referring now to the figures , several embodiments or implementations are hereinafter described in conjunction with the drawings , where like reference numerals are used to refer to like elements throughout , and where the various features are not necessarily drawn to scale . the present disclosure provides apparatus 2 and coil assemblies 4 thereof having an active turn 10 that couples with one or more fillet hardening passive turns 20 , 21 and couples with a workpiece journal for hardening the journal 102 and also providing the energy to the two passive turns or coils to harden one or more undercut fillet areas 104 of the workpiece 100 . the disclosed approach advantageously locates the active turn crossovers 10 c , 10 e so as to inductively heat the journal 102 , and the active turn 10 can extend around a circumferential portion of the journal by up to about 180 degrees . the active turn 10 , moreover , is electrically connected to a power supply to conduct current from the supply for inductively heating the journal 102 via the crossovers . one or more inner passive turns 20 , 21 are provided in at least one space within and / or below the active turn 10 in certain embodiments , where the passive turns 20 , 21 are passively energized via electromagnetic coupling from the active turn 10 such that the passive turns 20 , 21 are not connected to the power supply , where the passive turns 20 , 21 each form a closed loop structure . a coupling portion 20 a , 21 a of each passive turn 20 , 21 is proximate the circumferential and / or other coupling portion 10 b , 10 f , 10 d of the active turn 10 so as to electromagnetically couple therewith , and a second portion 20 c , 21 c of the passive turn 20 , 21 is extendable at least partially into an undercut of a fillet 104 for heating thereof . in certain journal portions 104 of a crankshaft workpiece 100 having two undercut fillets 104 at two axially spaced sides of the journal 102 , each side can have one or more passive heating turns or turn portions for heating the associated fillet 104 , with the passive turns 20 , 21 of each side being axially movable at least partially into the undercut fillet areas 104 after radial approach of the coil arrangement 2 to or proximate the journal 102 . referring initially to fig1 - 11 , an exemplary induction coil apparatus 2 is illustrated , which may be advantageously employed for hardening undercut fillets 104 of a crankshaft or other workpiece 100 . one embodiment is best shown in fig1 - 4 , where the apparatus 2 has a coil assembly 4 including an active turn or coil 10 and two passive turns or coils 20 and 21 . the active turn 10 is coupleable using conductive couplings 11 and 12 to an electric power source to form a first conductive circuit therewith , where the active turn is formed of hollow copper tube structures to provide an internal fluidic path for circulation of coolant during operation via the couplings 11 and 12 . likewise , the first and second passive turns 20 and 21 are formed of conductive ( e . g ., copper ) tubes providing internal coolant passageways for coolant circulated via coolant couplings 40 . the active turn 10 of fig1 - 11 is formed of sections 10 a - 10 g including active coupling section 10 b , a first active crossover section 10 c , a second active coupling section 10 d , a second crossover section 10 e , another partial coupling portion 10 f and a final portion 10 g , where the crossover sections 10 c and 10 e are operative to heat all or a portion of the workpiece journal area 102 in a recess 110 thereof when the coil assembly 4 is positioned at or near the workpiece 100 , as shown in fig9 . the first passive turn 20 is electrically isolated from the active turn 10 and forms a closed conductive circuit including a circumferential coupling section 20 a , a crossover portion 20 b , a passive heating section 20 c , and a second crossover portion 20 d connecting back to the coupling portion 20 a . similarly , the second passive turn 21 forms a closed conductive circuit electrically isolated from the active turn 10 via coupling section 21 a , crossover sections 21 b and 21 d and an outlying heating section 21 c , and the passive turns 20 and 21 are separately cooled by fluid provided via tubes 40 . the active and passive turns 10 , 20 , and 21 are fitted with lamination keeper structures 30 for laminations 42 ( fig1 and 11 ) for electromagnetic coupling of the active turn 10 with the passive turns 20 and 21 , where any suitable magnetic material may be used for the laminations 42 , such as iron in one embodiment . other embodiments are possible using suitable magnetic material to form flux concentrator structures 42 , which need not be laminations . as shown in fig1 and 11 , moreover , the coil apparatus 2 further includes protective outer covers 50 and may include other suitable mounting and support structures for translation operation as described herein . referring also to fig5 - 9 , the passive turns 20 and 21 are translatable by any suitable mechanical translation devices ( not shown ) relative to the active turn 10 between a first position ( fig5 and 7 ) in which the passive heating sections 20 c and 21 c clear sidewalls 108 of a workpiece recess 110 as the coil assembly 4 is translated radially toward the recess 110 ( fig7 ) and a second position ( fig6 and 8 in which the passive coupling sections 20 a , 21 a are proximate to and electromagnetically coupled with the first active coupling section 10 b , 10 f , 10 d of the active turn 10 . as shown in fig8 , moreover , an insulator 26 may be provided in certain embodiments between the passive coupling section 20 a and the active coupling section 10 b , 10 f , as well as between the active coupling section 10 d and the passive coupling section 21 a of the second passive turn 21 , which may be any suitable insulator material , such as teflon in one example . in the second positions , moreover , the first passive heating section 20 c is located at least partially within a first undercut fillet area 104 of the workpiece recess 110 , and the second passive