Machinery's Handbook, 31st Edition
496 HEAT TREATING HIGH-SPEED STEELS 1100°F (538 to 593°C) for ordinary high-speed steels and from 1200 to 1300°F (649 to 704°C) for steels of the cobalt type. These furnace temperatures apply to tools of the class used on lathes and planers. Such tools, in service, frequently heat to the point of visible redness. Milling cutters, forming tools, or any other tools for lighter duty may be tem- pered as low as 850 or 900°F (454 to 4821°C). When the tool has reached the temperature of the furnace, it should be held at this temperature for from one to several hours until it has been heated evenly throughout. It should then be allowed to cool gradually in the air and in a place that is dry and free from air drafts. In tempering, the tool should not be quenched because quenching tends to produce strains that may result later in cracks. Annealing Tungsten High-Speed Steel.— The following method of annealing high- speed steel has been used extensively. Use an iron box or pipe of sufficient size to allow at least 1 ⁄ 2 inch (12.7 mm) of packing between the pieces of steel to be annealed and the sides of the box or pipe. It is not necessary that each piece of steel be kept separate from every other piece, but only that the steel be prevented from touching the sides of the annealing pipe or box. Pack carefully with powdered charcoal, fine dry lime, or mica (preferably charcoal), and cover with an airtight cap or lute with fire clay; heat slowly to 1600 to 1650°F (871 to 899°C) and keep at this heat from 2 to 8 hours, depending on the size of the pieces to be annealed. A piece measuring 2 3 1 3 8 inches (5 3 2.5 3 20.3 cm) requires about 3 hours. Cool as slowly as possible, and do not expose to the air until cold because cooling in air is likely to cause partial hardening. A good method is to allow the box or pipe to remain in the furnace until cold. Hardening Molybdenum High-Speed Steels.— Table 3 gives the compositions of sev eral molybdenum high-speed steels that are widely used for general commercial tool applications. The general method of hardening molybdenum high-speed steels resembles that used for 18-4-1 tungsten high-speed steel except that the hardening temperatures are lower and more precautions must be taken to avoid decarburization, especially on tools made from Type I or Type II steels, when the surface is not ground after hardening. Either salt baths or atmosphere-controlled furnaces are recommended for hardening molybde- num high-speed steels. Table 3. Compositions of Molybdenum High-Speed Steels
Molybdenum- Vanadium
Tungsten- Molybdenum
Molybdenum-Tungsten
Type Ia (Percent) 0.70-0.85 1.25-2.00 3.00-5.00 0.90-1.50 8.00-9.50
Type Ib a (Percent) 0.76-0.82 1.60-2.30 3.70-4.20 1.05-1.35 8.00-9.00 4.50-5.50
Type II (Percent) 0.70-0.90 3.00-5.00 1.50-2.25 7.50-9.50 …
Type III (Percent) 0.75-0.90 5.00-6.00 3.50-5.00 1.25-1.75 3.50-5.50
Element
Carbon Tungsten Chromium Vanadium Molybdenum
Cobalt See footnote a Cobalt may be used in any of these steels in varying amounts up to 9 percent, and the vanadium content may be as high as 2.25 percent. When cobalt is used in Type III steel, the vanadium content may be as high as 2.25 percent. When cobalt is used in Type III steel, this steel becomes susceptible to decarburization. As an illustration of the use of cobalt, Type Ib steel is included. This is steel T10 in the US Navy Specification 46S37, dated November 1, 1939. See footnote See footnote The usual method is to preheat uniformly in a separate furnace to 1250 to 1550°F (677 to 843°C), then transfer to a high-heat furnace maintained within the hardening temperature range given in Table 4. Single-point cutting tools, in general, should be hardened at the upper end of the temperature range indicated by Table 4. Slight grain coarsening is not objectionable in such tools when they are properly supported in service and are not sub jected to chattering; however, when these tools are used for intermittent cuts, it is better
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