Heat treating of magnetic iron powder

The invention concerns a method of compacting and heat-treating iron powders in order to obtain magnetic core components having improved soft magnetic properties. The iron powder consists of fine particles which are insulated by a thin layer having a low phosphorous content. According to the invention, the compacted iron powder is subjected to heat treatment at a temperature between 350.degree. and 550.degree. C.

This invention relates to a method of heat-treating iron powders. More 
particularly, the invention relates to a method in which iron composites 
are moulded and pressed. The pressed components are then heat treated. The 
method is particularly useful to make magnetic core components having 
improved soft magnetic properties. 
Iron-based particles have long been used as a base material in the 
manufacture of structural components by powder metallurgical methods. The 
iron-based particles are first moulded in a die under high pressures in 
order to produce the desired shape. After the moulding step, the 
structural component usually undergoes a sintering step to impart the 
necessary strength to the component. 
Magnetic core components have also been manufactured by such power 
metallurgical methods, but the iron-based particles used in these methods 
are generally coated with a circumferential layer of insulating material. 
Two key characteristics of an iron core component are its magnetic 
permeability and core loss characteristics. The magnetic permeability of a 
material is an indication of its ability to become magnetized or its 
ability to carry a magnetic flux. Permeability is defined as the ratio of 
the induced magnetic flux to the magnetising force or field intensity. 
When a magnetic material is exposed to a rapidly varying field, the total 
energy of the core is reduced by the occurrence of hysteresis losses 
and/or eddy current losses. The hysteresis loss is brought about by the 
necessary expenditure of energy to overcome the retained magnetic forces 
within the iron core component. The eddy current loss is brought about by 
the production of electric currents in the iron core component due to the 
changing flux caused by alternating current (AC) conditions. 
Magnetic core components are made from laminated sheet steel, but these 
components are difficult to manufacture to net shape for small intricate 
parts and experience large core losses at higher frequencies. Application 
of these lamination-based cores is also limited by the necessity to carry 
magnetic flux only in the plane of the sheet in order to avoid excessive 
eddy current losses. Sintered metal powders have been used to replace the 
laminated steel as the material for the magnetic core component, but these 
sintered parts also have high core losses and are restricted primarily to 
direct current (DC) operations. 
Research in the powder metallurgical manufacture of magnetic core 
components using coated iron-based powders has been directed to the 
development of iron powder compositions that enhance certain physical and 
magnetic properties without detrimentally affecting other properties. 
Desired properties include a high permeability through an extended 
frequency range, high pressed strength, low core losses and suitability 
for compression moulding techniques. 
When moulding a core component for AC power applications, it is generally 
required that the iron particles have an electrically insulating coating 
to decrease core losses. The use of plastic coating (see U.S. Pat. No. 
3,935,340 to Yamaguchi) and the use of doubly-coated iron particles (see 
U.S. Pat. No. 4,601,765 to Soileau et al) have been employed to insulate 
the iron particles and therefore reduce eddy current losses. However, 
these powder compositions require a high level of binder, resulting in 
decreased density of the pressed core part and, consequently, a decrease 
in permeability. Moreover, although the strength of pressed parts made 
from such powder compositions would generally be increased by sintering, 
the desired end-utility of the parts precludes such a processing step: the 
elevated temperatures at which sintering of the core metal particles 
normally occurs would degrade the insulating material and generally 
destroy the insulation between individual particles by forming 
metallurgical bonds. 
In brief the present invention provides a method of making a component 
having improved magnetic properties by compacting or die-pressing a powder 
composition of insulated particles of an atomized or sponge iron powder 
optionally in combination with a thermosetting resin and subsequently 
subjecting the compacted composition to heat treatment at a temperature 
preferably not more than 500.degree. C. 
