Machinery's Handbook, 31st Edition
494 HEAT TREATING HIGH-SPEED STEELS be directly compared with each other, except to note changes in the point of maximum cutting efficiency. Table 1. Relation of Hardening Temperature to Cutting Efficiency Hardening Temperature Typical Analyses of High-Speed Steels Deg. F Deg. C 18 – 4 – 1 14 – 4 – 2 18 – 4 –1 Cobalt 14 – 4 – 2 Cobalt 2200 1204 0.86 0.83 0.84 0.85 2250 1232 0.88 0.88 0.86 0.88 2300 1260 0.90 0.93 0.90 0.91 2350 1288 0.95 0.98 0.94 0.94 2400 1316 0.99 0.98 0.98 0.98 2450 1343 1.00 … 0.99 1.00 2500 1371 0.98 … 1.00 0.97 The figures in the table refer to tools heated in an oven-type furnace in which a neutral atmosphere is maintained. The available data indicate that a steel reaches its best cutting qualities at a temperature approximately 50°F (28°C) lower than the figures in the table if it is hardened in a bath-type furnace. It is, however, desirable to use a hardening tempera ture approximately 50°F (28°C) lower than that giving maximum cutting qualities to avoid the possibility of overheating the tool. Length of Time for Heating: The cutting efficiency of a tool is affected by the time that it is kept at the hardening temperature, almost as much as by the hardening temperature itself. It has been common practice to heat a tool for hardening until a “sweat” appears on its surface. This sweat is presumably a melting of the oxide film on the surface of a tool heated in an oxidizing atmosphere. It does not appear when the tool is heated in an inert atmosphere. This method of determining the proper heating time is at best an approxima tion and indicates only the temperature on the outside of the tool rather than the condition of the interior. As such, it cannot be relied upon to give consistent results. The only safe method is to heat the tool for a definite predetermined time, based on the size and the thickness of metal that the heat must penetrate to reach the interior. The values given in Table 2 are based on a series of experiments to determine the relative cutting effi ciency of a group of tools hardened in an identical manner except for variations in the time the tools were kept at the hardening temperature. The time given is based on that required to harden throughout a tool resting on a conducting hearth; the tool receives heat freely from three sides, on its large top surface and its smaller side surfaces. (The table does not apply to a disk lying flat on the hearth.) For a tool having a projecting cutting edge, such as a tap, the thickness or depth of the projecting portion on which the cutting edge is formed should be used when referring to the table. Table 2. Length of Heating Time for Through Hardening High-Speed Steel Tool Thickness Time in
High-Speed Steel Tool Thickness Inch Centimeter
Time in Furnace at High Heat, Minutes
High-Speed Steel Tool Thickness Inch Centimeter
Time in Furnace at High Heat, Minutes
Furnace at High Heat, Minutes
Inch Centimeter
2 3 4 5
7 8
5 6 8
18 20 25
1
1 ⁄ 1 ⁄ 3 ⁄
1 ⁄
0.635 1.27 1.905 2.54
3.81 5.08 7.62
12.7 15.24 20.32 25.4
4 2 4
2
2 3 4
12 15
1 30 The time periods given in Table 2 are based on complete penetration of the hardening effect. For very thick tools, the practical procedure is to harden to a depth sufficient to pro duce an adequate cutting edge, leaving the interior of the tool relatively soft. 10.16 10
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