Introduction to Refractory Metals

  • Overview
  • Molybdenum
  • Tungsten
  • Tantalum
  • Titanium
  • Rhenium
  • Niobium
Molybdenum element symbol

Molybdenum

Molybdenum is number 42 on the periodic table with a melting point of 2610 degrees C and a density of 10.22 gm/cc. Moly has many properties that make it an excellent candidate for fabricated parts and it's the most commonly used refractory metal.

Molybdenum is the lamp industry standard for mandrels and supports, usually in wire form. Several unique properties of molybdenum that satisfy more demanding industry requirements have increased the use of molybdenum as a material in applications requiring other mill forms. Molybdenum is machinable and is fabricated into high temperature furnace hardware and lighting components.

Tungsten element sysmbol

Tungsten

Tungsten is number 74 on the periodic table, in between tantalum and rhenium. Tungsten has the highest melting point (3410 degrees C) of the four common refractory metals. In addition, with a density of 19.3 gm/cc, it is only surpassed by rhenium and osmium in weight.

Tungsten has a long history of use for filaments in the lamp industry. It offers exceptionally high strength at very high temperatures. In fact, it has the best high-temperature strength of the four common refractory metals. Its high-temperature strength, combined with its good electrical resistivity have made it a popular choice for other applications in addition to filaments. Tungsten's high density and strength are utilized in aircraft counterweights, radiation shielding, weapon systems, golf clubs, high temperature furnaces and rocket nozzles.

Tantalum element symbol

Tantalum

Tantalum is number 73 on the periodic table. It has a melting point of 2996 degrees C and a density of 16.654 gm/cc. Tantalum is one of the refractory metals that offers a valuable combination of properties.

Tantalum is one of the most corrosion resistant metals available. It's used in chemical reactors, medical implants and highly acidic environments. Tantalum and its alloys are midway between tungsten and molybdenum in density and melting points. Tantalum can be worked easily at room temperature. Its thermal conductivity is one-fourth that of molybdenum and its coefficient of expansion is one-third greater. Its elevated temperature strength is low compared with tungsten and molybdenum.

Titanium

Titanium is number 22 on the periodic table. It has a melting point of 1668°C and a density of 4.54 gm/cc. Titanium properties and characteristics which are important to design engineers are excellent. Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.

The combination of high strength and low density results in exceptionally favorable strength-to-weight ratios for titanium-based alloys. These ratios for titanium-based alloys are superior to almost all other metals and become important in such diverse applications as deep well tubestrings in the petroleum industry and surgical implants in the medical field.

Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals. Titanium offers superior resistance to erosion, cavitation or impingement attack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.

Titanium and titanium alloys have proven to be technically superior, highly reliable and cost-effective in a wide variety of chemical, industrial, marine and aerospace applications. Titanium is utilized in many critical services due to its unique set of properties.

Rhenium

Rhenium is number 75 on the periodic table with a density of 21.02 gm/cc and a melting point of 3180 degrees C. It is second only to tungsten in melting temperature and to Osmium in density.

Rhenium is rare, expensive, and extremely difficult to machine. It is very limited in the sizes and shapes available. It is most commonly used as an alloying agent to improve the properties of other metals (mostly refractory metals).

Rhenium has several unique properties that are also imparted to its alloys. One of the most important of these is its high melting point of 3180 degrees C. This is second only to tungsten. The high melting point, coupled with its ability to maintain ductility even after recrystallization, offers many advantages.

Rhenium also offers high electrical resistivity over a wide temperature range. The resistivity is higher than tungsten. This characteristic, plus having a low vapor pressure, make it a good choice for high temperature filaments. This is especially true when considering ductility and the fact that it is not affected by the oxidation/reduction cycle as is tungsten. Rhenium and it's alloys are used in electronic components, gyroscopes and nuclear reactors.

Niobium

Niobium, also known as Columbium, is number 41 on the periodic table. With a melting point of 2468 degrees 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 weld-ability under a clean, dry inert gas or a vacuum.

Niobium can be found in electrolytic capacitors, superconductor alloys, gas tubings, vacuum tubes and nuclear reactors.

 

Molybdenum

Molybdenum uses

mo·lyb·de·num (1778 - Carl W. Scheele) A metallic element that resembles chromium and tungsten in many properties, is used especially in strengthening and hardening steel, and is a trace element in plant and animal metabolism. Molybdenum is derived from the Greek word "molybdenum", meaning "lead".

Molybdenum is number 42 on the periodic table. With a melting point of 2610°C, molybdenum has a density of 10.22 gm/cc. It has many properties that make it an excellent candidate for fabricated parts that must be made of a refractory metal.

Molybdenum has been used for many years in the lamp industry for mandrels and supports, usually in wire form. Today, several unique properties of molybdenum that satisfy more demanding industry requirements have increased the use of molybdenum as a material in applications requiring other mill forms.

Physical Properties of Molybdenum

PROPERTY
Atomic Number 42
Atomic Weight 95.94
Density (20°C) 10.22 g/CC
Melting Point 2896 K, 2610°C, 4753°Fm
Boiling Point 4912 K, 5560°C, 8382°F
Coefficient of Thermal Expansion (20°C) 4.9 x 10-6/°C
Electrical Resistivity (20°C) 5.7 microhms-cm
Electrical Conductivity 30% IACS
Specific Heat .061 cal/g/°C
Thermal Conductivity .35 cal/cm2/cm°C/sec
Modulus of Elasticity (20°C) 46 x 106 psi

Molybdenum Alloys

Molybdenum has several alloys. For the purpose of this discussion, only alloys that are predominantly molybdenum will be considered.

  • TZM (titanium, zirconium, molybdenum)

    Molybdenum's prime alloy is TZM. This alloy contains 99.2% min. to 99.5% max. of Mo, 0.50% Ti and 0.08% Zr with a trace of C for carbide formations. TZM offers twice the strength of pure moly at temperatures over 1300°C. The recrystallization temperature of TZM is approximately 250°C higher than moly and it offers better weldability.

    The finer grain structure of TZM and the formation of TiC and ZrC in the grain boundaries of the moly inhibit grain growth and the related failure of the base metal as a result of fractures along the grain boundaries. This also gives it better properties for welding. TZM costs approximately 25% more than pure molybdenum and costs only about 5-10% more to machine. For high strength applications such as rocket nozzles, furnace structural components, and forging dies, it can be well worth the cost differential.

    TZM is available in sheet and rod form in basically the same size range as moly with the exception of thin foil. Rembar is experienced in the fabrication of TZM.

  • Moly/30% Tungsten

    This is another moly alloy that offers unique properties. It was developed for the zinc industry. This alloy resists the corrosive effects of molten zinc. Mo/30W has also proved effective in rocket nozzles and has the potential of offering enhanced performance in applications where any erosive effects are a factor.

  • Moly/50% Rhenium

    This alloy offers the strength of moly with the ductility and weldability of rhenium. It is a costly alloy and it is only available in a very limited size range. It offers significant advantages in thin foil applications for high temperature delicate parts, especially those that must be welded. Note that, although this alloy is nominally 47% rhenium, it is customarily referred to 50/50 moly/rhenium. Other moly/rhenium alloys include moly/rhenium sheet with 47.5% and 41% rhenium. The moly/41%Re alloy does not develop sigma phase. This makes the material even more ductile after exposure to high temperatures.

Applications of Molybdenum

There is an increasing demand from the electronics and aerospace industries for materials that maintain reliability under ever-increasing temperature conditions. Because its properties meet these requirements, molybdenum also is experiencing an increasing demand.

Characteristics that support the demand for molybdenum in many electronics applications are its:

  • exceptional strength and stiffness at high temperatures
  • good thermal conductivity
  • low thermal expansion
  • low emissivity
  • low vapor pressure
  • electrical resistivity
  • corrosion resistance
  • purity
  • ductility and fabricability
  • machinability
Some combination of these properties and characteristics predict increased usage of molybdenum in such applications as rocket nozzles, jet tabs, high temperature dies, electrodes, boring bars, tools, brazing fixtures, electrical contacts, boats, heat shields and many others as well as high vacuum applications.

