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Tempering Colors On Carbon SteelsCategory: HIGH-SPEED STEEL 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 will make quite a difference and cause confusion. It is these possible sources of error, coupled with the ever present chance of human error, that makes it advisable to draw the temper of tools in an oil bath heated to the proper temperature as shown by an accurate high-temperature thermometer. Another table, by Gilbert and Barker, runs to much higher temperatures. Beyond 2,200 deg., however, the eye is very uncertain. TABLE 26.--COLORS FOR TEMPERING TOOLS ----------------------------------------------------------------------- Approximate color and Kind of tool temperature ---------------------------------------------------------------------- Yellow Thread chasers, hollow mills (solid type) twist drills 430 to 450 deg.F. centering tools, forming tools, cut-off tools, profile cutters, milling cutters, reamers, dies, etc. ---------------------------------------------------------------------- Straw-yellow Thread rolling dies, counterbores, countersinks. Shear 460 deg.F. blades, boring tools, engraving tools, etc. ---------------------------------------------------------------------- Brown-yellow Taps, Thread dies, cutters, reamers, etc. 500 deg.F. ---------------------------------------------------------------------- Light purple Taps, dies, rock drills, knives, punches, gages, etc. 530 deg.F. ---------------------------------------------------------------------- Dark purple Circular saws for metal, augers, dental and surgical 550 deg.F. instruments, cold chisels, axes. ---------------------------------------------------------------------- Pale blue Bone saws, chisels, needles, cutters, etc. 580 deg.F. ---------------------------------------------------------------------- Blue Hack saws, wood saws, springs, etc. 600 deg.F. ----------------------------------------------------------------------- Next: High Speed Steel Previous: The Effect Of Tempering On Water-quenched Gages
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