**1. Introduction**

50 Efficiency, Performance and Robustness of Gas Turbines

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The desire forever-greater efficiency and increased performance has driven the development in modern gas turbine engines. These engines require high performance materials to exhibit maximum efficiency by increasing their operating temperatures. The blades in modern aero, marine and industrial gas turbines are manufactured exclusively from nickel based superalloys and the compressor section components from titanium based alloys (Fig.1). Achieving enhanced efficiency for marine gas turbines is a major challenge as the surrounding environment is highly aggressive. This aspect depends not only on the design but also on the selection of appropriate materials for their construction. Between the two, selection of materials plays a vital role as the materials have to perform well for the designed period under severe marine environmental conditions. The marine environment makes the superalloys and / or titanium based alloys to undergo a process namely hot corrosion. Hot corrosion can be divided into two types i.e. type I which takes place between

Fig. 1. Significance of superalloys and titanium alloys in gas turbine engines

The Selection of Materials for Marine Gas Turbine Engines 53

being shown to its hot corrosion resistance. Further, it is not always possible to achieve both high temperature strength and hot corrosion resistance simultaneously because some alloying elements help to improve hot corrosion resistance while some may help to improve high temperature strength. It is rare that an alloying element leads to enhancement both in high temperature strength and the hot corrosion resistance simultaneously. This is further complicated for marine applications by the aggressivity of the environment, which includes sulphur and sodium from the fuel and various halides contained in seawater. These features are known to drastically reduce the superalloy component life and reliability by consuming the material at an unpredictably rapid rate, thereby reducing the load-carrying capacity and potentially leading to catastrophic failure of components (Fig.2) [2-4]. Thus, the hot corrosion resistance of superalloys is as important as its high temperature strength in marine gas turbine engine applications [5-8]. Recent studies have shown that the high temperature strength materials are most susceptible to hot corrosion and the surface engineering plays a key role in effectively combating the hot corrosion problem [9-13].

The selected superalloys for the investigation are presented in Table 1. It is to be noted that SU 263, SU 718, IN 738 LC and IN 792 superalloys contain no rhenium but sufficient amount of chromium. However, SU 263 contains 6% molybdenum and 20% cobalt, iron content is very high in SU 718 with 6% tungsten, 6.5% tantalum and reduced molybdenum 3%. Good amount of tantalum and cobalt 8.5% each and further reduction in molybdenum 1.75% make IN 738 LC. IN 792 contains very low content of tungsten, molybdenum, more amount of aluminium 7.6% and tantalum 5% while CMSX-4 superalloy has 3% rhenium and reduced chromium. The newly developed alloy contains 6.5% rhenium and a very small amount of chromium. The modified chemistry with 6.5% rhenium, 8.5% tantalum and 5.8% tungsten makes the new superalloy to exhibit very good high temperature strength

Superalloy Ni Cr Co W Al Ta Ti Mo Re Hf Fe Mn Si Nb

SU-263 Bal 20 20 - 0.6 1.3 2.4 6.0 - - 0.7 0.6 0.4 -

SU-718 52.5 18.5 9.0 6.0 0.5 6.5 0.9 3.0 - - 19.0 0.2 0.2 5.1

IN 738 LC Bal 16 8.5 2.6 3.4 8.5 3.4 1.75 - - - 0.2 0.3 0.9

IN 792 Bal 13.5 9.0 1.2 7.6 1.3 5.0 1.2 - 0.2 0.5 - - -

CMSX-4 Bal 6.5 9.0 6.0 5.6 6.5 1.0 0.6 3.0 0.1 - - - -

CM 247 LC Bal 8.1 9.2 8.5 5.6 3.2 0.7 0.5 - 1.4 - - - -

New alloy Bal 2.9 7.9 5.8 5.6 8.5 - - 6.5 0.1 - - - -

Table 1. The chemical composition of selected superalloys (wt%)

**2. Superalloys** 

properties [1].

800 and 9500 C and type II that takes place from 600 to 7500 C. At higher temperatures, there is no hot corrosion problem as the salt evaporates. Unlike oxidation, hot corrosion is highly detrimental. In fact, hot corrosion is a limiting factor for the life of components for marine gas turbines. Vanadium that is present in the fuel makes the marine environment further corrosive by forming low melting point chemical compounds. Therefore, selection of appropriate materials is paramount importance. An ideal construction material should be able to survive this harsh corrosive environment. Thus, in order to improve the efficiency of marine gas turbine engines significantly, either the existing materials / coatings which can exhibit very good hot corrosion resistance or the advanced materials with considerably improved properties are necessary. Efforts made in this direction made it possible to develop a new superalloy which exhibits excellent high temperature strength properties [1].

Fig. 2. Failed gas turbine blade due to type I and II hot corrosion

The majority of nickel based superalloy developmental efforts have been directed towards improving the alloy high temperature strength properties with relatively minor concern being shown to its hot corrosion resistance. Further, it is not always possible to achieve both high temperature strength and hot corrosion resistance simultaneously because some alloying elements help to improve hot corrosion resistance while some may help to improve high temperature strength. It is rare that an alloying element leads to enhancement both in high temperature strength and the hot corrosion resistance simultaneously. This is further complicated for marine applications by the aggressivity of the environment, which includes sulphur and sodium from the fuel and various halides contained in seawater. These features are known to drastically reduce the superalloy component life and reliability by consuming the material at an unpredictably rapid rate, thereby reducing the load-carrying capacity and potentially leading to catastrophic failure of components (Fig.2) [2-4]. Thus, the hot corrosion resistance of superalloys is as important as its high temperature strength in marine gas turbine engine applications [5-8]. Recent studies have shown that the high temperature strength materials are most susceptible to hot corrosion and the surface engineering plays a key role in effectively combating the hot corrosion problem [9-13].
