layer of green coal
(screened and dampened blacksmiths' coal) as a cover. When the
green coal on top has been coked the fire is ready for use. As
the fuel burns out in the center, the coke forming around the edge
is pushed inward, and its place taken by more green coal. Thus the
fire is made up of three parts; the center where coke is burning
and the iron heating; a zone where coke is forming, and the outside
bank of green coal.

STEEL WORKED IN AUSTENITIC STATE.--As a general rule steel should
be worked when it is in the austenitic state. (See page 108.) It
is then soft and ductile.

As the steel is heated above the critical temperature the size of
the austenite crystals tends to grow rapidly. When forging starts,
however, these grains are broken up. The growth is continually
destroyed by the hammering, which should consequently be continued
down to the upper critical temperature when the austenite crystals
break up into ferrite and cementite. The size of the final grains
will be much smaller and hence a more uniform structure will result
if the "mother" austenite was also fine grained. A final steel
will be composed of pearlite; ferrite and pearlite; or cementite
and pearlite, according to the carbon content.

The ultimate object is to secure a fine, uniform grain throughout
the piece and this can be secured by uniform heating and by thoroughly
rolling it or working it at a temperature just down to its critical
point. If this is correctly done the fracture will be fine and
silky. Steel which has been overheated slightly and the forging
stopped at too high a temperature will show a "granular" fracture.
A badly overheated or "burned" steel will have iridescent colors
on a fresh fracture, it will be brittle both hot and cold, and
absolutely ruined.

STEEL CAN BE WORKED COLD.--As noted above, steel can be worked cold,
as in the case of cold-rolled steel. Heat treatment of cold-worked
steel is a very delicate operation. Cold working hardens and strengthens
steel. It also introduces internal stresses. Heat-treatments are
designed to eliminate the stresses without losing the hardness
and strength. This is done by tempering at a low heat. Avoid the
"blue" range (350 to 750 deg.C.). Tempering for a considerable time just
under the critical is liable to cause great brittleness. Annealing
(reheating through the critical) destroys the effect of cold work.

FORGING

HIGH-SPEED STEEL.--Heat very slowly and carefully to fro