Firstly, niobium ingot was preheated, and glass powder was applied on the surface of the preheated ingot. The preheating is conducive to the adhesion of the glass powder on the surface of the niobium ingot and the surface of the niobium ingot. The preheating temperature is preferably 150℃ ~250℃, preferably 175℃ ~225℃, and the best choice is 200℃. In order to prevent hydrogen and oxygen absorption during hot forging process, glass powder is coated on the surface of niobium ingots after preheating. The thickness of the glass powder is preferably 1mm~1.5mm to ensure that the glass powder can be evenly coated on the surface of the niobium ingot.
After the surface of the ingots is coated with glass powder, the ingots are hot forged for the first time. According to the invention, in order to improve the plasticity in the forging process of niobium ingot, ensure the forging permeability of niobium ingot and the sufficiency of grain breakage, the invention selects a forging mode of hot forging. The specific steps of the first hot forging are as follows: the niobium ingot coated with anti-oxidation coating is heated to 365℃~472℃, and after holding for 2h~4.5h, the niobium ingot is forged with radial upsetting and radial drawing. It is well known to technicians in the field that the ingot is smelted by electron beam furnace, and the direction of smelting is axial, that is, the direction of the length of the ingot, and the direction of coarse crystals, dendrites and crystal belts is produced in the axial smelting process, which is also the direction of good plasticity. The axial upsetting process can break the cast coarse crystal, dendrite and other structures in the axial direction, and make the grain fine. Meanwhile, the axial upsetting and axial drawing of the elongated niobium ingot are easy to deform and not easy to crack.
After the first hot forging is completed, in order to avoid the influence of the vaporization of the anti-oxidation coating on the surface quality of the niobium ingot in the subsequent heat treatment process, the invention preferably pickles the niobium ingot after the first heat treatment to remove the glass powder on the surface of the niobium ingot. The pickling acid solution is preferably a mixed solution of hydrofluoric acid, hydrochloric acid and sulfuric acid. The hydrofluoric acid is a commercially available hydrofluoric acid with a mass fraction of 35.35wt%; The hydrochloric acid is commercially available hydrochloric acid with a concentration of 37wt%, the sulfuric acid is commercially available concentrated sulfuric acid with a concentration of 98wt%, the volume ratio of the hydrochloric acid, hydrofluoric acid and sulfuric acid is preferably 5:3:2, the temperature of the acid solution is preferably 55℃ ~75℃, preferably 60~70℃. In order to enable the glass powder on the surface of the niobium ingot to be washed clean, the pickling time is preferably 5min~10min.
In order to eliminate the residual internal stress inside the niobium ingot during processing, the invention heat treats the niobium ingot after the first hot forging is completed. In the forging process, the niobium ingot will produce work hardening, if the heat treatment is not carried out, it will increase the difficulty of the second hot forging, and even cause the processing cracking.
