onizing. The work is constantly
and uniformly exposed to a clean carbonizing atmosphere instead
of partially spent carbonaceous solids which may give off very
complex compounds of phosphorus, sulphur, carbon and nitrogen.

Originally this process was thought to require a gas generator but
it has been discovered that city gas works all right. The gas consists
of vapors derived from petroleum or bituminous coal. Sometimes the
gas supply is diluted by air, to reduce the speed of carburization
and increase the depth.

PREVENTING CARBURIZING BY COPPER-PLATING

Copper-plating has been found effective and must have a thickness
of 0.0005 in. Less than this does not give a continuous coating.
The plating bath used has a temperature of 170 deg.F. A voltage of
4.1 is to be maintained across the terminals. Regions which are
to be hardened can be kept free from copper by coating them with
paraffin before they enter the plating tank. The operation is as
follows:

Operation
No. Contents of bath Purpose
1 Gasoline To remove grease
2 Sawdust To dry
3 Warm potassium hydroxide solution To remove grease and dirt
4 Warm water To wash
5 Warm sulphuric acid solution To acid clean
6 Warm water To wash
7 Cold water Additional wash
8 Cold potassium cyanide solution Cleanser
9 Cold water To wash
10 Electric cleaner, warm sodium Cleanser to give good
hydroxide case-iron anode plating surface
11 Copper plating bath of copper Plating bath
sulphate and potassium cyanide
solution warm

There are also other methods of preventing case-hardening, one
being to paint the surface with a special compound prepared for
this purpose. In some cases a coating of plastic asbestos is used
while in others thin sheet asbestos is wired around the part to
be kept soft.

PREPARING PARTS FOR LOCAL CASE-HARDENING

At the works of the Dayton Engineering Laboratories Company, Dayton,
Ohio, they have a large quantity of small shafts, Fig. 40, that
are to be case-hardened at _A_ while the ends _B_ and _C_ are to
be left soft. Formerly, the part _A_ was brush-coated with melted
paraffin but, as there were many shafts, this