Using Illuminating Gas
: CASE-HARDENING OR SURFACE-CARBURIZING
: The Working Of Steel
The choice of a carburizing furnace depends greatly on the facilities
available in the locality where the shop is situated and the nature
and quantity of the work to be done. The furnaces can be heated with
producer gas in most cases, but when space is of value illuminating
gas from a separate source of supply has some compensations. When
the latter is used it is well to install a governor if the pressure
is likely to
luctuate, particularly where the shop is at a high
altitude or at a long distance from the gas supply.
Many furnaces are coal-fired, and although greater care is required
in maintaining a uniform temperature good results have been obtained.
The use of electricity as a means of reaching the requisite temperature
is receiving some attention, and no doubt it would make the control
of temperature comparatively simple. However, the cost when applied
to large quantities of work will, for the present at least, prevent
this method from becoming popular. It is believed that the results
obtainable \with the electric furnace would surpass any others; but
the apparatus is expensive, and unless handled with intelligence
would not last long.
The most elementary medium of carburization is pure carbon, but
the rate of carburization induced by this material is very low,
and other components are necessary to accelerate the process. Many
mixtures have been marketed, each possessing its individual merits,
and as the prices vary considerably it is difficult to decide which
is the most advantageous.
Absorption from actual contact with solid carbon is decidedly slow,
and it is necessary to employ a compound from which gases are liberated,
and the steel will absorb the carbon from the gases much more readily.
Both bone and leather charcoal give off more carburizing gases
than wood charcoal, and although the high sulphur content of the
leather is objectionable as being injurious to the steel, as also
is the high phosphorus content of the bone charcoal, they are both
preferable to the wood charcoal.
By mixing bone charcoal with barium carbonate in the proportions
of 60 per cent of the former to 40 per cent of the latter a very
reliable compound is obtained.
The temperature to which this compound is subjected causes the
liberation of carbon monoxide when in contact with hot charcoal.
Many more elaborate explanations may be given of the actions and
reactions taking place, but the above is a satisfactory guide to
indicate that it is not the actual compound which causes carburization,
but the gases released 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
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
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.
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
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
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 straightening them.
The most suitable material in most cases is steel boiler plate 3/8
or 1/2 in. thick, which can be made with welded joints and will
The sizes of the boxes employed depend to a great extent on the
nature of the work being done, but care should be exercised to
avoid putting too much in one box, as smaller ones permit the heat
to penetrate more quickly, and one test piece is sufficient to
give a good indication of what has taken place. If it should be
necessary to use larger boxes it is advisable to put in three or four
test pieces in different positions to ascertain if the penetration
of carbon has been satisfactory in all parts of the box, as it
is quite possible that the temperature of the muffle is not the
same at all points, and a record shown by one test piece would
not then be applicable to all the parts contained in the box. It
has been found that the rate of carbon penetration increases with
the gas pressure around the articles being carburized, and it is
therefore necessary to be careful in sealing up the boxes after
packing. When the articles are placed within and each entirely
surrounded by compound so that the compound reaches to within 1
in. of the top of the box a layer of clay should be run around the
inside of the box on top of the compound. The lid, which should
be a good fit in the box, is then to be pressed on top of this,
and another layer of clay run just below the rim of the box on
top of the cover.