided as cooling medium  14   a,  which was contained in a cylindrical container  18 , keeping the cooling medium  14   a  in contact with an outside surface only of lower cooling zone  29 . A top edge  21  of cylindrical workpiece  20  is shown between internal and external coils of device  19 . 
         [0035]      FIG. 4  is a top view of the workpiece  20  of  FIG. 3 , showing an opening in a thickened base  23 . 
         [0036]      FIG. 5  is a side cross section view of the workpiece  20  of  FIG. 3  side by side with the generalized workpiece  10  of  FIG. 2 , showing corresponding sections for both. Cylindrical side walls  22  extend to a thickened base  23 , where a heat treating zone  26  is shown corresponding in arrow  11 ′ to heat treating zone  11 , a transition zone  27  is shown corresponding in arrow  12 ′ to transition zone  12 , an upper cooling zone  28  is shown corresponding in arrow  16 ′ to upper cooling zone  16 , and a lower cooling zone  29  is shown corresponding in arrow  15 ′ to lower cooling zone  15 . Cooling medium  30  (medium  14   a  in  FIG. 3 ) corresponds to cooling medium  14 . 
         [0037]      FIGS. 6 and 7  are each a top view and side view of a vertical sections of cylindrical workpieces  10 A and  10 B, which are to that of  FIGS. 4 and 5  respectively. In the side views, actual test values of hardness after heating and simultaneous cooling according to the invention are shown in three vertical rows of small circles, showing a reduced mechanical properties zone, corresponding to the heating zone  11  of  FIG. 2 , and a transition zone, corresponding to the transition zone  12  of  FIG. 2 . 
         [0038]      FIGS. 8 and 9  are each a top view and side view of a vertical sections of cylindrical workpieces  20 A and  20 B, which are to that of  FIGS. 4 and 5  respectively. In the side views, actual test values of hardness after heating and simultaneous cooling according to the invention are shown in three vertical rows of small circles, showing a reduced mechanical properties zone, corresponding to the heating zone  11  of  FIG. 2 , and a transition zone, corresponding to the transition zone  12  of  FIG. 2 . 
         [0039]      FIG. 10  shows three views of an actual workpiece  10 A as in  FIG. 6 , where a left item is a side view of a first top half cross section, a central item is a side view of a second top half cross section, and a right item is a side view. In the center item, hardness values are shown in a column, showing hardness measurements at those vertical levels of the workpiece, where a cooling zone section comprises hardness values of 80.5 and four values of 81 (in a cooling zone), 73 (in a transition zone), and 55 and 60 (in a heating zone). Temperatures of 750, 850 and 900 degrees are marked on the outside surface of the workpiece on the right item, indicating temperature indicating materials which reacted to melting. 
         [0040]      FIG. 11  is a top view of the workpiece of the right item of  FIG. 10 , showing internal walls and temperature indicator substances after the invention heat and cooling treatment. 
         [0041]    In the invention process with workpieces such as those in  FIG. 3 , induction coils are arranged with the workpiece on a turn table so that it can be rotated with reference to the induction coils. Three temperatures are been chosen to be measured with melting indicator materials for 750°, 800° and 850°. The base of the workpiece shall be placed in the center of a container 3″ tall and 10″ in diameter, with dry ice filling a space between the container and the outside surface of the workpiece. 
         [0042]    Induction coils are activated and the workpiece is carefully examined so that, during rotation, a highest temperature in the heating zone is maintained well below a melting temperature for aluminum, but still sufficient high to achieve the objects of the invention. 
         [0043]      FIG. 12  shows a table of hardness measurements at spots A, B, C and D for the aluminum workpiece shown and treated according to the objects of the invention. 
         [0044]    The measurements are shown for Brinnel testing. It is instantly appreciated that a transition zone exists at the elevation of the spot B, which is dramatically narrowed and focused as a result of the heat transfer properties of aluminum. 
         [0045]      FIG. 13  shows hardness testing results for the workpieces shown in  FIGS. 6-9  for sections of the workpieces  10 A,  10 B,  20 A and  20 B. 
         [0046]      FIG. 14  shows a prior art graph of thermal conductivity versus temperature for aluminum and other metals, demonstrating the efficacy of the present invention. The properties of aluminum at above 200 degrees F. clearly show the difficulty of heat treatment of aluminum, i.e., that each increment of temperature increase dramatically increases the rate at which aluminum will absorb heat, leading to inherent instability in heat treatment processes for that metal as compared with any other metal shown in  FIG. 14  other than brass. 
         [0047]      FIG. 15  is a top perspective view of the invention process being performed, with induction heating elements  50  and  51  heating a top portion of the workpiece  20  and dry ice  52  cooling a bottom portion of the workpiece  20 , where. 
         [0048]    The present invention provides a thermal “dam” at a relatively narrow region between heated and cooled sections of the heat treated piece, where the elevation location of the thermal “dam” is adjustable by way of relative heat transfers with the heated and cooled sections. For instance, if heat transfer to a heated section is increased while cooling to the cooled section is maintained at a constant rate, the narrow thermal “dam” zone will descend in a controlled manner. 
         [0049]    The prior art does not teach that a narrow thermal “dam” zone may be achieved by heat treatment of a heated section of a metal workpiece while intensely cooling a cooled end of that workpiece, thereby preserving the non-heat treated cooled section and endowing the narrowly separated heated section with desired heat treatment properties. 
         [0050]    It is significant that providing an intensely cooled section results in some protection of the heated zone from eutectic melting, as has been seen in some aluminum samples actually processed using the invention process, i.e., while substantial cooling of the heated section is blocked at the thermal “dam”, the heated section loses sufficient heat overall so that even higher temperature treatments of the heated section do not result in harmful eutectic melting in the heated section. Further, said overall cooling to the heated section at lower temperatures applied to the heated section results, in some actual samples processed using the invention process, have caused a desired reduction in yield limit of samples from the heated section (as would be expected with full annealing) without full annealing of the heated section. 
         [0051]    More specifically, in the invention process above, the transition zone is preferably less than 6 inches of the workpiece, more preferably four inches of the workpiece, and most preferably one fourth to one inch of the workpiece, where on either side of the workpiece the crystal structure of the cooled zone and heated zone are substantially the same. It is predicted that application of the present invention shall be effective the metal workpiece which comprises a metal or metals whose line slope of on a plot of thermal conductivity to temperature curve is equal to or greater than 0.004 watts/inch-degree F per degree F., similar to that of brass shown in  FIG. 14 . 
         [0052]    The invention process comprises a method where the the upper heat treating zone is heated to above 600 degrees F. and the lower cooling zone is maintained at a temperature no higher than 400 degrees F. The invention process also comprises a method where the the upper heat treating zone is heated to above 800 degrees F. and the lower cooling zone is maintained at a temperature no higher than 500 degrees F. 
         [0053]    The process of the invention comprises that after the upper heat treating zone is heated to over 600 degrees F. that workpiece is brought to ambient temperature so that the hardness of the lower cooling zone is thirty percent or less higher than that of the upper heat treating zone, preferably twenty five percent, fifteen percent or ten percent thereof. 
         [0054]    The present invention is useful for providing lower yield strengths to the heated section (as may be needed if it is securely engaged with carbon fiber connection to other structures but the cooled section is desired to maintain original higher yield strengths of the metal workpiece because it is not so engaged to a carbon fiber connection. 
         [0055]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.