Best  Practice  for  Design  and  Manufacturing  of  Heat  Treating  Inductors  

C.  Yakey,  V.  Nemkov,  R.  Goldstein,  J.  Jackowski  

Fluxtrol  Inc.,  1388  Atlantic  Blvd.,  Auburn  Hills,  Michigan  48326,  USA    

Abstract  

With  the  use  of  good  design  practices,  one  can  improve  coil  longevity  and  improve  production  quality.    By  eliminating  failure  points  in  the  initial  design,  proper  material  selection,  improved  cooling  and  proper  magnetic  flux  control,  induction  tooling  life  can  be  increased.    Computer  simulation  has  been  proven  to  be  an  effective  tool  for  predicting  not  only  electromagnetic  parameters  of  a  designed  system,  but  also  heat  patterns  in  a  given  part  and  in  the  induction  coil  itself.    When  a  coil  has  magnetic  flux  controllers  present,  their  influence  may  also  be  predicted  by  computer  simulation.    With  an  extensive  library  of  published  case  studies  in  induction  coil  design  and  performance  evaluations,  we  are  confident  with  the  use  of  these  tools  and  proper  coil  geometries  and  implementation,  production  life  and  quality  can  be  improved  on  most  induction  heat  treating  inductors.    These  design  practices  have  been  used  by  the  authors  for  over  20  years  with  proven  results.    A  case  is  examined  of  a  CVJ  stem  hardening  coil,  in  which  the  principles  discussed  can  be  applied  to  most  other  hardening  coils.  

Introduction  

The  quality  of  an  induction  coil  is  a  major  determinant  of  the  cost  to  produce  induction  heat  treated  components.    Oftentimes,  the  difference  between  a  well  designed  and  manufactured  inductor  and  a  poor  performing  inductor  is  not  readily  apparent.    However,  a  high  quality  induction  coil  can  lead  to  substantially  lower  component  manufacturing  costs  and  higher  profitability  for  the  induction  heat  treater.    The  additional  costs  of  a  poorly  designed  inductor  include:  

• Higher  per  component  tooling  costs  • Increased  time  for  change-­‐over  • Higher  energy  cost  per  component  • Increased  cycle  time  • Increased  unplanned  downtime  • More  frequent  part  inspection    • More  frequent  scrap  

There  have  been  many  papers  published  that  describe  the  source  of  induction  coil  failures,  good  design  practices  and  methods  for  increasing  coil  lifetime  [1-­‐6]  and  the  authors  encourage  induction  heat  treaters  to  read  these  along  with  many  other  articles  that  exist  on  this  topic.    Some  of  the  sources  of  induction  coil  failures  include:  

• Mechanical  impact  between  the  coil  and  part  • Arcing  between  different  areas  of  the  induction  coil  &  part  • Coil  component  cracking/melting/burning  due  to  overheating  • Coil  component  cracking/breaking/falling  off/deformation  due  to  mechanical  vibration  

With  the  proper  design,  coil  manufacturing  techniques  and  of  course,  preventative  maintenance,  inductor  lifetime  can  be  quite  long  and  per  component  tooling  costs  negligible  (even  if  the  coil  is  much  

more  expensive  than  a  poorly  built  one).    A  Constant  Velocity  Joint  (CVJ)  stem  hardening  coil  is  examined  to  demonstrate  how  subtle  changes  can  be  made  to  inductor  designs  and/or  manufacturing  techniques  that  result  in  substantial  improvement  to  coil  lifetime  and  a  dramatic  reduction  in  the  cost  to  produce  induction  heat  treated  components  for  the  part  supplier.    The  principles  discussed  can  be  applied  to  many  other  types  of  hardening  coils.  

CVJ  Stem  Coil  Design  Discussion  

Automotive  CVJ’s  are  a  component  that  is  frequently  induction  hardened  in  a  captive  heat  treatment  environment.    The  volumes  tend  to  be  quite  high  for  these  components,  so  the  induction  heating  power  densities  are  high  to  limit  the  cycle  time.    Due  to  this,  lifetimes  for  some  of  these  inductors  can  be  low  if  they  are  not  well  designed.      

For  CVJ’s,  an  induction  hardened  layer  is  required  in  the  stem  fillet,  shaft  and  spline.    The  most  common  induction  coil  style  used  for  hardening  this  type  of  component  is  a  single  shot  with  quench  in  place.  Magnetic  flux  controllers  are  usually  applied  to  critical  areas  to  increase  heat  concentration  to  meet  the  pattern  requirements  (Figure  1).  

