Hardening and Tempering Colors
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Just for those who have asked for it, here's a quick picture of the colors to look for when heating iron to harden & temper it:
I shot a piece of bright iron heated from one end to show the tempering colors. My source (a book from 1943) gives these temperatures for certain purposes:
- 230°C (446°F), light straw/yellow: Very hard cutting tools, engraving tools. Brittle.
- 260°C (500°F), dark yellow to brownish red: Lathe tools
- 270°C (518°F), reddish brown: drills, screwcutting taps
- 290°C (554°F), blue: springs, tools for stone and wood