Molybdenum can be furnished in many mill forms such as wire, ribbon, foil, plate, sheet, rod, billet, slab, bar, extruded shapes, tubes, and powder.

Glass-to-Metal Seals

Molybdenum has a straight-line expansion. The mean coefficient of expansion is 4.9 x 10-6 measured between 20°C and 500°C. Molybdenum is suitable for sealing to hard glass since it has approximately the same coefficient of expansion and a transition temperature below 700°C.

Sealing

Molybdenum oxides dissolve readily in glass. The adhesion between glass and this metal is very satisfactory and gives an absolutely tight seal.

It is essential for the surface of the metal to be correctly oxidized before it comes into contact with the glass. This is easily affected, provided that the surface is clean and free from grooves and cracks. The molybdenum supplied by Rembar is produced with extreme care to obtain a uniform oxide film.

The best method of oxidizing the surface is to heat it for a short time in an air-gas or oxygen flame. Excessive oxidation must be avoided since it results in incomplete absorption of the oxide in the glass. This can possibly render the seal to be porous.

Molybdenum should be oxidized by rapid heating, maintained at high temperature for a short period. The gas flame itself is a guard against excessive oxidation. This is indicated by a slight emission of smoke. Conversely, the reducing part of the flame provides insufficient oxidation and, therefore, must be avoided.

The most favorable sealing-in temperature depends upon the viscosity of the hard glass and lies between 1000°C and 1200°C. The pre-oxidized rod, after slight cooling, has a blue color, indicating a low oxide.

Molybdenum used for sealing glass is principally used in the form of wire and rod from about .040 inches in diameter and larger. Seals made with molybdenum are perfectly free from bubbles provided that the glass used is clear and free from bubbles. This is of special importance for high-voltage tubes because bubbles in the glass will reduce the dielectric strength of the seal.

Corrosion Resistance

Molybdenum provides corrosion resistance that is similar to tungsten. Molybdenum particularly resists non-oxidizing mineral acids. It is relatively inert to carbon dioxide, ammonia, and nitrogen to 1100°C and also in reducing atmospheres containing hydrogen sulfide.

Molybdenum offers excellent resistance to corrosion by iodine vapor, bromine, and chlorine, up to clearly defined temperature limits. It also provides good resistance to several liquid metals including bismuth, lithium, potassium, and sodium.

Chemical Cleaning

A cleaning process is designed to deal with one or more of the following:

  • surface scale
  • general contamination
  • removal of a basis metal

Of all the potential contaminants in wrought products, iron is of primary concern. Others, such as aluminum, carbon, calcium, copper, and nickel among others, may also be present as elements, but they are more frequently present in the form of oxides.

Removal of a controlled amount of basis metal may be desired to insure complete removal of contaminants. There are three main methods for cleaning molybdenum.

  1. Immerse the material in a glass cleaning etch composed of:
    95% H2SO4
    4.5% HNO2
    0.5% HF and Chromium Oxide
    (equivalent to 18.8gm/1)
  2. First, immerse in an alkaline bath composed of:
    10% NaOH
    5% KMnO4
    85% H2O
    The temperature of the bath should be kept between 150 - 180°F (66 - 82°C) with a soak duration of 5 to 10 minutes.
    When immersion in the alkaline bath is complete, immersion in a second bath is required to remove smut that may be formed by the first treatment. The second bath consists of:
    15% H2SO4
    15% HCl
    70% H2O
    6 - 10 wt. % per unit volume of chromic acid
    The second bath should also provide a soak duration of 5 - 10 minutes.
  3. The third method is generally applied to the molybdenum alloy TZM. The recommended procedure is:
    1. Degrease the material form 10 minutes in an appropriate solvent.
    2. Immerse in a commercial alkaline cleaner for 2-3 minutes.
    3. Rinse in cold water.
    4. Buff and vapor blast.
    5. Re-immerse in a commercial alkaline cleaner as above.
    6. Rinse in cold water.
    7. Electropolish in 80% H2SO4 at 120°F (54°C) with 8 -12 amps.
    8. Repeat the process beginning with step c., above.

Tungsten

Tungsten

tung·sten (1793 - Fausto de Elhuyar and Juan J. de Elhuyar) A gray-white heavy high-melting ductile hard polyvalent metallic element that resembles chromium and molybdenum in many of its properties and is used especially for electrical purposes and in hardening alloys (as steel). Tungsten means &Quot;heavy stone&Quot; in Swedish. The name came from medieval German smelters. Tungsten is also known as wolframium (formerly wolfram) thus the symbol W

Tungsten is number 74 on the periodic table, in between tantalum and rhenium. Tungsten has the highest melting point (3410°C) of the four common refractory metals. In addition, with a density of 19.3 gm/ cc, it is only surpassed by rhenium and osmium in weight.

Tungsten has a long history of use for filaments in the lamp industry. It offers exceptionally high strength at very high temperatures. In fact, it has the best hightemperature strength of the four common refractory metals. Its high-temperature strength, combined with its good electrical resistivity, have made it a popular choice for other applications in addition to filaments.

It is used for heating elements in vacuum furnaces that exceed the temperatures of molybdenum and tantalum as well as other heater applications. Tungsten has also gained wide acceptance as an essential material in electrical contacts, glass-to-metal seals, supports, and electrodes.

Tungsten's properties lend themselves to other metals when alloyed. Tungsten carbide has long been the choice for durable cutting tools. Tungsten's high density is used in conjunction with copper, nickel, iron, and cobalt to form heavy metal. This is an alloy containing 90%-97% tungsten and the other metals are used as a binder to keep the tungsten together and to give it machinable properties as well as to temper the brittleness of pure tungsten. Heavy metal is widely used for counter balances, radio isotope containers, and armor penetrations. Refer to "Heavy Metals".

Physical Properties of Tungsten

PROPERTY
Atomic Weight 183.85
Density 19.3 g/cc
Melting Point 3695 K, 3422°C, 6192°F
Boiling Point 6173 K, 5900°C, 10652°F

Tungsten has the best high-temperature strength of all the refractory metals. For this reason, it is used in very high temperature vacuum furnaces, i.e., those that operate at 2000'C and above. It is also widely used in arc lamps for both the cathode and anode. Its coefficient of expansion makes it a good material to choose in making hermetic glass-to-metal seals. Its stiffness at elevated temperatures makes it excellent for high-temperature support components in lamps.

Tungsten is very difficult to machine and fabricate. With experience, it can be turned. Milling is all but impossible. It is only done with great difficulty and high cost by those most experienced with it. Forming must be done at very high temperatures and with careful stress relieving. Welding is not recommended and riveting is difficult at best. Extreme care must be exercised when designing a component from tungsten. Rembar will readily provide assistance during the design stage upon request. Rembar offers tungsten in powder, sheet, wire, rod, and pressed/ sintered forms. If it is possible to fabricate the part needed from tungsten, Rembar can do it for you.

Applications of Tungsten

There is an increasing demand from the electronics, nuclear, and aerospace industries for materials that maintain reliability under ever-increasing temperature conditions. Because its properties meet these requirements, tungsten also is experiencing an increasing demand.

Characteristics that support the demand for tungsten in many electronics applications are its:

  • strength and stiffness at high temperatures
  • good thermal conductivity
  • low thermal expansion
  • low emissivity

Tungsten is also relied upon in high temperature furnace applications because of its properties that provide:

  • strength and stiffness
  • thermal expansion
  • low vapor pressure
  • electrical resistivity
  • emissivity
  • productivity and fabricability

Glass-to-Metal Seals

Tungsten has a coefficient of expansion approximating that of hard glass. For this reason, it is used extensively in glass-to-metal seals in hard glass lamp and electronic applications. Under special conditions, it may also be used with quartz.

All rod intended for sealing purposes is processed and inspected to produce split-free material with no surface imperfections.

Supports

Since tungsten rod has a high degree of strength at elevated temperatures, it is utilized structurally to hold or support high temperature sources such as filaments and heaters for lamp and electronic uses.

