Refractory metals such as Wah Chang's (Albany, OR) C-103 (Niobium + Hafnium + Titanium) provide design engineers with a suite of unique and wonderful properties. These alloys have high strength in continuous operation to 270O°F (148O°C) or higher, making them attractive for a variety of rocket-propulsion components and industrial-process assemblies. They are relatively impervious to exposure to high-temperature propellant gases (also important in propulsion applications). They have low ductile-to-brittle transition temperatures for withstanding high-frequency vibration; can be made into forgings, plates, sheets, and tubes; and are readily welded into complex assemblies.

These materials present special challenges to machinists, however. They impose new constraints on cutting tool design, coolants, and speeds and feeds, compared to more conventional materials. If you are searching for a manufacturer of critical refractory-metal components or assemblies, evaluating a candidate supplier's capability to perform such work, or planning for an in-house fabrication activity using refractory metals, there are several critical areas to evaluate.

These fall in the general areas of development of machining processes, tool definition, and process-control discipline. Our comments on these subjects are based on more than 20 years of experience in the design, process development, and manufacture of rocket propulsion components (hot gas control valves) made from C-103 and other refractory alloys that must meet the most exacting quality and reliability requirements. Typical machining operations on C-103 bar stock and forgings in our plant consist of turning, drilling, boring, milling, and threading fairly thick-walled body sections. Because machined parts will later be welded into higher-level assemblies, we require close-tolerance weld interfaces and fine surface finishes with no voids, tears, or inclusions. Typical tolerances for diametral features are ±0.001'' (0.03 mm), with positional tolerances running as close as 0.002'' (0.05 mm). Surface-finish callouts of 16 RMS are typical, as are closely controlled filet radii of 0.005'' (0.13 mm).

Development of machining processes is not straightforward. The unique properties of refractory metals present special challenges to the machine shop that require the development of special machining methods, tools, and coolants.

For example, C-103 is a highly ductile, soft, and stringy material. (One of our machinists has likened cutting it to "trying to machine an old shingle.") It has a high abrasive-wear characteristic that breaks down tool edges, and creates high heat buildup. This alloy is prone to tearing, galling, and chip welding to the tool face. Because of the high cutting forces and the tough, stringy chips produced when machining C-103, chipbreakers are completely ineffective.

During our development of turning processes, standard HSS tool profiles with zero or negative top rakes produced ragged, torn, poorly sized diameters that were unacceptable. Tool wear was tremendous as well, with extensive cratering of the cutting edge, and in some cases tools were broken off at the shank. Tungsten carbide inserts with similar profiles also produced bad results.

Tools with high positive top rake angles of 5, 10, 15, and 20° were tried until a favorable result was established. Turning tools of HSS and carbide with a positive top rake of 15-20° are able to machine C-103, providing that proper feeds, depthsof-cut, and speeds are used. Due to the abrasive nature of C-103, cutting speeds must be reduced to approximately 250 fpm (76 m/min) for carbide and 75 fpm (23 m/min) for HSS and cobalt-alloy tools. Typical feed rates for turning C-103 are approximately 0.005 ipr (0.131 mm/rev). Due to the unavailability of off-the-shelf carbide inserts with the required high positive top rakes, special toolholders and modified inserts are used throughout the shop.

One of the most important things that we learned about the machining of C-103 relates to the size of cuts that can be taken to produce the desired size and finish. In that it tears and does not abrade, C-103 is similar to copper; therefore, it's very difficult to form a chip less than 0.001'' (0.03-mm) thick. Rough and finish cuts must be adjusted to the proper ratio in order to produce a satisfactory size and finish.

Boring of C-103 poses some interesting problems. Standard boring bars of both HSS and carbide have geometries that will not allow regrinds of 15-20° of positive top rake without severely weakening the tool. The unsupported lengths of these boring bars create excessive deflection and chatter. Many different shapes were tried without success. One machinist suggested that we try a two-flute end mill, with one flute ground away for clearance. The results were miraculous. We got excellent chip control, with a beautiful surface finish, and we were able to hold size. Today we use both HSS and carbide single-flute router bits for almost all of our boring requirements.