from the compound.

Until the temperature of the muffle reaches about 1,300 deg.F. carburization
does not take place to any useful extent, and consequently it is
advisable to avoid the use of any compound from which the carburizing
gases are liberated much before that temperature is reached. In
the case of steel containing nickel slightly higher temperatures
may be used and are really necessary if the same rate of carbon
penetration is to be obtained, as the presence of nickel resists
the penetration.

At higher temperatures the rate of penetration is higher, but not
exactly in proportion to the temperature, and the rate is also
influenced by the nature of the material and the efficiency of the
compound employed.

The so-called saturation point of mild steel is reached when the
case contains 0.90 per cent of carbon, but this amount is frequently
exceeded. Should it be required to ascertain the amount of carbon
in a sample at varying depths below the skin this can be done by
turning off a small amount after carburizing and analyzing the
turnings. This can be repeated several times, and it will probably
be found that the proportion of carbon decreases as the test piece
is reduced in diameter unless decarburization has taken place.

[Illustration: FIG. 42.--Chart showing penetration of carbon.]

The chart, Fig. 42, is also a good guide.

In order to use the chart it is necessary to harden the sample
we desire to test as we would harden a piece of tool steel, and
then test by scleroscope. By locating on the chart the point on
the horizontal axis which represents the hardness of the sample
the curve enables one to determine the approximate amount of carbon
present in the case.

Should the hardness lack uniformity the soft places can be identified
by etching. To accomplish this the sample should be polished after
quenching and then washed with a weak solution of nitric acid in
alcohol, whereupon the harder points will show up darker than the
softer areas.

The selection of suitable boxes for carburizing is worthy of a
little consideration, and there can be no doubt that in certain
cases results are spoiled and considerable expense caused by using
unsuitable containers.

As far as initial expense goes cast-iron boxes are probably the
most expedient, but although they will withstand the necessary
temperatures they are liable to split and crack, and when they
get out of shape there is much difficulty in straightenin