Preparation of rare earth permanent magnets

A method for preparing a rare earth permanent magnet to be exposed to a refrigerant and/or lubricant for an extended period of time, comprising the steps of:

• furnishing a mother alloy based on R, T and B, wherein R is neodymium or a combination of neodymium with one or more rare earth elements, T is iron or a mixture of iron and cobalt, and B is boron, said mother alloy consisting essentially of 17 to 33.5% by weight of neodymium, 26.8 to 33.5% by weight of the entire R, 0.78 to 1.25% by weight of B, 0.05 to 3.5% by weight of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mn, Sn, Mo, Zn, Pb, Sb, Al, Si, V, Cr, Ti, Cu, Ca and Mg, the balance being T and incidental impurities, and an auxiliary alloy consisting essentially of 28 to 70% by weight of R′ wherein R′ is at least one rare earth element, 0 to 1.5% by weight of B, 0.05 to 10% by weight of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mo, Al, Si, V, Cr, Ti and Cu, the balance being a mixture of iron and cobalt and incidental impurities,
• hydriding and crushing the mother alloy in an oxygen-free atmosphere of argon, nitrogen or vacuum,
• mixing 85 to 99% by weight of the crushed mother alloy with 1 to 15% by weight of the auxiliary alloy, followed by comrninution, compacting under a magnetic field, sintering and aging, thereby yielding a sintered magnet having an oxygen concentration of up to 0.8% by weight, and magnetic properties including a residual flux density Br of 12.0 to 15.2 kG and a coercive force iHc of 9 to 35 kOe,
• cutting and/or polishing the magnet to give a sintered magnet with a finished surface, and
• heat treating the sintered magnet with a finished surface in an argon, nitrogen or low-pressure vacuum atmosphere having an oxygen partial pressure of 10−6 to 100 torr for 10 minutes to 10 hours at a temperature of 200 to 1,100° C.