Effect of heat treatment on the structure and properties of tungsten cathodes produced by pressing

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In vacuum metallurgy, electron beam guns (EB guns) are widely used, in which tungsten thermal cathodes (W-cathodes) are the source of electrons. The problem of improving technological processes for producing W-cathodes from the point of view of improving the cost/efficiency criterion remains relevant. The purpose of the work is to study the effect of vacuum heat treatment (HT) on the structure and properties of flat W-cathodes obtained by hot pressing tungsten wire blanks [1]. Comparative studies of the structure (by optical and scanning electron microscopy), mechanical (microhardness measurements) and operational properties (during tests in EB guns) have been carried out. The “as delivered” W-cathodes are plates 0.6 mm thick, 3 mm wide and 100 mm long. It was found that the microstructure of the cathodes in the delivered state is elongated grains, the average width is 3...5 µm, the length is 20...40 µm, the microhardness is at the level of 5.4...5.6 GPa. To study the recrystallization process in the W-cathodes, vacuum HT was carried out in the range of 800...1200°C for a duration of 20 to 120 minutes. It has been established that vacuum HT in the range of 800...1150°C does not have a significant effect on the structure and properties of W-cathodes. Starting from 1200°C, a mixed microstructure is formed, which is a mixture of initial and equiaxed recrystallized grains, the number and size of which increases with increasing exposure time at this temperature. Microhardness decreases to 4.7...4.9 GPa. It has been established that to improve the performance characteristics of W-cathodes, a mixed structure must be formed in them: up to 10-30% - equiaxed recrystallized grains, and at least the volume of the cathode material must retain the original structure. The optimal HT regime has been determined. The durability of cathodes heat-treated according to this mode is at least 50 hours, which corresponds to the level of durability of the best foreign cathodes.