TECHNICAL DATA

Table of Contents

• Refractory Metals: Typical Analysis
• Typical Properties of Molybdenum, Tantalum, Tungsten
• Charts (Molybdenum)
  • Specific Heat Chart for Molybdenum
  • Thermal Conductivity Chart for Molybdenum
  • Electrical Resistivity Chart for Molybdenum
  • Thermal Expansion Chart for Molybdenum
  • Strength/Temperature Chart
• Charts (Molybdenum and Tungsten)
  • Thermal Expansion/Temperature Chart
  • Strength/Temperature Chart
  • Specific Resistance/Temperature Chart
  • Strength/Diameter Chart
• Corrosion Data
• Chemical Reactivity of Molybdenum
• Chemical Reactivity of Tungsten
• Chemical Reactivity of Tantalum
• Corrosion Resistance Chart
• Comparative Machinability Ratings of Some Refractory Metals and Other Difficult Materials
• Vacuum Furnaces
• Radiant Shield Data for Molybdenum
• Machining and Welding Nickel-Iron Alloys
• Melting Points of Metals
• Densities of Metals
• Brazing Filler Metals

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.

  Data on Molybdenum and Tungsten

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.

R = Resistant.
VR = Variable Resistance depending on temperature and concentration.
NR = Non-resistant.

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:

• low absorption
• absorption
• cleanliness; the material must not be prone to dusting which could be injurious to the vacuum pumps.
• low heat storage to facilitate cooling
• light weight
• high strength

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

• Shield Pack
  a series of non-contacting metal sheets.
• Insulation Pack
  an inner metallic shield supporting a blanket insulation against a metallic outer shell.
• Self-facing Insulation
  such as rigidized alumina-silica fibers and graphite felt.

The advantage of Radiant Molybdenum Shielding over other materials is:

• Cleanliness
  Molybdenum will not flake off particles that could contaminate the work or pumping system.
• Heat
  Heat absorption of molybdenum is reflective to the radiant energy. This is the only way that heat can be transferred in a vacuum.
• Outgassing
  Molybdenum does not absorb gases as do the other materials. Therefore it does not outgas them during heat up to avoid prolonged pump down times.
• Low Heat Storage
  Molybdenum does not hold temperatures as long as the other materials and, therefore, allows for faster cooling.

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.

• Maximum rigidity of tool and workpiece must be obtained to insure smooth cutting.
• Machine speed should be kept low enough to provide sufficient torque or force at the cutting edge to prevent deceleration.
• Tools must be kept sharp with a high degree of surface finish on the rake face.
• Tool material should be either high speed steel or tungsten carbide.

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.

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.

• It is preferable to take a deep cut with a light feed rather than a light cut with a heavy feed.
• Tool relief angles should be kept to a minimum in order to provide maximum support for the cutting edge.
• To insure adequate chip disposal when roughing, it may be necessary to vary the values for back or side rake angles slightly. This is to have the chip curl over and break on the workpiece in advance of the tool.

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.

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

• HSS twist drills with a high degree of flute finish should be used.
• Drills should be reground as soon as they show signs of dulling.
• A large amount of cutting fluid should flow onto the cutting edge of the drill.
• When hand feeding, sufficient pressure should be applied to keep the cutting edge beneath the work surface to prevent work-hardening the material.
• When drilling into a previously worked surface, it may be necessary to thin the drill web and increase the point angle from a nominal 118° to 150°.
• Peripheral speeds should be of the order of 50 surface feet per minute with feeds not less than 0.004 in/rev (0.1 mm/rev).

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

| Molybdenum | Tungsten | Tantalum | Titanium | Rhenium | Niobium |

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