HEAT TREATING HIGH-SPEED STEELS Machinery's Handbook, 31st Edition
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Heat Treating High-Speed Steels Cobaltcrom Steel.— A tungstenless alloy steel or high-speed steel that contains approxi mately 1.5 percent carbon, 12.5 percent chromium, and 3.5 percent cobalt. Tools such as dies and milling cutters made from cobaltcrom steel can be cast to shape in suitable molds, the cutter teeth being formed so that it is necessary only to grind them. Before the blanks can be machined, they must be annealed; this operation is performed by pack annealing at the temperature of 1800°F (982°C), for a period of from 3 to 6 hours, according to the size of the castings being annealed. The following directions are given for the hardening of blanking and trimming dies, milling cutters, and similar tools made from cobaltcrom steel: Heat slowly in a hardening furnace to about 1830°F (999°C), and hold at this temperature until the tools are thoroughly soaked. Reduce the temperature about 50°F (28°C), withdraw the tools from the furnace, and allow them to cool in the atmosphere. As soon as the red color disappears from the cooling tool, place it in quenching oil until cold. The slight drop of 50°F (28°C) in temperature while the tool is still in the hardening furnace is highly important to obtain proper results. The steel will be injured if the tool is heated above 1860°F (1016°C). In cooling milling cutters or other rotary tools, it is suggested that they be suspended on a wire to ensure a uniform rate of cooling. Tools that are to be subjected to shocks or vibration, such as pneumatic rivet sets, shear blades, etc., should be heated slowly to 1650°F (899°C), after which the temperature should be reduced to about 1610°F (877°C), at which point the tool should be removed from the furnace and permitted to cool in the atmosphere. No appreciable scaling occurs in the hardening of cobaltcrom steel tools. Preheating Tungsten High-Speed Steel.— Tungsten high-speed steel must be hardened at a very high temperature; consequently, tools made from such steel are seldom hardened without at least one preheating stage to avoid internal strain. This requirement applies especially to milling cutters, taps, and other tools having thin teeth and thick bodies and to forming tools of irregular shape and section. The tools should be heated slowly and carefully to a temperature somewhat below the critical point of the steel, usually in the range of 1500 to 1600°F (816 to 871°C). Limiting the preheating temperature prevents the operation from being unduly sensitive, and the tool may be safely left in the furnace until it reaches a uniform temperature throughout its length and cross section. A single stage of preheating is customary for tools of simple form that are not more than from 1 to 1 1 ⁄ 2 inches (25 to 38 mm) in thickness. For large, intricate tools, two stages of preheating are frequently used. The first brings the tool up to a temperature of about 1100 to 1200°F (593 to 649°C), and the second raises its temperature to 1550 to 1600°F (843 to 649°C). A preheating time of 5 minutes for each 1 ⁄ 4 inch (6.4 mm) in tool thickness has been recommended for a furnace temperature of 1600°F (871°C). This is where a single stage of preheating is used and the furnace capacity should be sufficient to maintain practically constant temperature when the tools are changed. To prevent undue chilling, it is common practice to insert a single tool or a small lot in the hardening furnace whenever a tool or lot is removed rather than to insert a full charge of cold metal at one time. Preheating is usually done in a simple type of oven furnace heated by gas, electricity, or oil. Atmospheric control is seldom used, although for 18-4-1 steel a slightly reducing atmosphere (2 to 6 percent carbon monoxide) has been found to produce the least amount of scale and will result in a better surface after final hardening. Hardening of Tungsten High-Speed Steel.— All tungsten high-speed steels must be heated to a temperature close to their fusion point to develop their maximum efficiency as metal-cutting tools. Hardening temperatures ranging from 2200 to 2500°F (1204 to 1371°C) may be needed. The effects of changes in the hardening temperature on the cut ting efficiency of several of the more common high-speed steels are shown in Table 1. The figures given are ratios, the value 1.00 for each steel being assigned to the highest observed cutting speed for that steel. The figures for different steels, therefore, cannot
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