0.5 Mn, 0.5 Si

Fig. 6.7. Effect of Al + Ti Content on Strength of Nickel Based Alloys at 1600° F12

6.2.2 Iron-Nickel Based Superalloys

The iron-nickel based superalloys originally evolved from austenitic stainless steels with at least 25% nickel required to stabilize the FCC austenitic matrix. Most of them contain 25-45% nickel and 15-60% iron. Chromium from 15 to 28% is added for oxidation resistance at elevated temperature, while 1-6% molybdenum is added for solid solution strengthening. Titanium, aluminum and niobium are added for precipitation hardening. In the iron-nickel based alloys,1 the most common precipitate is y' as in the alloy A-286, which contains 26% nickel. In alloys containing niobium, such as Inconel 718, the y'' precipitate Ni3Nb is the predominate strengthener. Due to the lower temperature stability of the y'' precipitate compared to the y' precipitate, the maximum use temperature of Inconel 718 is about 1200° F; however, Inconel 718 is still the most widely used of all of the superalloys. It is one of the strongest at low temperatures but rapidly loses strength in the 1200-1500° F range. The iron-nickel based family also contains some low expansion alloys, such as Incoloy 903, which are important in applications requiring closely controlled clearances between rotating and static components. Some iron-nickel alloys are strengthened primarily by solid solution hardening, such as 19-9DL, which is essentially 18-8 stainless steel with slight chromium and nickel adjustments. As a class, the iron-nickel based alloys have useful strengths to about 1200° F.

6.2.3 Cobalt Based Superalloys

Unalloyed cobalt has a hexagonal close-packed matrix at temperatures below 780° F that then transforms to an FCC structure at higher temperatures; however, nickel alloying additions are used to stabilize the FCC austenitic structure between room temperature and the melting point. As a class, cobalt based superalloys are much simpler than the nickel based alloys. Cast cobalt alloys contain about 50-60% cobalt, 20-30% chromium, 5-10% tungsten and 0.1-1.0% carbon. Wrought alloys contain about 40% cobalt and high nickel contents (~20%) for increased workability. Unfortunately, no precipitates that result in a large strength increase have been found for the cobalt based alloys, and therefore they must rely on a combination of solid solution and carbide strengthening, which limits their use in many applications. However, a fine dispersion of carbides contributes significantly to the strength of these alloys. In general, there are three main classes of carbides: MC, M23C6 and M6C. In MC carbides, M stands for the reactive metals titanium, tantalum, niobium and zirconium. In the M23C6 carbides, M is mostly chromium but can also be molybdenum or tungsten. When the molybdenum or tungsten content exceeds about 5 atomic percent, M6C carbides can form. The cobalt alloys display good stress rupture properties at temperatures higher than 1830° F but cannot compete with the nickel based alloys for highly stressed parts, so are used for low stress long lived static parts. They also have superior hot corrosion resistance at elevated temperature, probably due to their quite high chromium contents. Examples of important cobalt based alloys are the wrought alloys Haynes 25 (L605) and Haynes 188 and the cast alloy X-40.

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