Cobalt-base alloys compositions

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 %...

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. 

The compositions of the alloys evaluated in Phase I are summarized in Table III. These alloys represent most classes of high-temperature iron-, nickel-, and cobalt-base alloys that could be considered for coal gasification service. Pack aluminized and chromized coatings on AISI 310 and IN-800 were also evaluated in the test program. 

The nickel base alloys are produced from a group of alloys which have chemical compositions generally over 50 % nickel and less than 10 % iron. They are mainly strengthened by intermetallic precipitation in an austenitic matrix. The cobalt base alloys have a high Co content (40 to 70 %X high Cr (over 20 %), high W (7 to 15 %) and they are strengthened by a combination of carbides and solid solution hardeners. 

TABLE 1—Nominal compositions of selected cobalt-base alloys (wt%). 

The materials currently used in the production of medical devices include stainless steels, cobalt-base alloys, titanium-base alloys, platinum-base alloys, and nickel-titanium alloys. Steels were the first modern metallic alloys to be used in orthopedics and initial problems with corrosion were overcome by modifying the composition of the steel with the addition of carbon, chromium, and molybdenum. Carbon was added at low concentrations (ca. 0.03-0.08%) to initiate carbide formation, while the addition of chromium (17-19%) facilitated the formation of a stable surface oxide layer and the presence of molybdenum (2.0-3.0%) was found to control corrosion. The compositions of stainless steels used can vary widely. Table V shows the limits for the chemical compositions of three different alloys containing eleven different elements together with the mechanical properties for the samples after annealing and cold working. 

There are at least four compositions of cobalt-base alloys in use which are similarly designated by code numbers such as F75, F90, F562, and F563. Again, these differ in the relative composition of the following elements manganese, silicon, chromium, nickel, molybdenum, carbon, iron, phosphorus, sulfur, tungsten, titanium, and cobalt. These alloys are used because of their superior... 

Table 2-13. Approximate chemical composition of wrought cobalt-based alloys.

TABLE E.13 Chemical Compositions of Nickel-, Nickel-Iron-, and Cobalt-Base Alloys... 

Table 5. Compositions of Cobalt-Base Wear-Resistant Alloys, wt...

Alloy Compositions and Product Forms. The nominal compositions of various cobalt-base wear-resistant alloys are Hsted in Table 5. The six most popular cobalt-base wear alloys are Hsted first. SteUite alloys 1, 6, and 12, derivatives of the original cobalt—chromium—tungsten alloys, are characterized by their carbon and tungsten contents. SteUite aUoy 1 is the hardest, most abrasion resistant, and least ductile. 

Alloy Compositions and Product Forms. SteUite 21, an early type of cobalt-base high temperature alloy, is used primarily for wear resistance. The use of tungsten rather than molybdenum, moderate nickel contents, lower carbon contents, and rare-earth additions typify cobalt-base high temperature alloys of the 1990s as can be seen from Table 5. 

Table 4-15 lists base materials Elliott has tested. This list, which is continually being expanded, includes low alloy steels, high alloy iron base, nickel base, cobalt base materials, and odiers. Table 4-16 shows some of the coatings Elliott has tested. The list indicates die supplier, coating designation, and major components of the coating composition. 

However, under more realistic test conditions Hancock and Islam showed that in burner rig tests with contaminant flux rates greater than about 0-1 mgcm h" the corrosion rate of nickel- and cobalt-base superalloys was largely independent of alloy composition in the temperature range 7(X)-850 C. However, in burner rig tests at 6(X) C, simulating diesel engine combustion, Saunders et reported that Nimonic 80A (20% Cr) had superior resistance to Stellite 6 (Co-28%Cr) and EN 52 (Fe-8%Cr-3%Si). 

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