|Title of Invention||
A METHOD OF IMPROVING THE MAGNETIC PROPERTIES OF POWDER METALLURGICALLY PRODUCED SOFT MAGNETIC COMPOSITE COMPONENTS
|Abstract||The present invention concerns a method of improving the properties of powder metallurgically produced SMC compacted body consisting of a soft magnetic material of insulated powder particles and an organic lubricant, to a stress relieving heat treatment in a furnace until the component has reached a temperature of at least 400 °C in an oxygen containing atmosphere having a CO content is less than 0.25 % by volume.|
HEAT TREATMENT Or IRON-BASED COMPONENTS
FIELD OF THE 'INVENTION
The present invention concerns soft magnetic composite components. Particularly the invention concerns' a method of improving the properties of such components by controlling the conditions during heat treatment of the soft magnetic composite components.
BACKGROUND OF THE INVENTION
Soft magnetic materials are used for applications, such as core materials in inductors, stators, rotors, electrical machines, actuators and sensors. Traditionally soft magnetic cores, such as rotors and stators in elec-cric machines are made of stacked steel laminates. Soft Magnetic Composite, SMC, materials are based.on soft magnetic particles, usually iron- based, with an electrically insulating coating en each particle. SMC parts are made by compacting insulated particles together with lubricants, and/or binder using the traditionally powder metallurgy process. By using such powder metallurgically produced materials a higher degree of freedom in the design of the SMC component is permitted than by using the steel laminates as the SMC material can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained by the compaction process.
However, compaction cf the insulated powder particles to a SMC component induces stresses, especially when the component is compressed to higher densities. These stresses have a negative influence of magnetic properties, such as permeability and hysteresis losses. Heat treatment will have a stress relieving effect and will hence partially restore the permeability and hysteresis
losses. The heat treatment must, however, not result in the deterioration of the insulating layer /coating as then metal to metal contact occurs -and*the eddy current losses increase. Additionally, ir. order to avoid cold welding between the iron particles and to maintain the continuous coating during the pressing operations, it is recommended to add lubricants the insulated powder.
A problem encountered when heat treating the powder metallurgically produced SMC components is that the magnetic properties tend to vary depending on the conditions of the heat treatment. This is particularly the case in industrial production. Ancther problem, which has also been observed in industrial production, is that the component surface is stained.
OBJECTS OF THE INVENTION
An object of the invention is to provide a method which results in components wherein the magnetic properties are improved and more consistent.
Another object of the invention is to provide a. % method which results in components without stained surfaces.
SUMMARY OF THE INVENTION
In brief it has been found that these objects as well as other objects which willsbe apparent from the following can be obtained by controlling the furnace atmosphere wherein the SMC component is heat treated. Specifically it has been found that the"" CO content of the furnace atmosphere should be controlled.
DETAILED DESCRIPTION OF THE INVENTION The SMC components are suitably prepared from ferromagnetic powders, the particles of which are provided with an electrically insulated coating. Before compaction the powders are mixed .with an organic lubricant. The compacted component is subsequently heat treated in an oxygen containing furnace atmosphere such as air.
The ferromagnetic powders especially contemplated according to the present invention are based on base powders which essentially consist of pure iron and could be e.g. a commercially available water-atomised iron powder or a sponge iron powder with round, irregular or flat particles. Typical examples of irregular, water-atomised powders which car. be used are the powders of the ABC 100 and ASC 100 series available from Hoganas AB, Sweden. The particle size of the base powder depends on the intended final use of the powder and is generally less than 500 pm. For higher frequencies, particles sizes below 45 ]im are preferred. These base powders are provided with an oxygen coating or barrier, and it is a distinctive feature that the amount of oxygen of the powders is only slightly elevated as compared with that of the base powder. More specifically the amount of oxygen in the powder is at most 0.2%, preferably at most 0.15% by weight higher than in the base powder. The insulating coating is applied on the base powder by treating the base powder with phosphoric acid in an organic solvent as discribed in the US patent 6,348,265 which is hereby incorporated by refextinct:. xnus, tne invention is particularly directed to soft magnetic powders wherein the insulated powder particles consist of a base powder of essentially pure iron having a very thin insulating oxygen- and phosphorus-containing barrier.
