More particularly, the invention relates to soft magnetic composites having improved strength. These composites which combine good soft magnetic properties with high strength are particularly useful as components in electrical machines.

Currently used components of soft magnetic compo- sites prepared from pressure compacted coated iron powder have a relatively low compressive strength. This is due to the fact that these materials cannot be subjected to the usual method of improving the strength i. e. sin- tering, since the high temperature required for sintering damages the insulating coating around the powder par- ticles. Today soft magnetic composites are heat treated at a temperature below the sintering temperature in order to improve the magnetic characteristics. Also the com- pressive strength of the component can be somewhat improved by such a heat treatment. Thus the W095/29490 discloses a method of making a component having improved magnetic properties by compacting or die-pressing a powder composition of insulated particles of an atomised or sponge iron powder optionally in combination with a lubricant and in some cases a binder and subsequently subjecting the compacted composition to heat treatment in air at a temperature preferably not more than 500°C. The strength of components prepared according to this patent is in the range 50-100 MPa, the higher strength being achieved at the cost of poorer magnetic properties. This strength is comparatively low and insufficient for cer- tain applications.

The Japanese Patent Publication 51-43007 discloses a method of manufacturing iron-based machine parts whereby an iron powder is pressure-compacted to obtain a green compact and the green compact is heated under an oxi-

dising atmosphere including vapour at 400-700 °C. The purpose of this known method is to form iron oxide onto the surface of each iron grain. This procedure replaces the two steps involving dewaxing, i. e. the removal of lubricant, which usually is carried out at a temperature of at least 400°C, and sintering, which is carried out at a temperature of at least 1100°C to form bonds between the metal particles. The Japanese publication also teaches that sizing of the body can be avoided because of the fact that the compacted and heat treated parts have high dimensional accuracy. The Japanese publication does not concern magnetic materials.

It has now been found that if uncoated iron powder particles, i e iron particles which are not provided with an insulating layer, are compacted and subsequently treated with vapour the strength of the material will in- crease but the energy loss in the material will be un- acceptably large. When it comes to the coated iron powder particles used for magnetic applications it was found that the energy loss in coated material increases with increasing frequency and this tendency is even larger for vapour treated material than for coated material heated in air. During extensive studies it was however found that for frequencies less than 1000, preferably less than 300 Hz it is possible to prepare soft magnetic composites having improved strength and a low energy loss.

Accordingly, the present invention concerns com- pacted, soft magnetic composites for AC applications which have improved strength in combination with low energy losses and which composites essentially consist of compacted electrically insulated particles of a soft magnetic material. A distinguishing feature of the inven- tion is that the compacted composite material is sub- jected to vapour treatment.

The soft magnetic material might be any type of known material, such as essentially pure iron powders, e. g. atomised or sponge iron powders or prealloyed iron-

based powders containing e. g. Ni, Si, Al or Co having a low carbon content.

Furthermore, the particles of the soft magnetic material must be coated or provided with an electrically insulating layer to minimise the eddy current loss in the compacted part. The type of insulating coating is not critical as long as metal to metal contact and cold wel- ding between the particles are avoided and the coating is stable during the compaction and subsequent heat treat- ment. The coating might be based on phosphorous oxides or phosphate, silicon oxide or polymers, such as polyamides.

It is preferred that the coating is very thin in order to have as little effect on the density of the compacted part as possible.

A specific example of an atomised iron powder with a suitable insulation is ABM 100.32 available from Hoganas AB, Sweden and disclosed in the publication WO 95/29490, which is hereby incorporated by reference. According to this publication particles of atomised or sponge iron are treated with an 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 and then the powder is dried. A suitable insulated sponge iron powder is SCM 100.28, which is also available from Höganäs AB.

Before compaction the powder of the electrically insulated particles is normally mixed with a lubricant.

The compaction could however also be carried out in a lubricated die. A combination of lubricant in the mixture and the use of a lubricated die is also possible. The compaction pressure normally is generally below 1000 MPa and varies preferably between 400 and 800 MPa. The amount of lubricant is normally less than 1 % by weight of the powder composition and varies preferably between 0.05 and 0.8 % by weight. Various types of conventional lubricants can be used, such as metal soaps, waxes and polyamides.

The temperatures for the vapour treatment usually vary between 400 and 700°C. The preferred temperatures varies between 420 and 580°C. According to a preferred embodiment the compacted composite material is first heated in a furnace with an atmosphere consisting of air. When the desired elevated temperature has been reached the vapour is introduced into the furnace. The vapour treatment is then carried out at atmospheric pressure or slightly above atmospheric pressure. The vapour treatment time should normally be between 5 and 60 minutes, preferably between 10 and 45 minutes.

The invention is further illustrated by the fol- lowing non limiting examples.

Example 1 ABM100.32, an atomised iron powder available from Hoganas AB, Sweden was mixed with 0.5% by weight of the lubricant Kenolube and compacted at 800 MPa to magnetic rings (toroid rings with an inner diameter of 45 mm, an outer diameter of 55 mm and a thickness of 5 mm) and TRS- bars (dimensions approximately 30x12x6 mm) used to measure the bending strength.

The sample was vapour treated at 500°C for 30 minutes. Another sample was treated at 500°C for 30 minutes in air for comparison. The samples were removed from the furnace and cooled to room temperature. The bending strength after this treatment was 205 N/mm2, and the energy losses measured at different frequencies are listed in table 1.

Example 2 Somaloy"500 which is available from Höganäs AB, Sweden, and is atomised powder with an insulating layer, was compacted at 800 MPa and then treated in the same way as ABM 100.32 in example 1. The bending strength after this treatment was 130 N/mm2, and the energy losses measured at different frequencies are listed in the following table. MaterialBendingDensityLossatLossatLossat (heat-strengthof50Hz100Hz300Hz treatment[N/mm~]toroids1,5T1,5T 1,5T at[g/cm3][W/kg][W/kg][W/kg] density [g/cm3]) ABM100.32 205 7, 33 30 100 590 (vapour(7,31) 500°C 30min) ABM100.32507,261530120 (air500°C(7, 35) 30min) Somaloy"5001307,332050180 (vapour(7,35) 500°C 30min) Somaloy"500457,32153090 (air500°C(7, 33) 30min) ABC.100.30*135 7. 36 50 1701220 (vapour(7.36) 500°C 30min)

* Powder without insulation for comparison The above table illustates the effect of vapour treatment on components of coated iron powders compared with conventional heat treatment in air and with an uncoated iron powder ABC 100.30 (available from Hoganas AB, Sweden). The difference between the coated powders on one hand and the uncoated powder on the other hand is very clearly demonstrated in figure 1, wherein"Uncoated" refers to the powder ABC. 100.30, coating 1 refers to the powder ABM 100.32 and coating 2 refers to the powder Soma 10YT'500.

Additionally, as can be seen from the enclosed figures 2 and 3, the bending strength (TRS) and the losses vary not only with the type of insulation but also with the temperature. The optimum time and temperature is specific to each insulated powder.