Stellite alloys

Stellites are hard-facing and wear-resistant alloys. They have been developed since 1920 and today more than 20 different qualities are on the market. There are two main groups  

TABLE 8.5. Examples of Master Alloys Used in Superalloy Production (concentrations in wt%) [8.6] 

Special hard-facing qualities contain in addition small percentages of Si and B. Typical analyses of tough, medium, and hard alloys are given in Table 8.6 including hardness at ambient and high temperature. 

The microstructure is defined by hard carbide particles in a Co-rich solid solution matrix. The wear characteristics of the matrix are influenced by a phase change from a strain-induced cubic to a hexagonal form. Toughness and abrasion resistance are due to the volume fraction of carbides and their morphology. 

They are prepared fi om Co-Cr-W master alloys by addition of the other components in pure quality. 

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It is alloyed with iron, nickel and other metals to make Alnico, an alloy of unusual magnetic strength with many important uses. Stellite alloys, containing cobalt, chromium, and tungsten, are used for high-speed, heavy-duty, high temperature cutting tools, and for dies. 

Stellite (alloy of eobalt, tungsten, ehromium, and molybdenum) used for surgieal instruments. 

In recent years, the spraying process has been adapted for hard facing, using the chromium-nickel-boron alloys which have become known as Colmonoy. More recently still, the cobalt-base Stellite alloys have also been used. These materials in powder form are sprayed on to the surface in the usual way. The deposit is afterwards heat treated by a torch, so that fusion takes place. The process is often known as spray-welding. Such coatings are primarily used for hard facing under wear conditions, but as the Anal surface is nickel-chromium or cobalt-chromium they exhibit very high anticorrosive properties. 

Considerable improvement in wear resistance can be achieved when dissimilar metals are coupled, and this is especially true for steels coupled with silicon bronze and Stellite alloys. The wear data further suggest that improvement in wear resistance can be achieved by altering the surface characteristics, such as by surface treatment or by adding a coating.75... 

TABLE 8.6. Analyses and Hardness of Stellite Alloys (Co content is balance) [8.5]... 

Many interesting correlations have been established between the critical surface tension of materials (or other approximations of surface free energy) and protein adsorption, cell adhesion, and thrombus formation (41-48). Unfortunately, very few studies in which a biological response has been related to a specific surface chemistry exist. One study in which such a relationship was established, demonstrated the power of the contact angle method in analyzing surface structure related to blood compatibility (40). The blood compatibility of Stellite alloy heart valves was not due to the alloy itself, but to the closely packed methyl group structure associated with a tallow polishing compound used to finish the valve. Very recently, the power... 

Models 2400 (aortic) and 6400 (mitral) Starr-Edwards composite track valve prostheses are closed single-cage hollow metallic ball valve prostheses. The cage struts, poppets, and metallic closure supports on the inner aspect of the base ring are made of Haynes alloy No. 21 (Stellite alloy No. 21) and are easily seen radiographically. The inner aspect of the cage struts has no cloth covering and hence no metal-cloth contact. 

NB For stellite alloys the Young s or elastic modulus is between 180 and 249 MPa the bulk or compression modulus is between 152 and 296 MPa the Coulomb s or shear modulus is between 90 and 97 MPa. 

Reference Collective (1970) Properties of Deloro Stellited Alloys. Deloro Stellite, Belleville, ON, Canada... 

Haynes Stellite alloy 25 C 0.15 Cr 20.0 Wrought sheet mill annealed 63 140 60 244 ... 

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