It has now been found that the CO content of the furnace atmosphere, which should preferably contain at leas.t.10 % by volume of oxygen, plays an important role for the properties of the final SMC compact. The CO content of the furnace atmosphere varies depending on the type and amount of lubricant used as well as the degree of decomposition of the lubricant during the heat treatment in the furnace. As high as up to 5 % by volume of CO may be obtained in the furnace atmosphere. By controlling the CO content to a value below 0.25 % by volume it has not only been found that more consistent magnetic properties can be obtained but it has also been found that magnetic properties, such as losses and frequency stability of the initial permeability, can be improved. These advantages are more pronounced the lower CO content in the furnace atmosphere. It is thus preferred that CO content is below about 0.1 or even below 0.05 % by volume. Without being bound to any specific theory it is believed, that high levels of CO impair the surface coatings of the insulated powder particles and as a result the frequency stability is lower for the material heat treated at high concentrations of CO. Furthermore we have found that a decrease in CO concentration results in a decrease of the total losses. Thus, by controlling the CO-content of the atmosphere, it is possible to improve the magnetic properties of the SMC parts.
In practice the inventive method may suitably be performed by measuring the concentration of CO in at least one point of the heat treatment, furnace during the whole heat treatment cycle, and the measured value of the CO concentration is used fcr controlling the furnace atmosphere. The CO content may thus be adjusted by controlling the air flow through the furnace. Furthermore, the
furnace temperature may be set at a value above the maximum intended component temperature. The temperature of the SMC component is -then measured and the heat treating cycle is terminated when the temperature of the component reaches the intended component temperature. The heat treatment may thus be terminated when the component has reached a temperature of at least 400° C. Preferably, the heat treatment is performed until the component has-reached a temperature between 450 and 650°C, and most
preferably between 450 and 600°C. Suitable temperature settings for the furnace are then about 450 to 1000°C. The heat treatment process can be followed by measuring the component temperature and is interrupted when the final component temperature has been reached. The period, during which-the component is subjected to the heat treatment in the .furnace varies depending on the size of the component and the desired final temperature of the component and may easily be determined by the man skilled in the art.
An additional advantage of the invention is that residues of organic lubricants present on the surface of the component subjected to the stress relieving heat treatment can be eliminated by utilizing the possibility of using higher furnace temperatures in combination with shorter dwell times enabled by measuring the component temperature.
The subsequent cooling of "he heat treated component is preferably made in air, but furnace cooling or cooling in other media is also possible-
BRIEF DESCRIPTION 0? THE DRAWINGS
Fig 1 shows initial permeability as a function of the frequency at different CO-ccntents.
Fig 2 shows core losses as a function of the frequency at an induction of 1 Tesla at different CO-contents.
Fig 3 shows component temperature as a function of dwell time at different furnace temperatures.
Fig 4 shows initial permeability as a function of the frequency heat treated at different temperatures and dwell times.
Fig 5a-c shows surface appearance of heat treated components. ^ t ^~~
The invention will be further illustrated by the following examples:
Magnetic rings with an inner diameter of 45 mm, an outer diameter of 55 mm and a height of 5 mm were produced by compaction x>f a pure iron based powder with a continuous coating, Somaloy 500™, together with 0.5% of the lubricant Kenolube™. The compaction pressure was 800 MPa and a green density of 7.35 g/cm3was obtained. The rings were heat treated in air at 500°C in a continuous production furnace at different CO concentrations obtained by adjusting the flow of air through the furnace.
The initial permeability was measured as a function of the frequency. The ability of the obtained SMC component to maintain the initial permeability at higher frequency is referred to as frequency stability.
