Temper Color

the iridescent color that appears on the smooth surface of hot steel as a result of the formation of a very thin oxide film. The thickness of the film depends on the reheating temperature; films of different thickness reflect light rays in different ways, which accounts for the different temper colors. Typical temper colors for carbon steel are straw (220°C), brown (240°C), purple (260°C), dark blue (300°C), and light gray (330°–350°C). The same temper colors appear on alloy (especially highalloy) steels at higher temperatures. The colors are also affected by the holding time of the steel at a given temperature, the illumination, and other factors. Before the advent of pyrometers and suitable testing and measuring instruments, the temperature of reheating was judged according to the temper color.