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Steel coils of surface color
The table below shows the temperatures and the associated colours required when tempering steel for particular uses.The tempering colors can be used to judge the final properties,tool steel is often tempered in the light to dark straw range, whereas spring steel is ofter tempered t the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. The oxide film will also increase in the thickness over time. Therefore, steel that has been held at 400。F for a very long time may turn brown or purple, even though the temperature never exceeded that needed to produce a light straw color. Other factors affecting the final outcome are oil film on the surface and the type of heat source used.
Steel coils of surface color 
steel coil of surface color

tempering color temperature( ℃) typical use
pale yellow 230 brass
dark yellow 240 mild steel
brown 250 wood tools
brown/purple 260 wood working tools
purple 270 Axes
duck purple 280 cold forging tools
blue 300 spring
particular usage of tempered steel strip
tempered color required Tem.(℃) finished surface
red 500-900 bull red---deep red---light red
Orange 950-1000 orange
yellow 1000-1100 yellow
yellow/white 1100-above yellow/white