Niobium, also known as columbium, is number 41 on the periodic table. With a melting point of 2468°C, it qualifies as a refractory metal. Niobium has a density of 8.57 gm/cc. It has many properties that make it an excellent candidate for fabricated parts that must be made of a refractory metal. Niobium offers good ductility and weldability under a clean, dry, inert gas or a vacuum.

Physical Properties of Niobium

PROPERTY
Atomic Weight	92.9064
Density	8.57 g/CC
Melting Point	2750 K, 2468°C, 4490°F
Boiling Point	5017 K, 4927°C, 8571°F
Coefficient of Thermal Expansion (20°C)	7.1 x 10-6/°C
Electrical Resistivity (20°C)	15 microhms-cm
Electrical Conductivity	13.2% IACS
Specific Heat	.126 cal/g/°C
Thermal Conductivity	.523 cal/cm2/cm°C/sec
Crystal Structure	bcc
Thermal Neutron Cross Section	1.1 b

Niobium, discovered in 1801 by C. Hatchett, an English Chemist, is a metal that is closely related to tantalum. It offers similar corrosion resistance and yet is formable, weldable, and easier to machine. However, neither niobium or tantalum are considered to be easy to machine.

Niobium is very similar to tantalum and several alloys are available in the arc-cast and wrought condition. It has the lowest melting point of all the refractory metals covered, the lowest modulus of elasticity and thermal conductivity, and the highest thermal expansion. It also has the lowest strength and lowest density of the refractory metals.

Niobium's ductile-to-brittle transition temperature ranges from -150° to -250°F (-101° to -157°C). This metal also has the low thermal neutron capture cross-section required for nuclear applications.

Its high melting point warrants its use at temperatures above the maximum service temperatures of the iron base, nickel base, and cobalt base metals. It has excellent ductility and fabricability. Pure niobium has a recrystallization temperature range of 1800 to 2000°F (982° to 1093°C).

Niobium offers nearly the corrosion resistance of tantalum and nearly the melting temperature of molybdenum. Yet, its cost is about 1/6th that of tantalum and 25% more than molybdenum.

Niobium was used as an alloy for many years. Nb/1%Zr was, and still is, used in nuclear reactors as the tubing for the fuel pellets because of its resistance to neutron bombardment. As C-103 alloy, it has been used for rocket nozzles and exhaust nozzles for jet engines and rockets because of its high strength and oxidation resistance at a low weight. Recently, it has been gaining favor in its pure form for semiconductor equipment components and corrosion resistant parts.

Heat treating in a vacuum at 1200°C for one hour causes complete recrystallization of material cold worked over 50%. This must be performed in a high vacuum (1 x 10^-4 minimum) or in a clean, dry, inert gas.

Niobium can be bent, spun, deep drawn, and formed at room temperature up to its maximum work hardening. Machining is somewhat more difficult. High speed tooling with a proper lubricant will allow machining of niobium.

However, note that tool wear is high and the cost of machining is high in comparison to conventional metals. Tools will wear fast and high rake angles should be maintained. Tool maintenance must be taken into consideration when costing niobium parts. Nonetheless, this metal is an ideal candidate for a lower cost alternative when tantalum is being considered. Rembar supplies niobium in powder, sheet, rod, and wire forms. If your needs require fabrication of niobium, Rembar has the experience to produce the part to your specifications.

Niobium's combination of strength, melting point, resistance to chemical attack, and low neutron absorption cross-section promotes its use in the nuclear industry. It has been identified as the preferred construction material for the first reactors in the space power systems programs.

Niobium mill products are used in the fabrication of corrosion resistant process equipment including reaction vessels, columns, bayonet heaters, shell and tube heat exchangers, U-tubes, thermowells, spargers, rupture diaphragms, and orifices.

The corrosion resistance of niobium is more limited than tantalum and this must be taken into consideration. The limitation stems from its sensitivity to most alkalis and certain strong oxidants.

Media	Concentration	Temp.	Nb
Acetic Acid	50%	Boiling	Nil
Bromine	Dry	200°F	Nil
Chlorine	Wet	220°F	Nil
Chromic Acid	50%	Boiling	1 mpy
Hydrochloric Acid	5%	200°F	1 mpy
	30%	200°F	5 mpy
Nitric Acid	65%	Boiling	<2 mpy
Sodium Hydroxide	10%	Room	*
Sulfuric Acid	40%	Boiling	20 mpy
	98%	Boiling	attacked

*Material may become brittle due to hydrogen attack

However, niobium is totally resistant to such highly corrosive media as wet or dry chlorine, bromine, saturated brines, ferric chloride, hydrogen sulfide, and sulfur dioxide as well as nitric and chromic acids. It is also resistant to sulfuric and hydrochloric acids within specific temperature and concentration limits.

Niobium is also resistant to attack by many liquid metals such as: Li <1000°C, Na, K + NaK <1000°C, ThMg <850°C, U <1400°C, Zn <450°C, Pb <850°C, Bi <500°C and Hg <600°C.

Niobium has the ability to form stable, passive oxides and therefore, it can provide unique solutions to many corrosion problems. However, niobium cannot be used in air at temperatures exceeding 200°C. Refer to the table entitled Corrosion Resistance for additional information.

The cold working properties of niobium are excellent. Because of its bcc crystal structure, niobium is a very ductile metal that can undergo cold reductions of more than 95% without failure. The metal can be easily forged, rolled or swaged directly from ingot at room temperature.

Annealing is necessary after the cross-sectional area has been reduced by approximately 90%. Heat treating at 1200° C for one hour causes complete recrystallization of material cold worked over 50%. Note that the annealing process must be performed either in an inert gas or in a high vacuum at pressures below 1 X 10^-4 Torr. Of the two methods, the use of a vacuum is preferred.

Niobium is well suited to deep drawing. The metal may be cupped and drawn to tube but special care must be taken with lubrication. Sheet metal can also easily be formed by general sheet metal working techniques. The low rate of work-hardening reduces springback and facilitates these operations.

Annealed	Ultimate Tensile Strength	195 M Pa (28 ksi)
	Yield Strength	105 M Pa (15 ksi)
	% Elongation	30%+
	% Reduction in Area	80%+
Cold Worked	Ultimate Tensile Strength	585 M Pa (85 ksi)
	% Elongation	5%
Hardness	Annealed	60 HV
	Cold Worked	150 HV
Poisson's Ratio		0.38
Strain Hardening Exponent		0.24
Elastic Modulus	Tension	103 G Pa (15 x 10-6psi)
	Shear	37.5 G Pa (15 x 106psi)
Ductile Brittle Transition Temperature *		<147°K
Recrystallization Temperature		800 - 1100°C

* Significantly affected by increasing interstitial contents.

To properly clean niobium, the following steps are recommended:

• Degrease
• Immerse in commercial alkaline cleanser for 5 - 10 minutes.
• Rinse with water
• Immerse in 35 - 40% HN0₂ for 2 - 5 minutes at room temperature.
• Rinse with tap water, follow by a rinse with distilled water.
• Force air drysupport design, manufacture and supply.

The REMBAR Company, Inc.
P.O. Box 67
67 Main Street
Dobbs Ferry, NY 10522

Tel.: (914) 693-2620
FAX: (914) 693-2247