The  term  "heat  treatment "  has  a  significance  far  too  wide
to  carry  in  itself  any  clear  notion  of  the  several  processes
to  which  the  material  must  be  subjected  to  arrive  at  the
greatest  physical  strength  of  the  steel.  The  procedure  may
conveniently  be  divided  into  the  following  sections  :—

Normalizing. — Here  we  have  uniform  heating  of  the
steel  to  a  temperature  exceeding  its  upper  critical  point
(Ac  3)  by  at  least  50  deg.,  followed  by  cooling  in  the
air  only.  The  excess  of  temperature  beyond  the  critical
range  may  vary  between  50  and  100  deg.  without  appreci-
ably influencing  the  result.
Softening  or  Annealing. — A  steel  that  has  already  under-
gone normalizing  should  be  still  further  improved  by
annealing,  the  main  objects  of  which  are  the  entire  elimina-
tion of  internal  strains  and  the  removal  of  hard  areas  with
a  view  to  subsequent  cutting,  turning,  and  drilling.
Hardening  or  Stiffening,  with  Toughening. — This  kind  of
hardening  is  not  identical  with  the  processes  of  the  tool
maker,  but  may  be  regarded  as  a  modification  of  them  upon

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ELEMENTARY   THERMAL   PROCESSES          91

a  different  material.  It  consists  of  uniform  heating  to
about  100  deg.  above  the  critical  point  (Ac  3),  to  be  immedi-
ately followed  by  quenching  in  (1)  water  at  from  80  to
100  deg.,  or  (2)  in  oil,  from  60  to  80  deg.  ;  a  light-bodied
oil  may  be  used  at  80  deg.,  or  a  heavier  oil  at  50  to  60  deg.
Tempering. — When  the  steel  is  reheated  after  stiffening
or  hardening  the  degree  of  heat  to  which  it  is  subjected
should  be  just  above  the  critical  range  of  the  particular
material  ;  that  is,  the  Ac  1  point  must  not  be  passed  if
the  maximum  hardness  or  toughness  is  to  be  retained.