DE 34 39 397 discloses a method for a powder metallurgical preparation of 
soft magnetic components. According to this method iron particles are 
enveloped by an insulating phosphate layer. These particles are then 
compacted and subsequently heated in an oxidizing atmosphere. Before the 
compacting step the phosphate insulated iron particles are optionally 
mixed with a resin, preferably an epoxy resin. In order to obtain low 
hysteresis losses heating temperatures above 500.degree. and below 
800.degree. C. are recommended. Furthermore this heat treatment should 
preferably be carried out stepwise with alternating reduced and normal or 
increased pressures and with stepwise increased temperatures for different 
periods of times. The advantages of this known process are experimentally 
disclosed for a heat treatment wherein the final step is carried out at a 
temperature of at least 600.degree. C. 
In view of this teaching it was quite unexpected to find that a remarkable 
improvement of the soft magnetic properties is obtained if the heat 
treatment is carried out at a temperature well below 600.degree. C. 
According to the present invention it is thus critical that the heat 
treatment is carried out at a temperature between 350.degree. and 
550.degree. C., preferably between 400.degree. and 530.degree. C. and most 
preferably between 430.degree. and 520.degree. C. Furthermore there is no 
need for alternating pressures and stepwise increasing temperatures as is 
recommended in the known process. The period of heat treatment according 
to the present invention is not critical and usually this period could 
vary between 20 minutes and 2 hours. Essentially the same improvements are 
obtained when heating for 0.5 h as when heating for 1 h. Furthermore and 
in contrast to the process disclosed in DE 34 39 397 the present invention 
can be carried out with a phosphorous acid treatment without any 
environmentally detrimental organic solvents. 
Another feature of this known invention is that the phosphate insulating 
layer should constitute between 0.1 and 1.5% by weight of the iron 
particles. As discussed below the insulating "P-layer" is an important 
feature also for the present invention, according to which lower amounts 
of P are used. 
More specifically the method according to the invention comprises the 
following steps. 
Particles of an atomized or sponge iron powder are treated with an aqueous 
phosphoric acid solution to form an iron phosphate layer at the surface of 
the iron particles. The phosphorous acid treatment is preferably carried 
out at room temperature and for a period of about 0.5 to about 2 hours. 
The water is then evaporated at a temperature of about 90.degree. to about 
100.degree. C. in order to obtain a dry powder. According to another 
embodiment the phosphoric acid is provided in an organic solvent such as 
acetone. 
The phosphorous layer should be as thin as possible and at the same time 
insulate the separate particles as completely as possible. Thus the amount 
of phosphorus must be higher for powders with a larger specific surface 
area. As sponge powders have a higher specific surface area than atomized 
powders, the amount of P should generally be higher for sponge powders 
than for atomized powders. In the first case the P amount may vary between 
about 0.02 and 0.06, preferably between 0.03 and 0.05 whereas in the 
latter case the P amount might vary between 0.005 and 0.03, preferably 
between 0.008 and 0.02% by weight of the powder. It was quite unexpected 
that the very thin-insulating layer, which is characterized by a very low 
P-content could withstand the heat-treatment according to the invention 
without degradation. 
The dried P-coated powder could optionally be mixed with a thermosetting 
resin. This is particularly the case if it is required that the final 
component should have relatively high tensile strength. According to a 
preferred embodiment a phenol-formaldehyde resin is used as thermosetting 
resin. An example of a commercially available thermosetting resin is 
Peracit.RTM. from Perstorp Chemitec, Sweden. The resin particles which 
preferably should have a fine particle size are mixed with the P-coated 
iron powders. When Peracit.RTM. is used curing temperatures of about 
150.degree. C. are convenient, and the curing period might be about an 
hour. 
Before the compacting step the P-coated iron powder or the P-coated iron 
powder containing the resin is mixed with a suitable lubricant. 
