Firmetal Co., Ltd.

General:
Niobium has a body-centered cubic structure. It has moderate density -- about half that of tantalum and about 10% greater than that of iron. In cold working, such as rolling, niobium can be reduced as much as 90% in cross sectional area before annealing becomes necessary.

Niobium reacts with atmospheric oxygen at temperatures as low as 230°C (446°F), although the reaction rate at this temperature is relatively slow. The metal should not be exposed to the atmosphere above 370°C (700°F) for longer than a few minutes.

At lower temperatures, a thin adherent oxide film forms on the surface. Long exposures at low temperatures or short expose at high temperatures results in a thick film that spalls, thereby exposing fresh metal to oxidation. Oxygen diffuses into the base metal to cause embrittlement.

Niobium reacts with most gases at higher temperatures, including nitrogen, hydrogen, water vapor, carbon dioxide and carbon monoxide. Annealing operations use a high vacuum (10-4 Torr min.) or an inert atmosphere.

Machining:
All of the normal machining techniques can be used for niobium. The metal has a strong tendency to gall, and special attention must be given to tool design and lubricant use.

In lathe turning, the metal behaves very much like soft copper. The use of high speed tooling requires adequate lubrication and cooling using water soluble oil or vegetable oil.

Although carbide tools can be used, the tendency to gall is more pronounced that with high speed steel. In turning, the metal should be removed in a shaving action and the chip allowed to slide off the tool surface. When build-up of the chip occurs, the resulting pressure breaks the cutting edge of the tool.

A minimum surface speed of 80 ft./min. is important. Slower speeds will cause the metal to tear, particularly annealed stock. Unannealed metal is preferred for lathe operations.

Drilling:
Standard high speed drills can be used with good results. The peripheral lands of the drill should be checked often for excessive wear to prevent drilling undersized holes.

Thread Cutting:
Standard techniques for thread cutting can be used. Sufficient lubricant is required to eliminate galling and tearing. In threading larger diameters, it is better to cut the threads on a lathe rather than with a threading die. When dies or taps are used, they must be kept free of chips and cleaned frequently.

Grinding:
Grinding niobium is difficult. Most grinding wheels have a tendency to "load", and silicon carbide wheels such as Carborundum 120-T (for rough grinding) and 120-R or 150-R (for finishing) should be used. An adequate supply of cooling water is desirable.

Cleaning:
Prior to annealing or joining, the metal must be cleaned to remove all traces of lubricant, soil, and oxide. Niobium reacts with common gases as well as contaminants such as oil, grease, and even residues from degreasing. Parts should be degreased in hydrocarbons or alkaline cleaners and rinsed in distilled or deionized water.

Pickling:
Immediately following degreasing, but just prior to finishing operations, parts should be pickled in 2% HF + 40% HNO3, balance distilled or deionized water at room temperature to remove surface oxidation. This should be followed by rinsing in deionized or distilled water and drying. The resultant clean surface will yield better welds with reduced carbon and/or oxygen dissolution in the molten pool and less porosity.

Clean surfaces entering annealing will prevent diffusion of carbon and oxygen, as well as control mechanical properties. After the parts have been cleaned, they should be handled with lint-free cotton gloves so that the body oils will not contaminate the surfaces.

Resistance Welding:
Resistance welding of niobium to niobium, and ceratin other metals can be done with conventional equipment and techniques. Because of its high melting point and relatively low electrical resistance, niobium requires a high power input to obtain a sound weld. Weld duration should be kept to a minimum, preferably one to 10 seconds (60 Hz), to prevent excessive heating of the weld area. If possible, the work should be flooded with water. In seam resistance welding, the work should be submerged in water, both to exclude air from the heat-affected zone and to cool the metal as rapidly as possible.

RWMA Class 2 welding electrodes are recommended and should be water cooled. Any copper pickup can be removed in nitric acid, which will not attack the niobium.

Fusion Welding:
Strong, ductile niobium welds can be made using TIG welding. Because niobium reacts with air above 230°C, certain modifications to the TIG process are required. It is best to weld in a chamber using argon or a mixture of argon and helium as a cover gas. If chamber welding is not practical or not available, welding in normal atmosphere can be done with proper fixturing to provide an inert gas atmosphere for the molten zone. Trailing shields are necessary to protect the fusion zone during cooling, and the metal must not be exposed to air until the temperature has dropped to 230°C (446°F). The back side of the weld zone also must be protected by an inert gas shield during both the welding and cooling cycles.

Sheets with thicknesses of 0.050 in. (1.27 mm) or less can be welded without using a filler rod. Cleanliness of the material to be welded and the filler rod is essential. Heavier sheet often requires use of a filler rod; bare rod should not be used. Use of a coated rod or flux is not recommended since molten niobium reacts with all of the known fluxes.

Electron Beam Welding:
Electron Beam (EB) welding is commonly used to join thick sections, but can also be advantageous for very thin sections. EB welds in thick sections, up to 0.75 in. (19 mm) are narrower and deeper than those produced by other methods. When joining thin sections, the narrow weld zone helps reduce distortion.

Other than care in welding and cleanliness of parts, normal electron beam welding procedure are adequate.

Blanking and Punching:
Dies and punches made of steels generally used for punching and blanking are satisfactory for niobium. Allowance of 6% of the metal thickness for clearance between the punch and die is recommended. Light oils or similar lubricants should be used to prevent scoring the dies.

Forming and Stamping:
Beryllium copper, aluminum bronzes, and steel may be used for form stamping tools. The techniques use for stamping of steel are adequate. Tools should be polished to prevent galling. Light oils or similar lubricant should be used.

Deep Drawing:
Beryllium copper, aluminum bronzes, and steel may be used for drawing. Single draws where the depth of the draw does not exceed the diameter of the blank can be accomplished. If more than one draw is to be made, the first draw should have a depth greater than 40% of the blank diameter.

Intermediate vacuum annealing may be desirable with multiple draws. Sulfonated tallow and Johnson's 150 Drawing Wax are acceptable.

Spinning:
Niobium can be spun by conventional techniques using wood formers and steel roller wheels in conjunction with an adequate lubricant such as sulfonated tallow or Johnson's 150 Drawing Wax. Spinning is done at room temperature. Beryllium copper or aluminum bronzes are satisfactory for the tooling. The metal should be worked in small steps or stages with long sweeping strokes using light pressure rather than a few heavy strokes.