Electrodes

Tungsten rod that is specially processed and manufactured for welding rod applications is used extensively in such processes as inert-gas-shielded arc welding and atomic hydrogen arc welding.

Other types of tungsten rod are used for electrodes. These types, both regular and thoriated, are used for electrodes in vacuum melting processes, resistance welding, and electro-discharge machining.

The resistance welding of most refractory metals and their alloys is not normally done for several reasons :

  • The resulting cast, coarse grained microstructure has essentially no ductility or toughness.
  • The coefficients of thermal expansion are much lower than most common metals, that results in cracking upon cooling if dissimilar metals are joined. 
  • The Refractory Metals are also chemically reactive and will oxidize unless heated in inert gas or vacuum. Some Electron Beam Welding is perform to join these metals for low stress applications.

Cathodes/Anodes

For tube applications, especially flash and xenon tubes, tungsten is used either as pure or thoriated at 1% and 2% for greater emissivity.

Working Characteristics

Because of Tungsten's high hardness and low ductility, it is a difficult material to fabricate. The best method for machining that involves metal removal is E.D.M. For parts such as tubes, crucibles and other small, thin-walled items, C.VD. is an effective, but costly, method of achieving them. There are times when neither of these methods will suffice and then conventional methods must be used.

Cleaning of Tungsten

A cleaning process is designed to deal with one or more of the following:

  • surface scale
  • general contamination
  • removal of a basis metal

Of all the potential contaminants in wrought products, iron is of primary concern. Others, such as aluminum, carbon, calcium, copper, nickel, etc., may also be present as elements, but they are more frequently present in the form of oxides.

Removal of a controlled amount of basis metal may be desired to insure complete removal of contaminants.

There are four main processes used to clean tungsten:

Molten Salt
This is one of the most common cleaning processes, requiring simple immersion in a molten bath containing oxidizing agents. This process will not attack the basis metal.

Aqueous Alkaline Solutions
This process works well on oxidized (yellow tungsten) surfaces. Reduced or intermediate oxides (brown, purple, etc.) will react more slowly to this process, if at all.

A tightly adherent black scale, with or without carbon, is commonly found on tungsten that has been worked at high temperature. Despite the fact that it is predominantly W03, normally yellow in bulk, it is only slowly attacked even by a hot, concentrated solution. This is probably due more to its dense, fused, physical state than because of its chemical nature.

This process is similar to the use of molten salts in that it will not attack the basis metal and it requires an oxidizing agent to work.

Acid Solutions
Tungsten is much less reactive to individual acids than most common metals. HCI, HF, and H2SO4 have essentially no effect. When tungsten is treated with acid solutions, it frequently is stained by residual oxides even if rapid and thorough rinsing is used.

Electrolytic Methods
Electrolytic etching is the removal of basis metal by an applied voltage in a medium capable of dissolving the products of the electrolytic reaction. This may be done in molten salts or aqueous solutions. Electrical current and time determine the amount of metal removal.

There are five primary cleaning methods for cleaning tungsten:

  • Immerse the material in a 20% solution of potassium hydroxide that has been brought to a boil.
  • Etch the material in a 20% solution of potassium hydroxide.
  • Etch the material in a 50 vol. % HN03 - 50 vol. % HF solution.
  • Immerse in molten sodium hydroxide.
  • Immerse in molten sodium hydride.

For rapid attack of heavy scale, molten salt is far superior to the other methods. In addition, if no oxidizer is present, it can be performed with no fear of basis metal loss.

If appreciable sizes or volumes of material are to be processed, particularly with significant basis metal removal, acid solutions present a disposal, as well as an operational, problem.

The utility of electro-etching is more dependent on geometry than the other methods. It will work well for treating continuous lengths of wire, however there is a contact problem if the cleaning is to be performed on many small parts.

For all systems involving basis metal attack, rate control must be achieved by definition of concentration, temperature, and time of exposure.

Porous Tungsten

Porous Tungsten is a new product that can replace wrought tungsten for many applications. It has similar characteristics but yields less strength because it has a sintered structure rather than a worked structure. The starting powder is compacted, then sintered in a hydrogen atmosphere at high temperatures. Because of the high sintering temperature, this material can be used for applications requiring the material to see temperatures as high as 2000°C without ill effect. For applications requiring higher temperatures the material can be processed to accommodate. The starting powders are 99.9% pure tungsten and because of the strict processing controls we have in place the billets are also of high purity. The density of these billets can vary from 80 –90 % dense depending on the requirements of the application. For further details or to discuss your application please contact Rembar.

Our manufacturing process also lends itself to the compaction and sintering of large billets. Billets can also be made close to finish size reducing costs by reducing waste material and cutting times. Often times a billet can be made large enough to make parts that would normally have to be made from two or more pieces and somehow held together. This can reduce assembly times and costs. With the sizes available, parts can now be manufactured where tungsten has been restricted to available sizes.

Heavy Metal

Heavy Metal is made possible by P/M techniques. This is a technique where tungsten powder is mixed with nickel, iron or copper powder. It is then compacted and liquid phase sintered. The result is a very high-density machinable material having a homogeneous structure with no grain direction. This provides a material with unique physical properties and applications.

Applications of Heavy Metal

Because of the physical properties of heavy metal, it is often used as both a weight and structural member. Weights and counterbalances for aircraft control surfaces and rotor blades, guidance platforms, balancing of flywheels and crankshafts, vibration damping governors, and fuse masses and weights for self-winding watches are typical applications. Other specific applications include:

Radiation Shielding

Because of the absorption characteristics of heavy metals, approximately one-third less material is required as compared to lead.

Heavy metal is used for source shielding on oil wells and industrial instrumentation as well as for collimators and shielding in medical therapy and detection equipment.

Rotating Inertia Members

Due to the material's unique combination of physical properties and high density, it can be rotated at extremely high speeds.

This aspect makes it ideal for use in gyroscope rotors, flywheels, and rotating members for governors.

Projectiles

Properties such as elongation and hardness make heavy metal advantageous for use in kinetic energy penetrators. These properties can also be varied by manufacturing technique and additives.

Heavy metal is used in squares, spheres, and projectile shapes for hypervelocity armor penetrating applications.

Boring Bars and Grinding Quills

Heavy metal is used for &Quot;chatter-free&Quot; boring and grinding. It is used where rigidity and minimum vibration are desirable. Heavier cuts and a better finish can be achieved with tools made of heavy metal. Longer tool extensions, with ratios up to 9:1 are also possible depending on the diameter of the tool.

Other benefits include:

  • Longer tool life due to the lower amount of heat generated as a result of the minimum chatter and high thermal conductivity that heavy metal provides.
  • Heavy metals do not anneal during brazing, therefore carbide can be brazed directly with no effect on the shank. This allows continued use.
  • Higher accuracy and trouble-free grinding are achieved when used as grinding quills. This is due to the vibration damping effect and rotating inertia characteristics of heavy metals.

