Patent Publication Number: US-6989062-B2

Title: Heat treatment of iron-based components

Description:
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. 
     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 electric 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 on 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 of the insulated powder to a SMC component induced stresses, especially when the component is compressed to higher densities, which has 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, in 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 and the size of the component. This is a particularly the case in industrial production. Another problem, which has also been observed in industrial production, is that the component surface is stained by residues of incompletely decomposed lubricants. 
     It has now surprisingly been found that powder metallurgically produced SMC components having a high quality surface without stains can be obtained by subjecting a compacted body consisting of a soft magnetic material of insulated powder particles and a lubricant, to a stress relieving heat treatment in a furnace until the component has reached a temperature of at least 400° C. in an atmosphere having a CO content less than 0.25% by volume. Preferably, heat treatment is performed until the component has reached a temperature between 450 and 650° C., and most preferably between 450 and 550° C. The heat treatment is performed in an oxygen containing atmosphere, preferably in air. 
     According to a preferred embodiment the method may 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 that the measured value of the CO concentration is used for 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 invention will be further illustrated by following example: 
    
    
     EXAMPLE 1 
     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 of 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/cm 3  was 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 of 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 80 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 to 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.  FIG. 5  shows that the component heat treated at 600° C. and 28 minutes has a better surface finish compared with the components heat treated at 500° C. The surface finish of the component heat treated at 500° C., 50 min. was acceptable and much better than the surface finish of the component heat treated at 500° C., 28 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.