enter
of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in.
box; and the longer the time required to heat to the center, the
more uneven the carburizing.

The work is packed in the box surrounded by materials which will
give up carbon when heated. It must be packed so that each piece
is separate from the others and does not touch the box, with a
sufficient amount of carburizing material surrounding each. Figures
27 to 31 show the kind of boxes used and the way the work should be
packed. Figure 31 shows a later type of box in which the edges can
be easily luted. Figure 30 shows test wires broken periodically to
determine the depth of case. Figure 28 shows the minimum clearance
which should be used in packing and Fig. 29 the way in which the
outer pieces receive the heat first and likewise take up the carbon
before those in the center. This is why a slow, soaking heat is
necessary in handling large quantities of work, so as to allow
the heat and carbon to soak in equally.

While it has been claimed that iron below its critical temperature
will absorb some carbon, Giolitti has shown that this absorption
is very slow. In order to produce quick and intense carburization
the iron should preferably be above its upper critical temperature
or 1,600 deg.F.,--therefore the carbon absorbed immediately goes into
austenite, or solid solution. It is also certain that the higher
the temperature the quicker will carbon be absorbed, and the deeper
it will penetrate into the steel, that is, the deeper the "case."
At Sheffield, England, where wrought iron is packed in charcoal and
heated for days to convert it into "blister steel," the temperatures
are from 1,750 to 1,830 deg.F. Charcoal by itself carburizes slowly,
consequently commercial compounds also contain certain "energizers"
which give rapid penetration at lower temperatures.

The most important thing in carburizing is the human element. Most
careful vigilance should be kept when packing and unpacking, and the
operator should be instructed in the necessity for clean compound
free from scale, moisture, fire clay, sand, floor sweepings, etc.
From just such causes, many a good carburizer has been unjustly
condemned. It is essential with most carburizers to use about 25 to
50 per cent of used material, in order to prevent undue shrinking
during heating; therefore the necessity of properly screening used
material and carefully inspecting it for foreign substances before