| 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 |
| Degrees Fahrenheit | High temperatures judged by color | |
|---|---|---|
| 430 | Very pale yellow | Visible in full daylight |
| 460 | Straw-yellow | |
| 480 | Dark yellow | |
| 500 | Brown-yellow | |
| 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°, however, the eye is very uncertain.
| Approximate color and temperature | Kind of tool |
|---|---|
| Yellow 430 to 450°F. | Thread chasers, hollow mills (solid type) twist drills centering tools, forming tools, cut-off tools, profile cutters, milling cutters, reamers, dies, etc. |
| Straw-yellow 460°F. | Thread rolling dies, counterbores, countersinks. Shear blades, boring tools, engraving tools, etc. |
| Brown-yellow 500°F. | Taps, Thread dies, cutters, reamers, etc. |
| Light purple 530°F. | Taps, dies, rock drills, knives, punches, gages, etc. |
| Dark purple 550°F. | Circular saws for metal, augers, dental and surgical instruments, cold chisels, axes. |
| Pale blue 580°F. | Bone saws, chisels, needles, cutters, etc. |
| Blue 600°F. | Hack saws, wood saws, springs, etc. |
CHAPTER X
HIGH-SPEED STEEL
For centuries the secret art of making tool steel was handed down from father to son. The manufacture of tool steel is still an art which, by the aid of science, has lost much of its secrecy; yet tool steel is today made by practical men skilled as melters, hammer-men, and rollers, each knowing his art. These practical men willingly accept guidance from the chemist and metallurgists.
A knowledge of conditions existing today in the manufacture of high-speed steel is essential to steel treaters. It is well for the manufacturer to have steel treaters understand some of his troubles and difficulties, so that they will better comprehend the necessity of certain trade customs and practices, and, realizing the manufacturer's desire to cooperate with them, will reciprocate.
The manufacturer of high-speed steel knows and appreciates the troubles and difficulties that may sometimes arise in the heat-treating of his product. His aim is to make a uniform steel that will best meet the requirements of the average machine shop on general work, and at the same time allow the widest variation in heat treatment to give desired results.