LITMIR.BIZ

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2.35 3.5 5 3 0.2 W — 2.6 4 — 5 Mo 0.3 1.8 4 4.3 0.4 Ta — 1.7 0.05 — 8 C 0.09 0.1 0.17 0.08 — Fe 1 0.1 0.18 — — B 0.003 0.01 0.1 0.006 — Zr 0.07 0.1 0.06 0.06 — Nb — 0.9 — — 0.1 Si 1.5 Trace Trace — — Cu 0.2 — — — — Mn 1 Trace Trace — — Re — — — — 6 Hf — — — — 0.3

Due to economic and environmental demands, the efficiencies of gas turbine engines have been steadily and systematically increased by design modifications, such as air cooling of components, to handle higher turbine inlet to firing temperatures. The gas turbine engines of today are more energy efficient and more reliable, as well as have cleaner burning capabilities, than their predecessors. The increase in operating temperatures, however, has led to the decrease in the life of components and increase in costs of replacement. This is serious because the technologies are becoming obsolete in a very short time, while the world's commercial fleet of aircraft or utilities is getting older. Around 80% of the large frame industrial/utility gas turbines operating in the world today were installed in the mid‐1960s to early 1970s, and they are now old and getting replaced. Consequently, there are now greater opportunities to repair and refurbish older models (Natole 1995; Valenti 1999). It is estimated that 50–70% of the operating and maintenance cost is due to the repair and/or replacement of hot section components made of nickel‐base superalloys. Repair or replacement of damaged engine components is not therefore a question of academic importance.

Figure 2.17 Dependence of strain age cracking susceptibility based “weldability” on the contents of aluminum and titanium. A general increasing trend for γ′ solvus temperature of the alloys is also shown; chemical compositions for alloys shown as solid points are given in Table 2.2.

      Source: N. K. Sinha.

      The repairability of a superalloy component is determined by a number of factors, such as chemical composition, grain size, grain‐boundary characteristic, substructural details of the matrix, prior heat treatment, history of service life, welding parameters, and welding processes. A full understanding of the interrelationship between the material characteristics, welding parameters, and the performance of the welded junctions requires knowledge of the physical chemistry of the parent and the filler materials involved.

The same features that improve high‐temperature performance of nickel‐base superalloys also make them vulnerable to structural degradation, such as coarsening of the γ′ precipitates and the oxidation of nickel and carbon producing subsurface grain‐boundary defects during service life, and also make them difficult to repair by welding (Jones and Westerman 1965; Thamburaj et al. 1983). Liquidation zone cracks (LZC) and heat‐affected zone (HAZ) cracks develop due to thermal stresses generated by volumetric changes associated with the dissolution and reprecipitation of γ′‐phase during welding (Thamburaj et al. 1983). The propensity for weld induced cracking (both LZC and HAZ) increases as the concentration of γ′ forming elements, such as aluminum and titanium, increases in the alloy, and hence as the volume fraction of γ′ increases. Figure 2.17 shows a few commonly used nickel‐base superalloys along with the demarcation line (dashed) proposed by Prager and Shira (1968) to differentiate between weldable and nonweldable alloys. They examined the weldability on the basis of strain age cracking susceptibility. Most of the popular “polycrystalline superalloys” being used today were introduced between 1960 and 1985 (Stephens 1989), but observations confirm that alloys containing more than about 4 wt.% total content of Ti and Al are in general difficult to weld (Ikawa et al. 1974; Kelly 1990).

While investigating the weldability improvement techniques of cast nickel‐base superalloys, the present author was frustrated by the fact that it was extremely difficult to keep track of the chemical compositions of different commercial alloys to develop a mental picture of their differences, which are often very small. There are excellent publications and books on phase transformations in metals and alloys showing complex phase diagrams (Betteridge and Heslop 1974; Ross and Sims 1987; Porter and Easterling 1992). The chemical compositions are, however, always given either in rows of elements with wt.% in the brackets and separated by commas or tables with columns of elements and rows of wt.%. In order to develop a visual representation to facilitate quick recognition of the differences, the author tried different approaches. One such graph is given in Figure 2.18. Here, the chemical compositions of three nickel‐base superalloys, Nimonic 90, IN‐738LC, and René 80 from Table 2.2,

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