ctically the whole of it may be
recovered by tempering at a sufficiently high temperature, but when
the swelling exceeds this amount the steel assumes a permanently
strained condition, and at the most only 0.0014 in. can be recovered
by tempering.

TEMPERING COLORS ON CARBON STEELS

Opinions differ as to the temperature which is indicated by the
various colors, or oxides, which appear on steel in tempering.

The figures shown are from five different sources and while the
variations are not great, it is safer to take the average temperature
shown in the last column.

TABLE 25.--COLORS, TEMPERATURES, DEGREES FAHRENHEIT
----------------------------------------------------------
| _A_ | _B_ | _C_ | _D_ | _E_ | Average
------------------|-----|-----|-----|-----|-----|---------
Faint yellow | 430 | 430 | 430 | 430 | 430 | 430
Light straw | 475 | 460 | 450 | ... | 450 | 458
Dark straw | 500 | 500 | 470 | 450 | 470 | 478
Purple (reddish) | 525 | 530 | 520 | 530 | 510 | 523
Purple (bluish) | ... | 555 | 550 | 550 | 550 | 551
Blue | 575 | 585 | 560 | 580 | 560 | 572
Gray blue | ... | 600 | ... | 600 | 610 | 603
Greenish blue | ... | 625 | ... | ... | 630 | 627
----------------------------------------------------------

TABLE 26.--ANOTHER COLOR TABLE
----------------------------------------------------------
Degrees |
Fahrenheit | High temperatures judged by color
------------|---------------------------------------------
430 | Very pale yellow \
460 | Straw-yellow |
480 | Dark yellow |
500 | Brown-yellow > Visible in full daylight
520 | Brown-purple |
540 | Full purple |
560 | Full blue |
600 | Very dark blue /
752 | Red heat, visible in the dark
885 | Red heat, visible in the twilight
975 | Red heat, visible in the daylight
1,292 | Dark red
1,652 | Cherry-red
1,832 | Bright cherry-red
2,012 | Orange-red
2,192 | Orange-yellow
2,372 | Yellow-white
2,552 | White welding heat
2,732 | Brilliant white
2,912 | Dazzling white (bluish-white)
----------------------------------------------------------

These differences might easily be due to the difference in the light
at the time the colors were observed. It must also be remembered
that even a thin coating of oil