Note: Descriptions are shown in the official language in which they were submitted.
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1. Field of the Invention
The invention relates to metallurgy and, more particu-
larly, to a method for treating articles made from magnetically
soft alloys.
The present invention has most effective application in
manufacture of parts of radio-electronic, relay and switching
apparatus of optical-and-mechanical and automation systems.
In addition, the present invention may be employed for
manufacturing parts from magnetically soft alloys for computer,
aircraft and space equipment.
15 2. Description of the Prior Art.
Present-day instrument making imposes strict requirements
for physico-chemical properties of parts of magnetic systems
from magnetically soft alloys. Thus, magnetically soft alloys
are required to have high and stable magnetic properties along
with a high electric resistivety, a high corrosion resistance
in moist atmospheres, in marine climates, in industrial at-
mospheres, in acid solutions and in fungous media and, in some
applications, both high hardness of the surface and wearability
thereof.
The existing magnetically soft alloys fail to meet the
above set of conditions, thus lowering the reliability and
durability of electromagnetic instruments and devices.
Such a variety of physico-chemical properties cannot be
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obtained by integral alloying, as acquisition of some proper-
ties is accompanied by a loss of o~hers. Neither can this
problem be solved by other currently available processing
~neans, such as vacuum annealing (or annealing in an atmosphere
of hydrogen, argon or dissocia~ed amlnoni~) with subsequent
application of chemical coats, galvanizing and electroplating
c~rf~ts m~de
of itcmD from alloys based on iron, nickel and cobalt; nor by
vacuum annealing or vacuum annealing with subsequent -thermal
G?~c/~s
oxidizing of iterns from iron-silicon and iron-nickel alloys.
'~he above kinds of annealing a~fect only the structurally
sensitive magnetic properties (magnetic permeability, coercive
force) and fail to provide the necessary set of physico-chemi-
cal properties, since, for example, annealing lowers resistance
of parts to corrosion and their wearability.
Vacuum annealing with subsequent thermal oxidizing is
effective with respect only to parts made of thin and extra-
thin rolled alloy products which can be provided with a protec-
tive oxide film inhibiting their further oxidation (for example,
when processing parts of iron-silicon and iron-nickel alloys).
Galvanizing, chemical coating and electrochemical plating em-
ployed subsequently to one of the l~inds of annealing with a
view to improving the corrosion and the wear resistance of
parts fail in some instances to provide coats of required resis-
tance to corrosion and wear, the coats possessing a poor conti-
nuity of deposited layers and poor adhesion strength, whereas
high residual stresses in the coats cause their cracking and
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peeling in service. In addition, the above coats lower thestructurally sensitive magnetic characteristics, stability of
properties and substantially increase manufacturing cycle.
There is known a method for diffusion chromizing of
structural steels and alloys, employed with a view to enhancing
wearability and corrosion resistances, consisting in heating
the articles in a powder mixture of chromium, aluminium oxide
and ammonia, exposing the articles to temperatures between 800
and 1200C for over an hour, followed by subsequent cooling
thereof.
However, the known method for diffusion chromizing
fails to improve the magnetic properties of magnetically soft
alloys and stability thereof, since a necessary set of physico-
chemical properties in parts from magnetically soft alloys is
possible only at appropriate rates of heating and cooling, which
are not provided for by the existing methods of diffusion chro-
mizing.
There is known a method for diffusion chromizing of
parts from permalloy, heated to 800C and above in an atmosphere
of a chromium halide compound and gaseous hydrogen with subse-
quent cooling of the articles (Japan, patent application no.
45-123347, filed 31.XII.70).
However, notwithstanding, intricate processing requir-
ing complicate and costly equipment, the above method fails to
provide the necessary set of physico-chemical properties. For
example, the method cannot provide a necessary wearability of
parts subject to intensive deterioration, since it necessarily
involves the use of a gaseous hydrogen atmosphere where articles
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decarburize readily. This produces a solid solution of chro-
mium in iron in the surface layers of the articles, the wearabi-
lity of which is rather poor. In addition, this method fails
to ensure high corrosion resistance in chlorine ion media, as
the thickness of the diffusion layer is small.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to improve
the magnetic characteristics of articles made from magnetically
soft alloys and the stability thereof.
