INVESTIGATION OF THE STRUCTURE AND FEATURES OF THERMOMAGNETIC TREATMENT OF THE YuNDK35T5 ALLOY MICROALLOYED WITH BORON

V. Kh. DAMMER, E. M. BUTKEVICH

(Presented by Prof. A. N. Dobrovidov)

The alloy YuNDK35T5, GOST 9575-60 (35% Co, 15% Ni, 8% Al, 5% Ti, 4% Cu, Fe—the balance), possesses high coercive force and residual induction and is widely used in radio electronics and special machine building. Because of its high brittleness, permanent magnets made from this alloy are manufactured only in the form of shaped castings followed by grinding. In this process, during grinding, even at small feeds, chipping of the magnet edges and tearing out of individual grains are observed. To improve grindability, additional alloying of the alloy with such components as S, Ce, Ca, V, and others is used. However, in most cases the indicated alloying additions significantly reduce the magnetic properties of the alloy, which limits their use.

In the present work, the mechanical and magnetic properties of the YuNDK35T5 alloy additionally alloyed with boron (0.003–0.8%) were investigated.

The alloys for the investigation were melted in a high-frequency furnace in packed crucibles made of electrocorundum with a capacity from 3 to 15 kg, sintered with a graphite template. The melts were carried out under cryolite. Boron was introduced as the last of the charge materials in the form of an iron–boron master alloy or potassium fluoroborate ($\mathrm{KBF_4}$).

As is known (1), heat treatment of the YuNDK35T5 alloy is associated with heating to a temperature at which the alloy has the structure of a solid solution, followed by cooling in a magnetic field at a controlled rate. In this case, in the upper temperature range (1200–900°C) the specimens are cooled at a rate of more than 100°/min in order to avoid high-temperature $\gamma$-decomposition, while in the lower range (850–700°C) the cooling rate is 20–40°/min.

A number of authors [1, 2] recommend the use of additional isothermal holds in the region of 800–700°C for more complete occurrence of the $\beta—\beta_2$ decomposition, which forms high magnetic properties. After thermomagnetic treatment, the specimens are usually subjected to multistage tempering, which increases the coercive force with some decrease in residual induction.

Additional alloying of the YuNDK35T5 alloy with boron changes its structure and properties in a definite manner.

In studying the microstructure of cast specimens of experimental heats on an unetched polished section, inclusions are observed, the number of which depends on the boron content in the alloy. The microhardness of individual inclusions lies within the range from 1200 to 2900 kg/mm².

The high hardness of the inclusions, their characteristic location in the region of grain boundaries, and their direct dependence on the boron content make it possible to consider these inclusions to be simple or complex borides based on Fe, Al, and Ti. X-ray structural analysis also confirms the presence of borides of the FeB, TiB₂, and AlB₁₂ types. In as-cast specimens with a boron content above 0.06%, after etching, precipitates of a second phase are observed mainly along grain boundaries; at a boron content above 0.1%, they are also observed inside grains. In contrast to the high-temperature γ-phase, the observed second phase does not have a characteristic acicular structure; its amount does not increase during holding in the temperature range of the most intense γ-decomposition (1100–1150°C), and during holding at temperatures above 1220°C, sufficient for complete dissolution of the γ-phase, the phase under investigation does not disappear. In all probability, the observed phase is a boride eutectic.

Experience has shown that when more than 0.08% boron is introduced into the YuNDK35T5 alloy, a continuous network of boride eutectic is formed. At the same time, the temperature at which melting of the specimens begins decreases from 1260°C (without boron) to 1230°C (0.08% boron).

In this work, the dependence of the alloy hardness on the boron concentration was investigated. The tests were carried out by two methods: the hardness of the specimens was measured on a Vickers instrument under a load of 20 kg, and the microhardness of the grain and of individual phases was measured on a PMT-3 instrument under a load of 100 g. To equalize the structure, the specimens were preliminarily annealed at 1220°C for two hours. As was to be expected, the hardness measured on the Vickers instrument (650–690 kg/mm²) proved to be intermediate between the microhardness of the grain (700–780 kg/mm²) and that of the boride phase (490–700 kg/mm²) located along the boundaries.

To determine the effect of boron on the magnetic properties, specimens of experimental heats were subjected to the thermomagnetic treatment standard for the YuNDK35T5 alloy according to the following schedule: heating to 1220°C, holding for 20–30 min, cooling in air (300–350°/min) to 850–840°C, and then in a special chamber at a rate of 20–30°/min in a magnetic field to 600°C. After quenching, the specimens were tempered at 640°C for 4–6 hours. The magnetic properties fall within the limits of GOST 9575—60 at a boron content up to 0.1% and decrease sharply with a further increase in the boron concentration.

In most experiments, thermomagnetic treatment was carried out according to a two-stage schedule, including quenching of the specimens in oil from a temperature of 1220–1240°C to fix the solid solution, followed by reheating for heat treatment in a special furnace chamber with application of a magnetic field. The highest magnetic properties were obtained with the following heat-treatment schedule: heating to 840°C, holding for 3 min, cooling at a rate of 30°/min to 800°C, holding at 800°C for 10 min, cooling to 750°C, holding at 750°C for 20 min, cooling to 600°C, and then in air. Specimens treated according to this schedule, after additional tempering at 640°C for 6 hours and at 550°C for 20 hours, had magnetic properties (Br = 0.8 T, Hc = 124 kA/m) significantly exceeding GOST 9575—60. Figure 1 shows the dependence of the residual induction and coercive force on the boron concentration in the alloy.

The effect of the heating temperature for quenching on the magnetic properties was also investigated. A temperature range from 1180 to 1240°C was selected. The low level of magnetic properties at a heating temperature of 1180°C

Table 1

is explained by the presence of a $\gamma$-phase in the structure, which is confirmed by metallographic analysis. A temperature of 1220°C, apparently, is likewise insufficient, since raising the temperature to 1240°C on specimens of melt 132 produced an increase in the magnetic properties. In view of the possible melting of specimens at a higher heating temperature, the optimum temperature for specimens with a boron content from 0,003 to 0,008% should be considered to be 1230–1240°C.

As noted above, the YuNDK35T5 alloy has very poor machinability. According to GOST 9575–60, the infeed during rough machining must be no more than 0,05 mm/pass. Final grinding must be carried out with an infeed;

equal to 0.01 mm/pass. In the work carried out, the specimens were ground both in the as-cast state and after thermomagnetic treatment. The tests were performed on a 3G71 surface-grinding machine, with an SM-grade abrasive wheel of grit size 46, under the following conditions: wheel speed—30 m/sec, table speed—15 m/min. The test results are presented in Table 1. As can be seen from Table 1, increasing the boron concentration from 0.007 to 0.08% significantly improved grindability.

Fig. 1. Dependence of the coercive force and residual induction of the YuNDK35T5 alloy on boron concentration

Analyzing the results obtained, the following conclusions may be drawn.

1. The effect of small additions of boron, introduced in amounts from 0.003 to 0.8%, on the grindability and magnetic properties of the YuNDK35T5 alloy was investigated.

2. A boron concentration range from 0.003 to 0.01% was established, within which machinability improved by a factor of 2–3 without a decrease in magnetic properties.

3. Thermomagnetic treatment of the boron-alloyed YuNDK35T5 alloy was experimentally developed.

LITERATURE

1. Ya. M. Dovgalevskii, Cast Magnets from Magnico Alloys. Moscow, Mashinostroenie, 1964.

2. A. A. Shekalov. New alloys for cast permanent magnets. Leningrad, series “Physical Metallurgy and Heat Treatment,” issue 3, LDNTP, 1963.