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Niobium
ni·o·bi·um (1801 - Charles Hatchett) A lustrous light gray ductile metallic element that resembles tantalum chemically and is used in alloys. Also known as Columbium Cb.
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.
Applications of Niobium
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.
Corrosion Resistance
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.
Working Characteristics
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.
Mechanical Properties of Niobium
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.
Cleaning Niobium
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 dry.
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