Refractory Metal Technical Information

Table of Contents


Please note. We have provided this technical information on refractory metals to our customers for several years without charge. It has been compiled from what we believe are reliable sources. Indeed, we use it in our daily decision making. However, if you use this in your decision making, you must independently verify that it is correct and that it properly applies to your intended use.


Refractory Metals: Typical Analysis

Element Maximum % Molybdenum Maximum % Tungsten Maximum % Tantalum Maximum % Niobium
Aluminum 0.001 0.002 --- 0.005
Calcium 0.003 0.003 --- ---
Chromium 0.005 0.002 --- ---
Copper 0.001 0.002 --- ---
Iron 0.005 0.003 0.010 0.01
Lead 0.002 0.002 --- ---
Magnesium 0.001 0.002 --- ---
Molybdenum 99.95 Min --- 0.010 0.01
Manganese 0.001 0.002 --- ---
Nickel 0.001 0.003 0.005 0.005
Silicon 0.003 0.002 0.005 0.005
Tin 0.003 0.002 --- ---
Titanium 0.002 0.002 0.005 ---
Tantalum --- --- 99.90 Min 0.2
Tungsten --- 99.95 Min 0.030 0.05
Carbon 0.005 0.005 0.0075 0.01
Oxygen --- --- 0.020 0.025
Nitrogen --- --- 0.0075 0.01
Hydrogen --- --- 0.0001 0.0015
Niobium --- --- 0.050 99.9


Typical Properties of Molybdenum, Tantalum, Tungsten

(Ranges only: Data will vary with type of sample and previous work history)

Molybdenum Tungsten Tantalum
Property Atomic Number 42 74 73
Atomic Weight 95.95 183.86 180.95
Atomic Volume 9.41 9.53 10.90
Lattice Type Body centered cube Body centered cube Body centered cube
Lattice Constant;
20°C, A
3.1468 3.1585 3.3026
Isotope (Natural) 92, 94, 95, 96, 97, 98, 100 180, 182, 183, 184 186 181
Mass Density at 20° C gm/cc 10.2 19.3 16.6
Density at 20° C lb/in 3 0.368 0.697 0.600
Thermal Properties Melting Point, °C 2610 3410 2996
Boiling Point, °C 5560 5900 6100
Linear Coefficient of Expansion per °C 4.9 x 10-6 4.3 x 10-6 6.5 x 10-6
Thermal Conductivity at 20°C, cal/cm2/cm°C/sec. 0.35 0.40 0.130
Specific Heat, cal/g/°C; 20°C 0.061 0.032 0.036
Electrical Properties Conductivity, % IACS 30% 31% 13%
Resistivity, microhms-cm; 20°C 5.7 5.5 13.5
Temperature Coefficient of Resistivity per °C (0-100°C) 0.0046 0.0046 0.0038
Mechanical Properties Tensile Strength at room temperature, psi 100,000-200,000 100,000-500,000 35,000-70,000
Tensile Strength-500°C psi 35,000-65,000 75,000-200,000 25,000-45,000
Tensile Strength-1000°C psi 20,000-30,000 50,000-75,000 13,000-17,000
Young's Modulus of Elasticity; lb/in2
Room Temperature 46 x 106 59 x 106 27 x 106
500°C 41 x 106 55 x 106 25 x 106
1000°C 39 x 106 50 x 106 22 x 106
Spectral Emissivity (Wave Length approx. 0.65) 0.37 (1000°C) 0.45 (900°C) 0.46 (900°C)
Working Temperature 1600°C 1700°C Room
Recrystallizing Temp 900-1200°C 1200-1400°C 1000-1250°C
Stress Relieving Temp 800°C 1100°C 850°C
Metallography Etchant Hot H2O2; 6% sol HF-NH; F sol Alk.K3FE(CN) sol
Polishing Alumina - Rouge to finish
Note: Etch and polish repeatedly until grain boundaries appear.


Data on Molybdenum

 


Data on Molybdenum and Tungsten

Corrosion Data

General

Corrosion resistance of the refractory metals is second only to that of the noble metals. Unlike the noble metals, however, the refractory metals are inherently reactive.

Such reactivity is a decided plus for corrosion resistance. On contact with air or any other oxidant, refractory metals immediately form an extremely dense, adherent oxide film. This passivating layer prevents access of the oxidant to the underlying metal and renders it resistant to further attack.

Unfortunately, these oxides spall or volatize at elevated temperatures, leaving the metals susceptible to oxidation at approximately 300 to 500 degrees C. For high-temperature applications under non-reducing conditions, the refractory metals must be protected by an applied coating, such as a metal silicide.

