For Milling Cutters And Formed Tools

: The Working Of Steel

FORGING.--Forge as before.--ANNEALING.--Place the steel in a pipe,

box or muffle. Arrange the steel so as to allow at least 1 in.

of packing, consisting of dry powder ashes, powdered charcoal,

mica, etc., between the pieces and the walls of the box or pipe.

If using a pipe close the ends. Heat slowly and uniformly to a

cherry red, 1,375 to 1,450 deg.F. according to size. Hold the steel at

this temperature until the hea
has thoroughly saturated through

the metal, then allow the muffle box and tools to cool very slowly

in a dying furnace or remove the muffle with its charge and bury

in hot ashes or lime. The slower the cooling the softer the steel.

The heating requires from 2 to 10 hr. depending upon the size of

the piece.

HARDENING AND TEMPERING.--It is preferable to use two furnaces

when hardening milling cutters and special shape tools. One furnace

should be maintained at a uniform temperature from 1,375 to 1,450 deg.F.

while the other should be maintained at about 2,250 deg.F. Keep the

tool to be hardened in the low temperature furnace until the tool

has attained the full heat of this furnace. A short time should be

allowed so as to be assured that the center of the tool is as hot

as the outside. Then quickly remove the tool from this preheating

furnace to the full heat furnace. Keep the tool in this furnace only

as long as is necessary for the tool to attain the full temperature

of this furnace. Then quickly remove and quench in oil or in a

dry air blast. Remove before the tool is entirely cold and draw

the temper in an oil bath by raising the temperature of the oil

to from 500 to 750 deg.F. and allow this tool to remain, at this

temperature, in the bath for at least 30 min., insuring uniformity

of temper; then cool in the bath, atmosphere or oil.

If higher drawing temperatures are desired than those possible

with oil, a salt bath can be used. A very excellent bath is made

by mixing two parts by weight of crude potassium nitrate and three

parts crude sodium nitrate. These will melt at about 450 deg.F. and

can be used up to 1,000 deg.F. Before heating the steel in the salt

bath, slowly preheat, preferably in oil. Reheating the hardened

high-speed steel to 1,000 deg.F. will materially increase the life

of lathe tools, but milling and form cutters, taps, dies, etc.,

should not be reheated higher than 500 to 650 deg.F., unless extreme

hardness is required, when 1,100 to 1,000 deg.F., will give the hardest