Temperatures To Use

: The Working Of Steel

As soon as the temperature of the steel reaches

100 deg.C. (212 deg.F.) the transformation begins, increasing in intensity

as the temperature is raised, until finally when the lower critical

range is reached, the steel has been all changed into the ordinary

constituents of unhardened steels.

If a piece of polished steel is heated in an ordinary furnace, a

thin film of oxides will form on its surface. The
olors of this

film change with temperature, and so, in tempering, they are generally

used as an indication of the temperature of the steel. The steel

should have at least one polished face so that this film of oxides

may be seen.

An alternative method to the determination of temper by color is

to temper by heating in an oil or salt bath. Oil baths can be used

up to temperatures of 500 deg.F.; above this, fused-salt baths are

required. The article to be tempered is put into the bath, brought

up to and held at the 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.



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


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.