Another object of the invention is to enhance the corro-
sion resistance of parts made from magnetically soft alloys.
Yet another object of the invention is to raise the
wear-resistant properties of articles made from magnetically soft
alloys.
It is also an object of the invention to reduce the pro-
cessing cycle and to lower manpower requirements for treating
the articles.
Still another object of the invention is to provide
an adequate roughness of the surface of the articles.
In addition, an object of the invention is a reduction
of the manufacturing cycle of parts made from magnetically soft
alloys.
A further object of the invention is to raise the relia-
bility and durability of parts made from megnetically soft
alloys.
An additional object of the invention is an economy of
production floorspace necessary for manufacturing articles made
from magnetically soft alloys.
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And finally, an object of the inventiOn is to cut down
the cost of manufacture of articles made from magnetically soft
alloys.
The above and other objects are attained in a method for
treating articles from magnetically soft alloys, comprising sa-
turating surfaces of the articles with chromium at a temperature
between 800 and 1200C and subsequently cooling them, wherein
according to the invention, the articles are heated in a powder
chromizing mixture at a rate not higher than that of recyrstal-
lization of a metal of the articles and that of diffusion pro-
cesses of saturation of the articles, whereas the articles are
cooled at a rate close to that of phase transformations and
formation of a magnetic struoture in the metal of the articles.
The present invention can improve the quality of the
articles made form magnetically soft alloys through an appropri-
ate choice of the rate of heating and that of cooling the arti-
cles which correspond to the formation of necessary structure
and refining of the metal in the protective coat application
process. This provides a basis for combining annealing of the
articles with the application thereon of a protective coat hav-
ing a high chromium content (65 to 85~ Cr), this appreciably re-
ducing the processing cycle. The conditions chosen for diffusion
chromizina substantially raise the magnetic permeability, lower
the coercive force and the magnetic ageing of the articles.
and enhance the corrosion resistance thereof under high moisture
conditions, sea fogs, in the tropics, in nitric acid solutions,
in industrial atmospheres containing sulfurous gas and in fungus
media. The formation then on the surface of the articles of a
carbonitride phase Me2 (N,C) lays the basis for a material in-
crease in the wear-resistant properties of the surface of the
articles. The present invention provides excellent uniformity
and low porosity of the protective coat, a negligible roughness
of the surface; reduces the requirements in industrial floor-
space and brings down the cost of manufacture of the articles.
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In addition, the present invention greatly improves
the reliability and durability of electromagnetic apparatus
and devices.
The present invention can find an effective use in the
manufacture of electromagnetic relays, step-by-step electric
motors, usual types of electric motors, switches, electromagne-
tic clutches, magnetic heads, screens and miscellaneous other
articles, both on small- or medium-lot and mass production
scales.
It is advisable to effect the heating of the articles
at rates of 200-400 degree/hour.
The choice of this range of heating rates provides appro-
priate conditions for the formation ofa necessary metallographic
and crystallographic structure, a necessary depth of refining
and for preparing a protective coat of good continuity and
density of diffusion layer.
When the articles are heated at a rate less than 200
degree/hour, on the one hand, the rate of dissociation of am-
monia is inadequate, and the rate of oxygen removal from the
reaction is low, so that the items are oxidized, and on the
other hand, the starting concentrations of chromium in the con-
tainer reaction space are low. Therefore, when the articles
are heated at the rate of less than 200 degree/hour, the pro-
tective coat formed on the surface thereof is highly porous,
the porosity of the diffusion layer increasing inversely to
the rate of heating.
