Hardening and Tempering Colors

by Jan-Eric Nyström

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:

Hardening and Tempering Color Chart, by Jan-Eric Nyström. Used with permission.

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