HIGH STRENGTH SOFT MAGNETIC ALLOYS

Abstract: The invention provides a soft magnetic alloy consisting essentially of: 45 % to 55 % cobalt; 0.75 % to less than 1.5 % vanadium; 0.15 % to 0.5 % niobium or tantalum or mixtures thereof; 0 % to 0.3 % manganese; 0 % to 0.2 % silicon; 0 % to 0.1 % carbon; and the balance iron, all percentages being by weight based on the total weight of the alloy. The alloys can be used to manufacture both rotor and stator components of electrical machines. Control of heat treatment conditions allows outstanding mechanical and/or soft magnetic properties to be achieved.

Description: HIGH STRENGTH SOFT MAGNETIC ALLOYS The present invention relates to improved soft magnetic alloys. The present invention also relates to processes for the preparation of such alloys. Specialised soft magnetic alloys are needed for electrical machine components moving at high speed. Examples of such machines are electrical generators and magnetic bearings. Higher operational speeds are sought in order to achieve improved performance and to widen the field of operation of these machines. As a consequence, soft magnetic materials having increased mechanical strength, and in particular increased yield stress, are required to withstand the stress levels associated with higher rotation speeds. Typically, a minimum yield stress of 600 MPa is specified. In many conventional electrical machines, both a stator and a rotor are constructed from laminations stamped from a strip of soft magnetic material. For simplicity, both the stator and rotor laminations are preferably stamped from the same soft magnetic alloy strip. For best electrical performance, the soft magnetic alloy should preferably exhibit high magnetic saturation and low eddy current losses. The main technology used hitherto for soft magnetic alloys in such applications is an alloy consisting essentially of 45-55% Co, 1.5-2.5% V, with the balance Fe. The preferred composition is 49/49/2 Fe/Co/V. The vanadium is added to improve ductility for cold working and also to increase resistivity, thus reducing eddy current losses. The mechanical and magnetic properties of such alloys are summarised in Table 2 below. GB-A-1523881 describes the addition of nickel to the 49/49/2 Fe/CoN alloy to produce a substantial increase in yield stress. Typically, the alloy has composition 44% cobalt, 3.5% nickel, 1.5% vanadium and the balance iron. These alloys have increased yield strength, but exhibit less favourable magnetic saturation and 400 Hz eddy current losses than the basic 49/49/2 alloys, as summarised in Table 2 below (Alloy A). GB-A-2207927 describes soft magnetic alloys comprising 33-55% cobalt, 0.15%-0.5% tantalum or niobium or mixtures thereof, and the balance iron. The use of tantalum or niobium instead of vanadium results in improved magnetic saturation as compared to 49Co/49Fe/2V alloys. See Alloy C in Table 2. US-A-3634072 describes a soft magnetic alloy containing 45-52% cobalt, 0.5-2.5% vanadium, at least one element selected from the group consisting of 0.02-0.5% niobium and 0.07-0.3% zirconium, and the balance iron. The preferred compositions and examples all contain at least 1.5% of vanadium, preferably 1.8- 2.2% vanadium. A version of such an alloy is commercially available under the registered trade mark HIPERCO Alloy 50HS from Carpenter Technology Corporation, and typically contains 49% cobalt, 1.9% vanadium, 0.3% niobium, and the balance iron and minor impurities. The magnetic properties of such an alloy are, however, inferior to those 49Co/49Fe/2V, as can be seen from the example (Alloy B) in Table 2 below. It has now been found that FeCo soft magnetic alloys containing niobium or tantalum, and containing quantities of vanadium substantially less than the conventional 2% by weight, have surprisingly improved mechanical and soft magnetic properties. Furthermore, these properties can be tailored for different applications by controlled heat treatment of the alloys. Accordingly, the present invention provides a soft magnetic alloy consisting essentially of: 45% to 55% cobalt; 0.8% to less than 1.5% vanadium; 0.15% to 0.5% niobium or tantalum or mixtures thereof; 0% to 0.3% manganese; 0% to 0.2% silicon; 0% to 0.1% carbon; and the balance iron, all percentages being by weight based on the total weight of the alloy. The term "consisting essentially of implies that the alloy consists of the stated components, plus incidental impurities present in amounts too small to influence the mechanical or magnetic properties of the alloy to any significant extent. Preferably, the total level of such incidental impurities is less than 0.3% by weight based on the total weight of the alloy. The soft magnetic alloy preferably comprises 48 to 51% cobalt, 0.9% to 1.3% vanadium, 0.25% to 0.4% niobium or tantalum, less than 0.15% manganese, less than 0.15% silicon, and less than 0.02% carbon. Preferably, the soft magnetic alloys contain less than 0.1% niobium, and more preferably they contain less than 0.02% of niobium, most preferably substantially no niobium. The soft magnetic alloy may be in the form of a powder, or an article such as a rod, strip or the like. Preferably, the soft magnetic alloy is in the form of a strip, whereby laminations for a magnetic machine can be stamped from the strip. The grain size and order/disorder state of the individual grains in the strip can be controlled by conventional metallurgical techniques such as melting, rolling, annealing, quenching and the like in known fashion. Preferably, the formed article has a yield strength of at least 500 MN/m 2 , more preferably at