According to one of the embodiments ofthe invention, the
articles are cooled at the rate of 20-200 degree/hour below
the Curie point (below the temperature of the magnetic struc-
ture formation) so as to avoid the appearance of high body
stresses and of an internal work hardening in the core of the
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metal, and avoid also the precipitation of secondary phases
when iron-nickel and iron-cobalt items are involved, and, there-
fore, to obtain high magnetic properties.
In another embodiment of the invention, it is advan-
tageous to saturate the articles in an inert gas atmosphere.
The saturation of the surface of the articles in an
inert gas atmosphere enchances the continuity of the diffusion
layer and the corrosion resistance of the articles, weara-
bility, magnetic properties and the stability thereof. This
is due to the fact that the articles are not oxicized in an
inert gas atmosphere at the intial stages of the process which
otherwise is unavoidable.
BRIEF DESCRIPTION OF THE DRAWING
These and other objects and features of the invention
become readily apparent from one embodiment thereof which
will now be described by way of example with reference to the
accompanying drawing, in which is illustrated a container for
carrying out the method according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A container 1 is made of steel and is a body with a
mouth closed by a cover 2. The interior of the container 1 is
filled with a chromizing mixture 3, wherein are placed the
articles 4 to be treated. An annular space A between the body
of the container 1 and the cover 2 is filled with a quartz
sand 5 with a layer of ground nitrosilicate glass 6 being
placed thereon. The quartz sand 5 and the nitrosilicate glass
6 form a fusible seal of the container 1.
The treatment of parts from magnetically soft alloys
includes the following steps:
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1. Preparation of a chromizing mixture.
The chromizingmixture 3 contains chromium or ferro-
chrome, aluminium oxide (which can be substituted by quartz
sand, kaolin, chromium oxide or magnesium oxide) and ammonia
(chloride, iodide, bromide, fluoride). The mixture is pre-
pared directly before use.
The prepared mixture is thoroughly mixed and calcinated
in the contianer 1, at the temperature of 1050-1100C.
2. Preparation of the surface of the articles 4. The
surface of the articles 4 to be chromized is chlorided, clean-
ed of traces of dirt, corrosion and scale.
3. Packing of the articles 4 to be treated in the con-
tainer 1. The articles 4 are placed in the container 1 and
interspersed with the chromizing mixture 3, so as to keep them
clear of one another and of the walls of the container 1.
Once the articles are placed into the container 1, the cover
2 thereof is put into pl~ce, and the annular space A between the
container 1 and the cover 2 is filled with the quartz sand 5
and the nitro-
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nace.
4. IIea-ting of the container 1 inside a furnace and c~ro-
mizing o~ parts.
~^f~
The container 1 with the itc~ 4 can be heated in any
thermal furnace with any kind of heating. The rate of heating
of the container should range between 200 and 400C/h.
A variation of the rate of heating of the container 1 in
chromizing, ranging from 25 to 800 degree/hour, exerts no
appreciable influence upon the magnetic properties o~ the
alloys, except ~or the ef~ect upon the magnetic properties o
alloys based on nickel when these are heated at a rate over
400C/h.
The adverse effect upon the magnetic properties of perm-
alloy, for example, is explained by that the necessary rate
of heating is governed by the rate of recrystallization o~
the alloy (the rate of recrystallization o~ the alloy is
a~f~c/e
~00-500 degree/hour). '~he rate o~ heating of an ~e~ in chro-
mizing has also a substantial influence upon the continuity
of the dif~usion layer, this being due to the effect the rate
of hea-ting has upon the rate of reactions and the diffusion
processes inside the container 1.
On heating, the air is displaced from the container 1 by
products of dissociated a~monia through the annular space A
between the cover 2 and the container 1 until the nitrosilica-te
glass 6 melts. Next, the glass 6 melts and seals off the contai-
ner 1.
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When the articles are heated at a rate less than 200
degree/hour, on the one hand, the ammonia dissociation rate is
insufficient and the rate of removal of oxygen from the reac-
tion space of the container is low, so that the articles oxi-
dize. On the other hand, when the articles are heated at a
rate less than 200 degree/hour, the concentration of chromium
is excessively low in the container reaction space, and, con-
sequently, a protective coat formed on the surface of the ar-
ticles is highly porous.
