Stellite alloys are a group of cobalt-chromium 'super-alloys' consisting of complex carbides in an alloy matrix predominantly designed for high wear resistance and superior chemical and corrosion performance in hostile environments. The combination of Cobalt and Chromium also results in an extremely high melting point making them perfect for a range of intriguing applications from extreme cutting tools to hot section alloy coatings in gas turbines. They may also contain molybdenum or tungsten and a small amount of carbon to offer even greater performance for specific applications.
The Stellite range of alloys were first developed by Elwood Haynes in the early 1900s as an alternative for cutlery that was susceptible to staining. Stellite is a trademarked name of the Deloro Stellite Company, now part of the Kennametal group.
Stellite alloys are non-magnetic and typically associated with high corrosion resistance and as with many alloys, they are adaptable and can be refined for a range of specific applications. Due to their incredibly hard material properties, Stellite alloys are inherently difficult and often expensive to machine therefore some very precise casting and grinding machining methods are often employed.
The carbides present in the Co-Cr-W-based stellite alloys are of chromium-rich M7C3 type. The trademark owners of Stellite claim that Stellite 6 is the most widely used of their range of Stellite alloys, offering a proven industry standard for general-purpose wear resistance, and high mechanical and chemical perfromance in hostile environments.
There are several types and variations of stellite superalloys containing varying levels of: titanium, silicon, sulfur, phosphorus, molybdenum, manganese, chromium, carbon, boron, aluminium, iron, nickel and cobalt in different quantities. Most of the stellite alloys contain four to six of these elements. The Carbon content (and hence carbide volume) of a Stellite alloy is incredibly influential on the materials performance. Therefore, it is possible to group Stellite alloys as follows:
Chromium is also an extremely important component of Stellite alloys, not only does it offer its high corrosion resistant properties to the alloy but it is also the predominant carbide former and it provides strength (as a solute) in the alloying matrix.
Typical Chemical composition of stellite 1:
|Chromium, Cr||28 - 32%|
|Tungsten, W||11 - 13%|
|Carbon, C||2 - 3 %|
The mechanical properties of stellite 1 are displayed in the table below:
|Density||8.69 g/cm3||0.314 lb/in3|
|Hardness, Rockwell C||50-58||50-58|
|Tensile strength||1195 MPa||173 ksi|
|Yield strength||1050 MPa||152 ksi|
|Modulus of elasticity||230 GPa||33.4x106 psi|
|Elongation at break||<1%||<1%|
Stellite alloys are produced by a range of different processes or methods including wrought or hot forging, hardfaced deposit, powder metal and casting depending on teh final appication. Stellite is more difficult to machine and grind than steel, and hence requires high performance processing equipment and specialized machining tools. Due to its incredible toughness Stellite is often machined by grinding rather than cutting.
Currently Stellite base material is rolled into bar, sheet and plate forms. During forming, the size and orientation of the matrix in the alloy is optimized, so that the material achieves higher strength than traditional cast material. The alloy is then cut into blanks of different sizes, which are then processed into finished parts.
Some of the major applications of stellite include the following: