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SHOP NOTES. 479
of the fluid iron on some part of the mould is liable to make the
sand yield a little and thereby cause the casting to be as large
as, or even larger than the pattern. All such things a practical
pattern maker takes into consideration when allowing for
shrinkage in patterns.
Case Hardening Wrought Iron and Soft Steel.
Bone dust specially prepared for the purpose, or burnt
leather scrap, is placed in a cast-iron box, together with the
article to be hardened. Cover the top of the box with plenty of
the hardening material in order to keep the air out. Heat the
whole mass slowly in a furnace to a red heat from two to five
hours in order that it may be uniformly and thoroughly heated
through. A few iron rods about 5
/\q inch in diameter are put in
when packing the box, one end of the rod reaching about to the
middle of the box, and the other end projecting out through the
hardening material on top. When the box appears to have the
right heat, these rods are pulled out one at a time, in order to
judge of the heat in the center of the mass. When the box
has been exposed to the fire the desired length of time, its
contents are quickly dumped into cool water.
Sieves of iron netting are laid on the bottom of the tub into
which the case hardening material is dumped so that the hard-
ened articles may be conveniently taken up from the water by
one of the sieves. The case hardening material itself is also
taken out by another sieve which is of very fine netting and
placed under the first one. The material is dried and used over
again, and a little new material is added when repacking the
boxes.
When articles are well finished before hardening, this pro-
cess gives a very fine color to both soft steel and wrought iron.
Case hardening may also be effected by packing the articles
in soot, but this process does not give a nice color.
Horn and hoof is also used for case hardening. Malleable
iron may also be case hardened, but it requires careful handling
in order to prevent its cracking and twisting out of shape.
Case Hardening Boxes
are made from cast-iron and are of various sizes. Small boxes
may be made nine inches long, five inches wide, and four inches
deep, and about one-fourth inch thick. They should be pro-
vided with legs at least one inch high so that the heat may get
under the bottom as at the top. An ear having a rectangular
hole through it should be cast under the bottom at each end of
the box. This gives a chance to handle the box with a fork
having flat prongs instead of taking it out of the hardening fur-
nace with a pair of tongs, which is liable to break the box,
as cast-iron is very inferior in strength when hot.
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