Tantalum

Tantalum is clearly the top of the line for corrosion resistance. 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, although 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 materials of construction. Tantalum is also used extensively to repair damage and flaws in glass-lined steel equipment.

Because of its high cost and lack of strength compared to its easy fabricability, tantalum is usually used as a lining over a stronger, less expensive base material.

Niobium (or Columbium)

Niobium can be a less-expensive alternative to tantalum. However, its corrosion resistance is more limited. This is because it is sensitive to most alkalis and certain strong oxidants.

Niobium does remain totally resistant to such highly corrosive media as wet or dry chlorine, bromine, saturated brines, ferric chloride, hydrogen sulfide, sulfur dioxide, nitric and chromic acids, and sulfuric and hydrochloric acids within specific temperature and concentration limits.

Even though the mechanical strength of niobium is less than that of tantalum, it can be used economically where the extreme inertness of tantalum is not required.

Molybdenum

Molybdenum provides corrosion resistance that is slightly better than that of tungsten. It particularly resists non-oxidizing mineral acids.

Molybdenum is relatively inert to carbon dioxide, hydrogen, ammonia and nitrogen to 1100 degrees C and also in reducing atmospheres containing hydrogen sulfide.

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

For more specific information on refractory metals and the affect of specific reagents, refer to the tables inluded herein.


Chemical Reactivity of Molybdenum

Reagent R VR NR Reagent R VR NR
Water X     Hydrogen X    
Hydroflouric Acid1 X     Nitrogen X    
Hydrochloric Acid (cold) X     Inert Gasses (all) X    
Sulfuric Acid (hot)   X   Carbon Monoxide (1400°C)-Carbide Formation     X
Nitric Acid (cold)   X   Carbon Dioxide (1200°C)-Oxidation     X
Nitric Acid (hot)     X Hydrocarbons (1100°C)-Carbide Formation     X
Aqua Regia (cold)   X   Aluminum (molten)     X
Aqua Regia (hot)     X Iron (molten)     X
Nitric/Hydroflouric mixture1     X Cobalt (molten)     X
Aqueous Ammonia   X   Nickel (molten)     X
Aqueous Caustic Soda/Potash X     Tin (molten)     X
Molten Caustic   X   Zinc (molten)   X  
Molten Caustics2     X Lead   X  
Boron (hot)-Boride fomation     X Cesium   X  
Carbon (1100°C)-Carbide Formation     X Gallium   X  
Silicon (1000°C)-Silicide Formation     X Potassium   X  
Phosphorous X     Lithium   X  
Sulfide Formation (440°C)     X Magnesium   X  
Iodine X     Sodium   X  
Bromine X     Mercury   X  
Chlorine X     Bismuth X    
Flourine (room temperature)     X KNO2, KNO3, KCLO3 (molten)     X
Oxygen or air (>400°C)   X   Molten Glass X    
Oxygen or air (>600°C)     X Al2O3, BeO, MgO, ThO2, ZrO2(<1700°C) X    
Notes: Key:
1. May be either hot or cold.

2. Molten Caustics are in the presence of KNO2, KNO3, KCLO3, PbO2.

R = Resistant.

VR = Variable Resistance depending on temperature and concentration.

NR = Non-resistant.


Chemical Reactivity of Tungsten

Reagent R VR NR Reagent R VR NR
Water X     Flourine     X
Water Vapor (red heat)-Oxidation     X Oxygen or air (<400°C) X    
Hydroflouric Acid X     Oxygen or air (>400°C)   X  
Hydrochloric Acid X     In air   X  
Sulfuric Acid   X   Hydrogen X    
Nitric Acid X     Nitrogen X    
Aqua Regia (cold) X     Carbon Monoxide (<800°C) X    
Aqua Regia (warm/hot)     X Carbon Monoxide (>800°C)   X  
Nitric/Hydroflouric mixture     X Carbon Dioxide (>1200°C)-Oxidation     X
Aqueous Caustic Soda/Potash X     Aluminum oxide-Oxidation     X
Ammonia X     Magnesium Oxide-Oxidation     X
Ammonia in presence of H2O2   X   Thorium oxide (>2220°C)-Oxidation     X
Ammonia (<700°C) X     Sodium Nitrite (molten)     X
Ammonia (>700°C)   X   Sulfur (molten, boiling)   X  
Carbon (>1400°C)-Carbide Formation     X Hydrogen/Chloride Gas (<600°C) X    
Iodine (at red heat)     X Nitric Oxide (hot)-Oxidation     X
Bromine (at red heat)     X Hydrogen Sulfide (red heat)   X  
Chlorine (>250°C)   X   Sulfur Dioxide (red heat)     X
Carbon Disulfide (red heat)     X In presence of KNO2, KNO3, KCLO3, PbO2     X
Mercury (and vapor) X            
Key: R = Resistant.
VR = Variable Resistance depending on temperature and concentration.
NR = Non-resistant.