Alternatively, the die is lubricated. The amount of lubricant should be as 
low as possible. One type of lubricant which is useful according to the 
present invention is Kenolube.RTM. available from Hoganas AB, Sweden, 
which can be used in an amount of 0.3-0.6% by weight of the powder. The 
compacting step is carried out in conventional equipment, usually at 
ambient temperature and at pressures between about 400 and 1800 MPa. 
In the final heat-treatment step the compacted mixture is subjected to a 
temperature between 350.degree. and 550.degree. C. Preferably the 
temperature varies between 420.degree. and 530.degree. C. and most 
preferably between 430.degree. and 520.degree. C. The heat treatment is 
preferably carried out in one step but alternatively the resin might be 
cured at the recommended curing temperature in a first step. For 
phenol-formaldehyde of the type discussed above the curing temperature is 
about 150.degree. C. and the curing period about an hour. 
The invention is illustrated in the following examples.

EXAMPLE 1 
Sponge iron powder and atomized powder were treated with aqueous phosphoric 
acid to form a phosphate layer on the surface. After drying the powder was 
mixed with 0.5% Kenolube and/or resin and compacted in a die at 800 MPa to 
form toroids with outer diameter 5.5 cm, inner diameter 4.5 cm and height 
0.8 cm. The component was then heated at 150.degree. C., alternatively 
500.degree. C., for 60(30) minutes in air. 
Materials operating at high frequency i.e. above 1 kHz require high 
permeability (.mu.), eddy current loss causes a rapid depletion of 
permeability with increasing frequency. Insulated iron powder cores can be 
produced with permeability values ranging from very low up to 90 at a 
frequency of 5 kHz. The use of heat treatment, according to this 
invention, to increase the permeability while maintaining an effective 
insulation layer for minimum eddy current losses results in permeability 
values as high as 130 at 5 kHz as illustrated in Table 1. 
TABLE 1 
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Sponge Sponge Atomized 
&lt;150 .mu.m &lt;150 .mu.m &lt;150 .mu.m 
Temper- 
+0.5% Peracit 
+0% Resin +0.5% Peracit 
ature +0.5% Kenolube 
+0.5% Kenolube 
+0.5% Kenolube 
______________________________________ 
150.degree. C. 
.mu. at 5% kHz = 75 
.mu. at 5 kHz = 77 
.mu. at 5 kHz = 73 
500.degree. C. 
.mu. at 5% kHz = 115 
.mu. at 5 kHz = 130 
.mu. at 5 kHz = 100 
600.degree. C. .mu. at 5 kHz = 42 
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The use of small particle size iron powder will extend the frequency range 
for which a stable permeability is achieved. A constant permeability of 
100 is maintained at 25 kHz when the particle size of the iron powder is 
reduced to &lt;40 .mu.m. 
The total loss is considerably reduced by the heat treatment procedure. In 
contrast to the conventional material of laminated steel the total loss of 
the insulated powder is dominated by hysteresis loss which is relatively 
high at low frequency. However due to the heat treatment, the hysteresis 
loss is decreased. As the insulation layer is surprisingly not degraded by 
the heat treatment the eddy current loss remains low. At higher frequency 
a large eddy current loss will result in a considerable increase in total 
loss. As illustrated in Table 2 the heat treatment reduces the hysteresis 
loss of the insulated powder resulting in a total loss of 13 W/kg for the 
atomized grade compared with 14 W/kg for the conventional laminated steel. 
TABLE 2 
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Ref Atomized 
Conventional Sponge &lt;150 .mu.m + 
&lt;150 .mu.m + 
Laminated 
Temper- 0% Peracit + 0.5% Peracit + 
Steel 1018 
ature 0.5% Kenolube 
0.5% Kenolube 
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P.sub.1.5/50 = 
150.degree. C. 
P.sub.1.5/50 = 
P.sub.1.5/50 = 
14 W/kg 25 W/kg 20 W/kg 
500.degree. C. 
P.sub.1.5/50 = 
P.sub.1.5/50 = 
20 W/kg 15 W/kg or 
13 W/kg with- 
out resin 
600.degree. C. 