heating section 21 c is located at least partially within a second undercut fillet area 104 of the workpiece recess 110 , as illustrated in fig8 . in addition , as shown in fig9 , the active turn crossover portions 10 c and 10 e are disposed to be proximate the workpiece journal portion 102 during induction heating when the apparatus 2 is translated toward the recess 110 . in the embodiment of fig8 , the first passive turn 20 is translatable relative to the active turn 10 in a first direction ( to the left in the figure ) generally parallel to a longitudinal workpiece axis 8 between the first and second positions , and the second passive turn 21 is translatable in a generally opposite axial direction ( to the right ) between its first and second positions , where any suitable mechanism ( s ) can be used to provide the translation of the passive turns 20 , 21 relative to the active turn 10 . the current in the closely coupled fillet hardening active turns 20 , 21 will be equal ( except for leakage flux ) and opposite to the current in the main active turn 10 in the illustrated embodiments . as a result , the current in the fillet hardening portions 20 c and 21 c is in the same direction as the main driving current of the active turn 10 , and the current induced in the fillets 104 is in the same direction as the main journal hardening current of the active turn crossover portions 10 c and 10 e so that the net electromagnetic configuration is that of a standard crankshaft hardening coil . with this arrangement , the fillet hardening passive sections 20 c and 21 c can be freely moved , at least partially , into and out of the fillet without any external electrical connection . as shown in fig1 , moreover , bumpers or guides 52 may be provided in the apparatus 2 for locating the apparatus 2 relative to a workpiece 100 , which will generally be rotated about the axis 8 during heating with the active turn 10 energized . in addition , the apparatus and the coil assembly 4 thereof may extend any circumferential distance about the workpiece 100 , such as about 180 degrees or less in various embodiments . referring now to fig1 and 17 , the translation of the passive turns 20 and 21 need not be strictly parallel to the workpiece axis 8 . in the embodiment of fig1 and 17 , the passive turns 20 and 21 are pivotally translatable between first retracted positions for insertion or removal of the apparatus 2 while clearing the sidewalls 108 ( fig1 ), and second extended positions for undercut fillet heating ( fig1 ). referring now to fig1 - 15 , another exemplary embodiment of the coil assembly 4 is illustrated , having a single passive turn 20 configured generally as described above , along with an active turn 10 similar to that shown in the above figures , but with the section 10 d being lowered to be proximate the journal 102 , such as approximately the same proximity as turn 20 c , in order to force the current to return through the journal 102 . this embodiment may be employed for hardening recesses 110 having only one undercut fillet 104 , which is heated via the translatable passive turn portion 20 c , while the journal 102 is heated via the active turn 10 as described in the above embodiments . the above described apparatus 2 is particularly suitable for various induction heating processes . the disclosure provides an exemplary process for hardening undercut fillets 104 of a workpiece 100 , which may employ the described apparatus 2 or which may be used in conjunction with other apparatus . the process includes translating a coil assembly 4 radially toward a recess 110 in the workpiece 100 such that a passive turn 20 of the coil assembly 4 clears a sidewall 108 of the recess 110 and at least one crossover section 10 c , 10 e of an active coil 10 of the coil assembly 4 is proximate a journal area 102 in the workpiece recess 110 . the process also includes translating the passive turn 20 relative to the active turn 10 such that a passive coupling section 20 a of the passive turn 20 is proximate to and electromagnetically coupled with an active coupling section 10 b , 10 f of the active turn 10 and a passive heating section 20 c of the passive turn 20 is located at least partially within an undercut fillet area 104 of the workpiece recess 110 , and energizing the active turn 10 with electrical power to heat at least a portion of a journal area 102 in the workpiece recess 110 using the at least one crossover section 10 c , 10 e of the active coil 10 and to induce electrical current in the passive turn 20 to heat at least a portion of the undercut fillet area 104 of the workpiece recess 110 using the passive heating section 20 c of the passive turn 20 . in certain embodiments , the translation of the passive turn 20 relative to the active turn 10 comprises translating the passive turn 20 in a direction parallel to a longitudinal axis 8 of the workpiece 100 . the above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure , where equivalent alterations and / or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings . in particular regard to the various functions performed by the above described components ( assemblies , devices , systems , circuits , and the like ), the terms ( including a reference to a “ means ”) used to describe such components are intended to correspond , unless otherwise indicated , to any component , such as hardware , software , or combinations thereof , which performs the specified function of the described component ( i . e ., that is functionally equivalent ), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure . in addition , although a particular feature of the disclosure may have been illustrated and / or described with respect to only one of several implementations , such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application . also , to the extent that the terms “ including ”, “ includes ”, “ having ”, “ has ”, “ with ”, or variants thereof are used in the detailed description and / or in the claims , such terms are intended to be inclusive in a manner similar to the term “ comprising ”.