    Heavy metal is often used in place of Tungsten Carbide boring bars because:

  • they have a higher density
  • they are readily machinable
  • they are less prone to chipping and breakage
  • lower cost is achieved both with material and finishing

Typical Properties of Heavy Metals

(Note: Properties may vary according to size and shape of part)

Material * 17 gm/cc
90% W
6% Ni
4% Cu
17 gm/cc
90% W
6% Ni
4% Cu/Fe
17 gm/cc
90% W
7% Ni
3% Fe
17.5 gm/cc
92.5% W
5.25% Ni
2.25% Fe
18 gm/cc
95% W
3.5% Ni
1.5% Cu
18 gm/cc
95% W
3.5% Ni
1.5 Fe
18.5 gm/cc
97% W
2.1% Ni
0.9% Fe
Density; lbs/in3 0.614 0.614 0.632 0.65 0.65 0.668
Mil. Spec. T-21014 D Class 1 Class 1 Class 2 Class 3 Class 3 Class 4
ASTM-B-459-67 Grade 1
Type II & III
Grade 1
Type II & III
Grade 2
Type II & III
Grade 3
Type II & III
Grade 3
Type II & III
Grade 4
Type II & III
Hardness; Rockwell C 24 25 26 27 27 28
Ultimate Tensile Strength; PSI 110,000 125,000 114,000 115,000 125,000 128,000
Yield Strength, .2% Offset; PSI 90,000 88,000 90,000 85,000 90,000 85,000
Elongation, % in 1" 8 14 12 7 12 10
Proportional Elastic Limit; PSI 45,000 52,000 46,000 45,000 44,000 45,000
Modulus of Elasticity; PSI 40 x 106 45 x 106 47 x 106 45 x 106 50 x 106 53 x 106
Coefficient of Thermal Expansion (x 10-6 1°C 20°-400°C) 5.4 4.8 4.6 4.4 4.6 4.5
Thermal Conductivity; CGS Units 0.23 0.18 0.20 0.33 0.26 0.30
Electrical Conductivity; %IACS 14 10 13 16 13 17
Magnetic Properties NIL Slightly Magnetic Slightly Magnetic Slightly Magnetic NIL Slightly Magnetic Slightly Magnetic

* Composition shown is typical and may change for manufacturing purposes or to meet physical and/or application requirements. If non-magnetic material is required, it should be specified.

Tantalum

Tantalum

tan·ta·lum (1802 - Gustav Ekeberg)  A hard ductile gray-white acid-resisting metallic element of the vanadium family found combined in rare minerals (as tantalite and columbite).

Tantalum is number 73 on the periodic table. It has a melting point of 2996°C and a density of 16.654 gm/cc. Tantalum is one of the refractory metals that offers a valuable combination of properties.

Tantalum and its alloys are midway between tungsten and molybdenum in density and melting points. Tantalum can be worked easily at room temperature.

Its thermal conductivity is one-fourth that of molybdenum and its coefficient of expansion is one-third greater. Its elevated temperature strength is low compared with tungsten and molybdenum.

Tantalum's corrosion resistance is unusually good in most commercial combinations of acids. Pure tantalum recrystallizes at approximately 2200°F (1204°C).

Tantalum has several unique properties that have made it essential to certain applications, making it well worth the high cost. It offers approximately the same corrosion resistance to most acids and caustics as glass.

In addition, it can be fabricated by bending, roll forming, and welding with relative ease by personnel experienced with the metal. Tantalum's ductility and density have made it popular with the military for armor penetration. Its density and nuclear stability make it a valuable material for containers of radioactive elements.

Physical Properties of Tantalum

PROPERTY
Atomic Weight 180.95
Density 16.6 g/cc
Melting Point 3290 K, 2996°C, 5462°F
Boiling Point 5731 K, 6100°C, 9856°F
Coefficient of Thermal Expansion (20°C) 6.5 x 10-6/°C
Electrical Resistivity (20°C) 13.5 microhms-cm
Electrical Conductivity 13% IACS
Specific Heat .036 cal/g/°C
Thermal Conductivity .13 cal/cm2/cm°C/sec

Tantalum, discovered by Ekeberg, a Swedish chemist in 1802, is a metal that is closely related to niobium. Tantalum works similarly to copper in forming operations. It can be cold-formed in both the grain direction and in the cross grain direction. It can be spun, drawn, and hydro-formed. Like copper, it work-hardens and when this occurs, it requires vacuum annealing before further working. Tantalum is readily weldable by EB and, if care is taken, by TIG in a dry-box. The welds are strong and can be stress-relieved by vacuum annealing. Welded tantalum can be further formed and even drawn.

Tantalum offers excellent "gettering" properties, making it popular in vacuum tubes to absorb products of out-gassing upon heat up of the tube components. It is also used to getter potential contaminants of niobium and its alloys as well as titanium during vacuum heat treating operations. Tantalum is used in vacuum furnaces where very high temperatures must be attained and where there can be no residual oxygen or hydrogen present during the cycle.

Tantalum also provides good thermal conductivity that, combined with its corrosion resistance, has made it the ideal choice for heat exchangers for acid processing equipment. It is superior to the nickel-based alloys in both these categories.

Tantalum also develops a stable oxide that is useful in electronics industry applications. It has gained acceptance as a suitable material for mass spectrometer filaments providing an alternative to rhenium, historically the only suitable material.

Tantalum is difficult to machine, however, and should be left to those experienced with it for machining operations. Mistakes in fabricating tantalum can prove very costly. Rembar only machines and forms refractory metals and has the necessary expertise to handle tantalum in the most cost-effective ways. Rembar stocks tantalum in sheet, rod, and wire forms. Powder is also available as a mill order.

Applications of Tantalum

Tantalum has gained wide acceptance for use in electronic components, chemical equipment, missile technology, and nuclear reactors. The electronics industry consumes the majority of tantalum produced (approximately 60%) for capacitors. Other industries concerned with corrosion, especially the chemical processing industry, are accounting for an increasingly large percentage of the market.

Tantalum can be used to fabricate valves for corrosive liquids and to manufacture heaters for acids and heat shields for rocket motors. It is also used as a component of ion implanters in the manufacture of semiconductors. Also, because tantalum does not have a low neutron absorption cross section, it is used for radiation shielding.

Tantalum 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.

Other properties of tantalum that are useful in its application are as follows:

  • Corrosion Resistance
    Tantalum is almost completely immune to attack by acids and liquid metals. It equals glass in resistance to acids and it is impervious to liquid metals up to 1650°F Only a few reagents - hydrofluoric acid, fuming sulfuric acid, and strong alkalis - will begin to break through tantalum's corrosion barrier. This ability to resist practically everything has won tantalum favor among manufacturers of chemicals, chemical equipment, instruments, heating elements, and surgical implants.

  • Good Thermal Conductivity
    Tantalum conducts heat better than the nickel alloys, ductile irons, and stainless and high temperature steels. Therefore, tantalum has become the efficient heat transfer surface, especially in acidic or corrosive environments.

    The corrosion-proof heating and cooling surfaces of tantalum remain smooth and clean under conditions that foul and scale so-called acid resistant materials. This is why a tantalum heat exchanger delivers a continuously high rate of heat transfer with practically no downtime.

  • High Melting Point: 5425°F (2996°C)
    Of the refractory metals, tantalum is outranked only by tungsten at 6170°F and rhenium at 5732°F. The high temperature strength of tantalum, combined with its workability, has resulted in the fabrication of superior heat shields, heating elements, electrodes, and other high temperature parts.

  • Superior Gettering Characteristics
    Tantalum absorbs surrounding gases and vapors extremely well at elevated temperatures. Electronic tube manufacturers have long-recognized the ability of tantalum to maintain high vacuum inside the tubes.

  • Good "Valve Action"
    Tantalum forms highly stable anodic films. This property, combined with tantalum's acid resistance, has been exploited by manufacturers of rectifiers, capacitors, lightning arrestors, surge suppressors and optical lenses.

  • Tough, Durable and Ductile
    Tantalum surpasses most of the other refractory metals in ductility, workability and weldability.  As an example, tantalum can be rolled, swaged, drawn, and formed cold. In drawing, stamping or spinning, it works like steel. When stamped, forged or annealed, tantalum will not spring back. It work-hardens slowly and will not age-harden.

Tantalum Alloys

Two alloys of tantalum are particularly well-suited for specific applications:

  • 97.5% Tantalum; 2.5% Tungsten
    This alloy is particularly useful in applications where low temperature strength is important along with high corrosion resistance and good formability.  This alloy offers higher strength than pure tantalum while maintaining the fabricability-characteristics. It is available in basically the same sizes and shapes as pure tantalum, at a comparable cost.

  • 90% Tantalum; 10% Tungsten
    This alloy should be considered when high temperatures of up to 4500°F and high strength in a corrosive environment are required.  The alloy has approximately twice the tensile strength of pure tantalum and yet retains tantalum's corrosion resistance and a good portion of tantalum's ductility. It is not available in as many forms, particularly thin-wall tubing, as pure tantalum or the alloy above. The cost is also somewhat higher than the above alloy.