P.sub.1,5/50 = 
27 W/kg 
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The use of large particle size iron powder is known to result in high 
permeability values. Insulation of the particles reduces the total loss. 
The use of heat treatment, according to this invention, on insulated iron 
powder with a particle size of &gt;150 .mu.m results in a low total loss of 
P.sub.1,5/50 =13 W/kg fully comparable with that achieved with &lt;150 .mu.m 
particles. However the maximum permeability of the &gt;150 .mu.m powder is 
500 compared to 400 when the particle size is &lt;150 .mu.m. 
At higher frequency the dominant eddy current loss in the conventional 
material will increase the total loss at a faster rate with increasing 
frequency. Surprisingly the heat treatment has not caused the insulation 
layer to disintegrate causing metal to metal contact. The low eddy current 
loss of the insulated material results in lower total loss with increasing 
frequency. This is illustrated by the example in Table 3 where the low 
eddy current loss of the insulated powder results in a total loss of 65 
W/kg for the atomized grade after heat treatment. The high eddy current 
loss of the conventional laminated steel results in a total loss of 115 
W/kg at 1000 Hz and 0.5 Tesla--a result which exceeds that of the 
insulated powder heat treated at 150.degree. C. 
TABLE 3 
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Ref Atomized 
Conventional 
Sponge &lt;150 .mu.m + 
&lt;150 .mu.m 
Laminated +0.5% Peracit +0.5% Peracit 
Steel 1018 +0.5% Kenolube +0.5% Kenolube 
______________________________________ 
150.degree. C. 500.degree. C. 
P.sub.0.5/1000 = 
P.sub.0.5/1000 = 
P.sub.0.5/1000 = 
115 W/kg 100 W/kg 75 W/kg or 
65 W/kg with- 
out resin 
______________________________________ 
EXAMPLE 2--Comparison Between the Process According to the German Patent 3 
439 397 and the Present Invention 
A water atomized iron powder ABC 100.30, available from Hoganas AB, Sweden 
was subjected to treatment with phosphoric acid and dried as described in 
example 1 of the patent. After drying for 1 h at 100.degree. C., the 
powder was compacted at 800 MPa and the compacted product was heated at 
500.degree. C. for 30 minutes. 
The obtained product was compared with a product prepared according to the 
present invention. This product was prepared from the same base powder ABC 
100.30, but subjected to a phosphoric acid treatment such that the 
P-content was 0.01% by weight. This was achieved by subjecting the powder 
to an 1.85% aqueous orthophosphoric acid solution which was added to the 
iron powder in a quantity of 8 ml/kg and mixed for 1 minute. The obtained 
mixture was dried at 100.degree. C. for 60 minutes and the powder was 
compacted at 800 MPa and the compacted product was heated at 500.degree. 
C. for 30 minutes in air. It is not clarified if the insulating layer 
actually is made up of phosphate. However, the layer is extremely thin 
and, so far, not identified as to chemical composition. A comparison 
disclosed that measured properties, such as flow, green strength and 
density, were superior for the product according to the present invention. 
The following is a comparison of the magnetic properties total losses and 
permeability: 
______________________________________ 
Total losses 
product according to 
product according to 
DE patent present invention 
______________________________________ 
P 0.5T/1000 Hz = 88 W/kg 
P 0.5/1000 Hz = 75 W/kg 
P 1.5T/1000 Hz = 850 W/kg 
P 1.5/1000 Hz = 700 W/kg 
Permeability .mu. at H.sub.max and 50 Hz/0.5T 
160 320 
______________________________________ 
The P-contents of the powder according to the DE patent and according to 
the present invention were 0.206 and 0.013 respectively. 
The above comparison discloses that the process according to the present 
invention, which, as compared with the process according to the German 
patent, is simplified, requires less energy and is environmentally 
advantageous and results in products having superior properties.