Carbon  Content  and  Critical  Range. — With  special  refer-
ence to  the  colour  method  of  detection  an  approximate  set
pf  temperatures  covering  the  upper  critical  point  (Ac  3)


ELEMENTARY  THERMAL  PROCESSES


93


of  various  carbon  steels  used  in  constructional  work  and  the
production  of  parts  will  at  this  stage  prove  useful ;  but
while  a  scale  of  this  kind  may  form  a  guide,  the  difference
in  the  appearance  of  red-hot  steel  at  1,300°  F.  from  that
at  1,500°  E.  is  scarcely  sufficiently  marked  to  enable  any
but  an  experienced  hardener  to  pick  out  any  temperatures
between  these  two  points.  LTntil  the  eye  becomes  familiar
with  the  appearance  of  heated  steel  some  kind  of  instru-
mental aid  should  be  at  hand  ;  and  for  constancy  and  a
fair  degree  of  accuracy  the  optical  instruments  furnish
a  degree  scale  that  probably  cannot  be  surpassed.
TABLE  OF  COLOUR  TEMPERATURES.
Carbon,  %.
Degrees  F.,          Appearance  of  the
Ac  3.                          Steel.
Degrees  F.,
Ar3.
Degrees  F.,
Tempering.
0-25

1,500

Very  bright  red

1,360

1,300

0-28

1,500

Very  bright  red

1,360

1,300

0-30

1,425       !  Bright  red

1,285

1,235

0-35

1,425       !  Bright  red

1,285

1,235

0-40

1,400          Red

1,260

1,200

0-50

1,375          Duller  red

1,235

1,175

0-60

1,340       •  Blood  red

1,200

1,140

0-70

1,340       !  Duller  red

1,200

1,140

0-80

1,330

Duller  red

1,190

1,130

Speed  of  Cooling  Determines  Hardness. — Hardness  is  a
relative  term.  As  applied  to  a  low  carbon  steel  it  might
more  appropriately  be  termed  stiffness  or  toughness.
