Stress cobalt-based alloys

Tables 10 and 11 list typical compositions of cast and wrought cobalt-base alloys, respectively. Stress—mpture properties of two wrought cobalt alloys, Haynes 188 and L-605, are compared to those of iron—nickel alloys ia Figure 10 (49). The cobalt alloys generally are inferior ia strength to the strongest cast nickel-base superaHoys. Tensile strengths at low and iatermediate temperatures are particularly deficient for the cobalt alloys.

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

Stress-Corrosion Cracking of Cobalt-Based Alloys. 98... 

Density is a particularly important characteristic of alloys used in rotating machinery, because centrifugal stresses increase with density. Densities of the various metals in Table 1 range from 6.1 to 19.3 g/cm. Those of iron, nickel, and cobalt-base superaHoys fall in the range 7-8.5 g/cm. Those alloys which contain the heavier elements, ie, molybdenum, tantalum, or tungsten, have correspondingly high densities. 

Superalloys are alloys that display a particularly excellent ability to resist deformation under stress at high temperatures along with good resistance to corrosion and great surface stability. Most often, a superalloy involves nickel, cobalt, or nickel-iron as the base alloying element. Superalloys have been used primarily in turbines and in the aerospace industry. 

The term superalloy is used for a group of nickel-, iron-nickel-, and cobalt-based high-temperature materials for applications at temperatures > 540 °C. It is useful to compare the main subgroups in terms of the strengthening mechanisms applied and stress-rupture characteristics achieved, as shown in Fig. 3.1-127. In this section iron-nickel- and nickel-based superalloys are covered whereas cobalt-based superalloys are dealt with in Sect. 3.1.6.3. Nickel-based superalloys are among the most complex metallic materials with numerous alloying elements serving particular functions, as briefly outlined here. 

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