least 600 MN/m 2 . Preferably, the soft magnetic alloy has a magnetic induction at field strength H = 1600 A m of more than 2.05T, preferably more than 2.10T, and most preferably at least 2.18T. Preferably, the strip or other body of soft magnetic alloy according to the present invention exhibits magnetic induction losses at B = 2T, strip thickness (t) of 0.35 mm, and 400 Hz, of less than 150 W/kg, preferably less than 100 W/kg. The present invention also provides a rotor for an electrical machine, wherein the rotor comprises a soft magnetic alloy according to the present invention, said alloy preferably having a yield strength of at least 600 MN/m 2 . The present invention further provides a stator for an electrical machine, comprising a soft magnetic alloy according to the present invention, said alloy preferably having a power loss at B = 2T, t = 0.35 mm, and 400 Hz of less than 100 W/kg. The present invention further provides an electrical machine comprising a rotor according to the invention and a stator according to the invention. Preferably, the soft magnetic alloy of the rotor and the soft magnetic alloy of the stator have the same chemical composition. This can be accomplished by stamping the rotor and the stator from the same soft magnetic alloy strip, followed by different heat treatments of the rotor and stator laminations to provide the desired mechanical and magnetic properties. The present invention further provides a process for the preparation of a soft magnetic alloy comprising the steps of: providing an alloy composition consisting essentially of: 45% to 55% Cobalt; 0.8% to less than 1.5% Vanadium; 0.15% to 0.5% Niobium or Tantalum or mixtures thereof; 0 to 0.3% Manganese; 0 to 0.2% Silicon; 0 to 0.1% carbon; and the balance iron; all percentages being by weight based on the total weight of the alloy; and heat treating said alloy composition under non-oxidizing conditions at a temperature of 700°C to 920°C. Preferably, the alloy composition is a composition according to one of the preferred embodiments of the present invention as hereinbefore defined. The alloy composition may be formed, for example, by vacuum induction melting of the metal components. The heat treatment step is typically carried out under vacuum, or under a reducing atmosphere such as hydrogen, or under an inert atmosphere such as argon, or under mixtures of such atmospheres. The heat treatment is preferably carried out for a time of from 0.5 to 8 hours, more preferably 1 to 3 hours. The heat treatment is preferably carried out either in a temperature range of 850°C to 920°C, or in a temperature range from 700°C to 760°C. The higher temperature range tends to give improved permeability and lower magnetic induction losses. However, heat treatment at the higher temperature tends to lower the yield strength of the resulting alloy. In contrast, carrying out the heat treatment in a temperature range of 700-760°C gives a material having a higher yield strength, but generally less good magnetic properties. Therefore, the lower temperature range is typically used for magnetic rotor components, where high yield strength is required. The higher temperature range is used for stator components of magnetic machines, since mechanical performance is less critical for the stator components that are not subject to rotational forces. It can thus be seen that, in preferred embodiments, both the rotor and the stator components can be formed from the same alloy composition, or even stamped from the same strip, followed by different heat treatment regimes to achieve the desired balance of mechanical and magnetic properties. The alloy composition is processed into an alloy body, preferably an alloy strip, prior to the step of heat treating. Preferably, this process comprises melting the alloy, hot working the alloy, reheating the alloy above its order/disorder temperature, followed by quenching the alloy to lock in the disordered state. Specific embodiments of the prior art and of the present invention will now be described in more detail, in and by the following examples. Comparative Example 1 For comparison purposes, a 49%Co/49%Fe/2%V alloy strip was prepared by vacuum melting, hot working to a 2.5 mm thick strip, reheating above the order/disorder temperature, water quenching and rolling to 0.35 mm final thickness. Magnetic and tensile test samples were stamped from the strip. The test samples were heat treated in dry hydrogen for 2 hours at 680°C and 760°C. The mechanical and magnetic properties of the resulting strips are shown in Table 1. TABLE 1 It can be seen that the 49Co/49Fe/2V strip annealed at 680°C has high yield strength, but relatively poor magnetic saturation and induction losses. In contrast, the sample heat treated at 760°C has good magnetic induction saturation and low losses, but a yield strength that is too low for use in electrical machine components subject to high operational loadings. Comparative Example 2 The following comparative alloys A and C were prepared and heat treated as described above for Comparative Example 1. The resulting mechanical and magnetic properties for a strip 0.35 mm thick are shown in Table 2. The data for comparative Alloy B are drawn from published sources. Alloy A 44% Co, 3.5% Ni, 1.5% V, balance Fe Alloy B 49% Co, 1.9% V, 0.3% Nb, balance Fe Alloy C 50% Co, 0.24% V, 0.28% Ta, 0.06% Mn, 0.06% Si, 0.002% C, balance iron TABLE 2 It can be seen that none of these comparative examples provides the best combination of high mechanical yield strength, and good magnetic properties that is needed for rotating components of magnetic