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 re crystallization 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.
Niobium may be machined using standard techniques. However, due to the tendency of the material to gall, special attention needs to be given to tool angles and lubrication.
Niobium also has 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.
Machining on a lathe is best performed with high speed steel tools. For cooling and lubricating, air, soluble oil, Rapid-TapTM or other suitable products may be used.
The metal turns very much like lead or soft copper. It must be sheared with the chip allowed to slide off the tool's surface. If any buildup of material occurs, the pressure will break the cutting edge, ruining the tool.
Carbide tooling should be used only for fast, light cuts to work efficiently (.010 to.015 inches deep). Tooling recommendations are given in Table 1. The data applies to both high speed steel and carbide tools unless otherwise noted.
|Approach Angle||15° - 20°|
|Side Rake||30° - 35°|
|Plan Relief Angle||15° - 20°|
|Nose radius||0.25" +/- .005"|
|Cutting Speed: (ft/minute)|
|High Speed Steel||60 - 80|
|Carbide||250 - 300|
|Roughing||.010" +/- .002"|
|Depth of cut||.030" - 0.125"|
Standard high speed drills, ground to normal angles, may be used. However, the peripheral lands wear badly so that care must be exercised to ensure the drill has not worn undersize.
Niobium may be screw-cut using a standard die- cutting head provided that an ample amount of lubricant is used. The use of sufficient lubricant prevents galling on the die resulting in the tearing of the thread. Roll threading is an alternate, and preferred, method.
With some minor modifications, normal techniques of metal spinning may be applied successfully to niobium.
It is generally better to work the metal in stages. For example, when spinning a right -angled cup from flat sheet, several formers should be used to perform the operation in steps of approximately 10'. Wooden formers may be used for the rough spinning, but a brass or bronze former is essential for finishing. This is because niobium is soft and readily accepts the contour of the former.
For small work, aluminum, bronze or narite tools should be used with a radius of approximately 3/8 inch. Note that if sharp angles are required, the tool must be shaped accordingly.
Suitable lubrication for this process may be either yellow soap or tallow, both of which must be continually cold-worked. The peripheral speed of the work piece should be approximately 500 feet per minute.
Niobium is prone to "thinning" during this process. This is avoided by working the tool in successive, long, sweeping strokes with light pressure instead of a few heavy strokes.
Niobium is a highly active metal. It reacts at temperatures well below its melting point with all the common gases, e. g. nitrogen, oxygen, hydrogen and carbon dioxide. At the melting point and above, niobium will react with all the known fluxes. This severely restricts the choice of welding methods.
Niobium can be welded to several metals, one of which is tantalum. This can be readily accomplished by resistance welding, tungsten-inert gas, plasma welding and electron beam welding.
Formation of brittle intermetallic phases is likely with many metals and must be avoided. Surfaces to be heated above 300°C should be protected by an inert gas such as argon or helium to prevent embrittlement.
It is critical to ensure that the metal is clean prior to welding. An acid pickle wash is recommended. For ambient temperature pickling, a typical solution is 25%-35% HF, 25%-33% HN02 with the balance H20. Coupons should be used before immersing the part to check the etchant rate. Removal of approximately .0001 inch is generally acceptable.
Niobium can be welded satisfactorily by applying standard gas-tungsten-arc (G.T.A.W.) heli-arc procedures. The resulting welds are superior to those made under similar conditions with an alternating current. The argon from the torch seems to provide better protection for a small pool.
The TIG method is the recommended procedure for welding niobium. However, some modifications to this method are required.
It is essential to completely cover the area of the molten pool and the heated zone with inert gas to avoid contamination of the weld metal. This protection must be given to both the back of the weld and the face. Several examples of butt joints with different size stock are given below:
When this metal is exposed to air at reaction temperatures, it acquires a relatively thick and adherent oxide film that is extremely difficult to remove.
Vacuum annealing, covered in the section entitled "Welding", will cause the oxide film to diffuse rapidly into the metal. This results in a hardening of the weld bead and the heat-affected zone.
Note that contamination-free welds can be produced under totally inert atmospheres compared to welds produced employing only inert shielding.
To properly clean niobium, the following steps are recommended: