Abstract:
An induction hardening heat treatment method comprises the steps of induction heating a metal part or component, particularly a part fabricated of a high strength material, and then exposing the part to a sequence or series of partial, intermittent or interrupted quenches. Such intermittent or interrupted quenching achieves the necessary change in surface temperature with time to achieve a martensitic transformation of the surface adjacent metal or material while minimizing surface to core temperature differentials during the process which could result in cracking of the part, especially if it is fabricated of high strength materials.

Description:
FIELD 
       [0001]    The present disclosure relates to a method of induction hardening of metals and more particularly to a method of induction heating and intermittent quenching of metals, particularly high strength materials, which reduces or eliminates cracking. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0003]    Post fabrication treatment of fabricated metal parts such as gears, shafts, sprockets, bearings and similar components is commonplace. The usual reason for such treatment is a desire or need to increase the strength and durability of the part and most processes involve heating the part, followed by controlled cooling. Due to the available excellent process control, induction hardening, which involves induction heating of the part followed by a controlled quench, is a preferred method of strength increase for many automotive parts such as gears, sprockets and shafts. Induction hardening provides a surface adjacent region or case of increased hardness which may be in the range of from 37 to 58 HRC, thus the frequently used term “case-hardened.” 
         [0004]    The strength, load and service requirements of a particular component may be such that even with post fabrication treatment, it must initially be fabricated of a high strength material whose strength is further increased by treatments such as the induction hardening process. The use of such high strength materials and the sophisticated and complex geometry of some parts presents an additional challenge, namely, the possibility or likelihood of cracking during the induction hardening quench. Such cracking is a direct result of the rapid temperature reduction of the part which is necessary to achieve the desired or necessary hardness and more particularly the temperature differential between the surface and the core of the part as it is quenched which generates internal stresses. Unfortunately, the rapid temperature reduction is the source or mechanism of strength increase, the cracking being but a highly undesirable side effect of such process. 
         [0005]    The present invention is directed to a method of post fabrication strength increase including induction hardening (heating and quenching) which minimizes or eliminates cracking of the treated part, especially parts fabricated of high strength materials. 
       SUMMARY 
       [0006]    The present invention provides an induction hardening heat treatment method comprising the steps of induction heating a metal part or component to at least its austenitic temperature, particularly a part fabricated of a high strength material, and then exposing the part to a sequence or series of partial, intermittent or interrupted quenches. Such intermittent or interrupted quenching achieves the necessary change in surface temperature with time to achieve a martensitic transformation while minimizing surface to core temperature differentials which could result in cracking of the part, especially if it is fabricated of high strength materials. 
         [0007]    Thus it is an aspect of the present invention to provide a method of induction hardening fabricated metal parts. 
         [0008]    It is a further aspect of the present invention to provide a method of induction hardening parts fabricated of high strength materials. 
         [0009]    It is a still further aspect of the present invention to provide a method of heat treating a fabricated metal part including the step of induction heating the part. 
         [0010]    It is a still further aspect of the present invention to provide a method of heat treating a fabricated metal part including the steps of induction heating the part and intermittently and repeatedly quenching the part. 
         [0011]    It is a still further aspect of the present invention to provide a method of heat treating a part fabricated of a high strength material including the steps of induction heating the part and exposing the part to a series of interrupted, partial quenches. 
         [0012]    It is a still further aspect of the present invention to provide a method of induction hardening a fabricated metal part including the steps of induction heating the part and intermittently and repeatedly quenching the part. 
         [0013]    It is a still further aspect of the present invention to provide a method of induction hardening a part fabricated of a high strength material including the steps of induction heating the part and exposing the part to a series of interrupted, partial quenches. 
         [0014]    Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0015]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0016]      FIGS. 1A ,  1 B and  1 C are schematic diagrams of a sequence of induction hardening steps according to the present invention which occur at a work station including an induction heating step, a partial spray quenching step and a dwell step, respectively; 
           [0017]      FIG. 2  is a flow diagram for an induction hardening process according to the present invention; and 
           [0018]      FIG. 3  is a time-temperature-transformation diagram of a typical steel alloy presenting temperature on the vertical (Y) axis, time on the horizontal (X) axis and data from a typical and representative intermittent quench operation according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0020]    With reference to  FIG. 1A , a work station for heat treating by induction hardening of parts or components such as gears, sprockets, shafts, bearings and the like is illustrated and generally designated by the reference number  10 . The heat treatment work station  10  includes a turntable or fixture  14  which is rotated at a desired speed by a motor and gear drive assembly  16 . It will be appreciated that the turntable or fixture  14  may take various forms and configurations which adapt it to and facilitate mounting or securement of variously shaped and configured parts or components  20  such as gears, sprockets, shafts, bearings or similar metal components thereto. Additional transport devices or mechanisms (not illustrated) may and typically will be utilized to load and unload the turntable or fixture  14 . 
         [0021]    The turntable or fixture  14  and the part or component  20  secured thereto are disposed and rotated within the heat treatment work station  10  which includes an induction heater  24  having one or more electromagnetic coils  28  which surround the part or component  20 . The electromagnetic coils  28  are supplied with alternating current at a power level and frequency that effectively and efficiently heats the part or component  20 . While different temperatures and temperature ranges will be appropriate for different materials and alloys as well as different sizes and configurations of parts or components  20 , a gear or sprocket having a diameter between about 80 and 150 millimeters (3.15 to 5.91 inches) and a thickness between about 10 and 20 millimeters (0.394 to 0.787 inches) with external teeth which is fabricated of a material such as D700 nodular iron per SAE standard J434 is preferably heated to a temperature in the range of 840° C. (1545° F.) to 950° C. (1740° F.). Given the variability of materials, parts and components  20 , shapes and sizes, it should be understood that specific temperatures both within and without this recited temperature range are within the purview of the present invention. 
         [0022]    Referring now to  FIG. 1B , after heating of the part or component  20  to the desired temperature in the induction heater  24 , electrical power to the electromagnetic coils  28  is terminated. The heat treatment work station  10  also includes a plurality or array of spray heads or nozzles  32  which are selectively supplied with a quenching solution from a controlled and pressurized source (not illustrated) of such solution which is primarily water and 3% to 9% polymer additive. The polymer additive may be one of many suitable known and available polymer quench additives. While the turntable or fixture  14  continues to rotate, a first, partial quench occurs as the part or component  20  is subjected to quenching by a spray of the quenching solution from the spray heads or nozzles  32  for one to five seconds. Continued rotation of the turntable or fixture  14  is desirable as it improves the uniformity of the partial quench. Given larger and heavier parts or components  20  or those fabricated of other materials, the first quench time may be longer and for smaller and lighter parts or components  20 , the first quench time may be shorter. 
         [0023]    Referring now to  FIG. 1C , the first, partial quench is interrupted by stopping the flow of the quenching solution to the spray heads or nozzles  32 . The quenching process then dwells for a first five to ten second interval. During this time, the turntable or fixture  14  and the part or component  20  may cease to rotate or they may continue to rotate. Once again, the recited dwell time interval may, in a given situation, be longer or shorter, depending upon the variables recited above. 
         [0024]    Referring again to  FIG. 1B , after the first dwell or interval, a second, partial quench is begun by restarting the flow of quenching solution to the spray heads or nozzles  32  and, if stopped, the turntable or fixture  14  and the part or component  20  are again rotated for a second ten to twenty second interval. 
         [0025]    Referring again to  FIG. 1C , after this second, partial quench, the supply of quenching solution to the spray heads or nozzles  32  is again terminated and a second interruption or dwell interval of ten to twenty seconds then occurs. Again, during this time, the turntable or fixture  14  and the part or component  20  may continue to rotate or their rotation may be stopped. 
         [0026]    Returning again to  FIG. 1B , the part or component  20  is next subjected to a third, partial quenching interval of ten to thirty seconds. This third, partial quench is achieved by again starting the flow of quenching solution through the spray heads or nozzles  32  and rotating the turntable or fixture  14  and the part or component  20  if it was stopped for the second dwell interval. For many parts and components  20 , the third, partial quench is sufficient to complete the quenching process. If the part or component  20  is then fully heat treated (induction hardened) as described more fully below, it may be released or removed from the turntable or fixture  14 . 
         [0027]    Once again, it should be understood that the quench and interrupt times recited above are nominal and effective values for the gear or sprocket described in Paragraph [0021], above, and that other time intervals and sequences including more or fewer and longer or shorter duration quenches and more or fewer and longer or shorter interruptions may, and typically will, be appropriate for other parts and components  20 . Furthermore, although  FIGS. 1A ,  1 B and  1 C illustrate a heat treatment work station  10  having both an induction heater  24  and a plurality or array of spray heads or nozzles  32 , the interrupted or sequential, partial quenching process of the present invention may also be accomplished in (1) a heat treating line in which a first station includes an induction heater and a second, separate station includes one or a plurality of spray nozzles or heads or (2) a heat treating line which includes an induction heater and one or a plurality of baths or tanks (not illustrated) in which the partial quenching steps occur. It should be understood that these and other quenching facilities are capable of providing the repeated, multiple partial quenches of the present invention. 