Fig 1 shows that the frequency stability is higher for the material heat treated at lower concentrations of
CO. For a concentration cf 0.25% CO, and below, acceptable values for the frequency stability were obtained.
The total losses were also measured and fig 2 shows that total loss for material heat treated at three different CO concentrations. Fig 2 shows a decrease in total losses when the CO concentration is decreased. Example 2
Cylindrical SMC components with the diameter of 80 mm, height of 30 mm and weight of approximately 1 kg were produced with the same iron-based powder mixture as in example 1 and the heat treatment was performed at two different furnace temperatures, 500 and 600°C, respectively. For the components heat treated at. 500°C the heat treatment was terminated after 30 minutes and 55 minutes, respectively. For the components heat treated at 600°C the process was terminated after 28 minutes.
Fig 3 shows the temperature profile of the components and it can be concluded that the -temperature* of the component heat treated at an furnace temperature of 600°C reached 550°C after 28 minutes.
Fig 4 shows that the same permeability is obtained for components heat treated at 500°C, 55 minutes and for components heat treated at 600°C, 28 minutes, whereas components heat treated at 500°C for 30 minutes have a lower permeability up to the frequency of about 8 0 kHz.
The frequency stability of the components heat treated at an furnace temperature of 600 C, 28 min and 500°C, 50 min is acceptable and as the permeability is higher below 80 kHz for these components compared with components heat treated at 500°C, 30 min the method of utilising a higher furnace temperature and a shorter dwell time is preferable.
The surfaces of the components were visually evaluated with respect to surface finish. Figure 5b shows that
the component heat treatec at 600°C and 28 minutes has a better surface finish compared with the components in fig 5a heat treated at 500°C 20 min. The surface finish of the component in fig 5c heat treated at 500°C/ 50 min was acceptable and much better than the surface finish of the component heat treated at 500°C/ 30 min. but less shiny compared with the component heat treated at 600°C, 28 min. An increased productivity can thus be obtained by using a higher heat treating temperature and a lower dwell time without deteriorating the magnetic permeability. A better surface finish can also be obtained.
1. Method of improving the magnetic properties of powder
metallurgically produced SMC components- by
- subjecting a compacted body consisting of a soft magnetic material of insulated powder particles and an organic lubricant, to a stress relieving heat treatment in a furnace atmosphere;
- controlling the furnace atmosphere to a CO content less than 0.25% by volume; and
- terminating the heat treatment when the component has
reached a temperature of at least 4 00°C.
2. Method according to claim 1, wherein the CO content in
the furnace atmosphere is less than 0.1% by volume pref
erably less than 0.05% by volume.
3. Method according to claim 1 or 2, wherein the insu-lated powder particles consist of a -base powder of essentially pure iron having an insulating oxygen- and phosphorus-containing barrier.
4. Method according to any one of the claims 1-3, wherein the heat treatment is terminated when the component has
reached a temperature between 450 and 650°C, preferably
between 450 and 600°C.
5. Method according to any one of the claims 1-4 wherein
the heat treatment is performed in a furnace atmosphere
containing at least 10 % by volume of oxygen;
at a furnace temperattu&e setting between 450 and 1000°C.
6. Method according to any one of the claims 1-5 wherein
that the concentration of CO is measured in at least one
point of the heat treatment furnace during the whole heat
7. Method according to any one of the claims 1-6 wherein
the CO content is reduced to a value below 0.25, prefera
bly below 0.1% and most preferably below 0.05% by volume
by controlling the air flow through the furnace.
|Indian Patent Application Number||697/CHENP/2005|
|PG Journal Number||28/2011|
|Date of Filing||21-Apr-2005|
|Name of Patentee||HOGANAS AB|
|Applicant Address||S-263 83 Höganäs|
|PCT International Classification Number||B22F 3/24|
|PCT International Application Number||PCT/SE2003/001631|
|PCT International Filing date||2003-10-22|