Machinery's Handbook, 31st Edition
HEAT TREATING HIGH-SPEED STEELS 497 to use the middle of the temperature range. All other cutting tools, such as drills, counter sinks, taps, milling cutters, reamers, broaches, and form tools, should be hardened in the middle of the range shown. For certain tools, such as slender taps, cold punches, and blanking and trimming dies, where greater toughness to resist shocks is required, the lower end of the hardening temperature range should be used. Table 4. Heat Treatment of Molybdenum High-Speed Steels
Molybdenum–Tungsten Types Ia and Ib a Temp.
Molybdenum–Vanadium Tungsten–Molybdenum
Type II Temp.
Type III Temp.
Heat-Treating Operation
Deg. F
Deg. C
Deg. F
Deg. C
Deg. F
Deg. C
Forging
1850–2000 1010–1093 1850–2000 1010–1093 1900–2050 1038–1121
Not below
1600
1600
1600
871
871
871
Annealing Strain relief Preheating Hardening b
1450–1550 1150–1350 1250–1500 2150–2250 a
1450–1550 788–843 1150–1350 621–732 1250–1500 677–816
1450–1550 1150–1350 1250–1550
788–843 621–732 677–816
788–843 621–732 677–843
2150–2250 1177–1232 2175–2275 1191–1246
1177–1232 a
Salt
2150–2225 1177–1218 2150–2225 1177–1218 2150–2250 1177–1232
Tempering
950–1100
950–1100
950–1100
510–593 510–593 a For similar working conditions, Type Ib steel requires slightly higher hardening heat than Type Ia. b The higher side of the hardening range should be used for large sections, and the lower side for small sections. 510–593 Molybdenum high-speed steels can be pack-hardened following the same practice as is used for tungsten high-speed steels but keeping on the lower side of the hardening range (approximately 1850°F or 1010°C). Special surface treatments such as nitriding by immersion in molten cyanide that are used for tungsten high-speed steels are also applica ble to molybdenum high-speed tools. When heated in an open fire or in furnaces without atmosphere control, these steels do not sweat like 18-4-1 steels; consequently, determining the proper time in the high- heat chamber is a matter of experience. This time approximates that used with 18-4-1 steels, although it may be slightly longer when the lower part of the hardening range is used. Much can be learned by preliminary hardening of test pieces and checking on the hardness fracture and structure. It is difficult to give the exact heating time because it is affected by temperature, type of furnace, size and shape, and furnace atmosphere. Rate of heat transfer is most rapid in salt baths and slowest in controlled-atmosphere furnaces with high carbon monoxide content. Quenching and Tempering of Molybdenum High-Speed Tools.— Quenching may be done in oil, air, or molten bath. To reduce the possibility of breakage and undue distortion of intricately shaped tools, it is advisable to quench in a molten bath at approximately 1100°F (593°C). The tool also may be quenched in oil and removed while still red, or at approximately 1100°F (593°C). The tool is then cooled in air to room temperature and tempered immediately to avoid cracking. When straightening is necessary, it should be done after quenching and before cooling to room temperature prior to tempering. To temper, the tools should be reheated slowly and uniformly to 950 to 1100°F (510 to 593°C). For general work, 1050°F (566°C) is most common. The tools should be held at this temperature at least 1 hour. Two hours is a safer minimum, and 4 hours is maximum. The time and temperature depend on the hardness and toughness required. Where tools are subjected to more or less shock, multiple temperings are suggested. Protective Coatings for Molybdenum Steels.— To protect the surface from oxidation during heat treatment, borax may be sprinkled lightly over the steel when the latter is heated in a furnace to a low temperature, 1200 to 1400°F (649 to 760°C). Small tools may be rolled in a box of borax before heating. Another method more suitable for finished tools is to apply the borax or boric acid in the form of a supersaturated water solution. The tools
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online