Improved Process for Hardening Machine Knives and Tools.
Year: 1899

page_text: Date of Application, 16th Mar., 1899-Accepted, 13th May, 1899
COMPLETE SPECIFICATION.
Improved Process for Hardening Machine Knives and Tools.
I, HERMANN LIHN, Manufacturer of Machine Knives, of Remscheid, Germany,
do hereby declare the nature of this invention and in what manner the same is
to be performed to be particularly described and ascertained in and by the follow-
ing statement:-
Hitherto it has been considered that the lowest and proper temperature for
hardening machine knives is that at which steel becomes capable of being hardened,
that is to say the temperature at which the carbon in the steel becomes hardening
carbon." If the steel be hardened at this temperature by immersion in water,
it will he too brittle for fine cutting tools (machine knives, paper cutting knives
and planes). If, in order to obviate this disadvantage, the machine knife which
has been heated to the temperature necessary for the hardening process is allowed
to cool in the air, the thin cutting edges, points and sharp corners will become
too cold whilst the thicker parts will remain too hot. The result of this uneven
cooling in the air and subsequent immersion in water is uneven hardening, as
shown above.
The present hardening process is intended to get rid of this difficulty. It
enables cutting tools to be hardened at a temperature considerably lower than
that at which steel is made capable by heating, of being hardened. The present
process has also the advantage that the return to a lower temperature is effected
with absolute uniformity. The knives, moreover, are finely hardened and cut well,
and retain their good quality in this respect until completely worn out.
According to this invention the knives which are to be hardened are heated to
the temperature at which the carbon in the steel becomes "hardening carbon."
They are then placed in a bath of molten metal or other heated liquid substances
which are good conductors of heat. At the beginning of the operation, the
temperature of the bath is that at which the steel becomes capable of being
hardened, or slightly lower. The bath is now cooled and thereby the cooling of
the knives which are in it is effected uniformly. The knives are then hardened
by immersion in water.
By this method of cooling, the knives can be hardened at a considerably lower
temperature than is possible with existing methods.
This hardening temperature, the lowest conceivable for the purpose, is s
considerably lower than that at which steel, by mere heating, becomes capable
of being hardened, that steel, which at a glowing bright red heat becomes capable
of being hardened, can be hardened under this process when still at a dark red
heat. For steel which becomes capable of being hardened either at a glowing
red or dark red heat, the lowest hardening temperature is correspondingly lower-
under this process it is as low as reddish black heat.
By this cooling process the following advantages are obtained.
Uniform heating of the knives which are to be hardened.
Uniform hardening of the same.
1.1.
and tananiter of the hordoned articles

page_text: This lowest conceivable and uniformly hardening temperature can only be
arrived at as follows:-The sieel must first be brought to the temperature
absolutely necessary for hardening, then cooled down to the lower temperaturo
in a bath of heated good heat conducting liquid, the knives being thereupon
hardened by immersion in water.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is:-
A process for hardening machine knives and tools, the distinguishing feature
of which is that the implements which are heated to the necessary degree for
effecting the hardening, are cooled down quite uniformly in a bath of hot good
heat conducting liquid, to a considerably lower temperature and then hardened
by immersion in water.
1
75
1
1006