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.

Table 10. Compositions of Wrought Cobalt-Base Alloys, wt %...

The cobalt deposition rate on new, replacement, or decontaminated recirculation piping surface has been reduced by pretreating the piping using an atmosphere of oxygenated wet steam to form an oxide film (25). Studies have been conducted for both PWRs and BWRs to reduce the cobalt content of materials used in the nuclear parts of the plants, particularly in hardened and wear surfaces where cobalt-base alloys ( 50% Co) are used (26). Some low cobalt materials have been developed however, the use of the materials is limited to replacement parts or new plants. 

Titanium alloy systems have been extensively studied. A single company evaluated over 3000 compositions in eight years (Rem-Cm sponsored work at BatteUe Memorial Institute). AHoy development has been aimed at elevated-temperature aerospace appHcations, strength for stmctural appHcations, biocompatibiHty, and corrosion resistance. The original effort has been in aerospace appHcations to replace nickel- and cobalt-base alloys in the 250—600°C range. The useful strength and corrosion-resistance temperature limit is ca 550°C. 

Cr C Cr C chromium iton(l l) [12052-89-0] CrFe (c phase), and chromium iron molybdenum(12 36 10) [12053-58-6] Cr 2F 36 o Q phase), are found as constituents in many alloy steels Ct2Al23 and CoCr ate found in aluminum and cobalt-based alloys, respectively. The chromium-rich interstitial compounds, Ci2H, chromium nitrogen(2 l) [12053-27-9] Ct2N, and important role in the effect of trace impurities on the... 

I. G. Wright, Oxidation of Iron-, Nickel-, and Cobalt-Base Alloys, MCIC Report 72-07, BatteUe-Columbus Laboratories, BatteUe Memorial Institute, 1972. 

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. 

With cobalt historically being approximately twice the cost of nickel, cobalt-base alloys for both high temperature and corrosion service tend to be much more expensive than competitive alloys. In some cases of severe service their performance iacrease is, however, commensurate with the cost iacrease and they are a cost-effective choice. For hardfaciag or wear apphcations, cobalt alloys typically compete with iron-base alloys and are at a significant cost disadvantage. 

Significant advances have also been made by forging titanium alloy (T1-6A1-4V) and cobalt chromium alloys cold working multiphase cobalt based alloys and by hot isostatically pressing cobalt chromium alloy powders. The property values claimed by the manufacturers are far in excess of the minimum values specified in the British, American and International Standards... 

The literature contains a number of studies on the susceptibility of the cobalt-based alloys to pitting corrosion. In-vitro studies conducted by Mueller and Greener , involving static conditions, revealed no evidence of pitting having occurred. Syrett and Wing ", utilising cyclic polarisation analyses, observed that neither as-cast nor annealed Co-Cr-Mo alloy demonstrated hysteresis loops in their cyclic polarisation curves. They... 

Lucas, L. C., Buchanan, R. A., Lemons, J. E., and Griffin, C. D., Susceptibility of Surgical Cobalt-Base Alloy to Pitting Corrosion , Journal of Biomedical Materials Research, 16, 799-810 (1982)... 

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