Overview

Improvements in the generator cycle efficiency are a universal method to achieve higher efficiencies in power generation and involve increasing the temperature of the power plant turbine generators. Inlet and exhaust temperatures of state-of-the-art gas turbines are approximately 1500°C and 600°C respectively and operate at around 50-60% cycle efficiency (see Figure 1). Operating a plant at higher temperatures (and with increasingly poorer quality fuels), poses extreme challenges to the materials used, whether in steam or gas turbine systems; in particular, creep, thermal fatigue cracking and stress corrosion cracking is of major concern.

Figure 1 Typical operating temperature range latest generation gas turbine (courtesy of SIEMENS)
Figure 1 Typical operating temperature range latest generation gas turbine (courtesy of SIEMENS)

    Developing such capability is the foundation for the OXIGEN project; through the development of materials and manufacturing method(s) to create components capable of operating at temperatures >1000°C. Oxide Dispersion Strengthened (ODS) alloys are a class of materials that offer exceptional high temperature strength, oxidation and corrosion resistance at temperatures exceeding 1,000°C (see Figure 2), along with outstanding resistance to radiation damage. Hence, these alloys are envisioned to be used in a number of future fossil energy and nuclear power applications.

    Figure 2 Temperature/strength/oxidation stability of high performance materials  (Source Plansee).
    Figure 2 Temperature/strength/oxidation stability of high performance materials  (Source Plansee).

     

    However, while the fundamental material properties are exceptionally well suited to power generation, the manufacture of components using ODS alloys are currently subject to a number of economic and technical barriers:

    • Currently available mechanical alloying processing equipment for production of ODS alloys makes alloy manufacturing time consuming and inefficient, leading to high production cost.
    • Low-volume supply chain lacks major industrial producers i.e. Special Metals and Plansee have stopped ODS alloy production.
    • Oxide particle coarsening as a result of conventional fusion (high heat input) joining techniques can lead to reduced high temperature creep strength,
    • ODS alloys are seen as difficult to repair for reasons given above,
    • Difficult to fabricate with traditional machining techniques (drilling, milling, grinding) due to their superior properties.
    • Superior creep strength in ODS alloys for very high temperature applications requires recrystallization which produces a coarse, usually high anisotropic grain structure.
    • Coarse grained ODS alloys can demonstrate significant component to component variability in creep life. Moreover, these alloys tend to be creep brittle (e.g. <1% creep elongation to failure), so there can be little warning of imminent failure using time averaging approaches, increasing the risk of unplanned downtime.