Improvements in and relating to the Annealing of Metals.
Year: 1892

page_text: Date of Application, 9th Feb., 1892-Accepted, 9th July, 1892
COMPLETE SPECIFICATION.
Improvements in and relating to the Annealing of Metals.
We, SAMUEL HENRY BROWN, Metal Worker of 45 Oliver Street, and MICHAEL
MCBARRON, Merchant, of No. 45 Oliver Street, both in the City of Boston, and
State of Massachusetts, United States of America, 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 following statement :-
The invention relates to an improved process of annealing metals, as aluminium,
copper and its alloys, and the precious metals, in a new annealing bath, and to a
process of restoring and maintaining said bath.
Heretofore in annealing metals, it has been difficult to bring all the parts of the
articles under treatment to a uniform heat and to heat the same to a proper
temperature without oxidizing the surface either during the heating or during the
subsequent cooling period.
It is the object of our invention to obviate these difficulties and expeditiously
and economically anneal the various metals and their alloys. To this end our
invention consists in subjecting metal to the action of a molten bath of a nitrite
of an alkali metal, and afterwards subjecting the same to the action of a cooling
bath. In practice, we subject the metal to be treated to a heat of about
1000° Fahrenheit, in a bath composed of the nitrite of potassium and afterwards
withdraw the same and plunge it into a cooling bath. We take the nitrate of
potassium in suitable quantity and heat it in a proper vessel to a temperature of
from 90° to 1,200° Fahrenheit, otherwise to a low red heat, at which temperature
it parts with a portion of its oxygen which if allowed to remain would oxidize the
surface of the metal under treatment. The bath is now in a limpid, molten
condition ready for the immersion of the articles to be annealed, and is essentially
a nitrite of potassium, which only gives off further oxygen slowly and then in
connection with nitrogen. The metal to be treated is now introduced to the bath
and when it has become heated to about the temperature of the bath, say in about
two minutes, more or less according to its dimensions and character, it is with-
drawn. It will then be found covered with a coating or film of the material of the
bath, which effectually protects it from the oxidizing effects of the atmosphere and
keeps it in condition to be subjected to the final cooling step of the annealing
process.
In the case of copper, its alloys, or the precious metals, the metal or alloy is
transferred immediately to the cooling bath. The cooling bath may be of any
suitable material, but we prefer to employ a bath of about 48 per cent. of glycerine,
49 per cent. of water, and about 3 per cent of starch. When the metal is plunged
into the cooling mixture, the film or coating is removed and the surface is left
smooth and free from scale.
To maintain the volume of the molten bath, as it becomes reduced by the with-
drawal of portions of the same, as a film or coating on the metal inserted and
withdrawn, additional nitrate of potassium is added in suitable quantities from
time to time. This maintains the bath in the condition of a nitrite.
Having now particularly described and ascertained the nature of our said
invention, and in what manner the same is to be performed we declare that
what we claim is :-
First. The process of annealing metals, which consists in fusing nitrate of
potassium, raising the temperature of the fused bath to about 1,000 degrees
Fahrenheit, maintaining the bath at this temperature a suitable period to drive off

page_text: a portion of the contained oxygen, placing the metal in the resulting bath until it
becomes sufficiently heated, and finally cooling the metal, as specified.
Second. The process of producing and maintaining an annealing bath of nitrite
of potassium, consisting in raising nitrate of potassium to a temperature of about
1,000 degrees Fahrenheit, maintaining the same at this temperature a suitable
period to drive off a portion of the contained oxygen and adding nitrate of
potassium at suitable intervals to the fused nitrite so as to supply the oxygen
necessary to prevent the nitrite from being reduced to an oxide, substantially as
described.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, Agents for the Applicants.