Chemical Reactivity of Tantalum

Reagent R VR NR Reagent R VR NR
Acetic Acid X     Methyl Sulfuric Acid X    
Acetic Anhydride X     Nickel Chloride X    
Aluminum Chloride X     Nickel Sulfate X    
Aluminum Sulfate X     Nitric Acid X    
Ammonia   X   Nitric Acid, fuming X    
Ammonium Chloride X     Nitric Oxides X    
Ammonium Hydroxide   X   Nitrous Acid X    
Ammonium Nitrate X     Nitrosyl Chloride X    
Ammonium Phosphate X     Organic Chloride X    
Ammonium Sulfate X     Oxalic Acid X    
Amyl Acetate or Chloride X     Perchloric Acid X    
Aqua Regia X     Phenol X    
Arsenic Acid X     Phosphoric Acid <4ppmF X    
Barium Hydroxide X     Pickling Acids1 X    
Bromine, dry (<200°C) X     Phthalic Anyhydride X    
Calcium Hydroxide X     Potassium Carbonate   X  
Calcium Hypochlorite X     Potassium Chloride X    
Chlorinated Brine X     Potassium Dichromate X    
Chlor. Hydrocarbons X     Potassium Hydroxide2   X  
Chlorine, dry (<175°C) X     Potassium Hydroxide3     X
Chlorine, wet X     Potassium Iodide-Iodine X    
Chlorine Oxides X     Silver Nitrate X    
Chloracetic Acid X     Sodium Bisulfate, molten     X
Chromic Acid X     Sodium Bisulfate, solution X    
Chrome Plating Solutions X     Sodium Bromide X    
Cleaning Solution X     Sodium Carbonate   X  
Copper Salts X     Sodium Chlorate X    
Ethylene Dibromide X     Sodium Chloride X    
Ethyl Chloride X     Sodium Hydroxide2   X  
Fatty Acids X     Sodium Hydroxide3     X
Ferric Chloride X     Sodium Hypochlorite X    
Ferric Sulfate X     Sodium Nitrate X    
Ferrous Sulfate X     Sodium Sulfate X    
Flourine     X Sodium Sulfide   X  
Formic Acid X     Sodium Sulfite X    
Fuming Nitric Acid X     Stannic Chloride X    
Fuming Sulfuric Acid     X Sulfur (<500°C) X    
Hydrobromic Acid X     Sulfur Dioxide X    
Hydrochloric Acid X     Sulfur Trioxide     X
Hydrocyanic Acid X     Sulfuric Acid (>160°C) X    
Hydrofluoric Acid     X Zinc Chloride X    
Hydrogen Bromide X     Zinc Sulfate X    
Hydrogen Chloride X    
Liquid Metals
Hydrogen Iodide X     Bismuth (<900°C) X    
Hydrogen Peroxide X     Gallium (<450°C) X    
Hydrogen Sulfide X     Lead (<1000°C) X    
Hypochlorous Acid X     Lithium (<1000°C) X    
Iodine (<1000°C) X     Magnesium (<1150°C) X    
Lactic Acid X     Mercury (<600°C) X    
Magnesium Chloride X     Sodium (<1000°C) X    
Magnesium Sulfate X     Sodium - Potassium Alloys (<1000°C) X    
Mercuric Chloride X     Zinc (<500°C) X    
Notes: Key:
1. Except HNO3-HF.
2. Dilute.
3. Concentrated.
R = Resistant.
VR = Variable Resistance depending on temperature and concentration.
NR = Non-resistant.


Comparative Machinability Ratings of Some Refractory Metals and Other Difficult Materials

  Carbide Tool   Machinability Ratings
Workpiece Material Hardness Surface Speed (ft/min) Cut Depth (in.) Feed (in/rev) Type Life (in3) Removal Rate (in3/min) Relative Removal Rate Relative Removal Cost
Steel
4130 200 BHN 445 0.12 0.019 C6 582 11.50 100.0 1
4130 54RC 90 0.12 0.004 C6 19 0.62 5.4 19
Superalloys
Rene 41 320 BHN 70 0.06 0.009 C2 23 0.47 4.1 25
Rene 41 365 BHN 70 0.06 0.009 C2 16 0.47 4.1 25
Refractory Metals
TZM 217 BHN 350 0.06 0.009 C2 99 2.30 20.0 5
Niobium 112 BHN 300 0.12 0.005 C2 151 2.20 19.0 6
Unalloyed, Wrought Molybdenum 223 BHN 275 0.10 0.010 C1 132 3.30 29.0 4
Note:
Ratings based on metal removal rate for 4130 steel at 100,000 psi tensile strength as 100; lower numbers indicates poorer machinability.


