Heating Time to Tempering Temperature for Tool Steel
Tempering temperature is normally quoted as a single figure. The time the tool needs to reach that temperature is not, and on a heavy section in a slow furnace the heat up can take longer than the hold. This page gives the published heating times for tool steel in a still hot air oven, in a circulating oven and in an oil bath, by section shape and section size.
The clock starts after the tool reaches temperature
Published practice is explicit that enough time must be allowed during tempering for the temperature to be distributed uniformly throughout the tool before time at temperature is counted. That requirement is strongest at low tempering temperatures and on tools with large sections, which are the two conditions where the centre of the section lags the surface by the largest margin.
If not enough time is allowed, the result is nonuniform tempering and possible damage to the tool. The surface can be at the nominated temperature and fully tempered while the core is still below it, so a hardness reading taken at the surface will not reveal the problem. The published guidance also rules out two shortcuts that are used in practice. Oxide film colour should not be used as a guide, because temper colours indicate the surface temperature of the tool and not its internal temperature, and grinding cracks in hardened tools may be caused by inadequate tempering.
Hot air oven without circulation
The first table covers a hot air oven without circulation, which is the slowest of the setups in the source. Both tables give minutes per millimetre, with the equivalent in minutes per inch in brackets.
| Temperature, °C | Temperature, °F | Cubes or spheres | Squares or cylinders | Average flats |
|---|---|---|---|---|
| 120 | 250 | 1.2 (30) | 2.2 (55) | 3.2 (80) |
| 150 | 300 | 1.2 (30) | 2.0 (50) | 3.0 (75) |
| 175 | 350 | 1.2 (30) | 2.0 (50) | 2.8 (70) |
| 205 | 400 | 1.0 (25) | 1.8 (45) | 2.6 (65) |
| 260 | 500 | 1.0 (25) | 1.6 (40) | 2.4 (60) |
| 315 | 600 | 1.0 (25) | 1.6 (40) | 2.2 (55) |
| 370 | 700 | 0.8 (20) | 1.4 (35) | 2.0 (50) |
| 425 | 800 | 0.8 (20) | 1.2 (30) | 1.8 (45) |
| 480 | 900 | 0.8 (20) | 1.2 (30) | 1.6 (40) |
Approximate heating time for a tool to reach the furnace temperature, for tools with dark or scaled surfaces. Source, Table 7 of Introduction to Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
Circulating air oven or an oil bath
The second table covers a circulating air oven and an oil bath, which the source gives as one set of figures.
| Temperature, °C | Temperature, °F | Cubes or spheres | Squares or cylinders | Average flats |
|---|---|---|---|---|
| 120 | 250 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 150 | 300 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 175 | 350 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 205 | 400 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 260 | 500 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 315 | 600 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 370 | 700 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 425 | 800 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
| 480 | 900 | 0.6 (15) | 0.8 (20) | 1.2 (30) |
Approximate heating time for a tool to reach the furnace temperature, for tools with dark or scaled surfaces. Source, Table 7 of Introduction to Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
How the numbers are used
The figures apply to the diameter or the thickness of the part, with the furnace maintained at the temperature in the first column. Multiply the figure by the controlling dimension of the part. For a round bar or a cylinder that is the diameter, and for a flat it is the thickness.
A worked example shows the spread. A 50 mm square block brought to 205 °C needs about 90 minutes to reach temperature in a still hot air oven, at 1.8 minutes per millimetre, and about 40 minutes in a circulating oven or an oil bath, at 0.8 minutes per millimetre. At 480 °C the same block needs about 60 minutes in the still oven and about 40 minutes in the bath.
What the shape and the furnace change
Three shapes are tabulated. A cube or a sphere heats fastest, a square or a cylinder sits in the middle, and an average flat is the slowest of the three, because the distance from the surface to the centre is what governs the heat up and a flat carries the largest such distance for its mass. At 120 °C in a still oven the three figures are 1.2, 2.2 and 3.2 minutes per millimetre.
The furnace matters more at low temperature than at high temperature. In a still hot air oven the figures fall as the tempering temperature rises, from 3.2 to 1.6 minutes per millimetre for a flat, while a circulating oven and an oil bath hold the same figures at every temperature in the table. A flat therefore needs about 2.7 times as long in the still oven at 120 °C and about 1.3 times as long at 480 °C.
Bright surfaces and full charges
The times in both tables are for tools with dark or scaled surfaces. If the surface has been finish ground or is otherwise bright, twice as much time should be allowed in a still hot air oven, and no extra allowance is needed in a circulating oven or in an oil bath.
For a charge of irregular shapes or of many pieces, the published guidance is to estimate the total size of the charge and apply the same allowance to the distance from the outside to the centre of the charge rather than to any single piece.
Oil baths are usually not used above 205 °C (400 °F).
Two tempers for high speed steel
High speed steels are tempered more than once, and the practice exists because of retained austenite. A minimum of two tempers is specified to ensure a consistent tempered martensitic structure and to overcome the uncertainty created by variations in the amount of retained austenite left in the as quenched condition. Those variations come from differences in heat chemistry, prior thermal history, hardening temperature and quenching conditions.
Three further points are published with the practice. An increase in the free matrix carbon content increases the amount of retained austenite in the as quenched condition. Retained austenite has a significant effect on the rate of transformation, particularly over short tempering cycles, so multiple tempering matters more when the tempering time is short. Cobalt works the other way, and in grades such as M42 tool steel it reduces the amount of retained austenite in the as quenched condition and accelerates its transformation during tempering.
The practical reading is that a short temper is a risk the tool carries after it leaves the furnace, and the risk is higher in the grades that carry no cobalt. The tempering windows by grade are gathered in the tool steel tempering chart, and the tool steel soak time chart covers the hold at temperature rather than the heat up.
What this means for an order
Tempering is done by the heat treater rather than at the steel mill, so a purchase specification for tool steel normally stops at the delivery condition and the recommended hardening and tempering range for the grade. The time in the furnace is the heat treater’s decision, and the useful thing for a buyer to carry into that conversation is how much section there is to heat.
On a tool with a heavy section, or on a job that is loaded as a full charge, the heat up is the part of the cycle most likely to be cut short. The grades where that shows up first are the cold work grades used in large blocks, such as D2 tool steel, and the hot work grades used in die blocks, such as H13 tool steel, and the same caution applies to any tool that is tempered at the low end of its range.
Before you use these times
This page is a reference summary of published practice and it is not an Aobo Steel specification. The figures are the values published for the specific furnace types and section shapes named in the source, and an actual cycle depends on the furnace, the fixture and the charge. Final parameters are set by the heat treater.
Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.