oduce ductility
and withstand shock. The operation is carried out by packing the
work to be carburized in boxes with a material rich in carbon and
maintaining the box so charged at a temperature in excess of the
highest critical point for a length of time to produce the desired
depth of carburized zone. Generally maintaining the temperature
at 1,650 to 1,700 deg. F. for 7 hr. will produce a carburized zone
1/32 in. deep.

Heating to a temperature slightly above the highest critical point
and cooling suddenly in some quenching medium, such as water or oil
hardens the steel. This treatment produces a maximum refinement
with the maximum strength.

Drawing to a temperature below the highest critical point (the
temperature being governed by the results required) relieves the
hardening strains set up by quenching, as well as the reducing
of the hardness and brittleness of hardened steel.

EFFECTS OF PROPER ANNEALING.--Proper annealing of low-carbon steels
causes a complete solution or combination to take place between
the ferrite and pearlite, producing a homogeneous mass of small
grains of each, the grains of the pearlite being surrounded by
grains of ferrite. A steel of this refinement will machine to good
advantage, due to the fact that the cutting tool will at all times
be in contact with metal of uniform composition.

While the alternate bands of ferrite and pearlite are microscopically
sized, it has been found that with a Gleason or Fellows gear-cutting
machine that rough cutting can be traced to poorly annealed steels,
having either a pronounced banded structure or a coarse granular
structure.

TEMPERATURE FOR ANNEALING.--Theoretically, annealing should be
accomplished at a temperature at just slightly above the critical
point. However, in practice the temperature is raised to a higher
point in order to allow for the solution of the carbon and iron to
be produced more rapidly, as the time required to produce complete
solution is reduced as the temperature increases past the critical
point.

For annealing the simpler types of low-carbon steels the following
temperatures have been found to produce uniform machining conditions
on account of producing uniform fine-grain pearlite structure:

0.15 to 0.25 per cent carbon, straight carbon steel.--Heat to 1,650 deg.F.
Hold at this temperature until the work is uniformly heated; pull
from the furnace and cool in air.

0.15 to 0.25 per cent carbon, 1-1/2 per cent nic