What are Stellite alloys?
Stellite is a series of cobalt-chromium alloys designed for wear resistance. The alloys may also contain tungsten or molybdenum and a small but significant amount of carbon.
Stellite is a trademark name of Kennametal Inc. Prior to that it was owned by Union Carbide, Stellite Division. Invented by Elwood Haynes in the early 1900s as a material for making cutlery that would not stain and did not need constant cleaning.
Composition
Stellite alloys are a series of cobalt-based alloys, with significant amounts of chromium (up to 33%) and tungsten (up to 18%). Some alloys also contain nickel or molybdenum. Most have a fairly high carbon content compared to carbon steels, although they contain less than 3% iron, and in stellite alloys the carbon is primarily associated with chromium to form hard chromium carbide particles dispersed in the cobalt-based matrix.
Properties
Stellite is a family of fully non-magnetic and corrosion-resistant cobalt alloys with different compositions optimized for different applications. The several alloys are formulated to maximize combinations of wear resistance, corrosion resistance or ability to withstand extreme temperatures.
Stellite alloys exhibit excellent hardness and toughness and are usually also highly corrosion resistant. Typically, a Stellite part is precision cast so that only minimal machining is required. Because of their very high hardness, many Stellite alloys are machined primarily by grinding because cutting operations in some alloys cause significant tool wear, even with carbide inserts. The alloys also have an extremely high melting point due to the cobalt and chromium content.
Applications
Typical applications include saw teeth, hardfacing and acid-resistant machine parts. Stellite was a major improvement in the production of poppet valves and valve seats for the valves, especially exhaust valves, of internal combustion engines. By reducing their erosion by hot gases, the interval between maintenance and resharpening of their seats was dramatically extended.
The first third of the barrel of the M2HB machine gun and the M60 machine gun (from the chamber) is lined with Stellite. In the early 1980s, experiments were conducted in the United Kingdom to make artificial hip joints and other bone replacements from precision-cast Stellite alloys. It is also widely used to make the cast structure of dental prostheses.
Although Stellite remains the material of choice for certain internal components in industrial process valves (hardfacing of valve seats), its use is discouraged in nuclear power plants.
In pipes that can communicate with the reactor, minute 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 half-life of five years releasing highly energetic gamma rays. This phenomenon is more problematic in boiling water reactors (BWR), since the steam is in direct contact with both the reactor and the steam turbine.
Designs for pressurized water reactors (PWR) are less sensitive. Although not a hazard to the general public, about one-third to one-half of the exposure of nuclear workers can be traced to the use of Stellite and to traces of cobalt in stainless steel. The industry has developed substitutes for Stellite, such as "NOREM" from the Electric Power Research Institute, which provide acceptable performance without cobalt. Since the nuclear power industry in the United States began replacing Stellite hardfacing of valve seats in the late 1970s and tightening the specifications of cobalt in stainless steel, worker exposure to cobalt-60 has decreased significantly.
Stellite Alloy Comparison
| Alloy | Mechanical Wear |
Corrosion | High Operating Temp |
| Stellite | Very Good | Very Good | Excellent |
| Deloro | Very Good | Satisfactory | Satisfactory |
| Tribaloy | Very Good | Very Good | Excellent |
| Nistelle | Low | Excellent | Very Good |
| Delcrome | Very Good | Low | Satisfactory |
| Stelcar™ | Excellent | Satisfactory | Satisfactory |
| Jet Kote™ | Excellent | Satisfactory | Satisfactory |
On the website of www.stellite.com you will find all types of Stellite alloys and related material types.
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Reference(s) ..
www.stellite.com
www.chemistrylearner.com
www.valvemagazine.com