 

                                                                             Figure  1.   Typical  CVJ  heat  treating  pattern  (left)  and  single  shot  coil  (right)  

The  most  common  cause  of  failure  for  this  type  of  induction  coil  is  overheating  of  a  section  on  the  lower  loop  that  drives  heat  into  the  fillet.  This  is  especially  true  when  frequency  is  low  (below  15  kHz)  and  the  fillet  radius  is  small  (less  than  4  mm).    The  overheating  typically  occurs  in  the  copper  nose  under  the  magnetic  flux  controller,  or  in  the  magnetic  flux  controller  just  on  the  nose  adjacent  to  the  shaft.    The  coil  will  typically  fail  by  one  of  the  following  methods:  

• Copper  cracking  in  the  overheated  area  due  to  thermal  fatigue,  which  results  in  water  spraying  on  the  component  or  an  arc  to  the  part  

• Melting  of  the  lower  loop  due  to  the  formation  of  a  vapor  barrier  between  cooling  water  and  the  copper  

• Loss  of  pattern  depth  in  the  fillet  due  to  concentrator  overheating  and  property  degradation.          

The  root  cause  of  all  of  these  failures  (even  the  concentrator  overheating)  is  high  temperature  of  the  copper  in  the  nose  of  the  inductor.      

Christopher J. Yakey� 7/1/2015 9:29 AMDeleted:  

For  this  type  of  application  (and  other  similar  ones),  there  are  some  basic  tips  and  pointers  to  help  the  life  of  this  specific  lower  loop.    We  will  start  with  something  as  basic  as  copper  selection  for  the  lower  loop.    Oxygen  free  copper,  where  available,  is  a  better  alloy  to  use  for  “loops”  on  inductors.    Oxygen  free  copper  has  better  thermal  and  electrical  conductivity  characteristics  than  stock  101  copper,  and  for  the  modest  price  increase  it  has  been  proven  to  be  worth  it.    We  can  also  make  some  changes  to  the  physical  design  of  the  inductor  to  increase  life  cycles.    Whenever  possible,  avoid  using  a  square  corner  in  the  bore  and  adding  a  radius  or  chamfer  to  reduce  the  current  load  on  that  edge.    This  will  prevent  failure  of  this  edge,  and  when  properly  designed  into  the  overall  inductor  will  not  affect  the  pattern.    In  most  cases  it  will  allow  for  a  closer  coupling  which  can  lead  to  better  fillet  patterns.    The  water  pocket  design  is  also  critical,  oftentimes  an  acute  angle  pocket  can  be  done  with  a  ball  nose  style  cutting  tool  to  match  the  outer  face.  This  can  make  for  a  better  flowing  pocket  and  increase  the  surface  area  for  heat  transfer  between  the  coil  copper  and  cooling  water  and  reduce  places  for  buildup  of  scale  and  impurities.  Figure  2  shows  a  comparison  of  the  two  different  lower  loop  designs  that  would  produce  nearly  identical  patterns  using  FEA  analysis  with  Flux2D.    The  results  show  the  difference  in  the  temperature  of  the  inductor  nose  is  approximately  25%  lower  if  only  these  minor  changes  are  made.      

 

Figure  2.                    Thermal  analysis  of  the  influences  concentrator  and  pocket  a)-­‐Typical,  but  improper  cooling  path  and  concentrator  design.  (b)-­‐Preferred  cooling  path  with  unchanged  concentrator.    (c)-­‐Preferred  cooling  path  and  concentrator  design.  

   In  practice,  the  authors  have  seen  many  instances  of  the  use  of  water  pocket  or  braze  seams  on  the  outside  diameter  of  the  head  (Figure  3).    These  “odd”  seams  or  braze  lines  can  cause  greatly  reduced  cooling  in  the  coil  head.    With  proper  placement  of  the  pocket  and  cover,  these  issues  can  be  eliminated  completely.        This  leads  to  brazing  of  the  covers  to  the  loop.    By  using  copper  filler  or  copper  brazing  rod  the  loop  temperatures  during  assembly  can  far  exceed  the  temperatures  needed  for  silver  brazing.    This  can  cause  the  copper  to  become  annealed  and  soft,  which  when  put  into  production  only  promotes  a  faster  failure  rate.    It  is  suggested  to  use  a  silver  (15%)  solder  to  braze  in  the  cover.    This  will  keep  the  loop  temperatures  down  during  construction.    When  the  coil  brazing  is  complete,  it  is  suggested  after  final  assembly  to  anneal  the  inductor  as  a  whole  to  relieve  work  stresses  built  up  during  sandblasting  and  calibration.    These  are  basic  steps  or  building  blocks  suggested  when  laying  out  the  copper  heads  of  a  single  shot  stem  coil.  