The porosity of the diffusion layer increases, as the
rate of heating slows down.
The temperature and the chromizing time are governed
by the required wearability, resistance to corrosion as well
as by magnetic and electric properties. The chromizing time
is counted from the moment the container 1 is heated to a spe-
cified tempreature.
5. Cooling and unpacking the container 1.
Once chromizing has been completed, the contianer 1 is
cooled at the rate of 20-200 degree/hour, i.e. at a rate close
to that of the phase transformations and of formation of the
magnetic structure in the metal of the articles, to a tempera-
ture below the Curie point. This avoids high body stresses
in the metal of the articles and of internal work hardening,
and the precipitation of secondary phases if the articles from
iron-nickel or iron-cobalt alloys are involved.
Upon cooling, the solidified glass 6 in the seal of the
container 1 is to be broken, and the chromizing mixture sieved
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and poured into a box especially provided for the purpose of
making the mixture reusable.
To enhance magnetic and corrosion-resistant properties,
the continuity and wearability of the diffusion layers, chromi-
zing should best be conducted in an inert gas atmosphere
(hydrogen, argon, dissociated ammonia, nitrogen-hydrogen mix-
ture), as then the articles are not oxidized on the initial
stages of the process, as is the case otherwise, this increas-
ing the degree of refining of the metal of the articles.
6. Cleaning chromized parts.
Once discharged from the container 1, the parts should
be flushed with hot water.
The effect of the rates of heating and cooling of
the articles 4 upon their magnetic properties, for example,
of the articles from Armco iron and permalloy are listed in
the Tables 1 and 2.
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Table 1
~ate of i~i~agnetic i~lield Coercive
heating, Allo permeabi- inten- iorce,
degree/ Y lity, sity, Oe
/hour Gs/Oe Oe
Armco iron 9100 84 61
Perrnalloy 290,000 1.1 1.05
Armco iron 9350 80 58
Permalloy 295,000 1.1 1.00
100 Armco iron 9270 85 59
Permalloy 285,000 1.2 1.06
200 Armco iron 9330 80 56
Permalloy 293,600 0.90 1.01
400 Armco iron 9360 80 56
Permalloy 281,000 1.00 1.05
800 Armco iro~ 9200 85 58
Permalloy 85,000 1.81 1.25
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Ta~le 2
.. . . _ . . _ _ . _ , _ , . . . _ _ _ . _ , . . .
1,1aglnetic Field Coerciv~
Alloy Cooling conditions permeabi- inten- force,
lit,y, . sity, Oe
Gs/Oe Oe
.... . .. _ _ .. . . _ -- . . .. . .. . . .. . . .. .
~i~fusion chromizing by one of the known methods
Arrnco 50C/h down to 600C and
iron then in the air 5360 120 78.0
Armco Cooling in the air from
iron the isothermal tempera~ure
of the process 7500 91 91
Armco Cooling together with the
iron furnace 95 87 72
Diffusion chromizing by the method according
to the invention
Armco Cooling together with the
iron furnace to 700C and then
in the air 10,200 82 61
Arrnco Cooling together with the
iron furnace to 600C and then
in the air 11,500 78 57
Known method :~
Permalloy Cooling in the air from
the isothermal temperatu-
re of the process 60,000 2.52 2.61
Permallo~ Cooling together with the
furnace 225,000 0.95 0.91
Method according to the invention
Permalloy Cooling together with the
furnace to 400C and then
in the air 270,000 0.82 0.87 .
Permalloy Cooling together with the
furnace to 600C and then
in ~he air 324,400 0.805 0.841
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~ s is readily apparent frorrl 'l'ables 1 and 2, the rates
of heating and of cooling in di:~usion chromizing have a sub-
stantial in~luence upon the magnetic properties of rnagnetically
soft alloys.
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