Niobium Data Sheets: 

Consolidation and Processing
1.Finished ingot is obtained through a minimum of two arc-melts of electron beam melted Niobium with Hafnium and Titanium alloy additions to obtain desired alloy contents, and more important, to insure homogeneity throughout the ingot.

2.Machined ingot is hot reduced to sheet bar or round as an intermediate product.

3.Sheet bar is conditioned, pickled and recrystallized.

4.Working to final sheet size consists of warm-rolling to .250” thick followed by an intermediate anneal and further rolling at room temperature to obtain the desired dimensions. Final conditioning consists of hand-working to remove minor handling blemishes and pickling to insure surface cleanliness. Final annealing results in a completely recrystallized structure.

The three primary considerations which confront a design engineer are cost, weight and reliability. They are so related that it is impossible to separate them from each other. As designs become more sophisticated, and space missions more complex, component weight becomes very critical.

Cost analysis should not necessarily be confined to the cost of the relatively few number of missions and the durations the equipment must operate, but to the numerous daily design verification and qualification testing of the components and how many times a component can be used for these test purposes. For example, if a rocket engine is required to operate for 900 seconds with two restarts, inferior materials could be capable of withstanding this duty cycle. However, it is often found that material yields and weldments crack from thermal shock after testing, so that postfire material testing is impractical. On the other hand, if hardware can be built to allow a test unit to operate 5 to 10 times the duty cycle for test purposes, the actual material costs for the program can be greatly reduced. Also, such performance gives a realistic reliability analysis. To reduce costs, therefore, it is necessary to select a material and fabrication process that can withstand prolonged cyclic operation and then pass destructive tests after cyclic testing to meet the reliability requirements of man-rated space systems. The need for post test material testing is necessary to eliminate marginal designs and short cuts which could plague reliability.

According to free energy relationships, hydrofluoric acid should attack Niobium without the aid of nitric acid. However, some oxidizing agent is necessary to start the reaction. Hydrogen peroxide, hot sulfuric acid, or electrical current could be used to replace the nitric acid, but these chemicals do not react at a reasonable or controlled speed. Investigations indicate that improper amounts of sulfuric acid can over exaggerate the bright finish and give the Niobium a “plywood” appearance. For this reason, only two pickling compositions are recommended. These are:

Forming
C-103 can be formed into complex shapes by drawing, spinning, or bulge forming. Thin wall tubing can be drawn from sheet product. Bell contour configurations, such as nozzle extensions, can be roll-formed, welded and sized with no intermediate heat treat requirements.

Forming:Spinning
Spinning of C-103 follows the same basic parameters as 300 series stainless steel. Spinning over welds has been successful but the welds must be filed flush to the parent metal with no weld undercutting. Also, there has been little success in trying to “gather” material by spinning; spinning of Niobium alloys should be limited to stretching of the metal. If it is elected to anneal weldments before spinning, a furnace with a vacuum system capable of maintaining 5 x 5 10 -5 Torr or better at temperatures, is required. Wrapping all weldments in tantalum foil is also recommended. After heat treating, all material should receive a light pickle prior to forming. All forming on C-103 can be accomplished at room temperature. All shear spinning must be in strict compliance to the “Sine Law”, which states “the volume of material in the blank or pre-form and finished part must remain constant during all spinning operations, and each increment of volume must retain its radial distance from the center of rotation.” In equation form:

Forming:Machining
The machinability of C-103 and most Niobium alloys may be compared generally to 316 stainless steel in reference to tool wear due to heat build-up and to copper in reference to tool geometry. However, Niobium is highly ductile, soft, stringy and tends to gall, tear and weld to the face of the cutting tool. Chip breakers have not proven practical because the chip is not easily broken due to its tough, stringy nature, and the tool forces are greatly increased. To reduce cratering and abrasive wear, high positive rake angles should be employed at all times possible and a water soluble coolant with good lubricity and wetting characteristics. Except for grinding, the water soluble cutting oil, PMC 9305, is satisfactory for general machining purposes, but a heavy duty, extreme pressure type cutting compound such as PMC 9318 has proven most satisfactory for producing fine finishes and holding close tolerances, especially in jet or mist spray applications. For surface grinding and cylindrical I.D. and O.D. grinding, PMC 9305 or Cimcool is satisfactory when using coarse grit vitrified wheels. However, for fine finishes and thread grinding operations the standard oil type grinding compound PMC 9205, has proven to be superior. Care must be taken to prevent the metal surface from heating up during grinding to prevent a surface hydrogen pickup.

