CA1127514A - Method for treating articles made from magnetically soft alloys - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method according to the present invention is intended for treating articles for magnetically soft alloys employed in radio-electronic, relay and switching apparatus, in optical-and-mechanical and automation systems. The method consists in sa-turating the surface of the articles with chromium inside a powder chromizing mixture at a temperature in excess of 800°C, the articles being heated at a rate not higher than that of re-crystallization of a metal of the articles and the rate of dif-fusion processes of saturating the articles. The articles are cooled at a rate close to that of phase transformations and for-mation of a magnetic structure in the metal of the articles, for example, at the rate of 20-200 degree/hour below the Curie point. To enhance the magnetic properties and the stability thereof, as well as wearability and the resistance of the items to corrosion, chromizing of the article surfaces is conducted in an inert gas atmosphere.

Description

- llZ~S14 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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-~127S14 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 : -l~Z~514 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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llZ7514 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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1~2~514 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-`

~ 127514silicate glass 6, the container 1 tnen being put inside a iur-nace.

4. IIea-ting of the container 1 inside a furnace and c~ro-mizing o~ parts.
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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 11~75~4 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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~27514 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 11275~.4 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 1~27~14 ~ 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.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLU-SIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for treating articles made from magneti-cally soft alloys, comprising the steps of: chromizing the surface of the articles inside a powder chromizing mixture at a temperature of 800-1200°C. and cooling subsequently the articles at a rate not exceeding that of recrystallization of a metal of said articles and applying diffusion processes of saturation of said articles, said articles being cooled at a rate close to that of diffusion transformations and of formation of magnetic structure in metal of said articles.

2. A method as claimed in claim 1, wherein said articles are heated at a rate of 200-400 degree/hour.

3. A method as claimed in claim 1, wherein said articles are cooled at a rate of 20-200 degree/hour below the Curie point.

4. A method as claimed in claim 1, wherein said articles are chromized in an inert gas atmosphere.