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Larson Miller Parameter P = K(20 + log t)

Fig. 6.20. Effect of Heat Treatment on Mechanical Properties of Waspaloy34

Fig. 6.20. Note that the blade heat treatment (coarse grain size) results in better rupture strength at high temperatures, while the disk heat treatment (fine grain size) gives better tensile and stress rupture properties at lower temperatures.

6.8.3 Precipitation Strengthened Iron-Nickel Base Superalloys

Although some of the wrought iron-nickel based superalloys are strengthened by y', Inconel 718, the most widely used wrought superalloy, accounting for 45% of all wrought alloy production, is strengthened by y'' (Ni3Nb). The y'' phase precipitates in the range of 1300-1650° F and has a solvus temperature of about 1675° F. In Inconel 718, the 8 phase, which is present above the y'' solvus, can be used for grain size control. The 8 phase (Ni3Nb) precipitates between 1600 and 1850° F and has a solvus temperature of about 1850° F. For applications requiring a good combination of tensile/fatigue and stress rupture properties, Inconel 718 can be heat treated below the 8 solvus as follows:33

• Solution treat at 1700-1850° F for 1-2 h followed by air cooling,

• Age at 1325° F for 8 h followed by furnace cooling to 1150° F, and

• Age at 1150° F for a total aging time of 18 h followed by air cooling.

Higher strength levels can be obtained by forging below the 8 solvus, quenching after forging and conducting a double aging treatment without a solution treatment. The precipitation of the 8 phase is helpful in achieving a fine grain size during hot working and avoiding undesirable grain growth.35

In general, lower solution treating temperatures produce better strength, while higher solution treating temperatures provide better creep and stress rupture properties.

6.8.4 Cast Superalloy Heat Treatment

Heat treatment of cast superalloys includes homogenization, solution heat treatment, and aging heat treatments. Stress relief treatments are also used to reduce residual stresses resulting from casting, weld repair, and machining. Cobalt based castings, which are usually given stress relief or aging treatments, can be heat treated in air while nickel base alloys are always heat treated using inert atmospheres or vacuum. Cobalt based casting aging treatments are conducted at approximately 1400° F to promote formation of discrete Cr21C6 carbides. Some superalloy castings are put into service in the as-cast condition. However, stress relieving of non-precipitation hardening alloys may be used if the casting is extremely complex, or when dimensional tolerances are stringent. Weld repaired castings are always stress relieved after welding. When heating to high solution treating temperatures, slow heat-up rates are often used for castings to allow some homogenization to minimize the danger of incipient melting due to alloy segregation.

To obtain optimum properties after casting, precipitation-strengthened superalloys must be homogenized at temperatures between the y' solvus and the solidus. At these temperatures, although the y' phase dissolves relatively rapidly in the y matrix, a substantial time (many hours) is required to obtain a uniform distribution of alloying elements through diffusion.36 Residual stresses also develop during casting due to non-uniform expansion and contraction rates at different points within the casting. Residual stresses are a function of the superheat prior to casting (higher superheat creates higher residual stresses), casting complexity (complex castings produce higher residual stresses) and is also dependent on alloy composition.37

Many conventionally cast polycrystalline nickel based alloys are not solution heat treated but all DS (directionally solidified) and SX (single crystal) alloys are solution heat treated. Because polycrystalline alloys respond differently to solution heat treatment, some are only given an aging treatment. Solution treatments are performed at temperatures above or near the y' solvus for times ranging from 2 to 6h, with the objective of dissolving all of the phases into the as-cast structure. Since precipitation occurs during cooling from the solution treating temperatures, fast cooling rates (e.g., gas furnace quenching) are used to prevent coarsening of the y' precipitate which can degrade the mechanical

Cumulative Probability (%)

Fig. 6.21. Effects of HIP on Stress Rupture Life of Cast Nickel Based Alloy31

Cumulative Probability (%)

Fig. 6.21. Effects of HIP on Stress Rupture Life of Cast Nickel Based Alloy31

properties. If a vacuum atmosphere is used for solution treating, a partial pressure of inert gas, rather than a hard vacuum, is used to prevent surface depletion of chromium and aluminum. Slow heating rates to the solution temperature are used to allow time for casting homogenization. Cooling rates in excess of 100° F/min can be achieved by backfilling with cold argon and rapidly circulating the gas past the parts being cooled.10

Hot isostatic pressing can be used to improve quality by healing internal casting porosity. A typical HIP cycle would be 2175° F for 4 h at 15 ksi argon pressure. The beneficial effects of HIP on the stress rupture properties of a polycrystalline cast nickel based alloy (Inconel 738) are shown in Fig. 6.21.

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