Removal of Interstitial Elements (O, N, C, H) in Refractory Metals by Multiple Electron Beam (EB) Melting

Refractory metals (such as niobium, tantalum, molybdenum, and tungsten) are widely used in aerospace, superconducting technology, and the nuclear industry. However, these metals have extremely high affinities for interstitial elements (oxygen, nitrogen, carbon, and hydrogen). Even a very small amount of interstitial elements will significantly deteriorate the ductility, processability, and superconducting critical parameters of metals. Electron beam (EB) melting has become the core means of purifying these metals due to its extremely high vacuum degree and extremely high energy density.

In this paper, the mechanism and practical effect of multiple EB smelting on the removal of four main interstitial elements are discussed.

Physicochemical Mechanism: The Bottom Logic of Purification

The purification process of EB smelting mainly depends on the physical and chemical reactions that occur in the molten pool under a high-vacuum environment.

Deoxidation

Oxygen is evaporated mainly in the form of metal oxides (e.g., NbO, NbO₂), or it reacts with carbon in the matrix to form a CO gas.

Denitrogenation

The removal of nitrogen mainly depends on the composite desorption of atomic nitrogen on the surface of the molten pool, which escapes in the form of N₂molecules.

Decarburization

Carbon forms CO bubbles by combining with dissolved oxygen. If the oxygen content is insufficient, it is necessary to add oxidants or utilize residual oxygen.

Dehydrogenation

The solubility of hydrogen decreases rapidly with the increase of temperature, and its diffusion coefficient is extremely high, which can be basically removed by thermal desorption in the primary melting.

Dynamic Effect of Multiple Smelting on Interstitial Elements

Multiple smelting is not a simple repetition but a process in which the concentration gradient tends to the equilibrium limit continuously.

Oxygen: "Marginal Effect" after Continuous Decline

Oxygen removal is the highlight of EB smelting. In the primary smelting, the large-sized oxide inclusions are rapidly decomposed. As the number of smelting increases:

First two smelting: Oxygen content usually shows an exponential decline. For example, after secondary melting, the oxygen content of industrial grade niobium materials can be reduced from 500 ppm to less than 50 ppm.

After multiple smelting: The purification efficiency will encounter a "ceiling". When the oxygen concentration is very low, its volatilization rate is limited by the diffusion rate of atoms to the molten pool surface. At this time, if the number of melting times is increased (e.g. from 3 to 6 times), the decline in oxygen content will be significantly narrowed.

Nitrogen: the most difficult "hard bone" to gnaw

The removal kinetics of nitrogen is much slower than that of oxygen. The results show that the equilibrium partial pressure of nitrogen in liquid metals is extremely low and its desorption is hindered by kinetics.

Necessity of multiple smelting: Single smelting often fails to meet the nitrogen content levels required for superconducting niobium materials (RRR value requirements).

Effect: As the number of smelting increases, the high temperature residence time of the molten pool increases, providing more opportunities for the recombination of nitrogen atoms. Multiple smelting can steadily reduce the nitrogen content, but it is difficult to completely remove it to less than 1 ppm

Carbon : Chemical equilibrium dependent on oxygen content

The removal efficiency of carbon is highly dependent on the "carbon-to-oxygen ratio".

If there is an excess of oxygen in the metal, carbon can be continuously reduced with increasing number of reactions through the C + O→ CO↑.

If the oxygen content is reduced to a very low level by multiple smelting, the decarburization reaction will stop. Therefore, in the process of multiple smelting, it is sometimes necessary to artificially control the atmosphere to prevent the deceleration of carbon from stagnating.

Hydrogen: Efficient and rapid

Hydrogen is the best treatment. Due to its extremely high vapor pressure, more than 90% of the dehydrogenation work can be completed in the first smelting.

Subsequent multiple smelting mainly serves as a consolidation function, preventing the re-absorption of trace hydrogen in the environment during the condensation process.

Analysis of Advantages and Disadvantages of Multiple Smelting

Purity	Significantly improved	especially for materials with extremely high requirements for RRR value
Composition uniformity	Improve	multiple flip smelting makes the impurity distribution more isotropic.
Grain size	Increase	Repeated remelting may lead to coarse grains in the ingot, which requires subsequent processing and refinement
Cost/yield	Unfavorable	Every additional smelting, the evaporation loss of metal increases, and the cost of electricity consumption increases.

Conclusion

Multiple EB meltings have a nonlinear gain effect on removing interstitial elements. For oxygen and hydrogen, the first two smeltings contributed most of the purification amount; for nitrogen and carbon, multiple smeltings provide the necessary kinetic time.

In actual production, the number of smelting times is not "the more the better." Usually, 3 to 4 meltings are considered to be the balance point between purity and economy. For extremely demanding scientific research-grade materials, as many as 6 or more refining times may be required, combined with a slow melting rate, to ensure that interstitial elements diffuse to the surface and evaporate thoroughly.