TABLE 23.--TEMPERING TEMPERATURES FOR STEELS ---------------------------------------------------------------------------- Temperature | | Temperature | for 1 hr. | | for 8 min. | ---------------| Color |---------------| Uses Deg. F.|Deg. C.| |Deg. F.|Deg. C.| -------|-------|------------|-------|-------|------------------------------- 370 | 188 |Faint yellow| 460 | 238 |Scrapers, brass-turning tools, | | | | |reamers, taps, milling cutters, | | | | |saw teeth. 390 | 199 |Light straw | 510 | 265 |Twist drills, lathe tools, | | | | |planer tools, finishing tools 410 | 210 |Dark straw | 560 | 293 |Stone tools, hammer faces, | | | | |chisels for hard work, boring | | | | |cutters. 430 | 221 |Brown | 610 | 321 |Trephining tools, stamps. 450 | 232 |Purple | 640 | 337 |Cold chisels for ordinary work, | | | | |carpenters' tools, picks, cold | | | | |punches, shear blades, slicing | | | | |tools, slotter tools. 490 | 254 |Dark blue | 660 | 343 |Hot chisels, tools for hot | | | | |work, springs. 510 | 265 |Light blue | 710 | 376 |Springs, screw drivers. ---------------------------------------------------------------------------- It will be noted that two sets of temperatures are shown, one being specified for a time interval of 8 min. and the other for 1 hr. For the finest work the longer time is preferable, while for ordinary rough work 8 min. is sufficient, after the steel has reached the specified temperature. The rate of cooling after tempering seems to be immaterial, and the piece can be cooled at any rate, providing that in large pieces it is sufficiently slow to prevent strains. KNOWING WHAT TAKES PLACE.--How are we to know if we have given a piece of steel the very best possible treatment? The best method is by microscopic examination of polished and etched sections, but this requires a certain expense for laboratory equipment and upkeep, which may prevent an ordinary commercial plant from attempting such a refinement. It is highly recommended that any firm that has any large amount of heat treatment to do, install such an equipment, which can be purchased for from $250 to $500. Its intelligent use will save its cost in a very short time.