         [0028]    Turning now to  FIGS. 1 and 2 , a process flow chart of the induction hardening steps according to the present invention is designated by the reference number  50 . The process flow chart  50  includes an initial step  52  of fabricating a part or component  20 . The part or component  20  may by a conventional metal or metal alloy or it may be fabricated of a high strength material. As noted above, such high strength materials are often prone to cracking during induction hardening and thus the present invention is especially appropriate and beneficial when utilized with such materials. 
         [0029]    Next, the part or component  20  is heated in an induction heater  24  in a process step  54 . The part or component  20  is then subjected to a first partial quench in a process step  56 . The first partial quench of the process step  56  extends over approximately 5% to 10% of the nominal total quench time for the part or component  20 . Next is a first dwell step  58  that extends over approximately 30% to 35% of the total dwell time. A second partial quench occurs in the process step  62  which occupies approximately 30% to 50% of the nominal total quench time for the part or component  20 . A second dwell step  64  follows which extends over approximately 65% to 70% of the nominal total dwell time. A third partial quench step  66  occupies approximately 50% of the nominal total quench time. Assuming appropriate initial temperature and quench and dwell times for the nature of the part or component  20 , it will likely be heat treated and induction hardened at this time. 
         [0030]    Two final steps may be undertaken which are not, per se, inherent or necessary process steps but are primarily diagnostic, that is, optional steps that may be undertaken or performed to determine whether the process has produced a properly heat treated part or component  20 . Accordingly, a decision point  70  is entered which inquires if bainite has been created in the part or component due to the slowness of the quench. If it has, the decision point is exited at YES and a process step  72  is entered which designates the part or component  20  as unsatisfactory or out of specification. A process step  74  is then entered which adjusts the various quench cycles to increase the quench rate such that bainite is not formed. If no bainite is formed, the decision point  70  is exited at NO and a second decision point  76  is encountered which inquires whether a surface martensitic transformation has occurred in the part or component  20 . If it has not, the decision point  76  is exited at NO and the process flow  50  returns to the induction heating step  54  to repeat. If it has, the decision point  76  is exited at YES and the process concludes at the endpoint  78 . 
         [0031]    Referring now to  FIG. 3 , a time-temperature-transformation diagram generally illustrating the various phases of a typical steel alloy and the process of the present invention appear together. The vertical (Y) axis represents temperature and the horizontal (X) axis represents time with the values of both increasing from their point of intersection. On the right of the diagram, various phase regions such as austenite, pearlite, bainite and martensite are presented. To the left of the diagram, represented by the line  80  is the induction heating step  54 . The line  80  represents both the surface temperature and the core temperature of the part or component  20  and is therefore somewhat wide to indicate that the two temperatures may not, and typically will not, be the same. Although it is desirable that the surface and core temperatures of the part or component  20  be the same, or very nearly so, at the conclusion of the heating step  54 /beginning of the first quench represented by the point  82 , the austenitic temperature, this can typically be achieved only with significant engineering effort and development which is generally viewed as of marginal value and benefit. 
         [0032]    The stepped line  84  which descends from the point  82  represents the surface temperature of the part or component  20  during the intermittent, that is, quench and dwell, steps  56 ,  58 ,  62 ,  64  and  66  set forth above. The slightly irregular line  86  which is adjacent the line  84  represents the core temperature of the part or component  20 . Note, first of all, that the line  84  terminates in the martensitic region  88  indicating a martensitic transformation of a surface adjacent region of the part or component  20  and that the bainite region  92  has been avoided. Second of all, there is never a significant differential between the temperatures of the surface, line  84 , and the core, line  86 . This relative conformity between the surface and core temperatures of the part or component  20  during the quenching stages of the induction hardening process minimizes the generation of internal stresses and thus minimizes or eliminates cracking of parts or components  20 , especially those fabricated of high strength metal alloys and materials. For purposes of comparison, the line  94  represents the surface temperature of a part or component  20  undergoing a conventional, rapid and continuous quench. Note that during the latter portion of the quench, a significant temperature differential exists between the core temperature, the line  86 , and the surface temperature, the line  94 . 
         [0033]    It will be appreciated that the foregoing description has enabled and described the best mode contemplated by the inventors for practicing the invention. As noted above, because of the wide variations of possible metals, materials and sizes and configurations of parts and components  20  that will benefit from the heat treating process described herein, it should be understood that more or fewer partial quenching steps, interrupted by more or fewer sequential rest or dwell periods as well as different times for both the quench and dwell periods may, and likely will, be appropriate for other parts and components  20 . 
         [0034]    The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.