Automated Heat Treating of Tool Steels
Production heat treating of tool steel is done on automated salt bath lines rather than one tool at a time. The line carries the parts through preheat, high heat and quench on a fixed cycle, and the cycle time is set by the workpiece rather than by the operator. This page gives the stage sequence with its operating temperatures, and the published cycle times for twist drills and other tools.
How an automated tool steel line is arranged
The published description of a production tool steel line has the parts suspended on tong-type fixtures and carried through the process by a chain conveyor on carrier bars. Rotary transfer arms sit between the preheat and the high heat units and between the high heat and the quench units. Transfer arm placement is governed mainly by the production rate, but an arm is always required between the high heat and the quench, because the metallurgical part of the cycle depends on the transfer being quick.
The lines also have clear areas above the furnaces so that the tools can be air cooled in place. They are equipped for cycles ranging from less than one minute to ten minutes. Where the volume justifies it, a station for an isothermal nitrate quench is added after the neutral salt quench. That extra stage pulls the temperature of the tools down quickly and cuts the air cooling time from 24 times the time at the high-heat temperature to 6 times.
Process stages and operating temperatures
The stages run in a fixed order from the first preheat to the rinse. Each has its own working temperature range, and the range widens or narrows with the grade and the section being treated. The time column is written as a multiple of the time at the high-heat temperature rather than in minutes, because that base time depends on the workpiece and is set by the second table below.
| Process stage | Operating temperature, °C | Operating temperature, °F | Total time in furnace |
|---|---|---|---|
| First preheat | 650-870 | 1200-1600 | X |
| Second preheat | 760-1040 | 1400-1900 | X |
| High heat | 1010-1290 | 1850-2350 | X |
| Isothermal quench | 540-705 | 1000-1300 | X |
| Air cool | Room temperature | Room temperature | 6X, 12X or 24X |
| Wash, hot water | 80-95 | 180-200 | 6X |
| Rinse, hot water | 80-95 | 180-200 | X |
Relative process times and temperatures for an automated tool steel heat treating line. The time column is expressed as a multiple of the time at the high-heat temperature, which is set by the part and listed in the table below, so an X is one multiple of that time. Source, Table 2 of Automatic Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
Cycle times for twist drills and other tools
The published times for twist drills are given to the second and banded by diameter, so a shop can read the high-heat time straight off the diameter. Cups and end mills are treated as single workpieces at fixed times. These are the times at the high-heat temperature, and they are the X in the stage table above.
| Workpiece | Time in high heat |
|---|---|
| 2.54-4.78 mm (0.100-0.188 in.) twist drills | 1 min 30 s |
| 4.80-8.08 mm (0.189-0.318 in.) twist drills | 1 min 40 s |
| 8.10-12.90 mm (0.319-0.508 in.) twist drills | 1 min 50 s |
| 12.93-18.24 mm (0.509-0.718 in.) twist drills | 2 min 0 s |
| 18.26-23.32 mm (0.719-0.918 in.) twist drills | 2 min 20 s |
| 23.34-38.10 mm (0.919-1.500 in.) twist drills | 2 min 40 s |
| 102 mm (4 in.) diameter cups | 6 min |
| 64 mm (2½ in.) diameter end mills | 7 min |
| 76 mm (3 in.) diameter end mills | 10 min |
Cycle times for twist drills and other tools in the high-heat stage. The diameters are the bands used by the source, and the times are the scheduled times at temperature. Source, Table 3 of Automatic Heat Treating of Tool Steels, in ASM Handbook, Volume 4, Heat Treating.
For the smallest diameters the same source records the loading as well. Drills of 2.54 mm (0.100 in.) run at 160 pieces per tong, 480 pieces in the bath, which is 1.2 kg (2.6 lb) of work in a single charge.
Why the nitrate quench is kept away from the high heat furnace
The nitrate quench and the high heat furnace are separate units for a reason that is stated plainly in the source. If as little as 600 ppm of nitrate salts is allowed to enter the high heat furnace, extreme surface damage can be done to the tool being treated. The transfer arrangement and the separate stations exist to keep the two baths apart.
Salt baths also need periodic rectification, because neutral austenitizing salts pick up soluble oxides and dissolved metals in use. Once the bath turns oxidizing and decarburizing toward steel, it has to be rectified, and the higher the operating temperature the more often. Baths run above 1080 °C (1975 °F) call for rectification at least once a day. A file test on a hardened specimen, or an analysis showing more than 0.5% BaO, is the common check that a high-heat bath has drifted.
Which grades run on these lines
The high-heat range in the table, 1010 to 1290 °C (1850 to 2350 °F), is the high speed steel range, and twist drills, end mills, taps, hobs and reamers are the tools that fill those lines. The drills and end mills in the second table are typically made from M2 high speed steel, the general purpose molybdenum grade used for the bulk of cutting tools.
The full cycle for these grades, from annealing through austenitizing, quenching and multiple tempering, is set out in the high speed steel heat treatment detail page, which carries the temperatures and the tempering response that go with the high-heat times listed here.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The stages and times are the published figures for the tools named, and a different line, fixture load or section size will move them. Final cycle parameters are set by the heat treater.
Source: ASM Handbook, Volume 4, Heat Treating, ASM International, 1991.