This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
When steel and iron are heated to a high temperature they first become red, then orange, and then white. The temperatures may be approximately told by the color as follows:
First sign of red | 957° F. |
Dull red | 1290° |
Cherry red | 1655° |
Bright red | 1870° |
Dark cherry | 1470° |
Dull orange | 2010° F. | |
Bright orange | 2190° | |
White heat | 2370° | |
Bright white | 2550° | |
Dazzling white | 2730° to 2910° |
The following table represents the colors which appear on a polished steel surface when heated in the air.
Color | Approximate Temperature | Used in Tempering |
Very pale yellow | 430° F. | |
Straw yellow | 460° | Taps and dies |
Dark straw color | 480° | Reamers, knives, punches, and dies |
Brownish yellow | 500° | Drills, stone-cutting tools |
Yellow tinged with purple | 520° | Angles, draft pins |
Light purple | 530° | Cold chisels for steel, augers |
Dark purple | 550° | Springs, screw - drivers, cold chisels for iron, hatches |
Dark blue | 570° | Saws for wood |
Pale blue | 610° | |
Blue tinged with green | 630° |
 
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