Stellite alloys display astounding hardness and toughness, and are also usually very resistant to corrosion. Stellite alloys are so hard that they are very difficult to machine, and anything made from them is, as a result, very expensive. Typically a Stellite part will be very precisely cast so that only minimal machining will be necessary. Machining of Stellite is more often done by grinding, rather than by cutting. Stellite alloys also tend to have extremely high melting points due to the cobalt and chromium content.
Stellite has also been used in the manufacture of turning tools for lathes. With the introduction and improvements in tipped tools it is not used as often any more, but it was found to have superior cutting properties compared to the early carbon steel tools and even some high speed steel tools, especially against difficult materials such as stainless steel. Care was needed in grinding the blanks and these were marked at one end to show the correct orientation, without which the cutting edge could chip prematurely.
While Stellite remains the material of choice for certain internal parts in industrial process valves (valve seat hardfacing), its use has been discouraged in nuclear power plants. In piping that can communicate with the reactor, tiny amounts of Stellite would be released into the process fluid and eventually enter the reactor. There the cobalt would be activated by the neutron flux in the reactor and become cobalt-60, a radioisotope with a five year half life that releases very energetic gamma rays. While not a hazard to the general public, about a third to a half of nuclear worker exposures could be traced to the use of Stellite and to trace amounts of cobalt in stainless steels. Replacements for Stellite have been developed by the industry, such as the Electric Power Research Institute’s “NOREM”, that provide acceptable performance without cobalt. Since the United States nuclear power industry has begun to replace the Stellite valve seat hardfacing in the late 1970s and to tighten specifications of cobalt in stainless steels, worker exposures due to cobalt-60 have dropped significantly.