 

Figure  3.                Example  of  overheated  inductor  loop  due  to  poor  cooling  or  improper  concentrator  design  (encircled  in  red).  The  cross-­‐section  shown  is  similar  to  the  modeled  cross-­‐section.  

There  are  some  basic  steps  that  should  be  followed  when  designing  the  magnetic  flux  concentrator  as  well.    In  general  we  should  avoid  using  sharp  points  or  knife  edge  corners  in  any  concentrator  design.    This  can  be  avoided  by  proper  use  of  a  chamfer  or  radius.    This  can  even  work  to  your  advantage  as  a  place  for  epoxy/glue  build  up  for  proper  adhesion  to  the  copper.    Splitting  the  concentrator  into  “pie  wedge”  shapes  around  the  lower  loop  (Figure  4)  can  help  extend  the  coil  life  for  two  reasons.    First,  the  concentrator  material  will  hold  up  longer  without  cracking  from  the  flexing  and  vibration  of  the  coil  while  running.  Secondly,  a  space  between  concentrator  pieces  will  alleviate  the  heavy  loading  on  the  heat  face  of  the  loop  to  help  lower  the  overall  temperature.    There  has  been  evidence  of  this  already  in  use  by  customers  of  Fluxtrol.      

 

Figure  4.                Examples  of  improper  (a)  and  proper  (b)  magnetic  flux  controller  design  and  application  

The  last  major  point  to  address  on  best  coil  design  practices  is  the  water/coolant  flow  thru  the  inductor.    The  use  of  booster  pumps  in  problematic  situations  is  encouraged.  Yet  another  option  is  to  add  supply  lines  (two)  to  the  lower  loop,  as  shown  in  Figure  5.    By  adding  two  inlets  (preferable)  or  outlets  to  the  lower  loop  you  can  create  two  separate  water/coolant  circuits  to  the  inductor,  therefore  making  a  shorter  path  for  the  water  to  flow  and  help  remove  the  heat  faster.    Check  to  make  sure  that  the  water/coolant  is  being  fed  to  the  coil  before  going  to  other  components  of  the  system  and  sufficient  water/coolant  is  reaching  the  inductor.  

 

 

a   b  

 Figure  5.                Examples  of  improper  (a)  and  proper  (b)  water  circuit  design  for  increasing  flow  through  a  very  heavily  loaded  inductor  with  a  booster  pump  

Conclusions  

With  the  use  of  good  design  and  manufacturing  practices,  one  can  improve  coil  longevity  and  improve  production  quality.    An  automotive  CVJ  stem  hardening  coil  was  selected  as  an  example  of  an  inductor  that  in  some  instances  can  have  a  short  lifetime.    The  causes  of  failure  in  this  type  of  inductor  are  typically  related  to  overheating  of  the  nose  on  the  bottom  loop  due  to  high  power  density.    Design  guidelines  for  bottom  loop  geometry,  water  pocket  design,  materials,  magnetic  flux  controller  geometry  and  assembly  techniques  were  given.    The  best  practices  were  explained  and  illustrations  given.    The  authors  have  used  these  techniques  in  practice  to  increase  coil  lifetimes  from  thousands  of  pieces  to  hundreds  of  thousands  of  pieces  in  this  type  of  application.  

References

[1] R.C. Goldstein, W.I. Stuehr, and M. Black, Design and Fabrication of Inductors for Induction Heat Treating, ASM Handbook Volume 4C, pages 588-606, ASM International, 2014.

[2] W.I. Stuehr and D. Lynch, How to Improve Inductor Life, 23rd ASM Heat Treating Society Conference, September 25-28, 2005, Pittsburg, PA, USA.

[3] V.I. Rudnev, Systematic Analysis of Induction Coil Failure, Part 1-11, Heat Treating Progress Magazine, August 2005 – September/October 2007.

[4] R.C. Goldstein and V.S. Nemkov, Influence of Cooling Conditions on Induction Coil Temperatures, International Symposium on Heating By Internal Sources, 2007, Padua, Italy.

[5] H. Svendsen, and S.T. Hagen, Thermo-mechanical Fatigue Life Estimation of Induction Coils, International Scientific Colloquium on Modeling of Electromagnetic Processing, October 27-29, 2008, Hannover, Germany.

[6] K. Kreter, et. Al. Enhancing Induction Coil Reliability, Journal of Materials Engineering and Performance, December 2014, Volume 23, Issue 12, Pages 4164 – 4169.

 

a   b