Corrosion Resistance

Tantalum is clearly the refractory metal that is most resistant to corrosion. The only media that can affect it are fluorine, hydrofluoric acid, sulfur trioxide (including fuming sulfuric acid), concentrated strong alkalis, and certain molten salts.

The corrosion resistance of tantalum can be compared to that of glass. However, tantalum withstands higher temperatures and offers the intrinsic fabrication advantages of a metal.

Tantalum equipment is frequently used in conjunction with glass, glass-lined steel and other nonmetallic construction materials. Tantalum is also used extensively to repair damage and flaws in glasslined steel equipment.

Corrosion Resistance of Tantalum

Media Concentration Temp. Ta
Acetic Acid 50% Boiling Nil
Bromine Dry 200°F Nil
Chlorine Wet 220°F Nil
Chromic Acid 50% Boiling Nil
Hydrochloric Acid 5% 200°F Nil
30% 200°F Nil
Nitric Acid 65% Boiling <2 mpy
Sodium Hydroxide 10% Room *
Sulfuric Acid 40% Boiling Nil
98% Boiling <2 mpy
* Material may become brittle due to hydrogen attack.

Tantalum 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.

Tantalum has the ability form stable, passive oxides and therefore, it can provide unique solutions to many corrosion problems. However, tantalum cannot be used in air at temperatures exceeding 300'C. Refer to the table entitled Corrosion Resistance for additional information.

Working Characteristics

Tantalum is extremely workable. It can be cold-worked with standard equipment. Because of its bcc crystal structure, tantalum is a very ductile metal that can undergo cold reductions of more than 95% without failure.

It can be rolled, forged, blanked, formed and drawn. It is also machinable with high speed and carbide tools using a suitable coolant. Tantalum can also be resistance welded, electron beam or tungsten inert gas welded, brazed, and riveted. Tantalum does have a tendency to stick to tooling during metal forming operations. To avoid this, specific lubricant and die material combinations are required in high pressure forming operations.

Most procedures used in working and fabricating tantalum are conventional and can be mastered without very much difficulty. However, two important characteristics of tantalum must constantly be kept in mind:

  • Annealed tantalum, like copper, lead, stainless steel, and some other metals is "sticky". Therefore, they have a strong tendency to seize, tear and gall.
  • All forming, bending, stamping or deep drawing operations are normally performed cold. Heavy sections can be heated for forging to approximately 800°F.

Fabricating procedures should be planned so that a minimum of scrap results.

Mechanical Properties of Tantalum

Annealed Ultimate Tensile Strength 285 M Pa (41 ksi)
Yield Strength 170 M Pa (25 ksi)
% Elongation 30%+
% Reduction in Area 80%+
Cold Worked Ultimate Tensile Strength 650 M Pa (95 ksi)
% Elongation 5%
Hardness Annealed 90 HV
Cold Worked 210 HV
Poisson's Ratio 0.35
Strain Hardening Exponent 0.24
Elastic Modulus Tension 186 G Pa (27 x 10-6psi)
Shear
Ductile Brittle Transition Temperature * <75°K
Recrystallization Temperature 900 - 1200°C
* Significantly affected by increasing interstitial contents.

Forming and Stamping

Most sheet metal work in tantalum is performed on metal with a thickness ranging from 0.004 to 0.060 inch. The instructions given here apply to metal in this thickness range. When using metal of greater thickness, it is suggested that you contact the factory for recommendations.

Blanking or Punching

Blanking or punching presents no special difficulties. Steel dies are recommended for use. The clearance between the punch and die should approximate 6% of the thickness of the metal being worked. Close adherence to this clearance is important. The use of light oil is recommended to prevent scoring of the dies. A suitable lubricant is necessary.

Form Stamping

Form stamping techniques are similar to those used with mild steel except that precautions should be taken to prevent seizing or tearing of the metal. Dies may be made of steel except where there is considerable slipping of the metal. In this case, aluminum/bronze or beryllium/copper should be used. Low melting alloys such as Kirksite may be used for experimental work or short runs. Rubber or pneumatic die cushions should be used where required. Annealed tantalum takes a permanent set in forming and does not spring back from the dies.

Deep Drawing

Deep drawing is an operation where the depth of the draw in the finished part is equal to, or greater than, the diameter of the blank. For deep drawing operations, only annealed tantalum sheet should be used.

Note that tantalum does not work-harden as rapidly as most metals, and that work-hardening begins to appear at the top, rather than at the deepest part of the draw. If the piece is to be drawn in one operation, a draw in which the depth is equal to the diameter of the blank can be made. If more than one drawing operation is to be performed, the first draw should have a depth of not more than 40% to 50% of the diameter.

Dies should be made of aluminum bronze, although the punch may be steel if not too much slippage is encountered. Sulphonated tallow, chlorinated oil, caster oil or Johnsons No. 150 Drawing Wax are suitable lubricants.

Spinning

Spinning can be accomplished using conventional techniques. Steel roller wheels may be used as tools, although yellow brass may be used for short runs. Yellow soap or Johnsons No. 150 Drawing Wax may be used as a lubricant.

Annealing

Annealing tantalum is accomplished by heating the metal in a high vacuum to temperatures above 2000°F.

Cleaning

Tantalum parts must not be cleaned by hydrogen firing. Cleaning and degreasing present no special problems and conventional methods and materials may be used. However, note that hot caustics must be avoided. Electronic tube parts that must be chemically cleaned require more careful treatment.

The first step for tantalum parts that have been blasted with steel grit is to immerse the parts in hot hydrochloric acid to remove particles of iron. The hydrochloric acid may be used as hot and as strong a solution as desired; it will not attack the tantalum. The parts should then be thoroughly rinsed with distilled water. Tap water often contains calcium salts that may be converted to insoluble sulfates in the subsequent cleaning process. If the tantalum parts have not been grit-blasted, the hydrochloric acid cleaning may begin with the second step below.

The second step is a chemical cleaning process. A hot chromic acid cleaning solution, commonly used for cleaning glass, may be applied. A saturated solution of potassium dichromate in hot concentrated sulfuric acid may be used for this purpose. However, chromium trioxide is preferred to potassium dichromate because its use eliminates the possibility of potassium residues in crevices or elsewhere on the tantalum parts. The cleaning solution should be applied at approximately 110'C and should maintain its red color at all times. When the liquid becomes muddy or turns green, it should be discarded.

After the chromic acid wash, a third step is applied to rinse the parts. The preferred rinse is hot distilled water. If running distilled water is not available, three lip washes will suffice, but it is important that all cleaning solution be removed. The parts should be dried in clean, warm air, free from dust. The parts should not be wiped with paper or cloth and they should not be handled with fingers.

Grit Blasting

Tantalum parts for electronic tubes are often blasted with steel grit to provide a greater radiation surface. The recommended procedure is a blast of a few seconds with No. 90 steel grit, at a pressure of 20 to 40 pounds. Sand, alumina, silicon carbide or other abrasives should not be used because they become embedded in the tantalum and cannot be removed with any chemical treatment that would not also damage the tantalum. This is followed by a thorough cleaning in hot hydrochloric acid as previously described.

The purpose of grit blasting is to increase the amount of surface per unit of area. Therefore, the blasting should be performed in a manner that will produce fine "whiskers" rather than mere indentations on the surface. Sharp particles of grit will do this, while dull ones merely indent the surface. To achieve best results, the blasting nozzle should be held at an angle nearly tangential to the work, rather than perpendicular to the work.

Titanium

Titanium

ti·ta·ni·um (circa 1791 William Gregor) A silvery gray light strong metallic element found combined in ilmenite and rutile and used especially in alloys (as steel) and combined in refractory materials and in coatings.

Titanium is number 22 on the periodic table. It has a melting point of 1668°C and a density of 4.54 gm/cc. Titanium properties and characteristics which are important to design engineers are excellent.  Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.

The combination of high strength and row density results in exceptionally favorable strength-to-weight ratios for titanium-based alloys. These ratios for titanium-based alloys are superior to almost all other metals and become important in such diverse applications as deepwell tubestrings in the petroleum industry and surgical implants in the medical field.  

