Had these gages been formed with a plain cylindrical end projecting in front of the screw, the first two threads would have been prevented from increasing more than the rest. The gages would then have been fairly easily corrected by lapping after tempering at 220 deg.C. Practically no lapping would be required if they were tempered at 340 deg.C. There seems to be no advantage in going to a higher temperature than this. The same degree of hardness could have been obtained with considerably less distortion by quenching directly in fused salt. It is interesting to note that when the swelling after water quenching does not exceed 0.0012 in., practically 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) ----------------------------------------------------------