Cast nickel-based alloys

The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5... 

Table 2-11. Approximate chemical composition of cast nickel-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 greatest use of cubic boron nitride is as an abrasive under the name Bora2on, in the form of small crystals, 1—500 p.m in si2e. Usually these crystals are incorporated in abrasive wheels and used to grind hard ferrous and nickel-based alloys, ranging from high speed steel tools and chilled cast-iron to gas turbine parts. The extreme hardness of the crystals and their resistance to attack by air and hot metal make the wheels very durable, and close tolerances can be maintained on the workpieces. 

Fig. 2. Tool wear mechanisms, (a) Crater wear on a cemented carbide tool produced during machining plain carbon steel, (b) Abrasive wear on the flank face of a cemented carbide tool produced during machining gray cast iron, (c) Built-up edge produced during low speed machining of a nickel-based alloy.

Other members of the medium-alloy group do not have ACI designations but are listed in ASTM specifications (refer to ASTM A 494) such as Incoloy 825 and Hastelloy G-3 and G-30. Several nickel-based alloys that are normally only wrought have cast equivalents but are not listed in any specifications. Hastelloy G-3 contains 44 percent... 

Based on the chemical composition, corrosion-resistant nickel-based alloys consist of commercially pure nickel. Ni-Cu alloys, Ni-Mo alloys, Ni-Cr-Mo alloys, and Ni-Cr-Fe alloys.63 The cast versions of the nickel-based alloys do not have the same corrosion resistance as the corresponding wrought products, mainly due to the higher carbon and silicon contents and the anisotropic microstructure of the cast products. (Rebak)5... 

Superalloys are produced via master alloys. In case of nickel-based alloys they include NiW and more complex nickel-based alloys containing Cr, Ta, and Mo. Some examples are given in Table 8.5. All raw materials have to be of high purity. Melting (performed in induction furnaces), casting and cooling takes place in vacuum. Master alloys are supplied in small lumps or as powder. 

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