Superior Strength-to-Weight Ratios The densities of titanium-based alloys range between .160 lb/in3 (4.43 gm/cm3) and .175 lb/in3 (4.85 gm/cm3. Yield strengths range from 25,000 psi (172 MPa) commercially pure (CP) Grade 1 to above 200,000 psi (1380 MPa) for heat treated beta alloys.

Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.  Titanium offers superior resistance to erosion, cavitation or impingement at-tack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.

Titanium and titanium alloys have proven to be technically superior, highly reliable and cost-effective in a wide variety of chemical, industrial, marine and aerospace applications. Titanium is utilized in many critical services due to its unique set of properties.

Titanium can exist in two crystal forms. The first is alpha which has a hexagonal close-packed crystal structure and the second is beta which has a body-centered cubic structure. In unalloyed titanium, the alpha phase is stable at all temperatures up to 1620°F. (880°C.) where it transforms to the beta phase. This temperature is known as the beta transus temperature. The beta phase is stable from 1620°F. (880°C.) to the melting point.

Physical Properties of Titanium

PROPERTY
Atomic Number 22
Atomic Weight 47.90
Density 4.54 g/cc
Melting Point 1941 K, 1668°C, 3034°F
Boiling Point 3560 K, 3260°C, 5948°F
Electrical Resistivity (20°C) 56 microhms-cm
Specific Heat 502.440  J/(kg*K)
Thermal Conductivity 16.44   W/(m*K)

Titanium, discovered by William Gregor in 1791, titanium was first isolated and named after the powerful mythological first sons of the Earth - the Titans, Titanium is most commonly associated with jet engines and airframes, but the most recent media attention has been given to fittings for prosthetic devices and the artificial heart.

To produce titanium, the basic ore, usually rutile (Ti02) is converted to sponge in two distinct steps. First, Ti02 is mixed with coke or tar and charged in a chlorinator. Heat is applied and chlorine gas is passed through the charge. The titanium ore reacts with the chlorine to form TiCl4, titanium tetrachloride, and the oxygen is removed as C0 and C02. The resultant crude TiCl4, produced is a colorless liquid and is purified by continues fractional distillation. It is then reacted with either magnesium or sodium under an inert atmosphere. This results in metallic titanium sponge, and either magnesium or sodium chloride which is reprocessed and recycled.

Melting is the second step. Titanium is converted from sponge to ingot by first blending crushed sponge with the desired alloying elements to insure uniformity of composition, and then pressing into briquettes, which are welded together to form an electrode. The electrode is melted in a consumable electrode vacuum arc furnace where an arc is struck between the electrode and a layer of titanium in a water-cooled copper crucible. The molten titanium on the outer surface solidifies on contact with the cold wall, forming a shell or skull to contain the molten pool. The ingot is not poured, but solidifies under vacuum in the melting furnace. To insure homogeneity of the final ingot, a second or sometimes a third melting operation is applied.

Applications of Titanium

Titanium and its alloys have proven to be technically superior and cost-effective materials of construction for a wide variety of aerospace, industrial, marine and commercial applications. In North America, approximately 70% of the titanium consumed is utilized for aerospace applications. Due to the expansion of existing applications and the development of new uses, the greatest growth will occur in the industrial, marine and commercial sectors.

In the Industrial & aerospace industry, titanium is currently being utilized in: Gas Turbine Engines, Heat Transfer, DSA-Dimensional Stable Anodes, Desalination, Extraction of Electrowinning of Metals, Medical, Hydrocarbon Pressing, Marine Applications, Chemical Processing, Steam Turbines, Automotive, Airframes, Space Structures, FGD (Flue Gas Desulfurization), Nuclear Waste Storage, etc.

  • Corrosion Resistance
    The protective passive oxide firm on titanium (mainly TiO2) is very stable over a wide range of pH, potential and temperature and is especially favored as the oxidizing character of the environment increases. For this reason, titanium generally resists mildly reducing, neutral and highly oxidizing environments up to reasonably high temperatures. It is only under highly reducing conditions where oxide film breakdown and resultant corrosion may occur. Another major benefit to the designer is the fact that weldments, heat affected zones and castings of many of the industrial titanium alloys exhibit corrosion resistance equal to their base metal counterparts. This is attributable to the metallurgical stability of the leaner titanium alloys and the similar protective oxide which forms on titanium surfaces despite microstructural differences.

  • Chlorine Gas
    Titanium is widely used to handle moist or wet chlorine gas, and has earned a reputation for outstanding performance in this service. The strongly oxidizing nature of moist chlorine passivates titanium resulting in low corrosion rates. Proper titanium alloy selection offers a solution to the possibility of crevice corrosion when wet chlorine service temperatures exceed 155°F. (70°C.). Chlorine Chemicals and Chlorine Solutions Titanium is fully resistant to solutions of chlorites, hypochlorites, chlorates, perchlorates and chlorine dioxide. It has been used to handle these chemicals in the pulp and paper industry for many years with no evidence of corrosion.

  • Halogen Compounds
    Similar considerations generally apply to other halogens and halides compounds. Special concern should be given to acidic aqueous fluorides and gaseous fluorine environments which can be highly corrosive to titanium alloys.

  • Resistance to Acids
    Oxidizing Acids In general, titanium has excellent resistance to oxidizing acids, such as nitric and chromic, over a wide range of temperatures and concentrations. Nitric Acid Titanium is used extensively for handling nitric acid in commercial applications. Titanium exhibits low corrosion rates in nitric acid over a wide range of conditions. At boiling temperatures and above, titanium's corrosion resistance is very sensitive to nitric acid purity. Generally, the higher the contamination and the higher the metallic ion content of the acid, the better titanium will perform. This is in contrast to stainless steels which are often adversely affected by acid contaminants. Since titanium's own corrosion product (Ti+4) is highly inhibitive, titanium often exhibits superb performance in recycled nitric acid streams such as re-boiler loops.

  • Passivation with Inhibitors
    Many industrial acid streams contain normal constituents or contaminants (i.e. up-stream corrosion products) which are oxidizing in nature, thereby passivating titanium alloys in normally aggressive acid media. Metal ion concentration levels as low as 20-100 ppm can provide extremely effective inhibition.

Common potent inhibitors for titanium in reducing acid media include dissolved oxygen, chlorine, bromine, nitrate, chromate, permanganate, molybdate and cationic metallic ions, such as ferric (Fe+3), cupric (Cu+2), nickelous (Ni+2) and many precious metal ions. Figure 2 shows how the useful corrosion resistance of unalloyed titanium is significantly extended as the ferric ion concentration is increased in very small amounts. It is this potent metal ion inhibition phenomenon which permits titanium to be successfully utilized for equipment handling hot HCl and H2SO4 acid solutions in metallic ore leaching processes.

Although inhibition is possible in most reducing acids, protection of titanium from hydrofluoric acid solutions is extremely difficult to achieve. Hydrofluoric acid will generally cause rapid general corrosion of all titanium alloys, and should, therefore, be avoided.
Figure 2: Beneficial effect of minute ferric ion additions to corrosion resistance of unalloyed titanium in HCI and media.

Titanium Alloys

There are three structural types of titanium alloys:

  • Alpha alloys are non-heat treatable and are generally very weldable. They have low to medium strength, good notch tough ness, reasonably good ductility and possess excellent mechanical properties at cryogenic temperatures. The more highly alloyed alpha and near-alpha alloys offer optimum high temperature creep strength and oxidation resistance as well.

  • Alpha-Beta alloys are heat treatable and most are weldable. Their strength levels are medium to high. Their hot-forming qualities are good, but the high temperature creep strength is not as good as in most alpha alloys.

  • Beta or near-beta alloys is readily heat treatable, generally weldable, and capable of high strengths and good creep resistance to intermediate temperatures. Excellent formability can be expected of the beta alloys in the solution treated condition. Beta-type alloys have good combinations of properties in sheet, heavy sections, fasteners and spring applications.