Welding
Because Niobium alloys can be severely embrittled by high interstitial content, extreme care must be taken during the welding process not to increase the carbon, nitrogen, oxygen and hydrogen of the weldment over that of the material. While electron beam welding minimizes interstitial contamination, much hardware is too large to weld in existing equipment and the cost of larger equipment is prohibitive.

A pickling operation should be conducted before and after welding as well as before and after heat treating. If performed, welding of Niobium alloys can be readily accomplished by both the TIG and electron beam processes. It is recommended that all copper tooling should have a hard chrome plating approximately 0.002” thick without at nickel strike. This prevents copper contamination which causes brittle welds. For TIG, properly designed trailer shields and backup grooves with adequate gas flow can give weldments a quality equal to welds made with electron beam equipment. Where possible, the argon and helium should be passed through hot Zirconium chips, maintained at 800°F to gather minute impurities in the gas. Stake welds of any length can be made without mismatch or burn-out. Girth welds by the TIG process have been made up to 54”, matching .010” to .020” material. Welding of C-103 sheet is often accomplished without weld wire. Material over 0.100” thick should be welded by electron beam process.

Generally, weld wire is not used because of the high cleanliness requirements for welding; the weld wire often has slight serrations and laps that are difficult to clean. Sheet is welded within two hours after pickling, and it is not possible to pickle a roll of welding wire each time it is used. In addition, the high melting point of Niobium causes the wire to deflect and give an uneven weld bead and incomplete penetration. To eliminate these problems and still get a weld without an adequate crown and root, the mating edges to be welded are rolled up to form a “burned-down flange”. The material is then welded with back-up gas and a trailer shield at a very high welding speed. The gases are a combination of helium and argon. TIG welding of second generation Niobium alloy, C-129Y to C-103 has been very successful.

Further, five existing Niobium nozzle extension designs are using Titanium resistance seam and spot welding to the Niobium section. Where complex Niobium-Titanium combinations are needed, it is possible to seam weld Titanium to Niobium that has been given certain aluminide Niobium coatings.

A typical photomicrograph of a Niobium-Titanium seam weld yields approximately 65% penetration in the Titanium with no penetration in the Niobium. In resistance welding, the integrity of the welding process consistently yields failure in the parent metal of test specimens. Considerable successful work has been accomplished in TIG welding of Niobium to Titanium. In addition, Niobium-Titanium welded specimens have been successfully coated with an aluminide oxidation resistant coating. Table 4 shows mechanical properties of electron beam welded materials.

Coating
Diffusion coatings for C-103 have shown a marked improvement. Aluminide coatings can be applied to Niobium and exhibit 100 hour life in air at 2400°F.

The evolution of coatings for Niobium appears to have arrived at a simple slurry-diffusion system. The coating materials are mixed together and either toluene or lacquer is used as a carrier to spray or dip the hardware. The diffusion cycle can be worked out to duplicate a desired heat treat cycle. Most coatings can be diffused into the Niobium in 90 minutes and applied in a one-step operation.

Coating type selection should be based on the mission, environment, and temperature. Generally, for many restarts for temperatures up to 2600°F in an air or rocket engine environment, modified aluminides should be considered. For long-time operation, up to 2800°F; and for short times, up to 3100°F. Coating of hardware has been extremely successful with no signs of compressive cracking on complex shapes.

Besides protecting material from contamination, an important criterion that is sometimes overlooked is the effect of the coating process on the mechanical properties of the raw materials. When rigid design calculations are made, based on yield strength, elongation and grain size, care must be taken that the coating and processing will not degrade or embrittle the material.