Machinery's Handbook, 31st Edition
498 HEAT TREATING HIGH-SPEED STEELS are then immersed in the solution at 180 to 212°F (82 to 100°C), or the solution may be applied with a brush or spray. Pieces so treated are heated as usual and carefully handled to ensure good adherence of the coating. When properly applied, special protective coat - ings or paints have been found to be extremely useful. These materials do not fuse or run at the temperatures used and therefore do not affect the furnace hearth. When applying these coatings, it is necessary to have a surface free from scale or grease to ensure good adherence. Coatings may be sprayed or brushed on, and usually one thin coat is sufficient. Heavy coats tend to pit the surface of the tool and are difficult to remove. Tools covered with these coatings should be allowed to dry before they are charged into the preheat fur nace. After hardening and tempering, the coating can be easily removed by light blasting with sand or steel shot. When tools are lightly ground, these coatings come off immedi ately. Protection may also be obtained by wrapping pieces in stainless steel foil. Nitriding High-Speed Steel Tools.— Nitriding is applied to high-speed steel for the pur pose of increasing tool life by producing a very hard skin or case, the thickness of which ordinarily is from 0.001 to 0.002 inch (0.025 to 0.05 mm). Nitriding is done after the tool has been fully heat treated and finish-ground. (The process differs entirely from that which is applied to surface harden certain alloy steels by heating in an atmosphere of ni- trogen or ammonia gas.) The temperature of the high-speed steel nitriding bath, which is a mixture of sodium and potassium cyanides, is equal to or slightly lower than the temper- ing temperature. For ordinary tools, this temperature usually varies from about 1025 to 1050°F (552 to 566°C); but, if the tools are exceptionally fragile, the range may be reduced to 950 or 1000°F (510 to 538°C). Accurate temperature control is essential to prevent exceeding the final tempering temperature. The nitriding time may vary from 10 or 15 minutes to 30 minutes or longer, and should be determined by experiment. The shorter periods are applied to tools for iron or steel, or any shock-resisting tools, and the longer periods are for tools used in machining nonferrous metals and plastics. This nitriding process is applied to tools, such as hobs, reamers, taps, box tools, form tools, and milling cutters. Nitriding may increase tool life 50 to 200 percent, or more, but it should always be preceded by correct heat treatment. Nitriding Bath Mixtures and Temperatures: A mixture of 60 percent sodium cyanide and 40 percent potassium cyanide is commonly used for nitriding. This mixture has a melt ing point of 925°F (496°C), which is gradually reduced to 800°F (427°C) as the cyanate content of the bath increases. A more economical mixture of 70 percent sodium cyanide and 30 percent potassium cyanide may be used if the bath temperature is only 1050°F (566°C). Nitriding bath temperatures should not exceed 1100°F (593°C) because higher temperatures accelerate the formation of carbonate at the expense of the essential cyanide. A third mixture used for nitriding consists of 55 percent sodium cyanide, 25 percent potas sium chloride, and 20 percent sodium carbonate. This mixture melts at 930°F (499°C). Equipment for Hardening High-Speed Steel.— Equipment for hardening high-speed steel consists of a hardening furnace capable of maintaining a temperature of 2350 to 2450°F (1288 to 1343°C); a preheating furnace capable of maintaining a temperature of 1700 to 1800°F (927 to 982°C) and of sufficient size to hold a number of pieces of the work; a tempering (drawing) furnace capable of maintaining a temperature of 1000 to 1200°F (538 to 649°C) as a general rule; and a water-cooled tank of quenching oil. High-speed steels usually are heated for hardening either in some type of electric furnace or in a gas-fired furnace of the muffle type. The small furnaces used for high- speed steel seldom are oil-fired. It is desirable to use automatic temperature control and, where an oven-type of furnace is employed, a controlled atmosphere is advisable because of the variations in cutting qualities caused by hardening under uncontrolled conditions. Some furnaces of both electric and fuel-fired types are equipped with a salt bath suitable for high-speed steel hardening temperatures. Salt baths have the advantage of providing protection against the atmosphere during the heating period. A type of salt developed for commercial use is water-soluble so that all deposits from the hardening bath may be
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