As alloying elements are added to pure titanium, the elements tend to change the temperature at which the phase transformation occurs and the amount of each phase present. Alloy additions to titanium, except tin and zirconium, tend to stabilize either the alpha or the beta phase. Elements called alpha stabilizers stabilize the alpha phase to higher temperatures and beta stabilizers stabilize the beta phase to lower temperatures.

Titanium alloys exhibit modulus of elasticity values which are approximately 5O% of steel. This low modulus means excellent flexibility which has been the basis for its use in dental fixtures (braces, etc.) and human prosthetic devices (hip joints, bone implants, etc.). Titanium's excellent compatibility provides an additional incentive for titanium's rapidly expanding use in body prosthetics. Other applications include springs, bellows, golf club shafts and tennis racquets.

Titanium possesses a coefficient of expansion which is significantly less than ferrous alloys. This property also allows titanium to be much more compatible with ceramic or glass materials than most metals, particularly when metal-ceramic/glass seals are involved.

Corrosion Resistance

The environmental resistance of titanium depends primarily on a very thin, tenacious and highly protective surface oxide film. Titanium and its alloys develop very stable surface oxides with high integrity, tenacity and good adherence. The surface oxide of titanium will, if scratched or damaged, immediately reheal and restore itself in the presence of air or water.

The presence of common oxidizing background or contaminating species often maintain or extend the useful performance limits of titanium in many highly aggressive environments. These inhibitive species include air, oxygen, ferrous alloy corrosion products, other specific metallic ions, and/or other dissolved oxidizing compounds. Titanium's already wide range of application can be expanded by alloying with certain noble elements or by impressed anodic potentials (anodic protection).

Also titanium generally exhibits superior resistance to chlorides and various forms of localized corrosion. Titanium alloys are considered to be essentially immune to chloride pitting and intergranular attack; and are highly resistant to crevice and stress corrosion.

Titanium is used in chloride salt solutions and other brines over the full concentration range, especially as temperatures increase. Near nil corrosion rates can be expected in brine media over the pH range of 3 to 11. Oxidizing metallic chlorides, such as FeCl3, NiCl2, or CuCl2, extend titanium's passivity to much lower pH levels.

A possible limiting factor of titanium alloy application in aqueous chlorides can be crevice corrosion in metal to metal joints, gasket to metal interfaces or under process stream deposits. Given these potential crevices in hot chloride containing media, localized corrosion of unalloyed titanium and other alloys may occur depending on pH and temperature.

Working Characteristics

Once judged to be expensive, titanium, in life cycle costing, is now more often seen to be economical. The key to its cost-effective use is to utilize its unique properties and characteristics in the design rather than to substitute titanium for another metal. Titanium is the world's fourth most abundant structural metal. It is found in North America, South America, Europe, Africa and Australia in the forms of ilmenite, rutile and other ores. The most widely used means of winning the metal from the ore is the Kroll process which uses magnesium as a reducing agent. Sodium is also used as a reducing agent by some producers.

The Properties and characteristics which are important to design engineers are:

  • Excellent Corrosion Resistance
    Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.

  • Superior Erosion Resistance
    Titanium offers superior resistance to erosion, cavitation or impingement at-tack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.

  • High Heat Transfer Efficiency
    Under "in service" conditions, the heat transfer properties of titanium approximate those of admiralty brass and copper-nickel. there are several reasons for this:
    (1)Titanium's higher strength permits the use of thinner walled equipment,
    (2) There appear to be unusual and beneficial characteristics in titanium's inherent oxide   film.
    (3) The relative absence of corrosion in media where titanium is generally used leaves the surface bright and smooth for improved lamellar flow.
    (4) Titanium's excellent erosion-corrosion resistance permits significantly higher operating velocities.

  • Superior Strength-to-Weight Ratios
    The densities of titanium-based alloys range between .160 lb/in3 (4.43 gm/cm3) and .175 lb/in3 (4.85 gm/cm3). Yield strengths range from 25,000 psi (172 MPa) commercially pure (CP) Grade 1 to above 200,000 psi (1380 MPa) for heat treated beta alloys.

Properties of Titanium Grades

Commercially Pure
Designation Grade 2 Ti-6A1-4V
Chemical
Composition
(Max. values unless range is shown)
0.10 C; 0.30 Fe; 0.03 Ni; 0.250;
0.015 H (sheet) 0.0125 H (bar)
0.0100 H(billet)
.08C; 0.25 Fe;.05N0.20 0;5.50/
6.75 Al; 3.5/4.5 V; 0.0150 H(sheet)
0.0125 H(tbar): 0.0100 H(billet)
Mill Annealed Tensile Properties Guar.R.T
Min.
Guar.R.T
Min.
Ultimate Strength (psi) 50,000 130,000
Yield Strength (psi) 0.1% offset 40,000 120,000
Elongation in 2" (%)Sheet>0.025 thick 20 10
Reduction in Area (%) Bar 30 25
Mechanical Properties (Typical)   600°F 800°F
Stress to Rupture
in Time Shown
Stress (psi)
Time (Hrs.)
  98,000
1,000
60,000
1,000
Stress and Time to
Produce Elongation
Shown (creep)
Stress (psi)
Time (Hrs.)
Creep (%)
  70,000
1,000
0.1
32,000
1,000
0.1
Charpy V-Notch Impact(ft.-lbs.)@Rm.Temp. 25-40 10-14
Bend Radius Under .070" thk.
.070" and Over
2.0 x Thickness
2.5 x Thickness
4.5 x Thickness
5.0 x Thickness
Welded Bend Radius 2.0-3.0 x Thickness 6.0-10.0 x Thickness
Hardness RB80 RC36
Physical Properties
Beta Transus (F±25) 1675 1830
Coefficient of
Thermal Expansion
(10 - 6inƒinƒF)
32-212 4.8 5.0
32-600 5.1 5.3
32-1000 5.4 5.6
32-1200 5.6 5.9
32-1500 5.6 6.1
Density (lbs._cu.in) 0.163 0.160
Melting Point, Approx. (°F) 3020 3200
Electrical Resistivity@R.T. (Microhms cm) 56 171
Modulus of Elasticity - Tension (10-6 psi) 14.9 16.5
Modulus of Elasticity - Torsion (10-6psi) 6.5 6.1
Thermal Conductivity (Btu/hr./sq.ft./F/Ft.) 9.5 at Room. Temperature 3.9 at Room. Temperature
Specific Heat (Btu/Lb,/F)@ Rm.Temp. 0.124 0.135
Weldability Good Fair
Annealing
Temp. (°F)
Full
Stress Relief
1300/30 Min.-2 Hrs.;Air Cool
1000-1100/30 Min.-2 Hrs.;Air Cool
1300-1450/15 Min.-2 Hrs.;Air Cool
900-1200/1-4 Hrs.;Air Cool
Forging
Temp. (°F)
Blocking
Finishing
1600-1700
1500-1600
1750-1800
1650-1750
Available Mill Products Bar; Billet; Extrusions
Plate; Sheet; Strip
Wire; Pipe; Tubing
Bar; Billet; Extrusions
Plate; Sheet; Strip
Wire
Typical Applications For corrosion resistance in the chemical and marine industries, and where a higher strength level and ease formability is desired.  
Industry Specifications AMS 4902
ASTM B265 Gr2
ASTM B337 Gr2
ASTM B338 Gr2
ASTM B348 Gr2
ASTM B381 Gr2
ASTM F67   Gr2
ASTM F467 Gr2
ASTM F468 Gr2
AMS 4911
AMS 4928
AMS 4935
AMS 4965
AMS 4967
ASTM B265 Gr2
ASTM B348 Gr2
ASTM B381 Gr2
ASTM F467 Gr2
ASTM F468 Gr2

Titanium and its alloys possess many unique physical properties which make them ideal for equipment design, even when strength or corrosion resistance may not be critical. These unique properties include:

  • Low Density

  • Low Modulus of Elasticity

  • Low Coefficient of Expansion

  • High Melting Point

  • Non-Magnetic

  • Extremely Short Radioactive Half-Life

Titanium's low density, roughly 56% that of steel, means twice as much metal volume per pound. Particularly when combined with alloy strength, this often means smaller and/or lighter components. Although obviously the basis for aerospace applications, positive implications are also apparent for many types of rotating or reciprocating components such as centrifuges and pumps.

