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:



Temperature conversion for the colors listed above:


 * 600°C = 1112°F
 * 700°C = 1292°F
 * 800°C = 1472°F
 * 900°C = 1652°F
 * 1000°C = 1832°F
 * 1100°C = 2012°F

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