increase its longevity.

Sometimes it is advisable to cool the work in the pots. This saves
compound, and causes a more gradual diffusion of the carbon between
the case and the core, and is very desirable condition, inasmuch
as abrupt cases are inclined to chip out.

The most satisfactory steel to carburize contains between 0.10
and 0.20 per cent carbon, less than 0.35 per cent manganese, less
than 0.04 per cent phosphorus and sulphur, and low silicon. But
steel of this composition does not seem to satisfy our progressive
engineers, and many alloy steels are now on the market, these,
although more or less difficult to machine, give when carburized
the various qualities demanded, such as a very hard case, very tough
core, or very hard case and tough core. However, the additional
elements also have a great effect both on the rate of penetration
during the carburizing operation, and on the final treatment,
consequently such alloy steels require very careful supervision
during the entire heat treating operations.

RATE OF ABSORPTION

According to Guillet, the absorption of carbon is favored by those
special elements which exist as double carbides in steel. For example,
manganese exists as manganese carbide in combination with the iron
carbide. The elements that favor the absorption of carbon are:
manganese, tungsten, chromium and molybdenum those opposing it,
nickel, silicon, and aluminum. Guillet has worked out the effect
of the different elements on the rate of penetration in comparison
with steel that absorbed carbon at a given temperature, at an average
rate of 0.035 in. per hour.

His tables show that the following elements require an increased
time of exposure to the carburizing material in order to obtain
the same depth of penetration as with simple steel:

When steel contains Increased time of exposure
2.0 per cent nickel 28 per cent
7.0 per cent nickel 30 per cent
1.0 per cent titanium 12 per cent
2.0 per cent titanium 28 per cent
0.5 per cent silicon 50 per cent
1.0 per cent silicon 80 per cent
2.0 per cent silicon 122 per cent
5.0 per cent silicon No penetration
1.0 per cent aluminum 122 per cent
2.0 per cent aluminum 350 per cent

The following elements seem to assist the rate of penetration of
carbon, and the carburizing time may therefore be reduced as follows:

When steel con