Machinery's Handbook, 31st Edition
Heat Treatment of Steel 477 largely of martensite . On reheating to a temperature of about 300 to 750°F (150 to 400°C), a softer and tougher structure known as troostite is formed. If the steel is reheated to a temperature of 750 to 1290°F (400 to 700°C), a structure known as sorbite is formed that has somewhat less strength than troostite but much greater ductility. Tempering Temperatures.— If steel is heated in an oxidizing atmosphere, a film of oxide forms on the surface that changes color as the temperature increases. These oxide colors (see Table 1 ) have been used extensively in the past as a means of gaging the correct amount of temper; but, since these colors are affected to some extent by the composition of the metal, the method is not dependable. Table 1. Temperatures as Indicated by the Color of Plain Carbon Steel Degrees Color of Steel Degrees Color of Steel Fahrenheit Centigrade Fahrenheit Centigrade 430 221.1 Very pale yellow 510 265.6 Spotted red-brown 440 226.7 Light yellow 520 271.1 Brown-purple 450 232.2 Pale straw-yellow 530 276.7 Light purple 460 237.8 Straw-yellow 540 282.2 Full purple 470 243.3 Deep straw-yellow 550 287.8 Dark purple 480 248.9 Dark yellow 560 293.3 Full blue 490 254.4 Yellow-brown 570 298.9 Dark blue 500 260.0 Brown-yellow 640 337.8 Light blue The availability of reliable pyrometers in combination with tempering baths of oil, salt, or lead makes it possible to heat the work uniformly and to a given temperature within close limits. Suggested temperatures for tempering various tools are given in Table 2. Table 2. Tempering Temperatures for Various Plain Carbon Steel Tools Degrees F Degrees C Class of Tool 495 to 500 257 to 260 Taps, 1 ⁄ 2 inch (1.27 cm) or over, for use on automatic screw machines 495 to 500 257 to 260 Nut taps, 1 ⁄ 2 inch (1.27 cm ) and under 515 to 520 268 to 271 Taps, 1 ⁄ 4 inch (0.635 cm) and under, for use on automatic screw machines 525 to 530 274 to 277 Thread dies to cut thread close to shoulder 500 to 510 260 to 266 Thread dies for general work 495 257 Thread dies for tool steel or steel tube 525 to 540 274 to 282 Dies for bolt threader threading to shoulder 460 to 470 238 to 243 Thread rolling dies 430 to 435 221 to 224 Hollow mills (solid type) for roughing on automatic screw machines 485 252 Knurls 450 232 Twist drills for hard service 450 232 Centering tools for automatic screw machines 430 221 Forming tools for automatic screw machines 430 to 435 221 to 224 Cut-off tools for automatic screw machines 440 to 450 227 to 232 Profile cutters for milling machines 430 221 Formed milling cutters 435 to 440 224 to 227 Milling cutters 430 to 440 221 to 227 Reamers 460 238 Counterbores and countersinks 480 249 Cutters for tube- or pipe-cutting machines 460 to 520 238 to 271 Snaps for pneumatic hammers — harden full length, temper to 460 ° F (238 ° C), then bring point to 520 ° F (271 ° C )
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