Figs.  68  and  69  exhibit  clearly  the  result  of  quenching
direct  from  the  hot  box,  and  the  improved  .structure
plbtained  by  cooling  out,  reheating  and  quenching,


170  STEEL   THERMAL  TREATMENT

CASE  CARBURIZING.
The  term  "  case  hardening  "  in  general  use  is  often  loosely
applied.  A  great  deal  of  carburizing  is  done  independently
of  hardening,  upon  articles  never  hardened.  This  is  case
carburizing  simply.  If  the  article  is  taken  red-hot  from
the  carburizing  box  and  plunged  into  water,  it  is  both
case  carburized  and  case  hardened.
The  carburizing  process,  which  forms  the  basis  of  all
case  hardening,  has  for  its  object  the  enrichment  of  the
outer  zone  of  the  article  by  the  addition  of  a  certain  per-
centage of  pure  carbon  in  a  form  that  will  become,  if
required,  glass -hard  when  suddenly  cooled  from  the  red-hot
condition,  as  when  plunged  into  cold  water  or  oil.  But,  as
already  mentioned,  the  process  need  not  be  carried  so  far.
A  large  proportion  of  articles  treated  are  simply  carburized,
and  not  hardened.  This  is  an  effective  way  of  improving
the  toughness  and  strength  of  many  parts  of  machinery
that  for  different  reasons  it  is  not  desired  to  harden.  Not
the  least  of  these  reasons  has  reference  to  the  fact  that,  in
the  act  of  being  suddenly  cooled  for  hardening,  there  is
always  a  tendency  to  produce  deformation,  or  slight  distor-
tions that  might  have  the  effect  of  destroying  the  usefulness
of  a  part  when  assigned  its  place  in  a  machine.