Vacuum Furnaces

In the cold wall vacuum furnace design, heating is from within the vacuum vessel so the heat losses from the work area to the cold wall must be reduced. To be compatible with the vacuum system, the insulation must meet certain requirements. These include:

Molybdenum is an ideal material for this application. Rembar stocks all the materials normally used in vacuum furnaces and can do much of the fabrication of both new and replacement parts.

There are three basic insulation systems that will meet most of the above requirements. These systems can be classified as:

The shield pack insulation system is composed of a multi-layer design that is made up of metal sheets that are separated to form a series of reflective shields. The selection of shield material is dependent on the maximum use-temperature of the system. Most commonly used is molybdenum.

The advantage of Radiant Molybdenum Shielding over other materials is:


Radiant Shield Data for Molybdenum

Heat Shields
Furnace Temperature, x F 1832 1832 2012 2012 2400 2400
Cold Shell Temperature, x F 100 100 100 100 100 100
Number of Shields (1 - 10) 1 2 1 2 1 2
Avg. Shield Emissivity Factor (0 - 1.0) 0.60 0.60 0.60 0.60 0.60 0.60
Cold Shell Emissivity Factor (.9 typ) 0.90 0.90 0.90 0.90 0.90 0.90
 
Computed Shield Temperature IN x F
#1 Shield 1484 1646 1637 1811 1965 2167
#2 Shield   1250   1384   1672
 
Computed Heat Loss (KW/ft2) 4.0 2.4 5.5 3.3 9.8 5.9
 
Furnace Temperature, x F 2400 2400 2400 2400 2192 2192
Cold Shell Temperature, x F 150 150 150 150 100 100
Number of Shields (1 - 10) 3 4 5 6 1 2
Avg. Shield Emissivity Factor (0 - 1.0) 0.60 0.60 0.60 0.60 0.60 0.60
Cold Shell Emissivity Factor (.9 typ) 0.70 0.70 0.70 0.70 0.90 0.90
 
Computed Shield Temperature IN x F
#1 Shield 2247 2282 2305 2320 1789 1976
#2 Shield 1976 2087 2151 2194   1517
#3 Shield 1563 1832 1966 2047    
#4 Shield   1444 1723 1869    
#5 Shield     1354 1636    
#6 Shield       1282    
 
Computed Heat Loss (KW/ft2) 4.0 3.1 2.6 2.2 7.3 4.3

Machining and Welding Nickel-Iron Alloys

(ASTM F-15 and Similar Alloys)

In general, these alloys are not difficult to machine, provided it is noted that these materials work-harden readily. Also note that adequate care is taken on the choice of such factors as tool geometry and material, speeds, feeds, cutting fluids, etc. The following data is intended as a guide to proper selection of these parameters for machining Ni-Fe alloys.

General Machining

Cutting Fluids

A large amount of heat is generated in cutting this material. Consequently, machining is made easier by using a good cutting fluid.

For general machine work, a copious flow (approximately 1 gpm/HP used) of soluble oil is recommended. A chlorinated oil is suggested for use on automatic and semi-automatic machines where a neat oil is required.

Turning and Boring

The general set up for these operations is similar to that used for steel. The following principle should be adhered to as closely as possible.

The following tool geometry, speed and feed values are given as a general guide for use with tungsten carbide tools. The speed and feed figures should generally be reduced by approximately 30% for high-speed steel tools.

Detail Roughing
Value
Finishing
Value
Back rake angle 10°
Side rake angle
Front cutting edge
clearance angle
Slide cutting edge
clearance angle
Plan trail angle
Plan approach angle* 15° 20°
Nose radius 0.30 in
(0.8 mm)
0.05 in
(1.3 mm)

Speed and
Feeds
Roughing Finishing
Depth of cut 0.1 in
(2.5 mm)
<=0.010 in
(0.25 mm)
Feed (in-mm/rev) 0.015 in
(0.4 mm)
>=0.004 in
(0.10 mm)
Speed (SFPM) 90 120
*Where it is impossible or impractical to apply this value, a decrease in plan approach angle should be followed by an increase in side rake and a decrease in back rake.
Note that the secondary front cutting edge clearance angle should be to suit application.