The relatively high melting point of titanium has led to consideration of titanium for ballistic armor. The higher melting point tends to reduce susceptibility to armor melting and ignition (burning) during ballistic impact. Good toughness and light weight are additional factors for considering titanium alloys in this application.

Titanium is virtually non-magnetic, making it ideal for applications where electro-magnetic intencerence must be minimized. Desirable applications include electronic equipment housing and downhole well logging tools.

Titanium has an extremely short half-life, thereby permitting its use in nuclear systems. In contrast to many ferrous alloys, many titanium alloys do not contain a significant amount of alloying elements which may become radioactive.

Rhenium

Rhenium

Rhenium
rhe·ni·um (1925 - Walter Noddack, Ida Tacke and Otto C. Berg) A rare heavy metallic element that resembles manganese, is obtained either as a powder or as a silver-white hard metal, and is used in catalysts and thermocouples.

Rhenium is number 75 on the periodic table with a density of 21.02 gm/cc and a melting point of 3180°C. It is second only to tungsten in melting temperature and to Osmium in density.

Rhenium is rare, expensive, and extremely difficult to machine. Availability in size and shape is very limited. It is most commonly used as an alloying agent to improve the properties of other metals (mostly refractory metals).

Rhenium has several unique properties that are also imparted to its alloys. One of the most important of these is its high melting point of 3180°C. This is second only to tungsten. The high melting point, coupled with its ability to maintain ductility even after recrystallization, offers many advantages.

Physical Properties of Rhenium

PROPERTY
Atomic Weight 186.20
Density 21.02 g/cc
Melting Point 3459 K, 3180°C, 5767°F
Boiling Point 5869 K, 5596°C, 10105°F

Rhenium also offers high electrical resistivity over a wide temperature range. The resistivity is higher than tungsten. This characteristic, plus having a low vapor pressure, make it a good choice for filaments. This is especially true when considering ductility and the fact that it is not affected by the oxidation/reduction cycle as is tungsten.

Rhenium has several unique properties that are highly desirable:

  • a very high melting temperature
  • extreme resistance to chemical reactivity
  • ability to maintain ductility after recrystallization
  • high electrical resistivity over a wide temperature range
  • low vapor pressure
  • resistance to oxidation at very high temperatures
  • good shock resistance
  • low friction

These properties can be transferred to other metals with which it is alloyed. As a result, it is used to alloy with molybdenum and tungsten to improve on their properties.

One of rhenium's major uses is as a catalyst in the crack distillation of petroleum products. Another common use is as filaments in mass spectrometers. Rhenium is available in 99.97 (commercial) purity and 99.995 (zone refined) purity. These purity levels become most important in the mass spectrometer industry. Use for mass spectrometer filaments is a high demand area. These are normally stocked for quick delivery in purities of both commercial (99.97%) and zone-refined (99.995%) grades.

As an alloying agent, it is used to produce 50/50 Mo/Re for use primarily in power tube parts. Its ductility (especially after recrystallization), weldability, formability, and strength all contribute to making it an ideal candidate for heat chokes, cathode stem supports, and similar parts.

Rhenium makes tungsten more ductile and machineable. Alloyed with tungsten in 3 %, 5 % and 26% content, rhenium gives stable electrical properties enabling it to be used for thermocouple wire for very high temperature thermocouples.

Rembar supplies rhenium in powder, wire, sheet and rod form. Rhenium is available only in a very limited size range: about 3" wide for foil and sheet and only up to about 1/2" diameter in rod.

In addition, rhenium is not machinable by conventional methods such as turning, milling, etc. It can be ground and EDM machined. It can also be formed and welded. Rembar will advise you on the practicality of making a part out of rhenium and, if it can be made, Rembar can make it.

Rhenium Alloys

When rhenium is alloyed with molybdenum or tungsten, it imparts its properties to these materials and permits a cost-effective solution. This allows the advantage of these properties at a considerably lower cost than that of pure rhenium. The alloys are available in several ratios:

  • Tungsten/26 Rhenium
  • Tungsten/5 Rhenium
  • Tungsten/3 Rhenium
  • 50/50 Molybdenum/Rhenium

Note that, although this alloy is nominally 47% rhenium, the reference to this alloy is as above.

In addition, molybdenum / rhenium sheet is available in two standard alloys with 47.5 % and 41 % rhenium. The Mo 41 % Re alloy becomes even more ductile after exposure to high temperatures because it does not develop a sigma phase.

When formed as tubing, thin-walled Mo 47.5% Re provide the advantages of high ductility and a high melting point. This alloy is used for electronics, nuclear, and space applications. Rembar specializes in seamless tubing made to specifications less than .375" O.D.. Mo 47.5% Re tubing is produced as small as.020" O.D.

These choices allow the selection of the proper alloy for a particular application while optimizing cost-effectiveness.

Molybdenum/rhenium wire remains very ductile even after high temperature exposure. Tungsten/rhenium wire is used for heating elements in high temperature furnaces, thermocouples, and in the electronics industry. This alloy's advantage is its ability to maintain greater ductility compared to tungsten after exposure to extremely high temperatures.

Physical Properties of Rhenium
PROPERTY
Atomic Weight 186.2
Density 21.02 g/CC
Melting Point 3180°C
Boiling Point 5900°C
Coefficient of Thermal Expansion (20°C) 6.5 x 10-6/°C
Electrical Resistivity (20°C) 13.5 microhms-cm
Electrical Conductivity 13.9% IACS
Specific Heat .032 cal/g/°C
Thermal Conductivity .39 cal/cm2/cm°C/sec

Electrical Resistivity vs. Temperature

Ductility vs. Temperature

Ductility vs. Temperature

Creep-Rupture Chart

Stress-Rupture Chart at 2200°C

Stress-Rupture Chart at 1600°C

Mechanical Properties: Rhenium and Alloys of Rhenium

Property Rhenium Molybdenum - 50 rhenium Tungsten - 25 rhenium
Modulus of elasticity in tension (psi x 106) Wrought Recrystallized Wrought 95% Recrystallized As sintered Wrought
at -65°C - - 50.8 55.7 - -
at 20°C - 68 52.3 53.3 53.6 62.5
at 800°C - 55.5 - - - -
Ultimate tensile strength (psi x 103) Wrought 15% Recrystallized Wrought Recrystallized Wrought Recrystallized
at 20°C 280 155 240 150 310 190
at 1200°C 80 60 50 35 110 105
at 2000°C 18 18 - - - -
Elongation (% in 3 inches) Wrought Recrystallized Wrought Recrystallized Wrought Recrystallized
at 20°C 2 15-20 4 19 - 15-20
at 1200°C 1 2 4 18 - -
at 2000°C 1 2 - 17 - -
Yield strength, 0.2% offset (psi x 103) Wrought 15% Recrystallized Wrought 50% Recrystallized Wrought Stress-relieved
at 20°C 255 42 210 116-123 - 249-294
at 1200°C - - - - - 59-78
at 2000°C - - - - - 6-7

High Temperature Stress-Rupture Data

Test Temperature Test Property Rhenium Molybdenum - 50 - rhenium Tungsten - 25 - rhenium
1600°C Stress (psi x 103) 1.5 6 12 2 4 6 4.8 10 15
Rupture Time (hours) 239 35.3 2.24 232 9.19 1.12 198 15.1 3.3
Elongation at rupture (%) 2.4 11 6 56 48 109 44 41 53
Time to produce 2% elongation (hours) - - - - - - - - -
2800°C Stress (psi x 103) 0.6 1 2 - - - 0.65 - -
Rupture time (hours) 11.4 2.9 0.55 - - - 1.12 - -
Elongation at rupture (%) 11 6 7 - - - 8 - -
Time to produce 2% elongation (hours) - - - - - - - - -

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.