Machinery's Handbook, 31st Edition
478 Heat Treatment of Steel Tempering in Oil.— Oil baths are extensively used for tempering tools (especially in quantity), the work being immersed in oil heated to the required temperature, as indi cated by a thermometer. It is important that the oil have a uniform temperature through - out and that the work be immersed long enough to acquire this temperature. Cold steel should not be plunged into a bath heated for tempering, owing to the danger of crack- ing. The steel should either be preheated to about 300°F (150°C) before placing it in the bath or the bath should be at a comparatively low temperature before the steel is im - mersed, and then reheated to the required degree. A temperature of from 650 to 700°F (343 to 371°C) can be obtained with heavy tempering oils; for higher temperatures, either a bath of nitrate salts or a lead bath may be used. In tempering, the best method is to immerse the pieces to be tempered before starting to heat the oil so that they are heated with the oil. After the tempered pieces are taken out of the oil bath, they should be immediately dipped in a tank of caustic soda and, after that, in a tank of hot water. This will remove all oil that might adhere to the tools. The following tempering oil has given satisfactory results: mineral oil, 94 percent; saponifiable oil, 6 percent; specific gravity, 0.920; flash point, 550°F (288°C); fire test, 625°F (329°C). Tempering in Salt Baths.— Molten salt baths may be used for tempering or drawing op- erations. Nitrate baths are particularly adapted for the usual drawing temperature range of, say, 300 to 1100°F (150 to 595°C). Tempering in an oil bath usually is limited to tem peratures of 500 to 600°F (260 to 315°C), and some heat-treating specialists recommend the use of a salt bath for temperatures above 350 or 400°F (175 or 205°C), as it is considered more efficient and economical. Tempering in a bath (salt or oil) has several advantages, such as ease in controlling the temperature range and maintenance of a uniform tempera- ture. The work is also heated much more rapidly in a molten bath. A gas- or oil-fired muffle or semimuffle furnace may be used for tempering, but a salt bath or oil bath is prefera - ble. A salt bath is recommended for tempering high-speed steel, although furnaces may also be used. The bath or furnace temperature should be increased gradually, say, from 300 to 400°F (150 to 205°C) up to the tempering temperature, which may range from 1050 to 1150°F (565 to 620°C) for high-speed steel. Tempering in a Lead Bath.— The lead bath is commonly used for heating steel in con nection with tempering, as well as for hardening. The bath is first heated to the tempera - ture at which the steel should be tempered; the preheated work is then placed in the bath long enough to acquire this temperature, after which it is removed and cooled. As the melting temperature of pure lead is about 620°F (327°C), tin is commonly added to lower the temperature sufficiently for tempering. Reductions in temperature can be obtained by varying the proportions of lead and tin, as shown in Table 3. Table 3. Temperatures of Lead Bath Alloys
Melting Temperature Parts Lead Deg. F Deg. C
Melting Temperature Parts Lead Deg. F Deg. C
Melting Temperature Deg. F Deg. C
Parts Lead
Parts Tin
Parts Tin
Parts Tin
200 8 560 293 39 8 510 266 19 100 8 550 288 33 8 500 260 17 75 8 540 282 28 8 490 254 16 60 8 530 277 24 8 480 249 15
8 460 238 8 450 232 8 440 227 8 430 221
48 8 520 271 21 8 470 243 14 8 420 216 To Prevent Lead from Sticking to Steel.— To prevent hot lead from sticking to parts heated in it, mix common whiting (calcium carbonate) with wood alcohol and paint the part that is to be heated. Water can be used instead of alcohol, but, in that case, the paint must be thoroughly dry, as otherwise the moisture will cause the lead to “fly.” Another method is to make a thick paste according to the following formula: pulverized charred leather, 1 pound; fine wheat flour, 1 1 ⁄ 2 pounds; fine table salt, 2 pounds. Coat the tool with
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