Gas atomized T-15 high speed steel powder was blended with solution of 0-1% boric acid dissolved in methanol and dried at 50℃. The powder mixture was filled into pyrex glass mould, vacuum-degassed at 450℃ and sealed. The sealed specimen was embedded in alumina boat tightly filled with graphite powder. The boat with specimen was put into the tube furnace heated to a constant temperature between 1175℃ and 1225℃, and sintered for 16 hours in air. And the microstructures of the sintered specimens were studied.
For densifying above 94% of theoretical density by CAP process, the T-15 high speed steel powder was required to be fine below 325 mesh size and to be added more than 0.05% of boric acid, and the sintering temperature should be 1200℃.
Densification of the high speed steel powder, sintered at 1200℃ for 16 hours, was considerably accelerated with increase of boric acid content up to 1%. The saturation of densification and the growth of austenite grains and primary carbides appeared with increase of boric acid at the same sintering condition.
Boric acid was transformed into liquid boron oxide, $B_2O_3$, after vacuum degassing and the packing density of the spherical high speed steel powder was considered to increase with increase of liquid boron oxide because the softened pyrex mould pressed the powder compact by the atmospheric pressure. During sintering at high temperature, liquid boron oxide was considered to be reduced to metallic boron by saturated carbon in the high speed steel and to form a liquid phase. This liquid phase seemed to accelerate the densification.
Model experiment to confirm the liquid formation was performed by superposing a T-15 high speed steel powder compact with 10\% boric acid on the fully densified body and sintering at 1200℃ for 1 hour. It was confirmed that liquid phase of low dihedral angle was formed by the addition of excess boric acid and penetrated along the grain boundaries of the sintered high speed steel body. Composition analysis of the liquid phase by EDAX showed that this liquid phase had high content of carbide forming elements such as tungsten, chromium, vanadium, and iron. Therefore the liquid phase was considered to be the iron-carbide-boride eutectic liquid.
