Temper Color Time Chart for Tool Steel
Minutes at a constant temperature before a polished piece of steel reaches straw, brown, purple, dark blue and pale blue, from tests published in 1910. A reference summary of published practice and not an Aobo Steel specification.
What the figures measure
This is a time table rather than a temperature table. Similar pieces of steel were held in air, one fixed temperature for each run, and the minutes recorded are those needed to bring the surface to the full colour named at the top of the column. A temper colour is an oxide film on polished steel, and the film keeps thickening while the piece stays hot, so the colour moves on even when the temperature never changes.
The runs were made with free access of air. The results are reproduced in The Heat Treatment of Tool Steel, written by Harry Brearley in 1910 for tool and die shops, on the same page that records the discovery behind the temper colour habit, that the colour comes from surface oxidation and not from the temperature alone.
Minutes to reach each temper color
Minutes at one temperature, polished steel heated in air
| Temperature | Straw | Brown | Purple | Dark blue | Pale blue |
|---|---|---|---|---|---|
| 200° C | 6 | 49 | – | – | – |
| 220° C | 3 | 33 | 63 | – | – |
| 250° C | 1 | 10 | 39 | – | – |
| 275° C | 0.5 | 3 | 11 | 27 | 40 |
Source, the tempering table on The Heat Treatment of Tool Steel (Harry Brearley).
Minutes at one temperature before the full colour appears on a polished piece heated in air. A dash means the source gives no figure for that pair. The half minute at 275 degrees C is printed as a fraction in the source. The pieces tested were similar in size, so times scale with section size and with the furnace atmosphere.
Read the columns first
Every colour arrives sooner as the temperature rises. Straw takes 6 minutes at 200 degrees C and half a minute at 275. Brown takes 49 minutes at 200 and 3 minutes at 275. Purple does not appear at 200 in these runs at all, needs 63 minutes at 220, and arrives in 11 minutes at 275. Dark blue and pale blue are reached only in the hottest run, at 27 and 40 minutes.
The span between the first colour and the last is what matters at the bench. At 275 degrees C a piece goes from straw to pale blue inside 40 minutes. At 220 degrees C it takes an hour to get from straw to purple, and the two hottest colours are never reached. Reading a colour is therefore a check on how long the piece has been hot as much as on how hot it is, and the two cannot be separated by eye.
The same color is not the same temper
A given shade marks a different temperature in a different run. The same page carries a demonstration by Turner that a purple can be produced at 250 degrees C in a few minutes, that the same purple needs about an hour at 220 degrees C, and that it takes roughly twelve hours at 170 degrees C. Three temperatures, one colour.
That is why the source concludes that the tempering effect depends on the temperature and is largely independent of time, while the colour is only an empirical guide under definite conditions. On a tool steel the hardness that matters comes from the tempering temperature held for its prescribed time, not from the shade on the surface. Colour is a field check for a shop that has no pyrometer, and it is a coarse one, since a polished face colours quickly, a scaled or rough one slowly, and a draught across the part shifts the reading.
For work that has to hold a hardness window, use the schedule published for the grade. Our tool steel tempering chart gives hardening and tempering temperatures with the resulting hardness, and the tool steel heat treatment guide covers the full sequence from annealing to the final temper.
Where temper color is still used
The oxide colour method survives on simple carbon and oil hardening grades, where a small part is tempered in an open furnace and the operator judges the finish by eye. O1 tool steel is the usual grade for that, and D2 is the common cold work grade where the same colours are still seen on a polished face after tempering. For anything loaded in service, the temperature against colour table is a better guide than the time table, because it is read against the furnace rather than the clock.
Related reference data
The temperature against colour chart is on our steel heat color chart. For the hardness each tempering temperature produces on a specific grade, see the tempering chart and the carbon tool steel temper chart.
Temper color time chart, printable PDF
Minutes to straw, brown, purple, dark blue and pale blue at 200 to 275 degrees C, in one PDF with our contact details.
Source: The Heat Treatment of Tool Steel (Harry Brearley).