machines. Example 3 The following soft magnetic alloy strips according to the present invention were prepared and heat treated as described in Example 1. The resulting mechanical and magnetic properties are shown in Table 3. Alloy 1 : 50.1 % Co, 1.04% V, 0.32% Ta, 0.06% Mn, 0.09% Si, 0.014% C, balance Fe Alloy 2: 49.5% Co, 1.42% V, 0.30% Ta, 0.05% Mn, 0.08% Si, 0.006% C, balance Fe TABLE 3 It can be seen that for both alloys the heat treatment at 720°C results in yield strengths in excess of 600MN/m 2 . The heat treatment temperature of 740°C results in yield strengths in excess of 500MN/m 2 . The DC magnetic performance figures are comparable to these of the standard 49/49/2 alloy heat treated at 760°C [Table 1] and superior to those of the known high strength alloys [Table 2]. The AC loss performance figures for these alloys of the invention heat treated at 720°C and 740°C are inferior however to the conventional 49/49/2 alloy. This is not generally of importance for rotor lamination application since they normally operate under dc conditions. When the alloys of the invention are heat treated at higher temperature, i.e. 880°C, it can be seen from Table 3 that both the DC and AC properties improve to levels that are equivalent or superior to that of the conventional alloy. Whilst a loss in mechanical strength does occur when the heat treatment temperature is increased to 880°C this is not of great importance from a stator lamination application viewpoint since they are not subject to the rotational forces. The better loss performance required for the stator laminations can therefore be achieved by using the higher temperature heat treatment 880°C. It can be seen that the alloys according to the present invention annealed at 720°C provide not only a yield strength above 600 MN/m 2 making them suitable for use in high stress components such as rotors, but also exhibit excellent magnetic properties. The alloys according to the present invention annealed at 880°C exhibit lower yield strengths, combined with outstanding magnetic properties. The higher temperature annealed strips are therefore extremely well suited for use in stator components of electrical machines. The above examples have been described for the purpose of illustration only. Many other examples falling within the scope of the present invention will be apparent to the skilled reader.

Claims: CLAIMS: 1. A soft magnetic alloy consisting essentially of: 45% to 55% cobalt; 0.8% to less than 1.5% vanadium; 0.15% to 0.5% niobium or tantalum or mixtures thereof; 0% to 0.3% manganese; 0% to 0.2% silicon; 0% to 0.1% carbon; and the balance iron, all percentages being by weight based on the total weight of the alloy. 2. A soft magnetic alloy according to claim 1 in the form of an article having a yield strength of at least 500 MN/m 2 . 3. A soft magnetic alloy according to claim 2, wherein said article has a yield strength of at least 600 MN/m 2 . 4. A soft magnetic alloy according to any preceding claim having a magnetic induction of more than 2.05 T at field strength H = 1600 A/m. 5. A soft magnetic alloy according to any preceding claim having a magnetic induction of more than 2.10T at field strength H = 1600 A/m. 6. A soft magnetic alloy according to any preceding claim, wherein a strip of thickness 0.35 mm of said alloy exhibits magnetic induction losses of less than 150 W/kg, at B=2T and 400Hz. 7. A soft magnetic alloy according to any preceding claim, wherein a strip of thickness 0.35 of said alloy exhibits magnetic induction losses of less than 100 W/kg, at B = 2T and 400 Hz. 8. A rotor for an electrical machine, comprising a soft magnetic alloy according to any preceding claim. 9. A stator for an electrical machine, comprising a soft magnetic alloy according to any of claims 1 to 7. 10. An electrical machine comprising a rotor according to claim 8 and a stator according to claim 9, wherein the soft magnetic alloy of the rotor and the soft magnetic alloy of the stator have the same chemical composition. 11. A process for the preparation of a soft magnetic alloy comprising the steps of: providing an alloy composition consisting essentially of: 45% to 55% Cobalt; 0.8% to less than 1.5% Vanadium; 0.15% to 0.5% Niobium or Tantalum or mixtures thereof; 0 to 0.3% Manganese; 0 to 0.2% Silicon; 0 to 0.1%carbon; and the balance iron, all percentages being by weight based on the total weight of the alloy; and heat treating said alloy composition under non-oxidizing conditions at a temperature of 700°C to 920°C. 12. A process according to claim 11 , wherein said step of heat treatment is carried out for a time of from 0.5 to 8 hours. 13. A process according to claim 11 or 12, wherein said step of heat treatment is carried out at a temperature of 850°C to 920°C. 14. A process according to claim 11 or 12, wherein said step of heat treatment is carried out at a temperature of 700°C to 760°C. 15. A process according to any one of claims 11 to 14, further comprising the step of processing said alloy composition into a formed article prior to said step of heat treating. 16. A process according to claim 15, wherein said step of processing comprises melting said alloy, hot working said alloy, reheating said alloy above its order- disorder temperature and quenching said alloy. 17. A process according to claims 15 or 16, wherein said article is a strip.

Inventors: GEMMELL WILLIAM, MAJOR RODNEY, GEMMELL, WILLIAM, MAJOR, RODNEY
Applicants: GEMMELL WILLIAM, MAJOR RODNEY, TELCON LTD, GEMMELL, WILLIAM, MAJOR, RODNEY, TELCON LIMITED
Application year: 1999
Main classification: H01F1
IPC codes: C21D6, C22C30, H01F1