required temperature for a certain length
of time, and then cooled, either rapidly or slowly. This takes
longer than the color method, but with low temperatures the results
are more satisfactory, because the temperature of the bath can
be controlled with a pyrometer. The tempering temperatures given
in the following table are taken from a handbook issued by the
Midvale Steel Company.

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.