Planing Techniques

To enable only a light cut to be taken with the finishing tool, roughing cuts should be taken to within approximately 0.25 in. (0.635 mm) of the finished dimension.

A goose neck type of planer tool is recommended for smoother finishing cuts since its shape enables it to withstand the greater mechanical shock encountered when machining Ni-Fe alloys.

The following tool angles are given as a general guide for use with high-speed tools.

Detail Roughing
Value
Finishing
Value
Back rake angle 10°-15°
Side rake angle 15°
Front cutting edge
clearange angle
Slide cutting edge
clearance angle
Nose radius 0.125 in
(3 mm)
0.250 in
(6 mm)


Drilling

The following principles should be observed when drilling Ni-Fe alloys:

Precision Grinding

The methods for grinding Ni-Fe alloys are similar to those used with steel. However, certain conditions require that a slightly softer grade of wheel be used to prevent loading the wheel. A copious flow of lubricant should be used.

Where high permeability is required, final grinding (after annealing) should finish with approximately five cuts progressively decreasing from 0.002 in (0.05 mm) to 0.0002 in (0.005 mm).

Melting Points of Metals

High   Medium   Low
  °C °F   °C °F   °C °F
Tungsten 3410 6170 Rhodium 1966 3571 Neodymium 1024 1875
Rhenium 3180 5756 Chromium 1930 3506 Silver 961 1762
Tantalum 2996 5425 Zirconium 1857 3375 Germanium 947 1737
Osmium 2700 4892 Thorium 1845 3353 Lanthanum 920 1688
Molybdenum 2610 4730 Platinum 1773 3223 Barium 850 1562
Iridium 2454 4449 Titanium 1725 3137 Calcium 848 1558
Ruthenium 2450 4442 Vanadium 1710 3110 Cerium 815 1499
Niobium 2468 4379 Palladium 1549 2820 Arsenic 814 1497
Boron 2300 4172 Iron 1535 2795 Strontium 774 1425
Hafnium 2230 4046 Cobalt 1495 2723 Aluminum 660 1220
Yttrium 1490 2714 Magnesium 651 1204
Nickel 1455 2651 Antimony 630 1166
Erbium 1450 2642 Tellurium 452 846
Beryllium 1278 2332 Zinc 419 786
Manganese 1220 2228 Lead 327 621
Europium 1150 2102 Cadmium 321 610
Uranium 1133 2071 Thallium 302 576
Copper 1083 1981 Bismuth 271 520
Samarium 1072 1962 Tin 232 450
Gold 1063 1945 Selenium 217 423
Silicon 1410 2570 Lithium 179 354
Indium 156 313
Sodium 98 208
Potassium 62 144
Gallium 30 8
Mercury -38.8 -38

Densities of Metals

High   Medium   Low
  G/CC   G/CC   G/CC
Osmium 22.48 Bismuth 9.90 Gallium 5.97
Iridium 22.42 Erbium 9.16 Arsenic 5.73
Platinum 21.45 Copper 8.96 Germainium 5.32
Rhenium 21.02 Cobalt 8.92 Europium 5.24
Gold 19.30 Nickel 8.90 Selenium 4.81
Tungsten 19.30 Cadmium 8.65 Titanium 4.50
Uranium 19.05 Niobium 8.57 Yttrium 4.34
Tantalum 16.60 Iron 7.87 Barium 3.50
Mercury 13.55 Manganese 7.44 Aluminum 2.70
Hafnium 13.09 Indium 7.31 Strontium 2.60
Rhodium 12.44 Tin 7.30 Boron 2.34
Ruthenium 12.20 Chromium 7.14 Silicon 2.32
Palladium 12.02 Zinc 7.14 Beryllium 1.84
Thallium 11.85 Neodynium 7.00 Magnesium 1.74
Thorium 11.70 Samarium 6.93 Calcium 1.55
Lead 11.34 Cerium 6.78 Sodium 0.97
Silver 10.49 Antimony 6.68 Potassium 0.86
Molybdenum 10.20 Zirconium 6.50 Lithium 0.53
Tellurium 6.24
Lanthanum 6.19
Vanadium 6.11

Brazing Filler Metals

for Refractory Metals

Liquidus Temperature
Brazing Filler Metal °F °C
Ag 1760 960
Cu 1980 1052
Ni 2650 1454
Pd-Mo 2860 1571
Pt-Mo 3225 1774
Ag-Cu-Mo 1435 779
Ni-Cu 2460 1349
Mo-Ru 3450 1899
Pd-Cu 2200 1204
Au-Cu 1625 885
Au-Ni 1740 949